Local 481 GFCI Workshop Curriculum
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© 2012 Guy Holt ------- All Rights Reserved ------- May not be used or reproduced without written permission.
Introduction:
When Ground Fault protection was first introduced to our industry, there existed, and still exist, many questions surrounding the use of it on set. Questions surrounding its' use
on portable generators - such as, the system grounding requirements, and required neutral connections - as well as the relevant health and safety regulations pertaining to the use
of it on sets created a great deal of confusion and still does. For example, some people believe that an ungrounded electrical system is safe without them while others believe the
opposite. And, some people believe that Ground Fault protection devices will function regardless of the grounding arrangement of the power source, while others believe the
opposite. To compound the issue, the difference of opinion on grounding is reflected in the fact that there are two distinct types of electrical systems in manufacturers' designs for
portable power devices (ie, generators and Battverters.) And, although there are separate safety standards for grounding, generators, and Ground Fault protection, none addresses
the use of the three together and none addresses the unique problems and scenarios found in motion picture lighting. Given such widespread confusion, it was inevitable that
Ground Fault protection devices would be misapplied, resulting in unnecessary nuisance tripping and even outright failure. Consequently, the technology developed a reputation
of being overly sensitive and finicky. Unfortunately, now when Ground Fault protection is used on set and trip under legitimate Ground Fault conditions, these warnings are
quite often dismissed as yet another false alarm. Ground Fault protection devices have become the proverbial "boy who cried wolf" one too many times.
As a result of these early experiences, questions regarding the applicability of Ground Fault protection on film sets arose - so much so, that our industry sought an exemption
from the NEC requirement to use them in Article 527.6. That exemption is granted in NEC Article 590.6 Section 2 "Assured Equipment Grounding Conductor Program" or
AEGCP for short. Article 590.6 Section 2 exempts us from using Ground Fault protection if we follow a prescribed program (the AEGCP) of assuring an adequate ground fault
circuit. But, as we will see the Article 590.6 Section 2 exemption does nothing to protect us from electrical shock in many situations. Rather than remove any ambiguity, it only
created something of a moral dilemma for us. To understand the nature of this dilemma, let's start with a survey of the available technology and its' intended applications.
Ground Fault Protection:
There are three basic types of Ground Fault protection systems or leakage current protection devices: Immersion-Detection Circuit-Interrupters (IDCIs) that offer immersion
detection protection for appliances that are UL Listed in accordance with UL 1664, Ground Fault Protection Equipment (GFPE) that offer ground fault protection for equipment
that is UL Listed in accordance with UL 1053, and Ground Fault Circuit Interrupters (GFCIs) that offer Ground Fault protection for personnel and are UL Listed in accordance
with UL 943. The most important thing to understand about ground fault protection devices is that one type is to protect personnel and the other is to protect electrical
equipment, the latter having different trip levels for different types of protection.
Immersion-Detection Circuit-Interrupters:
An immersion-detection circuit-interrupter (IDCI) is a device intended to be used with electric appliances and designed to interrupt circuit to the load when an appliance is
unintentionally immersed in water. An IDCI can not replace a GFCI where a GFCI is required by the NEC and it does not take the place of the branch circuit over-current
protection device in the circuit.
Ground Fault Protection Equipment:
Ground Fault Protection Equipment (GFPE) can either consist of an equipment leakage current interrupter (ELCI) or ground fault sensing and relaying equipment. An ELCI is a
device intended to provide leakage current protection in electrical appliances and electrical utilization equipment. This device will open all ungrounded conductors of the supply
circuit to electrical equipment if the current in excess of the trip current occurs between live parts and the grounded enclosure or other grounded parts of the system.
Ground-fault sensing and relaying equipment is intended for use in power distribution systems rated at 600 volts maximum. These devices are considered to be equipment
protection devices and not personnel protection devices. They operate to cause a disconnecting means to function at a predetermined minimum value of ground fault in
accordance with the NEC. The trip ratings for these devices are usually in the 30 milliamp and higher range. Again, neither of these devices can take the place of the branch
circuit over-current protection device in the circuit.
Ground Fault Circuit Interrupters:
The basic definition of a Ground-Fault Circuit-Interrupter (GFCI) is a general-use device whose function is to interrupt the electric circuit to a load within an established period
of time (25-100 milliseconds (1/1000 of a second.)) There is a Class A GFCI that trips when a ground fault current exceeds 5 milliamps and there is a Class C GFCI that trips
when a ground fault current exceeds 20 milliamps. Another GFCI-Type device is an appliance leakage current interrupter (ALCI). It is a device intended to be used in
conjunction with an electrical appliance and is designed to interrupt the circuit when a ground fault current exceeds 6 milliamps. An ALCI can not replace a GFCI where a GFCI
is required by the NEC and it does not take the place of the branch circuit over-current protection device in the circuit. Since we're primarily concerned about protecting people
let's focus our attention on Class A Ground-Fault Circuit-Interrupters.
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In the simplest terms, a GFCI works by monitoring the current between the Hot conductor and the Neutral conductor. Normally, the difference in current between the Hot and
Neutral conductors is zero. If current leaks from either conductor, there will be a difference between the two conductors as they pass through the GFCI. When it senses a very
small difference in current between the two, typically five to six milliamps (1/1000 of an Amp), it trips by opening internal current conducting contacts in less time than it takes
to receive a harmful amount of current.
100A GFCI provides ground fault protection on wet locations
GFCIs range from simple 15A Hardware Store style in-line devices to sophisticated high Amperage (up to 800A) multiphase devices like the Film Style GFCIs pictured above by
Bender and Littlefuse that are designed specifically for motion picture lighting. Before we look at the different types of GFCIs available and how to use them, let's look at the
NEC and OSHA guidelines that require the use of GFCIs.
NEC and OSHA Guidelines for the use of GFCIs
The NEC developed and OSHA adopted a two-prong approach to affording protection to workers. Prior to 1996, NEC Article 305 and OSHA 1926.404(b) both permitted either
the use of the Assured Equipment Grounding Conductor Program (AEGCP) or the use of GFCI protection for all 125-volt, single-phase, 15- and 20-Ampere receptacle outlets
on construction sites. The concept of the AEGCP is that frequent and regular inspection and testing of all equipment grounding conductors, receptacles and attachment plugs,
will "assure" that the continuity of the Equipment Grounding Conductor (EGC) is maintained and that a low-impedance grounding path will protect workers sufficiently against
the hazards of electrical shock by facilitating the operation of over-current devices (fuses & breakers.)
In the 1996 NEC, two significant changes occurred in Section 305-6, "Ground-Fault Protection for Personnel." First, the scope of the GFCI requirements was greatly expanded
by removing the limitation to construction sites only. Prior to the 1996 NEC, the GFCI requirements for 15- and 20- ampere, 125-volt receptacle outlets, only applied to
personnel on construction sites. The 1996 NEC removed this limitation and expanded the scope of the provision to include all "temporary wiring installations utilized to supply
temporary power to equipment used by personnel during construction, remodeling, maintenance, repair, or demolition of buildings, structures, equipment, or similar activities
(namely us.)"
GFCI protection required for 15, 20, and 30 ampere, 125-volt receptacles on construction sites
The second significant change was the restrictions placed on the use of the AEGCP. Prior to the 1996 NEC, either the use of a GFCI or adherence to a AEGCP would meet the
requirements of this section. In the 1996 NEC, the use of the AEGCP was strictly limited to "other receptacles not covered in (a)." This meant that, for other than industrial
establishments, as illustrated above, all 15- and 20-ampere, 125-volt, single-phase receptacle outlets had to be provided with GFCI protection. Subsequent revisions to the code
have continued to expand the scope of GFCI protection to include 30-ampere, 125-volt, single-phase receptacle outlets. And, since fulfillment of this requirement is often
accomplished by the use of cord sets which incorporate a GFCI device, the NEC prescribes in Section 305-6(a) the type of cord-set permitted as follows: "cord sets or devices
incorporating listed ground-fault circuit interrupter protection for personnel identified for portable use."
There are two important points to this last provision relevant to our work. First, note that the cord set must utilize listed GFCI protection, not that the cord set must be listed.
This sentence structure is intentionally different from other NEC sections mandating similar requirements. That is because OSHA does permit employers to construct their own
extension cord sets. There are several conditions that must be met but the practice is acceptable. Requiring the cord sets to be listed would severely limit this provision.
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"Shop Made" devices using GFCIs intended for permanent installation are not permitted
Secondly, the cord sets must utilize GFCI protection identified for portable use. The reason for this is that such GFCIs include "Open Neutral" protection, which enhances
personnel safety where such devices are subject to the possibility of losing a Neutral connection. For this reason, it is not permissible to utilize standard GFCI receptacles (like
the one pictured below), intended for permanent installation only, as part of a "shop-made" cord set.
OSHA Construction Standards
Since the early 1970's, OSHA Construction Electrical Standards have been driven by the NEC. Current OSHA requirements parallel the 1984 edition of the NEC. In fact,
Section 1926. 404(a) Note, states, "If the electrical installation is made in accordance with the National Electrical Code ANSI/NFPA 70-1984, exclusive of Formal
Interpretations and Tentative Interim Amendments, it will be deemed to be in compliance with 1926.403 through 1926.408, except for 1926.404(b)(1) and 1926.405(a)
(2)(ii)(E), (F), (G), and (J)." Unfortunately, as we have just discussed, significant changes in the NEC have occurred which are not currently enforceable by OSHA. The
restriction on the AEGCP for example, does not exist within the OSHA regulations. The addition of the 30-ampere, 125-volt receptacle outlet is not included as well. The scope
of application for GFCI requirements for OSHA is still limited to construction sites only.
Summary
The regulatory guidelines as they stand today, create two dilemmas for us. First, there exists a discrepancy between the NEC requirements and the OSHA regulations: the NEC
restricts the use of an AEGCP and OSHA does not. Second, most of the cord connectors we use (Bates or GPC, Camlock, etc.) fall outside the confines of Section 305-6(a),
leaving us with having to make a choice between using the high amperage multi-phase GFCI equipment to be discussed, or to follow an AEGCP. From an enforcement point of
view and from a safety point of view this is troublesome. This fact, however, ought not change our strategies for protecting personnel on set from electrical shock. As we will see,
when deployed properly GFCIs provide a level of protection superior to that of the AEGCP and can be implemented in an easier and more cost-efficient manner for the
employer. OSHA regulations are a minimum safety standard for protecting workers. When it comes to safety, we should adhere to the stronger of the two standards and follow
the provisions of Article 305 that provide the highest level of personnel protection. By doing so, we can perhaps prevent something like this happening on one of our sets:
At this point I show a homemade video that demonstrates the dangers of wet work without Ground Fault protection. The
video, made by a father of his son washing a new Jeep (probably a present from his parents) in the drive of their house,
shows the young man soaping the hood of the Jeep and then reaching to turn on what looks to be a pressure washer. As soon
as he switches on the pressure washer, he receives a fatal shock and drops dead on camera.
The object of this workshop is to explore the issues surrounding the use of GFCI protection that are critical to its' proper application so that we can avoid tragedies like the one
depicted in this video. Since we routinely use portable power supplies, be it generators or Battery/Inverters, to provide power in situations that include working in, on, and
around water (to provide power on boats, beaches, and around pools, lakes and streams) not to take every precaution available to avoid tragedies like the one we just witnessed
is I think unforgiveable. Who among us wants to tell a parent that we could have saved the life of their child, but it was cheaper and easier to take the Article 590.6 Section 2
exemption (AEGCP), than to understand how to properly use and apply protective technology that could have saved their life.
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ScreenLight & Grip has developed a special production package we call the "HD Plug-n-Play Package" (HD P&P Pkg.) For our HD P&P Pkg. we have picked lights that offer
both the highest output (lumens/watt) and the most feature style production capability and combined them with proprietary distribution technology we have developed that
enhances the production capability of the new Honda Inverter Generators.
Our 60A Full Power Transformer/Distro provides 7500 Watts of power in a single 120v circuit
from the new Honda EU6500i Generator
Until now, to power HMI lights over 1.2kw or Quartz lights over 2kw required a large diesel movie generator. Movie generators are not only expensive to rent, but they come
with hidden costs that usually break the budget of independently funded HD projects. Our HD P&P Pkg. takes advantage of technological advances to power HMI lights up to
6kw or Quartz lights up to 5kw off of wall outlets or the new generation of portable Honda Inverter Generators.
Left: Distorted power waveform created by pkg. of Non-PFC HMIs on a conventional portable generator.
Right: Near perfect power waveform created by the same lights with PFC ballasts of our HD Plug-n-Play Pkg.
on our modified Honda EU6500is generator.
By eliminating the need for a tow generator in order to have feature style production capability, our HD P&P Pkg. saves you the expense of not only the generator, but also the
added expense of a rental house grip truck and truck driver required to tow it. Use this link for more details about our HD Plug-n-Play Package.
Call (781) 326-5088, or contact us at rentals@screenlightandgrip.com for more information. Or, use this link for an informative newsletter article that explains the electrical
engineering principles that make it possible for our system to oeperate bigger lights, or more smaller lights, on portable generators than has ever been possible before. This article
is cited in the 4th Edition of Harry Box's "Set Lighting Technician's Handbook" and featured on the companion website "Box Book Extras." Of the article Harry Box exclaims:
"Great work!... this is the kind of thing I think very few technician's ever get to see, and as a result many people have absolutely no idea why things stop working."
"Following the prescriptions contained in this article enables the operation of bigger lights, or more smaller lights, on portable generators than has ever been possible
before."
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How much shock can we tolerate?
Most people think that high voltage causes fatal shocks, this is not necessarily so. The amount of current flowing through the body determines the effect of a shock. A
mili-ampere (1 mA) is 1/1000th of an amp; a current of 1 mA through the body is just barely perceptible. Up to 8 mA causes mild to strong surprise. Current from 8 to 15 mA
are unpleasant, but usually the victim is able to free himself or to "let-go" of the object that is causing the shock.
Currents over 15 mA are likely to lead to "muscular freeze" which prevents the victim from letting go and often leads to death. Currents over 75 mA are almost always fatal;
much depends on the individual involved; how much muscle mass, body condition and condition of the heart. In the final analysis it doesn't take much current to kill you.
Left: Duracell ProPack 600W "Batt-Verter." Center: Honda Inverter Generators.
Right: Xantrax ProSine 20000W True Sine wave DC-to-AC Power Inverter
In the final analysis, anyone of the alternative energy sources pictured above, that we find ourselves using with greater frequency as lighting instruments become more powerful
and energy efficient, can kill you just as assuredly as the traditional Crawford Tow Plant pictured below. For this reason, it is critical that you understand how these alternative
energy systems are designed and what it takes for GFCI devices to operate reliably on them.
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A Brief History
Large Amperage Multi-phase 208-240V GFCI devices were, more or less, invented in 1996 for the production of the film "Titanic." Director Jim Cameron wanted the highest
level of reality, which meant literally hundreds of people in, around, and under the water, with hundreds of submerged practical lighting units. On top of that there was
assembled, what was to date, the largest lighting package ever used on a motion picture production consisting of 5,000 lighting units, requiring 50,000 amps of electrical power,
and over 140 miles of distribution cable.
Because "Titanic" required a combination of HMIs, incandescents, dimmers and 'specialized' lighting units, "Titanic" Gaffer, John Buckley, and Rigging Gaffer, Mike Amorelli,
realized that DC power would not accommodate all of the production's power needs. And, given the scale of "Titanic" traditional methods for handling AC around water (use of
distilled water) was insufficient. Realizing "Titanic" required a new approach to working in and around water, they turned to Bill Masten and Rick Prey who operated a company
called SMS Inc.
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Primarily known for their award-winning NiteSun products - portable generator trucks with 120 ft booms for 12k HMIs - these two had already begun work on a protoype of a
208-240V multi-phase device which did not exist at the time. When Rick Prey worked on "The Abyss" in 1988 they used the electrical equipment that was available at the time,
and as a Navy trained electrician, it scared him to death, Prey said, because, absent a Class A device, they "had protection, but not personnel protection."
Bill Masten (left) and Rick Prey (right) in front of their NiteSun truck
After working with the Academy Award winning 100A Shock Block (developed by Stephen J. Kay of the K-Tec Corporation) on several more shows involving water (including
"Crimson Tide"), Prey and Masten realized that high amperage multi-phase GFCI devices were technically feasible and were working on a prototype for such a device when
approached regarding "Titanic."
Recognizing that the magnitude of power needed for "Titanic" (50'000 Amps in all) was beyond the scope of K-Tec's 100A Shock Blocks alone, Prey and Masten began work on
developing ground fault protection devices capable of protecting circuits of 400 Amps, which did not exist at that time.
The components of the original advanced ground-fault current interrupter (GFCI)
developed by SMS Inc for production of "Titanic" included (from left to right)
a 3-phase GFCI for chainmotors, a 3-phase 400-amp GFCI service,
a 100-amp/120V GFCI, and 3-phase 200-amp GFCI.
Familiar with the Bender Corporation's efforts on the "MagLev Train", a proposed high-speed rail line between Los Angeles and Las Vegas using a Magnetic Levitation Train,
Prey and Masten thought that some of the same technology could be applied to their prototype and so they approached Marcel Tremblay at Bender with their schematics. With
the help of Bender, SMS built a total of twenty-eight 1200 Amp GFCIs, and a number of 3-Phase 100- and 200 Amp models that were used, along with the 100 Amp 120-volt
Shock Blocks from K-Tec, in the production of "Titanic." The proof of concept came when a high wind dragged a piece of heavy duty lighting equipment into the water, the 1200
Amp blocks SMS designed shut down power instantly - saving lives.
