Strand Lighting Inc.
6603 Darin Way - Cypress, CA 90630
Tel: 714-230-8208 Fax: 714-899-0042
E-mail: mlay@strandlight.com
Strand Lighting Inc.
Ampere Interrupting
Capacity Calculations
Or……
Why Are There Extra
Fuses In My Dimmer
Rack?
23-Apr-2005
Ampere Interrupting Capacity
Page 1 of 11 Pages
Strand Lighting Inc.
6603 Darin Way - Cypress, CA 90630
Tel: 714-230-8208 Fax: 714-899-0042
E-mail: mlay@strandlight.com
1.0
INTRODUCTION
When designing dimmer rack circuit protection for a new or renovated facility,
theatrical consultants typically specify a dimmer rack main circuit breaker (or
fused disconnect) capable of supporting the expected connected lighting load.
Typical Fused Disconnect & Molded Case Main Circuit Breaker
This may not be sufficient equipment (or personnel) protection since this device
only provides overload protection.
It is critical that the theatrical consultant confirm with the project electrical
engineer that the overall electrical system design provides the proper interrupt
protection to the dimmer rack as well.
The short circuit "interrupting" protection rating is different than the normal
running ampere rating of the main protective device. The interrupting rating is the
maximum fault current that would flow if there were a bolted short (like a wrench
falling against some buss bars) in the circuit.
The fault current rating is determined by calculating the AIC (Ampere Interrupting
Capacity) short-circuit rating of the electrical system. This calculation needs to
be performed by a qualified electrical engineer as there are many variables
ranging from conductor length, conductor constants, transformer KVA,
transformer impedance and the available short circuit value at the beginning of
the circuit. Once the fault current rating is known, the interrupt protection
requirement should be listed in the project bid specifications or drawings.
23-Apr-2005
Ampere Interrupting Capacity
Page 2 of 11 Pages
Strand Lighting Inc.
6603 Darin Way - Cypress, CA 90630
Tel: 714-230-8208 Fax: 714-899-0042
E-mail: mlay@strandlight.com
2.0
AMP TRAPS
Dimmer rack AIC ratings vary between 5,000 and 100,000 AIC. In order to
increase the AIC ratings of dimmer racks manufacturers sometimes add current
limiting fuses (commonly known as "Amp Traps") to the buss structure design of
their dimmer racks.
Amp Traps are special current-limiting (not overload-limiting) fuses that are
capable of opening a faulted circuit before the fault current has sufficient time to
reach its maximum value.
Amp Traps are always used in conjunction with another overload current
protection device such as a main circuit breaker, or fused disconnect.
Typical Amp Traps
23-Apr-2005
Ampere Interrupting Capacity
Page 3 of 11 Pages
Strand Lighting Inc.
6603 Darin Way - Cypress, CA 90630
Tel: 714-230-8208 Fax: 714-899-0042
E-mail: mlay@strandlight.com
3.0
DO I NEED AMP TRAPS IN MY DIMMER RACK?
Amp Traps installed in dimmer racks are typically optional, and are only added to
the dimmer rack assembly if the AIC requirement for the facility is known when
the project is bid. The additional buss structure added to a dimmer rack to
support Amp Traps will increase the cost of the rack. Before adding them to your
specification, be absolutely certain that they are required.
Strand recently visited an installation with 16 dimmer racks (with rack to rack
bussing) installed on two 2400A services. These particular racks had an AIC
rating of 50,000. However the facility had paid additional money to include amp
traps in the bill of materials to increase the AIC rating of the dimmer racks to
100,000.
Unfortunately, the fault current calculations by the facility electrical engineer
showed that even with a relatively large service, the available fault current was
only 43,000 AIC effectively rending the Amp Traps unnecessary. Had this
calculation been made before the tender, project cost savings could have been
achieved.
Amp Traps
Amp Traps In A Strand Lighting CD80SV Dimmer Rack
23-Apr-2005
Ampere Interrupting Capacity
Page 4 of 11 Pages
Strand Lighting Inc.
6603 Darin Way - Cypress, CA 90630
Tel: 714-230-8208 Fax: 714-899-0042
E-mail: mlay@strandlight.com
Amp Traps
Amp Traps Installed In A Strand Lighting C21 Dimmer Rack
4.0
DIMMER MODULE CIRCUIT BREAKERS
A related question that we are frequently asked is how a dimmer rack can be
rated for 100,000 AIC - but have a dimmer module branch circuit breaker that is
only rated for 10,000A AIC?
This is because the circuit breaker must be rated for the applied voltage,
continuous amperage load, and must also have an AIC rating equal to or greater
than the available current at the location in the circuit where it will be installed.
Since the dimmer module is located between the main and branch circuit, a
10,000 AIC breaker is appropriate in many dimmer module designs.
23-Apr-2005
Ampere Interrupting Capacity
Page 5 of 11 Pages
Strand Lighting Inc.
