Electrical Safety in Grounded,
Resistance Grounded and
Ungrounded Systems
A Detailed Guideline for Installers,
System Designers and Technical Personnel
BENDER Group
Introduction
Ground faults in modern power systems
An introduction into the basics
Introduction
Hazardous ground fault occur frequently in today’s electrical
wiring. There are multiple ways to describe
a ground fault. Please review some typical expressions below:
nA leakage current from a conductor to a frame or ground
measured in milli-amperes or amperes
nAn insulation breakdown measured in ohms or kilo ohms
n
A charging current leaking into the ground (capacitive leak)
Why do we have different terms to describe similar occurrences?
What amount of current can be expected in a power system?
What does the resistor do in a resistance grounded
power system?
These question are generated due to the variety of power
system employed in the field. There is a huge variety of relays on
the market to protect these systems. This booklet has been written to provide a brief introduction to the major power systems
and the devices manufactured by Bender company which are
suited best to protect these systems in case of a ground fault.
The calculation made in the following examples were based on
simple forms assuming test bench or ideal conditions. Values
were chosen randomly to support the basic ideas and fundamentals. The following information might be used as a guideline
for an integrator or as a reference for a system designer who
is facing the first hurdle of identifying a product for a specific
ground fault problem. The text was written in a way that both,
technical and non-technical personnel can benefit from the
information.
Datasheets available at www.bender-ca.com
What type of monitor does the Bender company
recommend for my system?
The information in this booklet was carefully prepared and is believed to be correct, but Bender makes no warranties respecting it and
disclaims any responsibility or liability of any kind for any loss or damages as a consequence of anyone’s use of or reliance upon such
information.
2
This right to modifications are reserved. ©
Table of Contents
General Information
Different technologies for different power systems
4
Grounded Systems
Basic power systems
The ground fault
5
6
Grounded AC systems 60 cycles - The ground fault device
GF interrupting - Shunt trip breaker
GF interrupting - Contactor
Grounded AC systems with VFDs
DC systems
Location ground faults
- with portable equipment - with a current clamp
- with fixed equipment
7
8
9
10
11
12
13
14
Resistance Grounded
Basic power systems
The ground fault
15
15
16
17
18
Resistance grounded AC systems
- GFGC (Ground Fault Ground Check)
- Ground Fault - NGR Monitor
Locating ground faults
Ungrounded (floating) Systems
Basic power systems
The ground fault
Insulation Monitoring Device IMD
- Passive IMD
- Active IMDs
Ungrounded systems
- Locating ground faults with portable equipment
- Story: Ground fault location in a 480V delta fed system
- Locating ground faults with fixed equipment
Ground Fault Location System
19
19
20-21
22-23
24
25
26
27
Offline Monitoring
Offline monitoring with Bender IR ... devices
The Offline IMD’s and their measuring voltages
28
29
Attachment
SELECTION GUIDE for Ground Fault Relays
30
Index
31
3
General Information
Different technologies for different power systems
Not every monitor or relay will work on any power system.
A ground fault relay (GFR) in combination with a current transformer (CT) will work on grounded or resistance grounded systems,
but will need very special consideration if employed in a floating
system. An Insulation Monitoring Device (IMD) will work very
nicely in a floating system, but will do nothing but false alarm in
a grounded system.
Case # 1
The Insulation monitoring device (IMD) installation below will not work out! The IMD (Online megger) will send a measuring signal
into a three phase system. The signal will immediately find the neutral ground bonding jumper and indicate a ground fault.
Supply side 480 / 277 V AC
L1
L2
L3
circuit breaker
Motor, 3 p h
L1
T1
L2
T2
L3
T3
Bender
IMD
Neutral Ground (NG)
Jumper
GND
WRONG
application!
AMP measuring mode
Schematic 01: The grounded system with IMD
Case # 2
The ground fault relay (GFR) installation below will not work out! The current transformer (CT) is expecting a serious amount of
ground fault current. A floating delta normally does not create the fault current magnitude needed since it does not provide a low
ohmic neutral ground return path. The device will never trip, not even if there would be a dead short to ground for a couple days.
L2
NO current
return path
L3
transformer ,
passiv
Motor, 3 p h
L1
T1
L2
T2
L3
T3
WRONG
application!
Bender
RCM420
GF relay
GND
Leakage
capacitance
L1
circuit breaker
Faul t
(isolation)
Supply side 480V AC
GF=i.e. dead short
only mA current
Schematic 02: The ungrounded system with RCM technology
4
Grounded Systems
Basic power systems - Grounded
Grounded systems are derived from a power source where the
neutral is solidly tied to ground via a ground neutral bonding
jumper (NG). Often encountered is the typical three phase
208/120V Y or 480/277V Y configuration. Another possibility is a
single phase transformer where the neutral is tied into ground
or sometimes, in very rare occasions, we might encounter
corner grounded deltas. The general population is very familiar
with solidly grounded systems due to the fact that nearly every
residency in the U.S. is derived from a 240/120V transformer with
Supply side 480 / 277 V AC
center tap. The center tap is bonded solidly to ground. As always,
there are advantages and disadvantages to the grounded power
system. One disadvantage is the high amount of possible fault
current in a ground fault situation. Fire damage or personnel
injury can occur. Nevertheless, a tripped over current breaker
or a GFCI will enable the electrician to quickly identify a faulty
branch. Action will often be taken after a fault has occurred.