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The Bender "Lifeguard" Line of Film Style GFCIs
Using the "Titanic" production as field tests, Bender Corporation worked out further refinements to SMS's basic design for a 1200 Amp device until they had a unit that would
meet the requirements for UL listing as a personnel protection device (pictured above.) Shock Block was subsequently acquired by the electrical manufacturing giant Littlefuse
which introduced its' own UL listed high voltage multi-phase devices (pictured below.)
The Littlefuse "Shock Block" Line of Film Style GFCIs
Basic Operating Principle
A high amperage GFCI consists of a number of components. The primary component is the Differential Current Transformer (CT) which is a ring of ferrous metal (usually iron)
that the current carrying wires pass through. An imbalance in current between the conductors running through the CT because of leakage to ground generates a net Magnetic
Flux in it. The amount of Flux is an indication of the amount of current traveling back to the source on the ground. This Magnetic Flux induces a current in a Pick Up Coil. The
strength of the current reflects the degree to which the conductors are out of balance. Control circuitry compares this current to a prescribed pre-set and activates a shunting
device which opens Switching Contactors, interrupting the supply of power, when the level of imbalance between current carrying wires exceeds the prescribed level (6mA in the
case of Class A devices.)
(1) Shunting Device, (2) Grounded Neutral Detector, (3) Control Circuitry Interface, (4) Current Transformer, (5) Ground Lug, (6) Switcing Contactors.
In addition to the Hot-to-Ground sensing components above, to meet UL Standards a GFCI must also de-energize a circuit if there exists a load side Neutral-to-Ground leak commonly called a "Grounded Neutral Fault". For this purpose, GFCI devices employ a Grounded Neutral Detection Circuit. A Neutral-to-Ground Fault is typically the result of
mis-wiring (common in production trailers) or a short circuit in a load that creates a low resistance path between the Neutral and Ground wires downstream of the GFCI. Since
the Neutral conductor is also grounded at the source, such double grounding of the Neutral conductor could create a situation where, if there were a fault from Hot to Ground, a
portion of the Ground Fault Current from the Hot conductor returns to the source through the Neutral Conductor. As a consequence, the current differential showing up in the
CT would not be truly indicative of the magnitude of the ground leakage current. This has the effect of desensitizing the GFCI. But since, UL requires that GFCIs trip with a 6mA
Ground Fault even when the Neutral and Ground are connected, rather than leave the circuit energized when the GFCI in incapacitated, as a pre-emptive measure manufacturers
design their devices so that they trip as soon as power is applied to the circuit. The Grounded Neutral Detection Circuit serves this purpose.
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As illustrated above, the Grounded Neutral Detection Circuit consists of a second CT placed downstream of the current sensing CT. This second CT acts in a reverse manner to
the current sensing CT: a small drive signal is injected into this second CT via a coil and the resulting Flux induces equal voltage on the Hot and Neutral wires passing through its
core. Current does not flow in either wire because under normal conditions (non-grounded neutral) there is not complete circuit. Only when there is a fault from Neutral to
Ground (a grounded Neutral) on the load side, will there be a closed circuit between the Neutral and Ground wire that will cause current to flow to the Ground wire and back to
the Neutral via the service head bond. Where this current originated in the Drive CT, the current will return to the drive CT via the system Neutral. Because the drive CT is
downstream of the Sensing CT, the current induced on the Neutral Conductor by the drive CT, passes through the Sensing CT in route back to it's source. Because there is no
similar current induced in the Hot Wire (because the Hot-Neutral circuit is open), the Sensing CT registers an imbalance that will trip the GFCI in the same manner as the usual
Hot-to-Ground short. In this fashion, the Grounded Neutral Detection Circuit prohibits the circuit from becoming energized when there exists a Grounded Neutral Fault that
would incapacitate it. In short, rather than create a potentially hazardous situation, it stops itself from being turned on. Because it trips itself immediately before the load is even
turned on, this is quite often misinterpreted as a nuisance trip, or that the GFCI is defective, when in fact, it is doing precisely what it is meant to do. To test that the GFCI is in
fact tripping because of a Neutral-to-Ground Fault in the load, and is not simply defective, swap out the load and try to energize the circuit again. If the GFCI does not trip
immediately, then there is likely a Grounded Neutral Fault in the load. If the GFCI does trip immediately then it is defective.
In addition to the Neutral-to-Ground sensing components above, to meet UL Standards a GFCI must also include a Test Circuit so that the user can ascertain if the GFCI is
operating properly. The Test Circuit consists of a switch (the test button) that closes the test circuit. A resistor in the circuit causes current to flow through the circuit conductor
as well as limits the amount of current flowing and so is called the Limiting Resistor. The circuit conductor passes through the CT only once - on the return after passing through
the Limiting Resistor - creating a Flux in the CT. The value (Ohms) of the Limiting Resistor is such that it generates sufficient Flux to activate the Shunt to open the Switching
Contacts to interrupt power - thereby demonstrating that the GFCI is operational.
Left: the guts of a High Amperage GFCI. Right: the corresponding Block Diagram
Of course, as we shall see shortly, there is more to the sophisticated high amperage devices we use. But, before we go there, let us explore the basic operating principles of GFCIs
in more detail. Regardless of the level of sophistication, all GFCIs work on the principle of Hysteresis, or the induction of Magnetic Flux in ferrous metal by the magnetic fields
surrounding current carrying conductors. Current traveling in wires create magnetic fields around those wires. How a CT works is very much like how your clamp on Amp meter
works. When you clamp the jaws of your Amp meter around a current carrying wire, the magnetic fields surrounding that wire creates a Magnetic Flux in the meter's jaw. A
Sensing Coil in your Amp meter, likewise, generates a current that reflects the strength of the Magnetic Field surrounding the cable and hence the amount of current passing
through it.
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The essential difference between an Amp meter and CT is that to take an Amp reading you must clamp your meter's jaws around just the one conductor you want to read. If you
put it around all three conductors (Hot, Neutral, & Ground) you would get no reading because the opposing Flux created by the magnetic fields generated by the returning
current on the Neutral and Ground wires cancel out the Flux created by the magnetic field generated by the going current on the Hot wire. Both Amp Meters and the CTs of
GFCIs work on the same princple, but to different ends. Where an Amp meter reads the Flux generated by a single current carrying wire, the CT of a GFCI reads the net Flux
after the opposing Flux generated by multiple current carrying wires running through it have canceled each other out.
Now, if we don't pass the ground wire through the CT, the net Flux in the CT is an indication of the amount of current traveling back to the source on the ground. In other words,
when there is a current leak, the returning current takes two paths back to its source - the Neutral running through the CT and the Ground running outside the CT. Since current
returning through the the CT is less than the going, there is now a difference in the resulting Magnetic Flux. The net Flux induces a current in the Pick-Up Coil that triggers the
control circuitry to open the circuit when it reaches the prescribed threshold. Likewise, when there is no current leak, the current is the same coming and going through the CT,
the opposing Magnetic Flux generated by the Hot and Neutral wires then cancel one another so there is no net Flux within the Differential Current Transformer (CT) to induce
current in the Pick-up Coil.
In this same fashion, by passing the Hot Conductors and the Neutral, but not the ground, of multiphase distribution systems through a CT, more sophisticated Film Style GFCIs
like those manufactured by Bender and Littlefuse, can sense current leaks in single phase, and three phase systems up to 800 Amps per leg. But, you may be asking, wouldn't a
3-Phase GFCI device trip immediately because of the severe imbalance created by the fact that, as illustrated below, there are three Hot conductors passing through the CT for
the one Neutral.
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In fact, the opposite is true: by passing the three Hot Conductors (L1, L2, L3) and the Neutral of a 3 Phase distribution through a CT, the Magnetic Flux induced by each current
carrying wire cancels each other out just as out of phase currents do on the Neutral return of a distribution system. As you may recall, with Linear Loads (incandescent lights)
operating on a distribution system, if we draw equal current on each leg, the return current on the Neutral will be nearly zero. That is because, as illustrated below, the Electro
Magnetic Fields on the phase legs are opposed (120 degrees out of phase), and so nearly cancel one another when combined on the Neutral return. The same is true of the
Magnetic Flux induced on the CT. If there is no ground leak, the Flux induced on the CT by each Hot leg partially cancels out another, and the Flux induced by the Neutral
cancels out whatever net Flux was left over after the partial cancellation of the three hot legs, so that in the end there is no Flux on the CT. Where that is the case, a leak to
ground on any one of the Hot legs or on the Neutral, would create an imbalance and result in a net Flux on the CT that would be picked by the Sensing Coil.
The principle by which large amperage multi-phase GFCIs work - the sensing of residual Flux generated by current imbalance - is fairly straight forward when it comes to 120V
Single and Three Phase distribution systems. But, is more difficult to grasp in 208-240V systems that are absent a Neutral conductor. Since, these systems consist of Hot, Hot,
Ground conductors only, and only the two hot conductors pass through the Current Transformer, how does the GFCI sense a current imbalance? It is not necessarily the case
that the Magnetic Flux created by one cancels out the Magnetic Flux created by the other because it is likely that only two legs of a 3 phase service are passing through the CT where the two legs are not 180 degree out of phase, you would not have complete cancellation on the CT, and so the GFCI would trip all the time. No, there must be some other
explanation.
To understand how Single Phase 220V GFCIs (like the one pictured above) work, think back to the Open Neutral display you may have seen at one of our IATSE Local 481
Electrical Training Seminars. If you recall it consists of a board with three rows of lighting bulbs - each row consisting of ten 100W light bulbs. Each row of bulbs is powered by
a different leg of the service (L1, L2, L3), they all share in the Neutral. For the first part of the demonstration the Neutral is lifted. Rather than going out, the bulbs all stay on
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because the current of one leg (L1) returns on another leg (L2). In other words, absent a Neutral the current on L1 returns on L2 and visa versa the current on L2 returns on L1.
It doesn't matter that they are not 180 degrees out of phase because the return current of L1 cancels the going current of L1 and likewise for L2 since they both pass through the
CT.
___________________________________________________________________________________________________
(Start Advertisement)
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Left: Distorted power waveform created by pkg. of Non-PFC HMIs on a conventional portable generator.
Right: Near perfect power waveform created by the same lights with PFC ballasts of our HD Plug-n-Play Pkg.
on our modified Honda EU6500is generator.
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"Great work!... this is the kind of thing I think very few technician's ever get to see, and as a result many people have absolutely no idea why things stop working."
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"Following the prescriptions contained in this article enables the operation of bigger lights, or more smaller lights, on portable generators than has ever been possible
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(End Advertisement)
___________________________________________________________________________________________________
UL943
The reputation that GFCIs have for unreliability is in part their own fault. The first generation of GFCIs (before the Bender Lifeguards and Littlefuse Shock Blocks) were prone
to nuisance tripping when powering motion picture lighting loads. Voltage surges and sags, transient notches and spikes, as well as the current harmonics caused by SRC
dimmers, and magnetic and electronic ballasts caused the first GFCIs to trip without a "hasardous" current leak. To address these and other concerns surrounding the first
generation of GFCIs, Underwriter's Laboratories revised their requirements for listing of GFCIs.
In effect, UL required manufacturers to make a better GFCI by mandating in UL943 specific improvements that must be implemented into any new GFCI manufactured as of
January 1st, 2003 in order to receive a UL listing. These changes included the prevention of line-load mis-wiring, more stringent voltage surge tests, more resistance to moisture
and corrosion and better resistance to line noise and false tripping. The means by which Bender and Littlefuse addressed the last two requirements - better resistance to line
noise and false tripping - greatly improved their reliability and are worth exploring in more detail.
One feature they incorporated to reduce nuisance tripping was to design their GFCIs to trip on an "Inverse Time Curve." An inverse time curve can be mathematically expressed
as I squared T where "I" is current and "T" is the time it takes to trip. Since this is a logarithmic equation, as you can see for the plot of in the typical response time of a GFCI
below, the plot of I verses T does not follow a straight line but introduces a delay that decreases as the magnitude of the current increases (as we will see shortly, even people
have I vs. T curves.)
In other words, Littlefuse and Bender purposely introduced a delay in the tripping action of their GFCIs so that it takes longer at lower leakage current levels and less time at
higher current leakage levels. By designing a longer response time at lower leakage current levels, the inverse time characteristic of their GFCIs greatly reduced the nuisance
tripping from transient voltage surges and sags, notches and spikes.
Another feature they incorporated to meet the requirements of UL943 was filtration of high frequency harmonic currents. Harmonic filters are required in GFCIs used in motion
picture lighting because a lot of the older HMI and Kino Flo lights we use in our industry use non-Power Factor Corrected Switch Mode Power Supplies (ballasts) with PFs
ranging from .54 -.65 that dump a lot of high frequency harmonics back into the power distribution system (see graph below.) On top of that, we sometimes use these lights on
high impedance power generators. The combined effect of the harmonic currents generated by the power supplies interacting with the high impedance of the generator leads to
Voltage Waveform Distortions like that pictured below.
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The voltage distortion seen above is created by the harmonic currents, or "noise", thrown back into the distribution system by non-Power Factor Correct HMI & Kino ballasts.
Artifacts of this noise, that is evident here and worth noting, is the square shape of the distorted voltage waveform and the zigzag saw tooth pattern as voltage ascends from zero
potential and descends to zero potential. What accounts for this shape is the fact that whenever current encounters an impedance it creates a voltage at the same frequency. The
flat top and zig zag to the distorted voltage waveform above is created by many many high frequency harmonic currents in the distribution system stacking one on top of another.
Sprectrum analysis of the high frequency Harmonic Currents that create a Square Wave
For example, a 5th harmonic current produces a 5th harmonic voltage, a 7th harmonic current produces a 7th harmonic voltage, etc. Because non-PFC ballasts return the
hundreds of harmonic currents depicted above, in addition to the fundemental, back into the distribution system, each of these currents flowing into an impedance results in
stacked voltage harmonics appearing at the load bus, a voltage drop, and distortion of the voltage waveform as seen above.
Harmonics making up a Square Wave.
As illustrated above, if we were to break out the components contributing to the generation of the square-wave we see on our scope it would be comprised of more than just the
three orders of current harmonics depicted in the example above. We would find that it is generated by the many high frequency harmonic currents charted below, each inducing
a voltage spike of its' own, stacking one on top of another. The accumulative effect of all these induced spikes in voltage stacking one on top of the other is the square wave with
zig zag pattern as voltage ascends and descends from the zero crossover point that we see in the oscilloscope shot above.
Of the harmonic currents that electronic ballasts generate, the odd harmonics (i.e. 3rd, 5th, 7th, 9th, etc.) are more of a concern because the even harmonics have a tendency to
still cancel out. Of these the 3rd harmonic, and odd multiples of the 3rd (9th, 15th, etc) are particularly troublesome. These harmonics are called the "Triplens." What makes
them troublesome is that the triplen harmonics dumped back onto each phase of the distribution system are all in phase with each other. For this reason, rather than cancel each
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other out on the neutral conductor, as the out of phase fundamentals normally do, they instead add up. By generating harmonic currents that stack one upon another, and
shifting the phase of the primary currents so that they don't entirely cancel, electronic square wave ballasts can create unusually high returns on the neutral of the distribution
system (see illustration below.)
The Triplen Harmonics Stack to create excessive current on the Nuetral of a distro system
One might think that the higher return on the Neutral that is characteristic of distribution systems with harmonic generating loads would cause an imbalance in the Magnetic Flux
induced in the CT - but that is not the case. Even though, as illustrated below, the Switch Mode Power Supplies of HMIs and Kino Flos pull current out of phase of voltage, and
generate Harmonics that stack on the Neutral, so that upwards of 130 percent of the average of the hot legs will return on the Neutral, because it all (the entire mess) passes
through the CT it all balances out for a zero net Flux on the CT if there is no leakage to ground. No, the problem harmonic currents create for GFCIs (that lead to unnecessary
nuisance tripping) is caused by "Capacitive Coupling" and induction between current carrying wires and the equipment grounding wire in what is called "Proximity Effect."
Before UL943, a common cause for nuisance tripping in GFCI was because manufacturers of many electronic devices capacitively couple high frequency harmonic currents to ground to reduce the
amount of RF signals emitted into the atmosphere. Harmonic currents capacitively coupled to ground do not return to the GFCI via the Neutral and so cause the GFCI to see a difference between the
current leaving on the "Hot" line and the current returning on the Neutral line. Because the trip level and trip time of Class A GFCIs is very low, such high frequency currents traveling on the equipment
grounding wire tripped early GFCIs.
Another cause of nuisance tripping before UL943 was caused by the mutual inductance of conductors arranged closely parallel to one another in what is called "Proximity Effect." At fundamental
frequencies, Proximity Effects are usually negligible, but can increase significantly with current frequency. The reason for this is because system capacitance has an impedance defined by 1/(2 x pi x f x
C). What this means in practical terms is that the high frequency harmonic currents generated by dirty loads reduces the impedance of the system, which results in an increase of current leakage due to
Proximity Effect. As a result, leakage of high frequency harmonic currents (in the 20 Khz range) may be 300 times larger than 60 Hz values.