6603 Darin Way - Cypress, CA 90630
Tel: 714-230-8208 Fax: 714-899-0042
E-mail: mlay@strandlight.com
5.0
BEWARE OF SIMPLISTIC FAULT CURRENT CALCULATIONS
By Keith Lane, SASCO
Specifying and installing underrated equipment can undermine your power
distribution design
Fault current calculations are a critical piece of the electrical design/engineering
puzzle for electrical distribution systems in commercial and industrial
installations. A fault current calculation determines the maximum available
current that will be available at a given node, or location, in the system. Once the
fault currents have been calculated, you can then select overcurrent protection
equipment, breakers, and fuses with a fault current rating equal to or greater than
those values (NEC 110.9). If a breaker or fuse isn't rated to handle the maximum
available fault current it might see, it may not operate properly and its internal
parts could fuse together or the device may blow up under the destructive
stresses of a fault condition, which can cause serious injury and or property
damage.
Fault current calculations are based on Ohm's Law (V=I×R). To determine the
maximum current available at any given point in a distribution system, the
equation is rearranged to solve for current (I=V÷R). In a short circuit condition,
resistance (R) gets very small and is
essentially based on the total
resistance in the electrical
distribution system, from the derived
source of power to the point of the
fault.
As noted above, basic point-to-point
fault current calculations are derived
using Ohm's Law. System
characteristics like voltage,
conductor length, conductor
constants, available short circuit
values at the beginning of the circuit,
and transformer percentage
impedance are used to find the fault
current at various locations within
the system.
23-Apr-2005
AIC calculations for this roof-mounted distribution
panel must include motor contribution factor.
Ampere Interrupting Capacity
Page 6 of 11 Pages
Strand Lighting Inc.
6603 Darin Way - Cypress, CA 90630
Tel: 714-230-8208 Fax: 714-899-0042
E-mail: mlay@strandlight.com
These calculations become more complicated when you consider that the
resistance (R) should actually be replaced with an impedance value (Z).
Impedance is calculated using the formula Z=√(R2+X2). For instance, instead of
using the percentage impedance for transformers, percent Z, percent X, and
percent R are used and converted to X and R values on a per unit basis. And
instead of using a defined constant for conductors, the impedance for conductors
within the electrical system are also broken down into X and R components of
the impedance.
The ratio of reactance to resistance, or X/R, determines the peak asymmetric
fault current. The total asymmetric current is a measure of the total DC
component and the symmetrical component. The DC component causes the
short circuit current to be asymmetrical. The asymmetrical component decays
with time and will cause the first cycle of a fault current to be larger in magnitude
than the steady-state fault current.
In addition, the decay of the DC component depends on the X/R ratio of the
circuit between the source and the fault. If the fault comprises all reactive
components, then the resistance value of the X/R ratio is zero and the DC
component will never decay. If the reactive component of the impedance is zero,
then the DC component decays immediately. In the real world, impedance is
neither all resistive nor all reactive. It's a combination of both.
As you can see, calculating asymmetric current can be very difficult. Accurate
calculations require you to know the rate of change of all reactive components in
the system. However, multipliers, based on the actual calculated X/R ratios, have
been developed to somewhat simplify this process. They're used in conjunction
with the symmetric fault current calculation to provide an asymmetric fault
current, which includes the DC offset component.
The issue becomes more confusing based on the fact that all low-voltage
protective devices are tested at predetermined X/R ratios. If the calculated X/R
ratio at any given point in the
electrical distribution system
exceeds the tested X/R ratio of
the overcurrent protective device,
then you must derate the effective
rating of the gear.
All low-voltage protective devices are tested at predetermined X/R ratios.
This can be a very critical issue if
your fault current calculation doesn't include the X/R ratio or the peak asymmetric
fault current. A fault condition with a high reactive component can potentially
require a de-rating of the gear to below the symmetric fault current value
23-Apr-2005
Ampere Interrupting Capacity
Page 7 of 11 Pages
Strand Lighting Inc.
6603 Darin Way - Cypress, CA 90630
Tel: 714-230-8208 Fax: 714-899-0042
E-mail: mlay@strandlight.com
calculated at the gear. Replacing installed gear that has been undersized
because these asymmetric calculations haven't been provided could certainly
ruin your day.
In addition, induction motors can contribute short circuit current back into the
system during a fault condition. The motor can act as a generator and contribute
“back EMF” from the inertia of the load and the rotor driving the motor after the
fault occurs. Because the flux is produced from the induction from the stator and
not the field windings, the motor contribution is quickly reduced and will only last
for a few cycles. The total motor contribution depends on many factors, including
motor horsepower, voltage, the reactance of the systems at the point of the fault,
and the reactance of the motor.
The following example illustrates a potential problem of installing underrated
electrical equipment if X/R ratios and motor contribution aren't considered in the
initial design.
Example design criteria:
* Utility transformer rating: 2,500kVA
* Utility transformer % impedance: 4.775%
* Service conductors: 10 sets of 600 MCM copper
* Available fault current at utility transformer secondary: 63,000A
* X/R ratio at the utility transformer secondary: 11
* Motor contribution: 400 hp
* Ampacity of service conductors: 4,000A
* Service gear tested X/R ratio: 4.9
A fault current of 62,321A is calculated at the switchgear. This value is based on
the 2,500kVA utility transformer with 4.775% impedance and minimal impedance
from the service conductors (11 feet of 10 sets of 600 MCM copper).