The graphs above illustrate the difference of varying harmonic numbers on Proximity Effect (and an analogous problem "Skin Effect") for 12 AWG and 4/0 cable (the cable spacing used to obtain the
Proximity Effect values is based on National Electric Code (NEC) insulation type THHN.) Comparing the graphs, it is immediately apparent that Proximity Effect can be significant in both small and
large cables. Within the range of the Triplen Harmonics alone (i.e. 3rd, 9th, 15th), Proximity Effect increases by 60% in 12 AWG Cable. In a large distribution system operating a number of dirty loads,
the difference between Hot and Neutral currents as a result of high frequency current leakage due to induction, when combined with the Capacitive Coupling of equipment, can exceed the allowed
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threshold and cause the GFCI to trip. This tripping is considered a nuisance because the current not returning via the Neutral is seen as a problem by the GFCI when in reality both the coupled and
induced currents return to its source in a safe and controlled fashion via the equipment grounding wire.
The red trace on the Frequency vs. Trip Current plot above shows the trip level as a function of frequency for a first generation GFCI. Recognizing that such nuisance tripping as a result of Capacitive
Coupling and induction at high frequencies is not a hazard, in UL943 Underwriter's Laboratories allowed for higher maximum leakage current levels for higher current frequencies (the blue trace
above.) The higher maximum leakage current levels allowed by UL943, allowed GFCI manufacturers to develop devices with Frequency vs. Trip Current curves approximating the UL guidelines. But,
to allow for the varying conditions that a GFCI must operate under, the goal of manufacturers was to design a device with a safety margin between its actual response and the UL maximum level
allowed. This ideal response is represented by the green curve on the Frequency vs. Trip Current plot above.
To deal with the harmonics that non-PFC Kino & HMI ballasts kick back into the power stream, and achieve something approximating the desired Frequency vs. Trip Current Curve above, Bender and
Littlefuse GFCIs incorporate harmonic filters with frequency responses up to 120 Hz into their GFCIs. The 3rd harmonics (the first bar on the graph above) are attenuated by 50%, and by 500 Hz are
down to 20%. Attenuated by the filter, the effective trip level of the harmonic currents generated by dirty loads is substantively increased thereby reducing the incidence of nuisance tripping.
But is that necessarily a good thing. Don't leaks of high frequency harmonic currents pose a potential hazard? After all, they can be 300 times larger than 60 Hz values? No, because the high leakage
current at high frequency is the result of system capacitance. The human body is a resistive load and as a result the shock current is a function of voltage and resistance only. Since the voltage is not
increasing in this case, there is no increase in the hazard. A GFCI with harmonic filtering and an inverse-time trip characteristic can reduce nuisance tripping substantially.
Other Features
Film Style GFCIs also incorporate diagnostic features necessary for trouble free ground fault protection on movie sets. To help you avoid unnecessary tripping, both the Littlefuse and Bender Film Style
GFCIs feature a chain of LED lights that display the level of leakage at any given moment. This feature is beneficial in a number of ways. Since almost all electrical devices leak some current, and these
small amounts of leakage can add up to a trip level, it is a good idea to test electrical equipment for leakage current before being used. This includes stingers, lights, cables, or any other equipment that is
to be used on set. By monitoring the LEDs while plugging and unplugging equipment, an electrician can discover the amount of current leakage due to a particular load. This way, if a particular load is
leaking badly it can be eliminated before the set-up.
The LED displays on will also tell the lighting technician when the total leakage is approaching the maximum allowed by the GFCI. This way the technician can avoid overloading the GFCI and causing
an unnecessary trip that will delay production. Finally, many GFCIs also monitor ground presence and power phasing. If either of these is not correct, the unit does not reset and identifies the problem
on its display.
100A GFCI provides ground fault protection on wet locations
A GFCI will not prevent a person who is part of a Ground Fault Circuit from receiving a shock, but it will open the circuit so quickly that the shock will be below levels which will inhibit breathing or
heart action, or the ability to "let-go" of the circuit. A GFCI will not protect against short circuits or overloads. The circuit breaker or circuit protector in the control panel, which supplies power to the
circuit, provides that protection. A GFCI is not a substitute for grounding or over-current protection - it should be considered a supplemental part of the circuit. But, if employed properly on set, Shock
Blocks can provide an unprecedented level of ground fault protection. Let's now look at how to properly deploy GFCIs on wet locations.
Practical Demonstrations:
We now move to the first of a series of demonstrations that I will use to demonstrate the inter-relationship of Neutral Bonding, Grounding, Ground Faults, and GFCIs. Before I explain the first
demonstration we should make note of the demonstration apparatus.
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Note: workshop demonstrations on grounding, Ground Faults, and Ground Fault Protection will be conducted using various set-ups consisting
of various portable power sources (generators and Battverters), grounding electrodes, GFCI pigtail, load, "Grounding Unit" and "Ground
Fault Simulator." Here are more detailed descriptions of the items used in the demonstrations:
The Load:
The load will consist of a 150W Fresnel light and it will be plugged into a Patch Box (pictured below) that will allow us to selectively leak
current to the Ground and Neutral conductors.
Patch Box:
Patching between Hot, Neutral, Equipment Ground, Earth Ground, and the Load will be acoomplished via two patch boxes. Each patch box
will have Banana Plug receptacles for Hot, Neutral, and Ground.
Fault Simulator
We will use a "Fault Simulator" to simulate current leakage to Ground. The Fault Simulator consists of a 2 Amp PEC 10-kOhm adjustable
rheostat and four 15-kOhm Ohmite resistors that can be added in series, as necessary, to give us a 10 kOhm range of variable resistance
anywhere between 0 Ohms and 70kOhms.
Switch Box:
We will use a Switch Box to switch "On" and "Off" Fault Currents and Grounding Connections.
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GFCI:
We will demonstrate grounding, Ground Faults, and Ground Fault Protection with two representative types of GFCIs:
1) Primelight 3-ft. 12/3 SJTW power cord with 15 Amps (1,875 Watts) inline GFCI with test and reset buttons. Triple tap
cord end has a power indicator light.
2) Shock Block SB100 100A/120V GFCI with 6mAmp trip level.
Meters:
We will use two different types of meters to mesaure Fault Currents:
1) In order to accomplish extremely accurate measurements of ground leakage in mAmps, we will use an in-line approach
and the Fluke 1587 Insulation Multimeter. In this type of measurement, you break the circuit and put the meter "in-line." As
illustrated below, the meter becomes part of the circuit
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2) To obtain less precise, but still extremely accurate, measurements of ground leakage in situations where we can't put a
meter "in-line" we will use the Megger DCM300E Clamp Style Ground Leakage meter pictured below. Similar in style to
your Amp Probe, this specialized meter is capable of detecting very small current differentials and so will give fairly
accurate measurements of leakage currents on ground wires.
Power Sources:
Five representative power sources will be used in the demonstrations:
1) Crawford Studio Generator
2) Honda EU6500is Generator
3) Duracell HD600 "Battverter" Power Pack
4) DCM Battery Pack with Xantrex ProWatt 2000 Inverter
5) Hammond 7.5 KVA 240V-to-120V Step-down Transformer with 60A bates receptacle
Left: Duracell ProPack 600W "Batt-Verter." Center: Honda Inverter Generators.
Right: Xantrax ProSine 20000W True Sine wave DC-to-AC Power Inverter
Grounding:
Grounding is established via five 5-foot copper ground rods. Three of the ground rods are driven at 26 feet apart in a triangular
configuration. Two of the three points of the triangle are connected by 2 AWG wire. The fourth ground rod is driven just a few feet from the
triangle point not wired to the other two. The fifth ground rod is driven a few feet from the fourth and connected to the buidling ground by 2
AWG wire.
Grounding Unit:
The "Grounding Unit" consists of 2-kOHm IRC variable resister set to give 1.2 kOhm resistance. The resister can be switched on and off by
means of a toggle switch. Each wire lead of the Grounding Unit is connected to a battery clamp.
Bench Demonstration:
To test our instruments and demonstrate how they operate we first conduct a "bench test." We first plug the Primelight GFCI into a quad box
powered from the house service. We then plug one of our Patch Boxes into the Primelight GFCI and our 150W Fresnel into the Patchbox. To
create a Ground Fault Circuit, as illustrated below, we attach a jumper cable from the hot pocket of the Patch Box to the input of our Fault
Simulator. We attach a second jumper from the output to the Fluke 1587 Insulation Multimeter and then a third jumper cable from the Fluke
1587 to the ground pocket of the Patch Box.
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With the rheostat on the Fault Simulator set for maximum resistance we get a reading of 4.9mAmps on the Fluke 1587. To test the accuracy of
the Megger DCM300E Clamp Style Ground Leakage meter, we clamp it around the jumper going from the Patch Box hot pocket to the Fault
Simulator and it reads the same. To test the operability of our Primelight GFCI, we use the rheostat on the Fault Simulator to gradually
reduce the resistance in the Ground Fault Circuit we have established. As we reduce the resistance in the circuit, the leakage current begins to
rise. When the leakage current reaches approximately 5.14mAmps, the Primelight GFCI trips - interrupting power to the 150W Fresnel. Our
equipment works.
Demonstration #1: GFCI Effectiveness
The first demonstration set up consists of an ungrounded Honda EU6500is generator on a wooden pallet with the Primelight 3ft power cord GFCI powering a load consisting of the 150 Watt light. A
Fault Simulator is connected between the Hot wire at the load and the Equipment Grounding wire via a Patch Box at the load. A second Fault Simulator is connected between the Hot wire at the load
and a grounding electrode via the patch box at the load. Both are set for maximum resistance so that they do not leak much current. We test that the GFCI is operational by pressing the test button with
the generator running - we receive a positive test.
For the first part of the demonstration, we simulate a Fault to the Equipment Grounding wire by leaking current to the grounding wire by gradually reducing the resistance of the Fault Simulator (by
turning the variable resistance rheostat) that is connected between the Hot wire and the Grounding Wire. The GFCI does not trip - not even when the Ground Fault Simulator reaches minimum
resistance.
For the second part of the demonstration, we then simulate a Fault to Earth Ground by leaking current to the ground rod by gradually reducing the resistance of the Fault Simulator that is connected
between the Hot wire and the ground rod. The GFCI does not trip - not even when the Fault Simulator reaches minimum resistance.
That the GFCI does not trip when we have two clear Faults (one to equipment ground the other to earth ground) raises a number of questions. We know that it is not defective because it worked during
the bench test, and it tested positive on the generator. The distribution system provides an equipment ground and the generator is grounded - so why isn't it working? Where the potential consequence of
a GFCI failing to trip, after testing positive in the field, can be deadly, we must be able to answer this question in order to provide reliable ground fault protection on set. To understand what is happening
(or not happening in this case), we must first thoroughly understand some basic electrical concepts - only then will we know how to deploy GFCIs so that they provide reliable ground fault protection.
Review of Basic Concepts
To provide ground fault protection requires a basic understanding of the principles of electricity and electrical distribution, and of the parlance of the electrical trade. For example, the word "Ground" is
used in four totally different ways by electricians.
1) Equipment Grounding: The U-shaped prong on an Edison plug is for the equipment grounding wire. Grounding wires are not meant to carry current under normal circumstances.
They carry current only when there is a Fault inside a piece of equipment causing the metal housing to become energized.
2) Grounded Neutral: The Neutral wire is sometimes called a "Grounded Neutral." The reason for this will become clear in a moment. Grounded Neutral wires are not to be confused
with grounding wires.
3) System Grounding: The Neutral buss of an electrical service is grounded to the earth by use of a grounding electrode (ground rod) sunk into the earth. The grounding electrode
conductor is the wire that makes this connection.
4) Ground Fault: The unintentional grounding that occurs when a live conductor accidentally comes into contact with the grounding system or earth. Contact with the system
grounding wire (Neutral) usually creates arcing and is extremely destructive. When a Ground Fault of this type occurs in a grounded system, over-current devices (fuse or circuit
breaker) will activate to stop the flow of current in the distribution system.
Since the nature of electricity is not easy to grasp, analogies can be helpful in understanding the basic concepts in their simplest form.
Current, Voltage, Resistance
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To understand the characteristics of electricity - Current, Voltage, Resistance - it helps to compare it to water. Like water, electricity flows. This flow is called Current, and is measured in Amperes. Like
water pumped up a hill, it will always flow back to its' source. That is, electricity always travels in a circuit or a loop. When the circuit is broken, so is the flow of electricity. Like water pressure,
electricity requires power to push it through a circuit. The power needed to push the flow of electricity through the circuit is Voltage. The flow of Electricity encounters friction in the form of electrical
Resistance measured in Ohms. Like plumbing, the bigger the diameter of the electrical pipe (wire), the smaller the resistance. The smaller the diameter of the electrical pipe (wire), the higher the
resistance. The higher the resistance, the bigger the pressure or Voltage needed to push the same Current the same distence.
Like water, when electrical current is given alternate paths to travel, most, but not all, of the Current will follow the path of least Resistance. The plumbing of electrical circuits, are conductors. Most
metals have low resistance to electricity and make excellent conductors. The minerals in water also conduct electricity making water a good conductor. Human bodies, made of 90% water, are good
conductors, especially when the skin is wet. Since, electricity only shows a preference to flow through the conductor with the least resistance, it will also simultaneously flow through conductors of
greater resistance, making it possible for human bodies to become part of an electrical circuit even when there is a low impedance equipment grounding wire present.
Circuits
Electrical current travels from a power source (such as a generator), passes through a load (such as a light), and returns to the power source to complete a circuit. If there is not a complete circuit - a
switch is open or an "Air Gap" exists from a missing link or connection - current does not flow anywhere in the circuit. The part of the circuit going out from the power source to the load is called the
Hot or "positive" side. The part of the circuit returning from the load to the power source is called the Neutral or "negative" side. Ideally, the current going out form the power source (Hot) should be
equal to the current returning (Neutral.) For obvious reasons, it is preferable that current travels only through electrical wire or other suitable conductors, not people.
Grounding
A source of confusion about grounding is the fact that there are two types of grounding in electrical systems: Systems Grounding and Equipment Grounding. Let's explore each in more detail
Systems Grounding: is accomplished by attaching one current carrying conductor of an electrical system to Ground at the source of power. Henceforth, this wire is called the
common leg, or Grounded Neutral, or simply the Neutral. The Neutral is a current carrying circuit conductor, not an equipment ground, and if it is earth grounded to stablize the
potential of the hot legs, it should be grounded at the source and nowhere else. It might seem odd to bond the equipment grounding wires to the Neutral, because the Neutral carries
current. It might seem like this would energize the entire equipment grounding system. As we shall see below, it does quite the opposite.
Equipment Grounding: is accomplished by attaching all of the non-current carrying metal parts of a system together and connecting them to the Systems Ground or Neutral at the
source of power (a Bonded Neutral system). An equipment ground is critical to operating electrical distribution systems (circuits) safely because current always return to it's source
one way or another. Under normal conditions a circuit is completed by electricity going from the source to the load (through the positive side) and back to the power source (through
the negative side). But, in the event of a leak, the leaked current must still return to its source - the generator windings - so it will take whatever means are available. Since, for safety
reasons, it is always preferable to have current travel on prescribed circuits, we establish the equipment ground so that current has a low impedance path back to its' source that it will
travel instead of returning to its' source via the Earth. It is when current goes to the Earth that there is the potential for someone receiving a shock. The Equipment Ground is then a
safety loop that works in conjunction with over-current protection to protect people against shock from a faulty piece of equipment that has developed a short (contact with housing)
by offering an better alternate path for current to return to its' source. If no grounding wire were connected, anyone who touched the fixture, would (as illustrated above) offer current
an alternate path for it to complete the circuit by traveling through their body and the ground, back to its' source. But, in order for the Equipment Ground Wire to offer an alternate
path for the current, it must be connected to the Neutral.
Faults
An electrical fault can be compared to a leak in a water pipe. A fault occurs when current partially leaks out of the intended path or circuit, using another path to return to the power source. As a
result the current flow at the hot side is not the same as the current flow at the neutral side. Almost all electrical equipment leaks some current, this includes stingers, lights, feeder cables, and
ballasts. Even dust that has settled inside a light housing can be a conductor and cause a current leak. If the leak is large enough it can cause a Fault. Common causes of Faults are frayed or
nicked electrical cords, deteriorated insulation in lights and ballasts, or moisture.
There are two types of Faults:
1) Current leaking from Hot to Neutral: when current leaks to Neutral, and there is no effective resistance, it produces a short circuit. In such a case, current rapidly
approaches infinity causing circuit breakers or fuses in the circuit to trip or blow, de-energizing the circuit before any damage can be done to the distribution equipment.
However, if there is resistance between Hot and Neutral resulting from, for example, current having to travel through earth as it goes from Hot to Neutral, the current may not
be sufficient to trigger overcurrent devices resulting in a potentially hazardous condition. Called a double fault because it consists of a Fault in the Hot conductor and a second
Fault in the Neutral conductor, such situations are typically caused by a combination of loose or non-watertight connections, nicks in cable, or a break-down in generator
insulation, to name just a few possibilities,
2) Current leaking to Ground: in a grounded system the current leak completes the circuit by going back to the generator's winding through the Ground (either an equipment
grounding conductor or the earth.) Such situations are typically caused by defective wiring in lamp housing where the Hot or Neutral conductor makes contact with the lamp
housing. Ground Faults, however, can also be caused by water or even excessive dust in the lamp housing creating a conductor from the lamp leads to the housing.
Absent effective equipment grounding, both types of Faults can be deadly. In the absence of an effective equipment grounding conductor, electrocution can occur when a technician touches a
faulty piece of equipment that is energized by a current leak and the current completes the circuit back to its' source using the individual and the earth as a conductor, and not the equipment
grounding system. A prime example is an aluminum ladder contacting an overhead power line - the fault returns the current to its' source using the ladder, the worker, and the ground as its'
conductor to complete the circuit.
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With a grounding wire connected to the housing, electricity seeks the path of least resistance, and the bulk of electricity completes the path back to its' source (completing the Ground Fault
Circuit) through the grounding wire instead of the individual. If the Neutral of the system is also bonded to the generator frame (a Bonded Neutral system), when a Ground Fault occurs, it can
create an over-current situation, which may trip the breaker and remove the Ground Fault from the circuit.
One way to think of equipment grounding is that it is the intentional connecting of all metal parts of a system together through a ground wire so that all exposed conducting surfaces have the same
potential. That way if someone touches any two metal surfaces they will not receive a shock because they will not experience any difference in potential. If the equipment grounding circuit is
intentionally isolated from the earth, by putting the generator on rubber tires, making sure tow chains do not touch the ground, and by not driving a grounding electrode (ground rod), the system is
said to have a "Floating Ground" system. Floating Ground Systems are relatively safe. They are safe because if a Ground Fault occurs, and there is another path back to its source (the Equipment
Ground), the Fault Current will not go to Earth Ground to complete the circuit. Someone touching an energized lamp housing therefore will not receive a shock. The safety of Floating Ground
Systems is only relative because if the system becomes grounded by having a second Fault in the circuit (a double fault situation), an indidividual touching an energized lamp housing will receive a
shock because the second Fault makes it possible for them to become an alternate route for the current to return to its' source - the generator windings.
Power Source Types:
Manufacturers of portable power sources take two different approaches to guarding against electrocution. In the case of generators, some manufacturers believe it is safest not to bond the
generator windings (the common or Neutral) to the frame (Grounding Conductor.) Other manufacturers bond the generator windings to the frame. The same goes for manufacturers of power
inverters for batteries: some believe it is safest not to bond the common or Neutral conductor to the Grounding Conductor, others do. Let's look at each approach in more detail as it pertains to
generators. In the first, the Neutral system (the common conductor) is not bonded to the frame of the generator or to the earth ground lead. Rubber mounts are used to isolate the generator
winding from the frame and the equipment grounding conductors. As a result, there is no specific Hot leg and Neutral leg wiring arrangement for the generator winding connection to the
receptacle. As a consequence, both legs on the receptacle are potentially Hot legs. For this reason, generators that do not bond the Neutral circuit to the frame are commonly called "Floating
Neutral" generators. The Floating Neutral configuration is common for applications such as connection to a recreational vehicle and connection to home power where the transfer switch does not
switch out the Neutral to Ground connection.
The other type of portable generator has the Neutral bonded to the frame of the generator (as illustrated above and so is called a "Bonded Neutral" generator. Since Bonded Neutral generators
offer a high degree of protection against ground faults (if there was a fault to the frame, the generator's circuit breaker would trip eliminating the fault) whether they require an earth ground is up
to the AHJ (Authority Having Jurisdiction.) The AHJ, depending on where the work is taking place, may be the local city electrical inspector, the fire marshal, or the studio's safety officer. The
AHJ is the ultimate authority for what practices will be allowed on set.
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As lighting instruments become more powerful and energy efficient, alternative energy sources, like the Kino Flo battery powered DC-to-AC Inverter System pictured above, can be an effective
means of powering a limited number of lights in situations, like car rigs, where a generator is not practical. Inverters come in a variety of shapes and sizes - some are Neutral bonded and some are
not. The following in a brief summary of the variety of electrical systems (Floating Neutrals vs. Bonded Neutrals) and grounding preferences (Floating Grounds vs Earth Grounds) that you are
likely to come across. To assure personnel safety, it is critical that you understand how each system is designed and what it takes for GFCI devices to operate reliably on them.
Movie Units: Typically used as a "Separately Derived System," Movie
Units will have bonded Neutrals unless the bonding jumper was lifted
during servicing and not reconnected. The purpose and objective of
bonding separately derived systems such as generators is to insure the
low impedance path necessary to clear phase-to-ground faults. Failure
to provide a low impedance ground-fault path (no neutral-to-ground
bond) for the separately derived system can create a condition where a
phase-to-ground fault cannot be removed. The result is that all metal
parts of the electrical system will remain energized with dangerous line
voltage if a phase-to-ground fault occurs.
Industrial Generators: Because of increasingly stringent municipal
sound ordinances, industrial generators have gotten quieter and so can
be an economical source of set power under certain situations. You
have to be cautious with industrial generators because the Neutral bond
may have been lifted on the previous job if it served as stand-by power
for a building with a transfer switch that does not switch the Neutral.
According to the NEC, systems (generators, UPS systems, etc.) where a
grounded (Neutral) conductor is not switched within a transfer switch is
not considered a separately derived system. Under this condition,
according to the NEC, a neutral-to-ground connection shall not be
made at the new system.
To check if the generator winding is grounded to the generator frame
and equipment grounding conductor place an Ohmmeter across the
Ground and Neutral pin of a 120-volt receptacle while the generator is
not running. The Ohmmeter will indicate open if the Neutral is not
grounded.
Inverter Generators: Most of Honda's generator product line do not
bond Neutral to Ground, that is because they are designed to serve as
standby power for homes and recreational vehicles. As long as these
generators are under 5500 watts, the circuit conductors are insulated
from the generator frame, and all other grounded surfaces (a Floating
Neutral), they are exempted from the National Electric Code (NEC)
Section 305-6 requiring 125 volt 15-, 20-, and 30- Ampere receptacles
to have GFCI protection. The reason they are exempted is because they
cannot serve in this capacity and have a Bonded Neutral and GFCIs
(see below for details.)
Industrial AVR Generators So that they can provide a generator that
will pass NEC and OSHA job site inspections, manufacturers like
Honda provide special industrial generator lines that bond the Neutral
to the Equipment Grounding System and use GFCI receptacles.
However, don't assume that all industrial style open frame generators
have bonded Neutrals. If the generator has GFCI protected outlets on it,
the Neutral is most likely bonded to the Equipment Grounding
Conductor, unless of course it was lifted so that the generator could
serve as standby power for a building or RV. To be safe, check all
industrial AVR generators with an Ohmmeter before using. The EB
generators are Honda's "Industrial Generators." The EB3800, EB5000,
and EB6500 generators are Neutral bonded and GFCI protected to
meet OSHA jobsite regulations.
DC-to-AC Inverters: As lighting instruments become more powerful
and energy efficient, alternative energy sources like battery powered
DC-to-AC Inverters can be an effective means of powering lights in
special situations. Inverters come in a variety of shapes and sizes - some
are Neutral bonded and some are not. The Xantrex "PROwatt" and
"PROsine" Sine Wave Inverters predominantly used in motion picture
lighting applications are Neutral bonded and equipped with GFCIs.
They can provide nearly unlimited true sine wave power from Deep
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Cycle batteries for car rigs when the battery/inverter system is tied into
the vehicle's alternator as was done on the feature "Shuttle" (see
pictures below.)
Battverters: A "Battverter" system consists of a 12V DC power
source, a 12V DC-to-120V AC Power Inverter, and a Battery Charger
encapsulated in a compact housing for easy handling. Battverters can
work great with highly efficient light sources like LEDs and
Fluorescents, but offer limited capacity and run time. To maximize their
running time attach a "jumper cable" between it and the leads of a car's
alternator. That way you can use the car as a generator to run the lights
during set up and rehearsals. When it comes time to shoot, shut off the
engine and continue to run the lights on the silent Battverter alone.
Running the vehicle engine between takes charges the batteries so that
they will run longer.
Given the variety of electrical systems (Floating Neutrals vs. Bonded Neutrals) and grounding preferences (Floating Grounds vs Earth Grounds), and what it takes for GFCI
devices to operate reliably on them, it is no wonder that GFCIs sometimes trip unnecessarily or fail to trip outright. It is not the fault of the GFCI, but how it was used.
A custom Battery-Inverter system was used to power four 4'-4 Bank Kino Flos on the interior & exterior
of an airport shuttle bus for the feature "Shuttle"
Analysis of Demonstration #1
Based on what we now know, how do we account for the fact that the GFCI did not trip when there were clear Ground Faults? The GFCI did not trip because there was, in fact,
no current leaking through the Ground Faults. Where this generator (a Honda EU6500is) is designed to primarily serve as home standby power it does not bond Neutral to
Ground. Where that is the case, Fault Current will not go to the Equipment Ground wire, even where there is a fault, because absent a Neutral-to-Ground Bond there is not a
complete circuit for Fault Current to travel back to the generator's windings. Likewise, absent a Neutral-to-Ground Bond, earth ground does not offer a complete circuit back to
the generator's windings either, so current will not go to earth ground even though the generator is grounded with a ground rod. Current will simply not go to Ground (either the
Equipment Ground or Earth Ground) if it does not offer a complete circuit back to the generator's windings.
Birds sitting on a power line are not electrocuted for this same reason. Even though their bodies receive voltage, current does not flow through them, since another part of their
body does not complete a Ground Fault Circuit. Since Fault Current will not go to earth or the Equipment Ground wire, no imbalance is created, and so the GFCI does not trip.
To demonstrate this to be the case, we meter (with the Megger DCM300E Clamp Style Ground Leakage meter) first the jumper we established between the Hot and the
Equipment Grounding wire, and then the jumper we established between the Hot and the ground rod, and find that there is only 1.15mAmps. As we gradually reduce the
resistance on the Ground Fault Simulator, no change on the meter is observed. In effect there is no current passing to either Ground even though both Fault Simulators are set for
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minimal resistance.
That we have no current traveling on the wire connected to the ground rod dispels a commonly held belief that "electricity wants to go to the ground." This common fallacy is
found in electrical safety training videos and even books published about electrical wiring. The basis of this belief - called the "Sump Theory of Ground" - is that the earth is
some kind of giant reservoir of electro-magnetic charge from which all electricity comes and to which it must ultimately return. As is evident in this demonstration, nothing could
be further from the truth.
If there were an ounce of truth to the Sump Theory of Ground, our GFCI would have tripped because of current leakage to this supposed giant sump that is planet earth. That
there is no ground leak in this case clearly demonstrates that, as previously stated, electricity wants to return to its source and nowhere else. It is attracted only to the singular
magnetic field, in the core of a transformer or the rotors of a generator, from which it was created. The two low impedance conductors that were designed to carry it safely are the
preferred rout it takes. Whenever current goes to ground, or any other ground loop, it is only because it offers an alternate means to return to its' source.
Where this is the case can we eliminate the GFCI all together since they are such a nuisance?
What's wrong with a Floating Neutral system? Since the Ground wire and the Neutral wire are not bonded at the generator bus, neither the Equipment Grounding wire, nor the
earth, offers a path for Fault Current to complete the circuit back to the generator windings. In effect an open circuit, current will not travel it. If current won't go to Ground,
there is no opportunity for an individual touching an energized housing to become part of a Ground Fault Circuit - there is no risk of electrocution from a Fault. In fact, this is
the precise argument made by small portable generator manufacturers, and why up until 2002 they were exempted from putting GFCIs on their generators under 5000kW. The
figure below, illustrates why Floating Neutral generators are quite safe under single fault situations.
1) A fault in a metal fixture energizes the entire housing as soon as the circuit is turned on.
2) Since the Ground wire and the Neutral wire are not bonded at the generator bus, the equipment grounding wire
does not offer a path for the fault current to complete the circuit back to the generator windings.
Therefore, the fault current does not go to the equipment ground wire
3) Absent a Neutral-to-Ground jumper, the generator frame is completely isolated from the ground (a Floating Ground),
and an individual making contact with the energized housing does not present an alternate path
for fault current back to the generator windings. An open circuit, the fault current does not go
through the individual and ground back to its source.
But, what will happen in the event of a double fault - one on the Hot, and a second in the Neutral - if GFCI protection is not present? Let's see.
___________________________________________________________________________________________________
(Start Advertisement)
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ScreenLight & Grip has developed a special production package we call the "HD Plug-n-Play Package" (HD P&P Pkg.) For our HD P&P Pkg. we have picked lights that offer
both the highest output (lumens/watt) and the most feature style production capability and combined them with proprietary distribution technology we have developed that
enhances the production capability of the new Honda Inverter Generators.
Our 60A Full Power Transformer/Distro provides 7500 Watts of power in a single 120v circuit
from the new Honda EU6500i Generator
Until now, to power HMI lights over 1.2kw or Quartz lights over 2kw required a large diesel movie generator. Movie generators are not only expensive to rent, but they come
with hidden costs that usually break the budget of independently funded HD projects. Our HD P&P Pkg. takes advantage of technological advances to power HMI lights up to
6kw or Quartz lights up to 5kw off of wall outlets or the new generation of portable Honda Inverter Generators.
Left: Distorted power waveform created by pkg. of Non-PFC HMIs on a conventional portable generator.
Right: Near perfect power waveform created by the same lights with PFC ballasts of our HD Plug-n-Play Pkg.
on our modified Honda EU6500is generator.
By eliminating the need for a tow generator in order to have feature style production capability, our HD P&P Pkg. saves you the expense of not only the generator, but also the
added expense of a rental house grip truck and truck driver required to tow it. Use this link for more details about our HD Plug-n-Play Package.
Call (781) 326-5088, or contact us at rentals@screenlightandgrip.com for more information. Or, use this link for an informative newsletter article that explains the electrical
engineering principles that make it possible for our system to oeperate bigger lights, or more smaller lights, on portable generators than has ever been possible before. This article
is cited in the 4th Edition of Harry Box's "Set Lighting Technician's Handbook" and featured on the companion website "Box Book Extras." Of the article Harry Box exclaims:
"Great work!... this is the kind of thing I think very few technician's ever get to see, and as a result many people have absolutely no idea why things stop working."
"Following the prescriptions contained in this article enables the operation of bigger lights, or more smaller lights, on portable generators than has ever been possible
before."
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(End Advertisement)
___________________________________________________________________________________________________
Demonstration #2: Shock Hazards in Double Fault Situations
For the second demonstration set up we remove the Primelight GFCI. To simulate a Double Fault situation (like what would happen if a defective cord was dragged through the
mud) let's first create a fault in the Hot by attaching a jumper cable from the Hot pocket of the Patch Box to the input of our Fault Simulator. So that we can obtain precise
measurements of leakage current, let's attach a second jumper from the output of the Fault Simulator to the input of the Fluke 1587 Insulation Multimeter, and then a third
jumper cable from the output of the Fluke 1587 to one of our ground rods. To create a second fault let's jumper from the Neutral pocket of the Patch Box to the input of a
Switch Box and another jumper from the output of the Switch Box to another ground rod. We open the switch on the Switch Box and close the Ground Fault Simulator
(maximum resistance) so that we can regulate the current leaking to earth.
When we fire up the generator and supply power to our circuit, we see immediately on the Fluke 1587 that there is 4.9mAmps leaking to earth. Clamping the Megger DCM300E
Leakage meter onto the jumper going from the Neutral pocket of the Patch Box to the second ground rod, we see that the Fault Current is returning to the generator's windings
through the second fault that we established on the Neutral. As we gradually reduce the resistance of the Fault Simulator by turning its' rheostat, the leakage current begins to
rise. When the Fault Simulator is all the way open (minimum resistance) we note that the breaker has not tripped even though we have a clear Ground Fault of 7.9mAmps
according to the readout of the Fluke 1587. Closing the switch on the Switch Box so that there is only one fault in our system, the current leakage to earth stops. Opening the
switch on the Switch Box, the leak begins again according to the readout of the the Fluke 1587.
Analysis of Demonstration #2
A double fault - one in the Hot, and a second in the Neutral - creates a potentially hazardous situation because a path (circuit) now exists for Fault Current to return to the
generator windings. If an individual comes into contact with this Ground Fault Circuit created by the two faults, Fault Current will travel through the individual on its' way back
to the generator's windings. Put another way, the individual now "grounds" the system. Since the individual/ground route is most likely now the path of least resistance, Fault
Current travels through the individual and the ground back to the generator windings through the second Neutral Fault - delivering a shock to the individual. And, as
demonstrated here, if either of the Faults is high resistance, the current will not be high enough to open the breaker, and the individual will receive a sustained shock that can be
potentially fatal. The Figure below illustrates why an individual receives a shock when there are two Faults in a distribution system.
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1) Current goes out on the hot (black conductor) to the light housing fault.
2) Because the equipment ground wire is not bonded to the generator windings, current will not
go to the ground wire. Instead, current travels through the worker's body into the earth back to the
generator windings.
3) Current enters the generator frame and goes back to the generator winding through the second Fault
on the Neutral side. If either of the faults is high resistance, the current will not be high enough to
open the breaker. However, the current will be high enough to give the individual touching the
housing a shock.
4) The generator's circuit breaker may trip in response, but only if enough current flows through the
second fault to create an over-current situation.
5) The worker, however, is exposed to electrical shock until the breaker operates because no GFCI is present.
Is the Ground Fault current of 7.9mAmps in this demonstration a hazard? If you refer back to the Shock Tables, it is on the threshold of being painful but not potentially fatal
because an individual experiencing 7.9mAmps still has muscular control and will be able to remove them selves from the source of the shock. But, will an individual coming into
contact with this ground fault receive 7.9mAmps? According to Ohm's Law the answer is "No".
Ohm's Law
Ohm's Law simply states that Voltage (E) = Current (I) X Resistance (R). From this equation we can calculate the resistive value of our Fault Simulator that generates 7.9mAmps.