The simple form of this calculation, based on infinite bus theory, is indicated
below:
2,500kVA÷(√3×480V)÷0.04775=62,975, or 63,000AIC at the utility transformer
secondary
However, the AIC is further reduced at the service to 62,321A, based on the
impedance of the service conductors. This assumes no contribution from the
motors in the system or from the asymmetric component.
23-Apr-2005
Ampere Interrupting Capacity
Page 8 of 11 Pages
Strand Lighting Inc.
6603 Darin Way - Cypress, CA 90630
Tel: 714-230-8208 Fax: 714-899-0042
E-mail: mlay@strandlight.com
If the switchgear is rated based on this information only (i.e. no X/R or motor
contribution), the switchgear could have easily been specified as 65,000AIC.
In this example, we're looking at a total contribution of 400 hp for the motor and
an X/R ratio of 11 at the secondary side of the utility transformer. The X/R ratio at
the switchgear is calculated at 10.37
based on the X/R ratio at the service
transformer provided by the utility and the
contribution of resistance from the service
conductors and the reactance from the
motors in the system. The electrical
service equipment has been tested and
rated at an X/R ratio of 4.9. The
calculation of this X/R value can be
performed using sophisticated software,
electronic spreadsheets, or by long perunit handwritten calculations. However,
this work is beyond the scope of this
article.
This 4,000A service switchgear requires
AIC bracing and overcurrent protection
in excess of 65,000AIC.
The motor adds a total of about 2,406A of
fault current over the first half cycle. The
motor contribution is based on the
characteristics of the individual motors,
but can be estimated by taking the total
horsepower contribution, multiplying it by
5, and then converting this number to
amps.
The high X/R ratio requires the de-rating of the gear by a factor of 1.139. The
1.139 is a multiplier factor equal to the asymmetric current at the calculated X/R
ratio divided by the asymmetric current at the tested X/R ratio.
In addition, the following formula can be used to calculate the multiplier factor
based on the calculated X/R ratio and the test X/R ratio for a given overcurrent
protection device:
In this case, the total fault current available, including the motor contribution,
would be 64,727A.
23-Apr-2005
Ampere Interrupting Capacity
Page 9 of 11 Pages
Strand Lighting Inc.
6603 Darin Way - Cypress, CA 90630
Tel: 714-230-8208 Fax: 714-899-0042
E-mail: mlay@strandlight.com
62,321A (symmetric fault current)+2,406A (motor contribution)=64,727A
The gear, based on asymmetric de-rating, would be rated at 57,068A.
65,000A (gear rating)÷1.139 (X/R derating factor)=57,068A
Assuming that the simplistic form of fault current calculations was initially used to
size and install the main switchboard, the switchgear would have a fault current
rating lower than the maximum available short circuit current and would therefore
have to be replaced.
More complicated issues can arise if closed transition paralleling (utility and
generator) gear, parallel redundant UPS systems with closed transition bypass,
or high-impedance grounding systems are used. In many cases you should rely
on a qualified professional and the use of advanced software to ensure that
electrical gear is properly rated to provide protection for personnel and property.
Keith Lane is a registered P.E., RCDD/NTS specialist, LC, LEED A.P. and serves
as vice president — engineering at SASCO Design/Build Electrical Contractors
and Consultants in Woodinville, Wash.
Keith Lane's article was originally published in EC&M magazine Sep 2004
23-Apr-2005
Ampere Interrupting Capacity
Page 10 of 11 Pages
Strand Lighting Inc.
6603 Darin Way - Cypress, CA 90630
Tel: 714-230-8208 Fax: 714-899-0042
E-mail: mlay@strandlight.com
Strand Lighting Offices
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Via Delle Gardenie 33
00040 Pomezia-Roma, Italy
Tel: +39 06 919631
Fax: +39 06 9147136
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Tel: +49 30 707 9510
Fax: +49 30 707 95199
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Buildings 3-6
12069 Moscow, Russia
Tel: +7 095234 42 20
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3 On Yiu Street
Shatin, N.T. Hong Kong
Tel: (852) 2757 3033
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Strand Lighting Inc.
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Cypress, CA 90630, USA
Tel: +1 714 230 8200
Fax: +1 714 230 8173
Strand Lighting (Canada) Inc.
Eclairages Strand (Canada) Inc.
2430 Lucknow Drive #15
Mississauga, Ontario
L5S 1V3, Canada
Tel: +1 905 677 7130
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Unit 3 Hammersmith Studios,
Yeldham Road, Hammersmith,
London W6 8JF United Kingdom
Tel: +44 (0) 1592 652 333
Fax: +44 (0) 208735 9799
23-Apr-2005
Ampere Interrupting Capacity
Page 11 of 11 Pages