Since in this case, we want to find the resistive value that will allow 7.9mA of current (I) to flow, we will restate the formula as follows: R = E/I. Where, in this case, the Voltage
generated by the Honda EU6500is is 126V, a resistance of 15,949 Ohms (15.95 kOhms) will allow 7.9mAmps of current to flow in this ground fault circuit (126V/0.0079A =
15,949 Ohms.) An empirical test of our Fault Simulator set for minimum resistance with a Multimeter confirms the absolute brilliance of George Ohm who figured it all out in
1825 while working as a high school science teacher.
Georg Ohm: OUR HERO
But what does Ohm's law tell us from a practical standpoint - after all we are not wire wrapped around a ceramic core and Ohm's Law is not just an abstract mathematical
formula. What it means from a practical standpoint, is that for there to be a shock hazard (I) there must be a voltage source (E) and a resistance (R) and a closed circuit (ground
fault circuit in this case); and that the degree of hazard (I) is proportional to the resistance of whatever it is that completes the circuit. In other words, a person can contact
voltage and not be shocked if there is high resistance in the circuit. For example, a Film Electric can be working outside with a defective light and not receive a shock when the
ground is very dry or he is isolated from a ground loop by the rubber soles on his boots. That same person with the same light could receive a painful shock under wet conditions
because the resistance of the ground loop is greatly reduced by water. To be more precise, the mineral ions in water that make it a good conductor will also make an individual
soaked by rain (real or manufactured) a part of a ground loop circuit. If the water running over his boots makes the Electric become part of a ground loop, according to Ohm's
Law the resistivity of his/her body will determine how much current he/she will generate and consequently the amount of a shock they will take. Put simply, in such
circumstances the human body is basically a resistor and its resistance in Ohms determines how much current will flow. The figure below depicts a body resistance model. The
resistive values are for an average person doing moderate work.
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Let's use as an example the hand-to-hand resistance of the body: 500 + 500 = 1,000 ohms. Using Ohm's law we can calculate the amount of current our Electric will generate
and the degree of shock he/she will take if they become part of a Ground Fault circuit by grabbing a bare hot wire with one hand and an earth ground (in a grounded system)
with the other hand. Since in this case, we are concerned about the effects of current (I) on the human body, we will restate the formula as follows: I = E/R. According to Ohm's
law the Electric would take a shock of 0.120 amps (I = 120/1,000 = 0.120 amps.) If we multiply 0.120 amps by 1,000 (this converts amps to milliamps), we get 120 milliamps
(mA). In other words the Electric would generate enough current to receive a potentially fatal shock, where the 15,949 Ohm value of our Fault Simulator only generated enough
current to receive a painful shock (if it were a sentient being.) To see under what conditions this shock would be fatal we can consult the graph below:
The graph above plots the current flowing through the chest area and the time it takes to cause the heart to go into ventricular fibrillation (arrhythmic heartbeat). Using our
example of the body resistance at 1,000 ohms allowing 120 mA to flow (follow the dark line vertically from 120 mA to the shaded area, then left to the time of 0.8 seconds), you
can see that it would only take 0.8 seconds to cause electrocution.
Hidden Faults:
How likely is it that a distribution system will have two faults? Since all equipment leaks some current, it is not uncommon to have two or more faults in a system. A defect in
the generator, a poorly insulated or defective extension cord, defective insulation in a lamp housing, or defective plug, can all produce faults (to name just a few causes.)
Generators do not even have to be defective to develop a fault. Physically large generator windings (greater than 5kW) generate a capacitive reactance effect where alternating
current at 60Hz passes between the case of the generator and the windings even though there is no direct electrical connection between the winding and the case.
Note: The 1.15mA we detected in our first demonstration when electrical power from the Floating Neutral generator was
switched on is in fact the ground current generated by capacitive reactance. Since an ungrounded "Floating Neutral"
generator has no bond between Ground and Neutral, and is isolated from Earth, the current generated through capacitive
reactance has no outlet and so in effect charges the generator housing as if it were a capacitor until enough potential
develops for it to discharge itself in an arc to the generator windings - creating a fault.
It is also important to understand that faults can quite often go unnoticed. A fault in equipment that is benign when plugged into mains power, can become dangerous when
plugged into a generator because such capacitive coupling can create a path for current to flow back to its source (a Ground Fault Circuit) where one did not exist under normal
circumstances. Likewise, as illustrated above, a fault in equipment that is benign under dry conditions can become hazardous in wet conditions. While there may be sufficient
resistance in a fault that it doesn't leak much current in dry conditions, wet conditions can change the situation dramatically. Unbeknownst to you, the rubber soles of your shoes
may be insulating you from a Fault Circuit in dry conditions, but in wet conditions they pose little resistance to making you become a part of that circuit. If you become part of a
Ground Fault Circuit, as we saw above, electricity follows a fundamental law of nature in which you are nothing more than a resistor. Wet conditions (even perspiring) can lower
the resistive values in a ground loop circuit to the point where common-use electricity (120 volts) can become fatal. What's even more frightening is that a Double Fault situation
like the one we created in our demonstration might exist and there is no way of knowing until it is too late. Remember a high resistance fault like the one we created leaks
milli-Amps of current. The light powered by the circuit leaking the current will not appear noticeably dimmer from the loss. So there is no way of knowing that such a Ground
Fault Circuit exists until someone takes the hit.
As we will see, Double Fault situations are hazardous regardless of the type of Neutral/Ground configuration of the generator, but Double Faults with Floating Neutral systems
are especially hazardous. Without a Neutral/Ground bond, as illustrated below, a multiple fault condition exposes an individual touching faulty equipment to 240 volt potential.
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These demonstrations also expose other hazards particular to Floating Neutral generator systems. As illustrated below, the accidental reversing of the Ground Conductor with the
Neutral Conductor in an extension cord or lamp cord can lead to a potentially hazardous condition.
1. Current goes out on the hot conductor (black) to light. Note: The grounding conductor
is not connected to the winding of the generator (no connection between the neutral and ground)
and a GFCI is not used.
2. Current returns on the neutral (white) (shown in red) to the cord connector.
3. Current transfers to the grounding conductor from the receptacle and goes to the generator
frame and into the earth.
4. Current goes through the earth, through the victim, to the light housing, to the light ground wire.
5. Current goes through to the cord connector and transfers to the neutral, and from the receptacle
back to the generator winding.
6. Worker is connected across the generator winding and so receives a potentially fatal shock.
Note: Amoung the improvements mandated by UL943 in any new GFCI manufactured after January 1st, 2003 was the
requirement that the GFCI trip and not reset if Hot and Neutral are crossed.
Finally, as we experienced in this demonstration, GFCI test circuits can be misleading when they are used on Floating Neutral generators, which can be a serious problem. On a
Floating Neutral generator, the test button will draw power from the Hot through the Differential Current Transformer and back through the transformer again to the Neutral via
a Current Limiting Resister. The discrepancy caused by the Current Limiting Resister in the test circuit will initiate the GFCI to trip even though there will, in fact, be no
Equipment Ground Circuit for Fault Current to go to if there were an actual Fault, leading to a hazardous situation in Double Fault conditions.
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The false positive generated by the GFCI test circuit on ungrounded Floating Neutral generators is a serious problem because it generates a false sense of security. It gives the
appearance of a safely protected system when, in fact, it is not.
___________________________________________________________________________________________________
(Start Advertisement)
ScreenLight & Grip has developed a special production package we call the "HD Plug-n-Play Package" (HD P&P Pkg.) For our HD P&P Pkg. we have picked lights that offer
both the highest output (lumens/watt) and the most feature style production capability and combined them with proprietary distribution technology we have developed that
enhances the production capability of the new Honda Inverter Generators.
Our 60A Full Power Transformer/Distro provides 7500 Watts of power in a single 120v circuit
from the new Honda EU6500i Generator
Until now, to power HMI lights over 1.2kw or Quartz lights over 2kw required a large diesel movie generator. Movie generators are not only expensive to rent, but they come
with hidden costs that usually break the budget of independently funded HD projects. Our HD P&P Pkg. takes advantage of technological advances to power HMI lights up to
6kw or Quartz lights up to 5kw off of wall outlets or the new generation of portable Honda Inverter Generators.
Left: Distorted power waveform created by pkg. of Non-PFC HMIs on a conventional portable generator.
Right: Near perfect power waveform created by the same lights with PFC ballasts of our HD Plug-n-Play Pkg.
on our modified Honda EU6500is generator.
By eliminating the need for a tow generator in order to have feature style production capability, our HD P&P Pkg. saves you the expense of not only the generator, but also the
added expense of a rental house grip truck and truck driver required to tow it. Use this link for more details about our HD Plug-n-Play Package.
Call (781) 326-5088, or contact us at rentals@screenlightandgrip.com for more information. Or, use this link for an informative newsletter article that explains the electrical
engineering principles that make it possible for our system to oeperate bigger lights, or more smaller lights, on portable generators than has ever been possible before. This article
is cited in the 4th Edition of Harry Box's "Set Lighting Technician's Handbook" and featured on the companion website "Box Book Extras." Of the article Harry Box exclaims:
"Great work!... this is the kind of thing I think very few technician's ever get to see, and as a result many people have absolutely no idea why things stop working."
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"Following the prescriptions contained in this article enables the operation of bigger lights, or more smaller lights, on portable generators than has ever been possible
before."
(End Advertisement)
___________________________________________________________________________________________________
Conclusion:
There is inherent risk in using Floating Neutral generators because of the fact that the Neutral of the generator windings is neither grounded to the generator frame nor to the
grounding pin of the receptacle. This deficiency makes operation of the protective device (breaker or fuse) unreliable because in a two fault situation, fault current has no definite
path as it would in a Bonded Neutral generator. Clearly, simply grounding a Floating Neutral generator with a ground rod is not sufficient. Without a Neutral to Ground bond, an
earth ground serves no purpose. And, as we saw at the outset, simply using a GFCI on an ungrounded Floating Neutral generator will not ensure a safe system, and can in fact be
misleading. A GFCI will not operate reliably if one side of the winding is not grounded to the generator frame because Fault Current has no path back to the windings to
complete the circuit. In other words, a complete circuit is required to create an imbalance and cause the GFCI to trip. For GFCIs to operate reliably then, the Neutral must be
bonded to Ground. Only when Neutral is bonded to Ground, will current go to Ground to complete the circuit when there is a current leak. But that is not all that is deceptive
about Floating Neutral generators. Floating Neutral generators are deceptive because they give the appearance of a safely grounded system when, in fact, they are not. Their
receptacles accept a plug with a grounding pin, but in the receptacle the grounding pin is connected only to the generator frame and not to the generator winding (Neutral). The
overall appearance of Floating Neutral generators give a false sense of security. Will bonding the Neutral to the Gquipment Ground of the generator create a safe system so that
GFCIs are not necessary? Let's see.
Demonstration #3: Floating Bonded Neutral generator provides protection against leaks to ground without GFCI
For this demonstration we first bond the Neutral to the Equipment Grounding Circuit of the ungrounded Honda EU6500is generator by plugging an Edison plug with a jumper
between the Ground and Neutral blades into one of the generator's receptacles.
Note: Bonding the Neutral of a portable generator to its' frame is not as simple as putting an external jumper between the
ground and neutral of an open receptacle as I have done here. According to Article 250.4 (5) "Effective Ground-Fault
Current Path," of our National Electrical Code (NEC) requires that the grounding system create "a permanent,
low-impedance circuit capable of safely carrying the maximum ground-fault current likely to be imposed on it...."
An Edison Plug end jumper (like the one pictured above) that is rated for no more than 15 Amps clearly does not meet this
requirement. In open frame AVR models, like the ES6500, you can simply add a Neutral Bond jumper from one of the
generator winding leads to the frame (as pictured above.) In an EU series inverter generator, bonding the Neutral to
Ground requires removing the main panel and inverter module to get behind the breakers, switches, sockets etc. - not easily
done or undone. According to the Honda Service Bulletins, it should only be done by a qualified Honda service technician.
We connect a Switch Box between the Hot pocket and the Equipment Grounding pocket of the Patch Box at the load. The switch on the Switch Box is set to "Off" so that it
does not leak current. We start up the generator and turn on the light. To simulate a Fault to the Equipment Grounding wire, we flip the switch on the Switch Box to "On". The
light dims out and the breaker on the generator trips.
Demonstration #3 Analysis:
Since electricity prefers the path of least resistance, the current goes to the Grounding Wire because its' impedance is less than that of the load. The Fault Current travels back to
it's source, completing the Ground Fault Circuit, through the Grounding Wire, the jumper we established at the receptacle, and the Neutral. Since the Neutral is now bonded to
the generator frame, the Ground Fault current travels to the Neutral side of the Generator windings creating a dead short with the Hot side of the windings. Current amperage
rapidly climbs to infinity creating an over-current situation, which trips the breaker and removes the Ground Fault from the circuit. Even if the current leak had energized the
lamp housing, if someone were to touch it they would receive little to no shock because the current prefers the low impedance path of the Equipment Grounding wire over the
path they present (the individual/earth route) since it has a much higher resistance than the Grounding Wire because the generator is not grounded. The illustration below
demonstrates why this is the case.
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1) A fault in a metal fixture energizes the entire housing as soon as the circuit is turned on.
2) Since, electricity seeks the path of least resistance back to its' source, the bulk of the
fault current will travel through the equipment grounding wire, instead of the individual making contact
with the housing, because it is of a much lower resistance than the individual. The individual offers
the current a higher resistance path because it can not complete a circuit through them since the system
is insulated from the earth.
3) Because the Ground wire and the Neutral wire are bonded at the generator bus with a grounding
jumper, the current carried by the Ground wire back to the generator bus creates a dead short
(over-current situation). The fuse or circuit breaker pops in response.
This, in fact, is the preferred set up according to the guidelines (see excerpt from safety bulletin #23 above) established by the Safety Committee of the Contract Service
Administration Trust Fund (CSATF), an industry wide administrative body (governed by the collective bargaining agreement by and between the Producers, The International
Alliance of Theatrical Stage Employees ("I.A.T.S.E."), the Moving Picture Technicians Artists and Allied Crafts of the United States, its Territories, and Canada
("M.P.T.A.A.C."); as well as the collective bargaining agreements by and between the Producers and the Basic Crafts Locals (Article 18)).
"Generators mounted on trucks or trailers shall be completely insulated from earth by means of rubber tires, rubber mats around metal stairways and
rubber mats under any type of lift gate or jacking device. Metal supports for trailers shall be insulated by means of wooden blocks. Safety tow chains
shall be secured so as to not touch the ground. If complete insulation is not possible, a grounding electrode system shall be installed per the National
Electrical Code, Article 250.52."
To some, it might seem that without an earth ground, an electical system could not deliver full line potential (120V) to a load. That is simply not the case. When a source of
electrical power is completely isolated from earth ground (not to be confused with equipment ground in this case), the voltage potential created between the wires coming out of
the alternator of the generator is fixed. In fact, it is the only fixed quantity in an isolated system. The voltage potential from any part of the circuit to ground is not defined. With
no reference to ground, the phases are floating, but still fixed relative to one another, so loads will operate normally. To use an analogy: it is like a ship floating on a gentle sea even though the ship rises and falls with the swells of the ocean, the relationship of the keel (a phase leg) to the deck (the neutral) doesn't change. Likewise, the relationship of
the tip of the pilot house (another phase leg) doesn't change to the deck (the Neutral) or the Keel (the opposing Phase Leg) for that matter. In such an arrangement, we know the
phase potential is 120V higher than the common, but we don't really know higher than what? Like the ship, the relation to the ground is floating (called a Floating Ground
arrangement), changing slightly as the waves gently ungulate up and down. And, just like a ship navigating the open sea, this arrangement (an isolated power source) can function
just fine, as long as nothing happens to make a connection between the Keel of the ship and the ground of the ocean floor.
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But, isolating the generator from earth is only permissible, according to the NEC and OSHA Guidelines for the safe operation of Portable Generators (above), when the
Equipment Grounding System, consisting of the non-current carrying metal parts of equipment and the equipment grounding conductor terminals of the receptacles, is bonded to
the generator frame and the Neutral conductor is also bonded to the frame.
Proponents of Floating Ground systems argue correctly that if you have a Bonded Neutral, and you can effectively isolate the distribution system from earth ground, there is
good reason not to drive a grounding electrode or ground rod. It is a common misconception that ground rods are there to protect you (the same is true of circuit breakers.) A
circuit breaker is there to prevent fire created by heat from an over-current or short-circuit and protect the equipment. The amount of current it takes to electrocute a person is
much smaller than the amount needed to trip a circuit breaker. Add the fact that a ground rod will never pass enough Fault Current to trip an over current device and you realize
that they are not there for personnel safety. Where this is so contrary to popular belief, allow me to explain.
As the Figure of a Utility Line-men above clearly demonstrates, even though there will be current traveling on the grounding electrode in the event of a Ground Fault, because of
the high impedance of the rod, not enough will travel on it to open a breaker. Using Ohm's law, if we calculate how much current a ground rod will fault to the source through
the earth at 25 Ohm's (the prescribed impedance of a grounding device according to the NEC) at 120 volts, we find that a ground rod will only allow for 4.8 Amps to flow
through it. 4.8 Amps will not trip any breaker we commonly work with.
An electrical shock current of one hundred milliamps (100mA or 0.1A) is a very serious shock capable of causing paralysis of the lungs and heart muscle. The smallest circuit
breaker we use is 20A - that's about two hundred times more current than is needed to kill you. So in the final analysis, a ground round will not pass enough fault current to trip
an over current device, that is the job of the Equipment Grounding Circuit conductor. As Article 250.4 (5) of the NEC explains, a continuous equipment grounding circuit is to
provide a path for Fault Current:
"Article 250.4 (5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become
energized shall be installed in a manner that creates a permanent, low-impedance circuit capable of safely carrying the maximum ground-fault current
likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be used
as the sole equipment grounding conductor or effective ground-fault current path."
In short, ground rods and circuit breakers provide almost no increase in safety for people. They do not provide equal potential nor do they clear faults. Installing a rod for a
generator with bonded Neutral will only provide a path for a person to get in between the source and return. In fact, a case can be made that the installation of a ground rod
decreases safety. In the event of a Ground Fault, there is no potential to the earth without the rod. But, as soon as you bond the system to the earth there is potential to earth.
Conclusion:
As long as the Neutral is bonded to the Equipment Grounding Circuit, offering a low impedance path for Fault Current, Floating Ground Systems are relatively safe. Safe
enough, in fact, to permit an exemption from the NEC's requirement that GFCI protection be used on branch circuits of larger than 30A on temporary power distribution systems
outdoors. But, NEC Article 590.6 Section 2 "Assured Equipment Grounding Conductor Program" only exempts the requirement of GFCIs if a prescribed program of assuring an
adequate Ground Fault Circuit is followed. According to this program we must rigorously test all equipment, including heads, distros, & cable, to make sure the equipment
grounding system is in tact (as we saw previously, without an intact Equipment Ground, Fault Currents will not go to the ground wire.) The NEC "Assured Equipment
Grounding Conductor Program" (AEGCP for short) requires the following:
(2) Assured Equipment Grounding Conductor Prgram: A written assured equipment grounding conductor program continuously enforced at the site
by one or more designated persons to ensure that equipment grounding conductors for all cord sets, receptacles that are not a part of the permanent
wiring of the building or structure, and equipment connected by cord and plug are installed and maintained in accordance with the applicable
requirements of 250.114, 250.138, 406.3(C), and 590.4(D).
(a) The following tests shall be performed on all cord sets, receptacles that are not part of the permanent wiring of the building or
structure, and cord-and-plug-connected equipment required to be grounded:
(1) All equipment grounding conductors shall be tested for continuity and shall be electrically continuous.
(2) Each receptacle and attachment plug shall be tested for correct attachment of the Equipment Grounding conductor.
The Equipment Grounding conductor shall be connected to its proper terminal.
(3) All required tests shall be performed as follows:
(a) Before first use on site.
(b) When there is evidence of damage.
(c) Before equipment is returned to service following any repairs.
(d) At intervals not exceeding 3 months.
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(b) The tests required in item 2(a) shall be recorded and made available to the authority having jurisdiction.
At first glance, following an Assured Equipment Grounding Conductor Program (as detailed above) and bonding the Neutral to the Equipment Ground of the
generator would appear to remedy the risks inherent in Floating Neutral systems, and eliminate the need for GFCI protection; but, only if we can completely
isolate the generator and distribution system from Earth. If we are successful on all three accounts, we create a low impedance Ground Fault Circuit in the form
of the Equipment Grounding wire for Faults Currents to travel back to the Neutral side of the generator windings, creating a dead short between Hot and
Neutral, causing an over-current situation that trips the breaker and eliminates the Fault. But what if we are not successful on all three accounts? How likely is it
that we can be successful on all three?
Requirements one and two are fairly straight-forward (though the inspection and record keeping required to maintain an AEGCP are probably too much for a
film crew under the constraints of budgets today. When was the last time you were given sufficient checkout time to test the continuity of the Equipment Ground
of every cable, light, and distro box for a show?) The third requirement, however, that we completely isolate our system from earth, is probably the most difficult
to accomplish. In wet conditions it is practically impossible to effectively isolate a portable generator from earth ground as required by the NEC and OSHA.
When you think about it, to effectively isolate a portable generator in wet conditions, you not only have to place a rubber mat or apple box under any metal
parts of the generator frame that touches the ground, and assure tow chains not touch ground as required by CSATF Bulletin #23 (above); but you must also
insulate the entire distribution system, equipment and loads, from earth ground.
This is made difficult by the fact that most of the distribution equipment we commonly use is not rated for outdoor use, which is not to say that it can't be used
outdoors - it just has no inherent protection against moisture. In this country the NEMA rating for equipment that provides protection against damage from water
is 3R or 4. Some Cam-Lock connectors are NEMA 3R- or 4 rated. Some companies make main Cam-lock disconnects and spiders having a NEMA 3R rating
(you can tell that they are 3R-rated, because both the output and input connectors are protected with flaps that spring closed.) But, Stage Pin (Bates)
connectors, Edison plugs, most distro boxes, and lamp heads provide no protection from water. Unless kept away from water and raised and wrapped to protect
from errant spray, splashes, or condensation, these connection points will leak current like a bucket full of holes in wet conditions. You must therefore make
sure distribution cable is free of cuts or nicks that would expose copper to getting wet. Cable connections must be wrapped so that they are watertight.
Distribution equipment like snack boxes must be placed on apple or "Swamp" boxes. Finally, light stands must be placed on rubber mats or cup blocks.
Even after all that, you are still likely to have ground leaks because all equipment leaks some current. Capacitive reactance in the generator, a poorly insulated
or defective extension cord, defective insulation in a lamp housing, or defective plug, all will leak a little current (to name just a few causes.) While these faults
may be small, over a sizable distribution system they can all run together, like many ground springs running together to form a stream, as they all return to the
same source - the generator. Under such circumstances, someone touching the generator will receive a shock while someone else touching a lamp with a fault up
stream will just experience a tingle. Where this is the case, the protection afforded by using a Floating Ground arrangement is precarious at best.
With so many potential faults in a distribution system, it is not likely that they all will be from Hot to the Grounding Wire. They are just as likely to be from
from Hot to Neutral with enough resistance that it does not create a dead short. The double fault situation in our previous demonstration is just such an example.
Let's see what happens in that situation now that the Neutral is bonded to the Grounding Wire and the system is floating (not earth grounded.)
Demonstration #4: Floating Bonded Neutral generator still hazardous with double ground fault without GFCI
We bond the Neutral to the Equipment Grounding Circuit of the ungrounded Honda EU6500is generator by plugging the Edison plug with a jumper between the
Ground and Neutral Blade into one of the generator receptacles. To simulate a Double Fault situation, let's first create a fault in the Hot by attaching a jumper
cable from the Hot pocket of the Patch Box to the input of our Fault Simulator. And, like we did before, let's attach a second jumper from the output of the
Fault Simulator to the input of the Fluke 1587 Insulation Multimeter so that we can obtain precise measurements of leakage current. Finally, to complete the
connection to earth ground, let's put a third jumper from the output of the Fluke 1587 to one of our ground rods. To create a second fault on the Neutral, we
jumper from the Neutral pocket of the Patch Box to the input of a Switch Box and another jumper from the output of the Switch Box to another ground rod as
we did before. We open the switch on the Switch Box and close the Ground Fault Simulator (maximum resistance) so that we can regulate the current leaking to
earth.
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When we fire up the generator and supply power to our circuit, we see immediately on the Fluke 1587 that there is again 4.9mAmps leaking to earth. As we
gradually reduce the resistance of the Fault Simulator by turning its' rheostat, the leakage current begins to rise. When the Fault Simulator is all the way open
(minimum resistance) we note that the breaker again has not tripped even though we have a clear Ground Fault of 7.9mAmps according to the readout of the
Fluke 1587. Closing the switch on the Switch Box so that there is only one fault in our system, the current leakage to earth stops. Opening the switch on the
Switch Box, the leak begins again according to the readout of the the Fluke 1587.
Analysis of Demonstration #4
In this situation, the second fault creates a potential for current to leak from the first fault. Because of the resistance of our Fault Simulator, a dead short is not
created in this case between Hot and Neutral, which explains why the over-current protective device on the generator (the circuit breaker) did not trip as it did
before. If this were an individual completing the Ground Fault Circuit, rather than our Fault Simulator, the Fault Current would be sufficient to cause
electrocution even though it is not sufficient to trip the over-current protection on the generator.
1) Current goes out on the Hot (black conductor) to a fault in the cable (perhaps a nick.)
2) Even though the Equipment Grounding wire is in good condition, current flows through
the worker's body into the earth seeking the path of least resistance back to its source
because the fault is to earth ground (caused by a nick in the Hot wire), and not to
the Equipment Grounding wire.
3) Current enters the generator frame through the second Fault on the Neutral and goes
back to the generator windings.
4) If the faults are of a high resistance, the current will not be enough to open the
breaker and the individual touching the housing will receive a shock.
5) Because no GFCI is present, the worker is exposed to a sustained shock and is
electrocuted.
The illustration above deomonstrates why an individual making contact with either fault will be electrocuted. When the individual touches the fault, they act
like a switch closing. The individual completes the circuit and the Fault Current returns to its source via a path consisting of the individual, earth, and the
Neutral via the second Fault (remember there is no Fault to the Equipment Grounding System in this case.) Since the Fault Current is high enough to create
muscle seizure, but not high enough to trip the breaker, the individual is not able to let go. Unable to pull away and open the Ground Fault Circuit they
established, the individual will receive a sustained shock and eventually die if not knocked free of the fault.
Conclusion:
While Bonded Neutral Systems with Floating Grounds, offer a great deal of protection in the event of faults to the Grounding Wire, they do not offer much
protection in double fault situations where one is on the Hot and the other is on the Neutral and there is resistance (the earth) between them. Since faults are just
as likely to occur at a lamp's plug end and on the lamp cord (double fault), as the lamp housing (fault to ground), protection against both types of faults is
required to make a distribution safe - following an AEGCP alone is not sufficient. For this reason, GFCI protection is required even under the best case scenario
when there is a bonded Neutral and assured equipment ground circuit. Now that we have determined that GFCIs are required for personal safety in Floating
Ground systems, let's determine the most effective placement for them.
Demonstrations #5.1-3: GFCI Placement
In this next series of demonstrations we try out different placements of a GFCI in the distribution system to see how effective they are in both Hot-to-Ground
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Wire Faults and Hot-to-Neutral Faults.
Demonstration #5.1
For this first demonstration we plug the Primelight power cord GFCI back into the generator receptacle (Position "A"). The Neutral is still bonded to the
Equipment Grounding Circuit with our plug end jumper. So that we can switch between one and two fault situations in this series of demonstrations by simply
flipping the switch on the Switch Box, the rest of the set up will be similar to that of our previous double fault set-ups. If you recall, in those set-ups, we first
attach a jumper from the Hot pocket of a Patch Box at the load to the input of our Fault Simulator. And, like we did before, let's attach a second jumper from
the output of the Fault Simulator to the input of the Fluke 1587 so that we can obtain precise measurements of leakage current. But, rather than make a
connection to earth ground, for this first demonstration we'll put the third jumper from the output of the Fluke 1587 to the Ground Pocket of the Patch Box so
that we can leak current to the Equipment Grounding System instead. To create a second fault on the Neutral that we can switch on and off at the load, we
jumper from the Neutral pocket of the Patch Box to the input of a Switch Box and another jumper from the output of the Switch Box to another ground rod as
we have done before.
For this first demonstration we close the switch on the Switch Box so there is only one fault (Hot-to-Equipment Ground) and close the Fault Simulator
(maximum resistance) so that we can regulate the current leaking to earth. When we fire up the generator and supply power to our circuit, we see immediately on
the Fluke 1587 that there is again 4.9mAmps leaking to earth. We test that the GFCI is operational by pressing the test button - we receive a positive test when
the power goes off. We reset the GFCI and gradually reduce the resistance of the Fault Simulator by turning its' rheostat. As we reduce the resistance in the
circuit, the leakage current begins to rise. When the leakage current reaches approximately 5.14mAmps, the Primelight GFCI trips - interrupting power to the
150W Fresnel.
Demonstration #5.2
For the second of this series of demonstrations, we flip the switch on the Switch Box to create a second fault at the load. We move the jumper going from the
Fault Simulator to the Ground pocket of the Patch Box to the second ground rod instead. We reset the Fault Simulator to maximum resistance so that we can
now regulate the current leaking to earth. Finally we reset the GFCI.
As soon as power is supplied to our circuit, we see on the Fluke 1587 that there is 4.9mAmps leaking to earth. As we gradually reduce the resistance of the Fault
Simulator by turning its' rheostat, the leakage current begins to rise. When the Fault Simulator is all the way open (minimum resistance) we note that the GFCI
has not tripped even though we have a clear Ground Fault of 7.9mAmps according to the read-out of the Fluke 1587. Why isn't the GFCI tripping now, where it
just did? Clamping the Megger DCM300E Leakage meter onto the jumper going from the Neutral pocket of the Patch Box to the second ground rod, we see that
the Fault Current is returning to the generator's windings through the second fault that we established on the Neutral.
Demonstration #5.3
For the last of this series of demonstrations, we first replace the Primelight GFCI at the generator with a Patch Box. We then run out a short AC Extension and
plug the Primelight GFCI into it. We plug a second Patch Box into the GFCI and then our load into the Patch Box. As we did in the demonstration before, we
create a fault in the Hot by attaching a jumper cable from the Hot pocket of the Patch Box to the input of our Fault Simulator. And, like we did before, we
attach a second jumper from the output of the Fault Simulator to the input of the Fluke 1587 Insulation Multimeter so that we can obtain precise measurements
of leakage current. Finally, we make the connection to earth ground, by putting a third jumper from the output of the Fluke 1587 to one of our ground rods.
For this demonstration we create the second fault on the Neutral at the generator by putting a jumper from the Neutral pocket of the Patch Box at the generator
to the input of a Switch Box and another jumper from the output of the Switch Box to the other ground rod. By doing so, we have now positioned the GFCI
between our two faults (Position B in the diagram above.) We open the switch on the Switch Box and close the Fault Simulator (maximum resistance) so that we
can regulate the current leaking to earth.
When we fire up the generator and supply power to our circuit, we see immediately on the Fluke 1587 that there is again 4.9mAmps leaking to earth. As we
reduce the resistance in the circuit, the leakage current begins to rise. When the leakage current reaches approximately 5.14mAmps, the Primelight GFCI trips interrupting power to the 150W Fresnel.
Analysis of Demonstration #5.1-3
When operating Floating Ground systems, the optimum placement of GFCI protection appears to depend on the type of fault. As we saw in demonstration 5.1, a
GFCI at the generator is optimum for detecting Hot-to-Equipment Ground Wire Faults because it will detect a fault to an Equipment Ground Wire anywhere in
the system. Under these circumstances, current carried on the Equipment Grounding Conductor reduces the current returning on the Neutral conductor. Since
the Neutral conductor passes through the CT of the GFCI, and the Ground conductor does not, the GFCI will sense the imbalance and interrupt power to the
circuit. A GFCI placed at the generator is also optimum because it not only protects the Lamp Operator at the lamp, but also the Genny Op at the generator.
However, as we saw in demonstration 5.2, a GFCI at the generator would appear to protect no one in the event of a double fault situation like the one in our
demonstration (where one is Hot-to-Earth and the other Neutral-to-Earth), or one where an individual makes contact with both the Hot and Neutral conductors
at the same time as illustrated below.
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The reason for this is because, as measured with our clamp meter, the Ground Fault Current returns to the Neutral, restoring balance to the system, before it
passes through the return side of the CT. Since balance is restored, the GFCI does not detect the existence of the Fault. The higher than acceptable level of Fault
Current leaked in demonstration 5.2 demonstrates that an individual coming into contact with the fault, could receive a sustained life threatening shock without
the GFCI ever registering an imbalance.
The same GFCI positioned at the load will, however, trip at its prescribed level every-time. The reason a GFCI will trip reliably when located at the load is that
in this position (B) the Neutral return passing through the CT is reduced by the amount of current leaking to earth even though it may eventually return to the
Neutral through the second fault and restore balance to the system. Sensing the imbalance at the load, the GFCI trips and shuts off power to the circuit. If it were
an individual creating the Ground Fault, rather than our Fault Simulator, they would receive only a momentary shock because the GFCI quickly interrupts the
circuit. A close examination of the illustration above shows why this is the case. Since Faults are just as likely to occur at a lamp's plug end and on the lamp
cord, as the lamp housing, protection against both types of faults is required to make a distribution safe.
Conclusion:
On Floating Ground systems, where the Neutral and Equipment Grounding wire are bonded but isolated from earth, it would appear that it is optimum to have
GFCIs both at the generator and at the lamp. Since, a GFCI placed at the generator is very effective at detecting faults to the Equipment Grounding Wire
anywhere in the system, but can fail to detect double fault situations, it is necessary to also place GFCIs at the lamps to protect against double fault situations.
Interlocking multiple GFCIs in this fashion offers protection against both types of Faults - Hot-to-Ground and Hot-to-Neutral. Called "Interlocking Zone Ground
Fault Protection," such a deployment of GFCIs will offer optimum protection to equipment and crew.
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Employing an Interlocking Zone Ground Fault Protection scheme is very much like employing a zone defense in basketball. The distribution system is broken up
into zones wherever there is a change in wire size or branch circuit. A GFCI sized for the over current device used to protect the branch circuit is positioned
downstream of the over current device. The result is a cascade of interlocking protective zones starting at the power source and ending at the load. As you can
see in the table below, there exist a wide variety of GFCI devices for this purpose - ranging from Class A devices with fixed 6mA trip levels and devices with user
adjustable trip settings from 5- to 50mA. Adjustable devices set for a trip level of 20mA meets the UL943 standard for Class C protection of equipment but is
not suitable for personnel protection (Note: devices with adjustable trip levels are capable of providing personnel protection to UL943 Class A specifications
when set for a trip level of less than 6mA and time delay of 100 milliseconds, but are not Class A devices because the UL Standard for Class A requires a fixed
threshold of 6mA).
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The forward zone 120V and 250V single phase GFCIs must employ Class A devices with trip settings of less than 6 mA to assure the safety of personnel. As
long as that is the case, the rear zone 208/240V multi-phase GFCI can employ higher trip settings (typically 10 or 20mA.) The choice whether to use a trip
setting of 6-, 10-, or 20mA depends on the circumstances and the level of protection desired - but some type of rear zone GFCI protection is required in Floating
Ground systems. The reason is that when the Neutral and Equipment Grounding wire are bonded but isolated from earth, minor non-hazardous faults can
generate transient overvoltage that can puncture insulation and cause catastrophic insulation failure in secondarily derived systems without an earth ground.
According to the IEEE Std. 242-1986 (Buff Book), "if a ground fault is intermittent or allowed to continue, [an ungrounded] system could be subjected to
possible severe overvoltage to ground, which can be as high as six or eight times phase voltage. This can puncture insulation and result in additional ground
faults." Such overvoltage is caused by repetitive charging of the system capacitance, or by resonance between the system capacitance and the inductances of
equipment in the system. A GFCI will interrupt power to the circuit before a repetitive minor fault can spiral out of control and lead to a potentially hazardous
catastrophic insulation failure.
Left: Bender 400A 3 Phase Adjustable GFCI. Middle: Shock Block 400A 3 Phase Adjustable GFCI. Right: 100A 3 Phase Adjustable GFCI.
In extremely hazardous situations such as when working in water or rain (real or manufactured) a multi-phase GFCI with a 6mA trip level and response time of
50 milliseconds (50/1000 of a second) will offer the maximum level of protection to both personnel and equipment, but will also be more prone to tripping.
Since the rear multi-phase GFCI protects the entire distribution system and not just an individual zone, it is subject to the cumulative current leaks of all the
zones taken together, which for a large distribution system can approach 6mA even under the best conditions. For this reason, in less hazardous situations, it
makes sense to set the multi-phase GFCI in the rear with a higher Class C trip setting up to 20mA, but with a quicker response time of 25 milliseconds. This
combination of trip level and response time still offers protection for personnel and will accommodate the cumulative current leaks of a typical distribution
system. It is highly recommended that when using multi-phase 208/240V GFCIs with Class C trip levels of 10-20mA in the rear, that you also use forward
placed Class A devices. Used alone, Class C trip levels with short response times offer "protection" to personnel in that they will prevent electrocution, but it is
not an experience you would want to inflict upon anyone. Over before inflicting any permanent injury, the shock will be extremely painful none-the-less.
Where the generator is earth grounded, it is an option to follow a ground fault protection strategy that only utilizes Class A devices at the branch circuits and no
multi-phase device on the main feeder trunk at the generator. That is because Neutral grounding (driving a ground rod) not only stabilizes the phase-to-ground
voltage under normal operation, but is also very effective in reducing the transient voltage build-up from intermittent ground faults discussed above. By reducing
Neutral displacement from ground potential and by reducing the destructive impact of repeated ground faults, driving a ground rod eliminates these reasons for
using a multi-phase GFCI on the main feeder trunk. Bear in mind, however, that with only forward placed GFCIs there exists no protection against faults
resulting from nicks or cuts in the main feeder trunk or if the feeder's insulation is cut abruptly by a vehicle driving over it.
A third strategy to ground fault protection is to use only one multi-phase GFCI to "blanket" the entire distribution system. However, since in this approach there
are not Class A devices on larger amperage circits (60- and 100- Amp Circuits), the multi-phase device on the main feeder trunk should be set for a trip level of
6mA and a response time of not more than 50 milliseconds to assure adequate protection for personnel. There are several drawbacks to this approach. In
addition to being more prone to tripping (for the reasons mentioned above), if it does trip the entire distribution system goes down leaving you figurtively "in the
dark" on where the leak occurred while the entire set is literally left in the dark. A major advantage to combining the blanket approach with the branch approach
in a tiered interlocking zone approach is that if a GFCI trips it doesn't leave the entire set in the dark and you know immediately what zone the leak occured in
and so are more able to isolate the source of the leak while work continues on set. In the final analysis, for a Ground Fault protection system to work it has to be
used. If the approach chosen only results in nuisance tripping because of too low trip thresholds or misplacement of the GFCIs than they will be pulled out of
service and offer no protection at all. So, it is better to over engineer a Ground Fault protection system than try to make do with too little.
Regardless of the strategy you use - Blanket, Branch, or Tiered Interlocking Zone - you must follow the NEC requirement that all 120V 15- and 20- Amp
receptacles have GFCIs. For this purpose both Littlefuse and Bender offer 100A Lunch boxes with five hospital grade duplex 20A Class A GFCI outlets. Bender
also offers Pigtails with a 20A Class A GFCI rated for portable use as well as a unit that provides Class A GFCI protection for six circuits in a Socapex Cable
(pictured below.)
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To Ground, or not to Ground:
To earth ground a system or not to earth ground is a complicated issue. On the one hand, if you have a Bonded Neutral system, and you can effectively isolate it
from earth, there is good reason not to ground it. Installing a rod for a generator with bonded Neutral only provides a path for a person to get in between the
source and return. Short of two Ground Faults, there is no potential to the earth without the rod. But, as soon as you bond the system to the earth there is
potential to earth. In fact, a case can be made that the installation of a ground rod decreases safety.
The opposing argument is that a ground rod will limit voltage surges from lightning strikes or unintentional contact with high voltage lines. Given the chance of
either one of these occurrences happening on set, and the subsequent consequences, you would probably die from the shock of surprise first. A better argument
for earth grounding generators is that it equalizes potential throughout the system. While a Floating Ground configuration of a Bonded Neutral generator offers a
high degree of protection against ground faults in equipment, it is less than ideal. For instance, if someone were to touch a grounded part of the equipment
housing while making good contact to Ground (while holding a metal railing), then they could receive a slight shock equal to the difference in potential between
the arbitrary floating power source and Ground. Connecting a grounding electrode to the Neutral bus of a power source gives the source a relation to ground - it
establishes zero-potential between Neutral and Ground. The Phase and Neutral wires are not just 120V from one-another the Phase is now 120V above Ground.
It is like draining the ocean, with the boat resting on the bottom, the relationship between the keel and deck are fixed relative to the ground. With the difference
in potential between the arbitrary floating power source and Ground eliminated there is no potential difference between the lamp housing and hand railing to
cause a shock. No appreciable current needs to flow through the grounding electrode conductor to establish this relationship, but once it is established, all
equipment connected to the power source has the same zero-potential relation to Ground - there are no differences in potential that can cause a shock.
Another good argument to stick the earth is that, as we determined previously, it is nearly impossible to effectively isolate portable generators from earth
(especially the Hondas as the CSAO study demonstrated.) As we determined, to effectively isolate a portable generator from earth requires insulating the entire
system - generator, distribution equipment, and loads - from earth. Even after all that, as the CSAO study illustrated, portable generators, like Hondas, can still
be inadvertently grounded by moisture and even high humidity. Where that is the case the protection afforded by using a Floating Ground arrangement is
precarious at best, and since the NEC Article 250.52 requires a grounding electrode system be installed if complete insulation is not possible, it makes sense to
Ground your system regardless.
Even though a ground rod will not pass enough current to activate an over-current device, grounding generators to earth offers some degree of protection from
electrical shock. Without a doubt, the existence of the additional Ground Fault circuit created by a grounding electrode, will lessen the current traveling on the
Ground Fault Circuit created when an individual comes into contact with a Fault. Absent the ground rod, in a double fault situation for instance, the individual
carries the entire Fault Current. The presence of a second Ground Fault Circuit will split the Fault Current according to its' resistance relative to the individual.
The figure of the Utility Line-men above illustrates this fact. The Line-men receives a lesser shock than he would have had the ground rod been absent, because
some of the Fault Current travels through the ground rod and earth instead of him. And, because of the voltage split that results from there being two ground
paths, he receive less of a shock than if there were only one Ground Fault Circuit.
In the end, whether you drive a ground rod or not, is ultimately not your decision, but is dictated to you by the AHJ (Authority Having Jurisdiction.) The AHJ,
depending on where the work is taking place, may be the local city electrical inspector, the fire marshal, or the studio's safety officer. Regardless, it is
worthwhile, I think, to understand the issues surrounding earth grounds and how they effect the operation of GFCIs because there seems to be some confusion
on this point as well. Some people believe that GFCIs will function regardless of the grounding arrangement of the power source, while others believe the
opposite. To settle that debate lets see what effect earth grounding the generator has on the operation of GFCIs.
Demonstration #6: GFCI Effectiveness & Grounding
For our final demonstration, we plug the Primelight GFCI into the generator and a Patch Box into the GFCI as illustrated below. We then run out a short AC
Extension and plug a second Patch Box into it. We plug our load into the second Patch Box. As we did in demonstration 5.3, we create a fault in the Hot by
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attaching a jumper cable from the Hot pocket of the Patch Box at the load to the input of our Fault Simulator. And, like we did before, we attach a second
jumper from the output of the Fault Simulator to the input of the Fluke 1587 Insulation Multimeter so that we can obtain precise measurements of leakage
current. Finally, we make the connection to earth ground, by putting a third jumper from the output of the Fluke 1587 to one of our ground rods.
We create a second fault, this time on the Neutral, closer to the generator by "hard wiring" without a Switchbox a jumper from the Neutral pocket of the Patch
Box plugged into the GFCI to a second ground rod. The Switch Box in this demonstration is jumpered onto the grounding lug of the generator and then we
jumper from its' output to a third ground rod. In this fashion, we have now positioned the faults downstream of the GFCI and have the ability to turn on and off
the generator grounding by flipping the switch on the Switch Box. As illustrated above, we begin this demonstration with the switch on the Switch Box in the
open position (no ground) and close the Fault Simulator (maximum resistance) so that we can regulate the current leaking to earth.
As soon as power is supplied to our circuit, we see on the Fluke 1587 that there is 4.9mAmps leaking to earth. As we gradually reduce the resistance of the Fault
Simulator by turning its' rheostat, the leakage current begins to rise. When the Fault Simulator is all the way open (minimum resistance) we note that the GFCI
has not tripped even though we have a clear Ground Fault of 7.76mAmps according to the read-out of the Fluke 1587. To confirm that the GFCI is not tripping
because balance is being restored to the system by the Fault Current returning to the Neutral via the fault we established on the Neutral before it goes through
the CT (as it did before), we clamp the Megger DCM300E Leakage meter onto the jumper going from the Neutral pocket of the Patch Box to the second ground
rod and find the 7.76mAmps traveling back to the generator's windings through the second fault that we established on the Neutral.
We now ground the generator by flipping the switch on the Switch box to the "On" position - the GFCI does not trip. We meter the jumper that is returning Fault
Current to the Neutral through the second fault with the Megger Clamp Meter and we get a reading of 3.43mAmps. We still read 7.76mAmps on the Fluke
1587, so where is the missing current going. We meter the jumper that connects the Generator frame to the grounding electrode with the Megger Clamp Meter
and find the missing 4.33mAmps of Fault Current returning to its' source via the ground rod. By grounding the generator, we have split the Fault Current
between two parallel ground loops - 3.43mAmps returns via the Neutral Fault loop, while the balance of 4.33mAmps returns via the grounding electrode loop.
Since, the amount of Fault Current returning without passing through the GFCI CT (the 4.33mAmps on the grounding electrode loop) does not exceed the
threshold of the GFCI, it does not trip. To see if we can trip the GFCI, we replace the Fault Simulator with one that has a lower range of resistances (0-10.49
kOhms verses the 15.60-25.7 kOhms of the first one) and flip the switch on the Switch Box back to "Off" so that the generator is not grounded. As soon as
power is supplied to the circuit, we see on the Fluke 1587 that with the lower resistance we have 11.35Amps leaking to earth. As we gradually reduce the
resistance of the Fault Simulator by turning its' rheostat, the leakage current begins to rise. When the Fault Simulator is all the way open (minimum resistance)
we have a whopping 222.8mAmps of Fault Current according to the read-out of the Fluke 1587. The GFCI still has not tripped which creates a potentially
hazardous situation if someone were to come into contact with this fault circuit. We ground the generator by flipping the switch of the Switch Box to "On" - the
GFCI trips immediately.
Analysis of Demonstration #6
Earth grounding of generators would appear to greatly increase the effectiveness of GFCIs in double fault situations. As illustrated below, the ground rod creates
a definite Ground Fault Circuit, splitting the current, and making it impossible for balance to be restored to the system by all of the Fault Current returning to
the Neutral through the second Fault before passing back through the GFCI. By diverting some of the Fault Current around the second fault, the ground rod
assures that there will be an imbalance in the current traveling through the GFCI on the return side that will make the GFCI trip. Increasing the effectiveness of
GFCIs in double fault situations is yet another argument for earth grounding generators.
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That the GFCI does not trip at first dispels another commonly held fallacy about electricity. If, as is commonly believed, electricity will "return to ground" via
any available grounding electrode then a current imbalance would have been created in the GFCI from current going to the building ground electrode next to the
generator ground electrode.
Since that is clearly not the case, this demonstration confirms that current will only return to its' source and not another source of electricity. In other words,
current will only return to the singular magnetic field from which it was born and none other.
Conclusion:
Based upon what we now know, it is the recommendation of the IATSE Local 481 Electrical Department Steering Committee that the following precautions be
taken to ensure a safe distribution system:
To ensure a safe system take the following precautions in prep:
1) Inspect all portable and vehicle-mounted generators, transformers, and Battverters to determine:
A. Whether the generator winding is grounded to the generator frame.
B. Whether the grounding pin of the receptacle is grounded to the generator frame or the Neutral.
Steps A and B may be easily checked by placing an ohmmeter across the ground and neutral pin of the 120-volt receptacle
while the generator is not running. The ohmmeter will indicate open if the neutral is not grounded.
2) Order the right Equipment:
A. Plan out your ground fault protection strategy and be sure to have the right Class A and/or Class C GFCIs to
implement it successfully.
B. Order only PFC HMI Ballasts.
C. Order only Banded Cable.
3) Prep equipment before installation:
A. Test all equipment, including heads, distros, & cable, to make sure the equipment grounding system is in tact. Without
an intact equipment ground, fault currents will not go to ground and GFCI equipment will not identify potential faults
B. Inspect cable for cuts or nicks that would expose copper to getting wet.
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C. Clean the inside of Lamp housings of dust and debris (dead bugs) that will conduct current from lamp leads to case
ground.
D. Individually test equipment for ground leaks. Eliminate equipment with excessive leakage.
To ensure a safe system take the following precautions on set:
1) Ground the generator winding to the generator frame as needed by connecting a jumper between the Neutral connection and Ground
connection at any convenient point inside the wiring compartment. So that it does not simply act as a fuse, the wire size of the jumper
should be sufficient to safely carry the entire Fault Current that could be imposed upon it.
2) Ground the generator to earth with a ground rod and monitor it with a leakage meter.
3) To eliminate current leaks that will aggregate to trip levels take the following measures:
A) Wrap cable connections so that they are watertight.
B) Elevate distribution boxes at least six inches from the ground using apple or "Swamp" boxes.
C) Place light stands on rubber mats or cup blocks.
D) Place Visqueen rain hats on lights and rubber mats over distro boxes.
4) Deploy GFCIs using an Interlocking Zone Ground Fault Protection arrangement.
5) Retest with the ohmmeter as in step No. 1 above with the generator not running.
6) Before starting the generator, test between the Hot and Ground pins of a receptacle with an ohmmeter to ensure that a ground fault
has not been inadvertently created.
7) Start the generator engine and test between the Hot and Neutral pins for proper voltage with a voltmeter.
8) With the generator running, test the operation of the GFCIs by pressing their test buttons. Perform this test each time the generator is
started. A spare GFCI should be kept on hand in case of failure.
9) Switch large Tungsten heads at distro breaker or GFCI.
10) Monitor the Set at all times so that no one mistakenly jeopardizes your Ground Fault Protection Plan .
Several of these recommendations warrant exploring in more detail:
___________________________________________________________________________________________________
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___________________________________________________________________________________________________
Precautions in Prep:
1) Test equipment:
As a matter of safety, and to avoid unnecessary nuisance tripping, all electrical equipment should be thoroughly tested for leakage current before being used with
GFCIs. This includes stingers, lights, cables, or any other electrical equipment that is to be used on a GFCI protected circuit. Prep time and manpower should be
allocated for testing.
Since, a Fault in equipment that is benign when plugged into mains power, can become dangerous when plugged into a generator (as discussed previously
capacitive coupling can create a path for current to flow back to its source (a Ground Fault Circuit) where one did not exist under normal circumstances),
equipment should be tested on a generator if it is to be used on a generator.
To test equipment, Bender has developed the "e-Cart", a mobile testing station equipped with every kind of connector that will provide current leakage
information for whatever is plugged into it. Absent the e-Cart, there are a number of ways to go about testing for leakage current in equipment.
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Current Leakage Meter:
Fluke, Megger, and others make clamp-on meters for leakage testing. These are specialized meters capable of detecting very small current differentials. They give
a read-out of the exact amount of leakage detected.
Inline GFCI (6mA):
A simple, practical test method for small lights and stingers is to plug into a 20A portable GFCI unit. The GFCI will trip if it detects greater than 6mA of leakage
current. This will not tell you anything about leakage smaller than 6mA, but it eliminates equipment that has a significant ground fault.
100A GFCI with series LEDs
You can use a 100A GFCI with series LEDs (like the 120V or 208-240V GFCIs pictured below) to test both large and small lights as well as smaller (under a
100 Amps) distribution equipment.
For instance the five element LED chain on the 120- and 208-240V 100A Shock Block (pictured below) can be used to weed out potentially problematic
equipment by simply monitoring the LEDs while powering equipment on and off. Since the LED chain will indicate both the presence of a leak as well as the
severity, it can be used to test both lighting loads as well as distribution equipment.
Test the lights first by plugging them directly into the appropriate 100A Shock Block (A 208-240V 100A Single Phase GFCI like the Bender pictures above right
can be used to test larger lights like Tungsten 20/24ks, or HMIs ranging from 6- to 18K.) Then use a smaller resistive load (like a tungsten 2k) that proved to
have no leaks to test distribution equipment by plugging and unplugging it into the distribution to be used one at a time. This way, if a particular piece of
equipment (whether a light or cable) is leaking badly it can be eliminated before the set-up.
These simple tests can be done either at the rental house during check out or on the electrical trailer during down time on location.
Insulation Test Meter:
Since many ground faults that are dormant under dry conditions, become problematic when resistance is lowered under wet conditions, it can be difficult to
entirely weed out leaking equipment beforehand or to isolate equipment that was problematic on set after the fact. If you are unable to identify on set, the piece
of equipment within a circuit that was tripping the GFCI that was covering that zone, an "Insulation Test Meter" will help you be able to identify it back on the
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trailer or in the shop.
Designed principally to test insulation breakdown in motors and electrical wiring that can lead to ground faults (as pictured below), an Insulation Test Meter can
also be used to measure insulation resistance and can thereby be used to identify a leak in cable, distro boxes, or lamp heads that will only be problematic when
resistance is lowered under wet conditions.
To understand how it works, let's go back to basics: in an electrical circuit, current is delivered by a conductor, does it's work and returns to the source on
another conductor. Insulation keeps the current contained within the system. When we test a system's insulation, we look for leaks that would allow current to
sneak back to the source via the ground fault circuit.
Insulation is like a water pipe. To look for leaks in water pipes we can carefully apply high steady pressure. This makes it easier to see even small leaks.
To look at leakage current, an insulation tester carefully applies a higher-than-normal test voltage. The meter limits the test current to protect the user. Since the
meter measures the precise current and voltage it can use Ohm's law to show insulation resistance and identify a breakdown in insulation that would otherwise
only be apparent when resistance is lowered under wet conditions.
2) Order Only PFC HMI Ballasts :
As we saw at the outset, many HMI and Kino Flo lights we use in our industry use non-Power Factor Corrected ballasts that dump high frequency harmonic
currents back into the power distribution system that can cause GFCIs to trip without there being a hazardous ground fault. As a result of UL943, modern GFCIs
incorporate Harmonic Filters to attenuate these high frequency harmonic currents and reduce the incidence of nuisance tripping. While Harmonic Filters in
GFCIs have proved to be an effective means of dealing with these high frequency Harmonic Currents, it is a good idea none-the-less to reduce the amount of
high frequency harmonic currents in the distribution system. For this reason, you should use only HMIs and Kino Flos with ballasts that are Power Factor
Corrected (PFC.) Not only will a PFC circuit realign voltage and current and induce a smoother power waveform at the distribution bus, but it will also cause
the ballast to use power more efficiently and not return harmonic currents and line noise into the distribution system.
You can't assume that the ballast that you will get from the rental house will be Power Factor Corrected. While it is true that all major manufacturers include
PFC circuitry in 6-18kw HMI ballasts (they do so by necessity.) The early line of Lightmaker electronic ballasts (some of which are still kicking around) proved
that PFC circuitry was absolutely necessary in large ballasts to reduce heat and returns on the neutral, and to increase ballast reliability. Because of the added
cost, weight, and complexity of PFC circuitry, ballast manufacturers in the US have offered PFC circuitry only as an option in medium-sized 2.5-4kw ballasts, so
there are a lot of non-PFC 2.5-4kw ballasts around. And, until very recently manufacturers did not offer PFC circuitry in HMI ballasts smaller than 2.5kw in the
US (in the EU PFC circuitry in mandatory in all HMI ballasts sold), so it is very likely that the 575-1200W HMI ballast that you will get from a rental house is
non-PFC.
3) Order Only Banded Cable:
Something as simple as making sure all your 60- and 100A Stage Pin (Bates) cables are the banded variety can make a big difference when you need to hunt
down a ground leak. If you use banded 60- and 100 Amp Stage Pin (Bates) cables, you will be able to more easily monitor ground leaks in branch circuits. This
can be particularly advantageous when using the blanket approach of a single multi-phase GFCI on the main feeder trunk as illustrated below.
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For instance, if the series LED chain on the multi-phase GFCI is running high, and it covers among other things a 6 x 100 Box, the ability to get the jaws of a
leakage meter around the ground wire of the 100 Amp Bates extensions coming out of the box will enable you to isolate the source of the leak to one of the
100A branch circuits. From there you should be able identify the piece of equipment with the ground leak and eliminate it from the set before it becomes the
proverbial "straw that broke the camels' back." If you compare the illustrations above and below showing the Amp Probe clamp points for each approach, you
can see that you lose the capability to drill down to the 3rd- or 4th tier of your distribtuition system to look for leaking equipment if your 60- and 100 Amp
Stage Pin (Bates) cables have the three conductors in a single jacket.
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Precautions On Set:
1) Wrap Cable Connections:
An effective way to protect cable connections that are not designed for wet locations is to wrap the connections with a silicone-based fusing tape such as Rescue
Tape. This tape is applied by stretching the tape and wrapping it so that it over-laps itself. The tape is elastic, and stretching it activates a process by which it
fuses to itself in about 60 seconds and creates a watertight seal. To remove the tape, it has to be cut with a knife, but it does not adhere to anything but itself, so
there is no sticky residue left on the cable. Rated for up to 8'000 Volts, it is ideal for insulating cable.
2) Switch Heads at Distro Breaker:
AC/DC switches like those on tungsten 10k lights have been known to trip GFCIs when switched. For that reason it is a good idea to leave the light switch on,
and turn it on and off from the distribution box or the GFCI itself.
3) Monitor the Set:
A member of the Electrical Department trained in Ground Fault protection, leakage detection, and in the effective use and operation of GFCIs must be
designated to monitor the set for electrical safety whenever the distribution system is energized. This is so that potentially hazardous situations can be eliminated
before they develop. For instance, a member of another department may decide to place a work light (not protected by GFCI) right next to a wet area. Or,
someone may try to plug into a GFCI protected circuit with a piece of untested equipment that might trip it. To avoid against unsafe work practices or
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unnecessary tripping of GFCI devices, the designated set monitor must be aware of their responsibility, be able to recognize potential hazards, and replace
themselves before leaving set.
If the equipment deployed by the electrical department has been thoroughly tested, an earth leakage that is tripping a GFCI must then be due to a piece of gear
used by the sound department, video playback, or effects departments. Experience has shown that it is very often homemade equipment and cables used by these
departments that cause problems. Practical lamps are also a common source of earth leakage current. Of course the personnel in those departments will swear up
and down that their equipment is not causing the problem: if you have a means of testing equipment handy, you can help them confirm their claim - or not.
Honda Portable Generators
Before we conclude, I would like to correct the impression created here that you should not use Honda Portable Generators because they use a Floating Neutral
system. So that they can provide a generator that will pass OSHA job site inspections, manufacturers like Honda provide special industrial generator lines that
meet the OSHA requirements for GFCI receptacles and Bonded Neutrals. The EB generators are Honda's "Industrial Generators." The EB3800, EB5000, and
EB6500 generators are Neutral bonded and GFCI protected to meet OSHA jobsite regulations.
The Honda EB6500 meets OSHA requirements, but is too loud and too prone to voltage waveform distortion
to be practical in motion picture production applications.
Unfortunately the Honda EB generators are AVR type (prone to voltage waveform distortion from dirty loads) and quite loud because of their open frame
design. For example the Honda EB6500 is twice as load (72 dBA sound level) as the comparable Honda EU6500is (60 dBA) under full load. Since the Honda
EU6500is is an Inverter type, it is less susceptible to voltage waveform distortion, and quite a bit quieter than the EB6500 under less than full load because its'
speed is load dependent. Unfortunately, the EU series of Inverter generators, are not Neutral Bonded and do not offer GFCI protection and so do not meet
OSHA guidelines for use on work sites. So what should a film electrician recommend for portable power when filming will take place in wet hazardous
conditions?
Before I explain how you can use the Honda EU6500is generators in a fashion that meets OSHA requirements, it is necessary to understand why Honda makes
most of their generators, the EU series included, with Floating Neutrals. Most of Honda's generator product lines are designed to serve as standby power for
homes and recreational vehicles as illustrated above. As long as these generators are under 5500 watts, the circuit conductors are insulated from the generator
frame, and all other grounded surfaces (a Floating Neutral), they are exempted from the National Electric Code (NEC) Section 305-6 requiring 125 volt 15- and
20-ampere receptacles to have GFCI protection. The reason they are exempted is because they cannot serve in this capacity and have a Bonded Neutral and
GFCIs.
The reason they cannot have a Bonded Neutral and GFCIs is that as illustrated in the figure above the NEC also requires the main service head (panel) of homes
to also have Neutral bonded to Ground. Where that is the case, if the generator Neutral is also bonded to Ground, two parallel paths back to the generator are
created, one using the Neutral wire and one using the Ground wire. The Neutral current will then flow through both the Neutral and Ground conductors. Since
the Hot and Neutral wires pass through the Ground Fault sensor but the Ground wire does not, a GFCI will sense current imbalance and trip. In the case of
home standby power, bonding the Neutral in the generator defeats GFCIs when the Neutral is bonded in the main service panel. This is why most of Honda's
generators are designed with Floating Neutrals. None of the inverter generators (the EU series and EM5000is) have their neutrals bonded, and they are not
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equipped with GFCIs, which means that they do not meet OSHA requirements for use on work sites as a "Separately Derived System." So what is a film
electrician to do when they have to operate a portable generator in wet hazardous conditions?
While the Neutral and Grounds of open frame generators are easily bonded (see illustration above), the EU inverter generators are not designed for easy bonding
of the Neutral to the Ground. Inverter generators operate in a completely different manner than conventional open frame AVR generators. Inverter generators use
micro-processer control modules to produce AC power with a "true" sine wave from high voltage DC power converted from multi-phase AC power generated by
a multi-pole alternator. Unlike the simple two-pole alternators of AVR generators, an inverter generator uses a core that consists of multiple stator coils and
multiple rotor magnets. Each full rotation of the engine produces more than 300 three phase AC sine waves at frequencies up to 20 kHz, which is considerably
more electrical energy per engine revolution than produced in conventional two pole AVR generators. A fixed diode bridge rectifier then converts the more than
300 three phase AC sine waves to a DC voltage (about 200 V). AC Output is then generated from the high voltage DC by an inverter module with voltage and
frequency set by micro-processer controlled switches using Pulse Width Modulation (PWM) control logic.
Because of the inverter unit, bonding the Neutral of a Honda EU6500is to the frame is not as simple as in AVR models where you simply add a Neutral Bond
jumper from one of the generator winding leads to the frame (illustrated above.) And since, according to Article 250.4 (5) "Effective Ground-Fault Current
Path," of our National Electrical Code (NEC) requires that the grounding system create "a permanent, low-impedance circuit capable of safely carrying the
maximum ground-fault current likely to be imposed on it ....", simply putting an external jumper (pictured above) between the Ground and Neutral of an open
receptacle won't cut it. In an EU series inverter generator, bonding the Neutral to ground requires removing the main panel and inverter module to get behind the
breakers, switches, sockets etc. - not easily done or undone. According to the Honda Service Bulletins, it should only be done by a Honda qualified service
technician. Even if you did go to the expense of having the Neutral Bonded in a Honda EU6500, you still do not have GFCI protection on the machine. And,
where you can't use a film style GFCI, like a Shock Block or Bender Life-Guard, on any of the generator receptacles, you are left with hardware style 20A
GFCIs that are prone to tripping when used with non-PFC HMI, Kino, & LED power supplies.
A 100 GFCI with our 60A Transformer/Distro meets OSHA requirements
for use of a Honda EU6500is generator on work sites.
One approach that meets OSHA requirements is to use the 240V output of a Floating Neutral generator like the Honda EU 6500is with a grounded Step-down
Transformer. As an Impedance, a transformer bonds the Neutral to Ground on its' secondary or load side. Using a step-down transformer as a distro box is in fact
identical to that of a bonded building service head fed by a home standby generator. With Neutral and Ground bonded only in the transformer/distro and not in
the Honda EU6500is, you have a complete Equipment Ground circuit on the load side of the transform/distro that creates a low resistance path (illustrated
below) for fault current back to the transformer windings. Build a breaker into the load side and you provide over-current protection that will trip in a Fault
situation.
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1) A fault in a metal fixture energizes the entire housing.
2) Since, electricity prefers the path of least resistance back to its' source, the
bulk of the fault current will travel through the grounding wire, instead of the
individual making contact with the housing, because it is of a much lower resistance
than the individual.
3) Because the ground wire and the Neutral wire are bonded on the secondary side of the
transformer, the current carried by the ground wire back to the transformer creates a dead
short (over-current situation), and the breaker trips.
4) If for some reason the breaker does not trip (the short is high resistance), enough current
flows through the ground wire to create an imbalance between Hot and Neutral and the GFCI,
as a fail safe system, trips in response - shutting off power to the distribution system.
Now, to completely comply with the OSHA requirements for the use of a EU6500is on work sites all you need to do is outfit the transformer with a 60A Bates
receptacle that will enable you to use film Style Shock Block or Bender GFCIs. With the Neutral and Ground bonded in the transformer, in the event of a Fault,
current will go to ground and GFCIs will operate reliably even when the power is being generated by a Floating Neutral generator like the EU6500is. And, to
assure safe distribution of power from the generator in the wettest conditions, use a transformer with a Nema 3R "all weather" rated housing.
The ability to use GFCI protection in wet conditions or locations has got to be one of the greatest benefits to using a transformer/Distro with the Honda
EU6500is Generator. Not only can you use a generator that is quiet and produces clean power, but it also makes it possible to use GFCI technology, like Shock
Blocks, that are specifically designed for motion picture applications.
__________________________________________________________________
About the Author
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Guy Holt presenting his nationally recognized curriculum on
"Electrical Hazard Protection for the Entertainment Industry" to members of IATSE Local 481.
Guy Holt has served as a Gaffer, Set Electrician, and Generator Operator on numerous features and television productions (for a partial list of credits see his
imdb listing). Guy Holt developed a nationally recognized curriculum on "Electrical Hazard Protection for the Entertainment Industry" that he teaches through
the IATSE Local 481 Electrical Department "TECs" Program (pictured above.)
Guy Holt presenting to the Electrical Department of IATSE Local 481 as part of the
ÒAdvanced Power and Generation for Set Lighting Technicians SeminarÓ
Guy Holt presented on Harmonics to the Electrical Department of IATSE Local 481 (pictured above) as part of the ÒAdvanced Power and Generation for Set
Lighting Technicians SeminarÓ offered by Russ Saunders of Saunders Electric (the provider of power generation services for the Academy Awards since 1952
and a recipient of a technical Emmy). Here is what industry leaders have to say:
Guy Holt is "among the 1% of film technicians world wide that truly understand the dynamics
of power generation and Harmonics." - Russ Saunders, Saunders Electric
Guy Holt demonstrates a "broad technical and scientific understanding, and (is) blessed with a nice real world practical point of view ... (he possesses
a) mix of knowledge and the ability to communicate it effectively. " - David Pringle, Chairman and CTO, Luminys Systems Corp, Manufacturers of
Softsun and Lightning Strikes lumnaries.
"Great work!... this is the kind of thing I think very few technician's ever get to see, and as a result
many people have absolutely no idea why things stop working." - Harry Box, Author "Set Lighting Technician's Handbook"
"Following the prescriptions contained in this article enables the operation of bigger lights, or more smaller lights, on portable generators than has ever
been possible before." - Harry Box, Author "Set Lighting Technician's Handbook"
This article is cited in the 4th Edition of Harry Box's "Set Lighting Technician's Handbook" and featured on the companion website "Box Book Extras" (click on
link below for more details.)
Guy Holt's other credentials include:
- IATSE Local 481 Certified Gaffer
- IATSE Local 481 Certified Generator Operator
- IATSE Local 481 Certified Lighting Balloon Operator
- Certificate Holder of the MQ Power "MQP Special Generator (Crawford) Technical Service Seminar"
Guy Holt participating in a panel discussion as part of IATSE Local 481's Advanced HMI & LED Lighting Seminar (Fred Horne, Former Arri Northeast Sales Rep pictured left)
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Guy Holt is also the owner of ScreenLight & Grip, a lighting and grip rental company in Boston, MA renting Honda, MQ, and Crawford generators for motion picture production for
18 years. Inquiries can be sent to the attention of Guy Holt at rentals@screenlightandgrip.com
Guy Holt preparing to modify a Honda EU6500is generator
__________________________________________________________________
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