Preventative maintenance is not necessarily associated with the
grounded system.
circuit breaker
Motor, 3 p h
L1
L1
L2
L2
L3
L3
V
T1
V12=480 V
V13=480 V
T2
V23=480 V
T3
GND
V
VNG=0 V
V3G=277 V
Neutral Ground (NG)
Jumper
Schematic 03: The grounded system
5
Grounded Systems
The ground fault
The magnitude of a ground fault current in a solidly grounded
system can be very high. Its magnitude depends on the system
voltage and the resistance of the ground fault causing part itself.
The ground fault current can easily reach a value which is multiple times higher than the nominal load current.
A simplified calculation will explain how the high amounts of
current are generated:
Please review the schematic on the bottom. The current IF is
defined as:
(1)
IF =
IF
RGF
V3G
+ RGR + RNG
It can be shown with this example calculation that a theoretical
fault current will be devastating if a dead short occurs. Nevertheless, a ground fault relay or over current protective devices
should trip immediately and interrupt power from the load.
How many Amps would flow if a human would touch the same
circuit? Answer: Replace the dead short value of 0.1 Ohm with
a more realistic figure for a human body part. Lets assume that
a person is resting on the frame of a motor. We assume 1000
Ohms of resistance between phase L3 and a human body.
IF =
277 V
= 0.277 mA
1000 Ω + 0.2Ω + 0.1Ω
The current is a multiple of 15 mA, which is considered to be the
let go value for humans. 50 mA is considered to be lethal.
= Fault Current
V3G = Voltage between faulted phase and ground
phase andRgro
ound
= Resistance value at shorted point
GF
ted point R = Resistance of ground path
GR
th
RNG = Resistance of neutral ground bonding jumper
ound bonding jumper
(Please note: The resistance values werre randomly chosen)
ance values werre randomly chosen)
IF =
277 V
= 692.5 A
0.1Ω + 0.2Ω + 0.1Ω
Supply side 480 / 277 V AC
L1
L2
Neutral Ground (NG)
Jumper
NG path
(resistance
assumed 0.1 Ohm)
L3
Fault 2 (GF)
i.e. dead shor t
Resistance = 0.1Ohm
circuit breaker
L1
T1
L2
T2
L3
T3
Motor, 3 p h
V
Fault current p ath via ground
(resista nce assumed 0.2 Ohm)
GND
V =277 V
3G
Fault 1
(break)
Schematic 04: The grounded system with single ground fault and broken Ground
6
Grounded Systems
Grounded AC systems 60 cycles - The ground fault device
Most technicians are very familiar with a current transformer
based ground fault current relay. Even non technical personnel
encounter them on a daily basis in public rest rooms protecting
a wall outlet in a wet area. The operating theory behind the
relay is as follows. A current transformer (CT) or “donut” is placed
around the power wires leading to the protected load. It is
important that hot and neutral wires are fed through the CT. This
goes for both, single phase and three phase systems. One might
come across a three phase system without a neutral, feeding a
pump or an industrial motor. In this case the three phases only
will be fed through the CT. Basic rule for three phase systems: If
the neutral is carried out to the load -> Feed it through the CT.
If there is no neutral -> Then do not worry about it. The current
transformer will always read zero current in a healthy system
even under a full load condition. In accordance to Kirchhoffs
laws, Incoming and Outgoing currents will cancel each other
out. Assume a 10A load connected to a 480/277VAC system. 10A
will be fed from the source into the load, therefore 10A will have
to return from the load back to the source. The CT will measure
both simultaneously since it is placed around all conductors.
The values were randomly chosen. Here is what the CT would see
at a specific moment in time. In accordance with the schematic
below (for a healthy system):
10 A - 5 A - 5 A = 0 A
A ground fault (lets assume 1A) will divert some of the current
from the arrangement and bypass the CT via the ground wire, a
frame or the building ground and return back to the source. The
new equation for the CT is now:
10 A - 5 A - 4 A = 1A
whereas 10A go into the load, 9A return to the source via the
phase L2 and L3 and 1 A returns to the source via the ground
wire. The CT will step the current (1A) down and forward it to
the Ground Fault Relay (GFR). The GFR will then alarm when its
set point has been increased. The GF relay in combination with
a zero sequence CT will work in resistance grounded systems as
well. It will run into its limitations in circuits where wave form
modifying equipment, such as Variable Frequency Drives (VFDs)
or rectifier components are installed.
(2)
Σ I1,I2 ,I3 ,...,I x = 0
Supply side 480 / 277 V AC
L1
L2
L3
circuit breaker
transformer ,
passiv
Motor, 3 p h
L1
I1=10 A
T1
L2
I1=5 A
T2
L3
I1=4 A
T3
Fault 1
(isolation)
GND
Bender
GF relay (1)
with
summation
transformer
Neutral Ground (NG)
Jumper
I1=1 A
Schematic 05: The grounded system with single ground fault and GF relay
(1) Relays: RCM 420 series
7
Grounded Systems
GF interrupting - Shunt trip breaker
Grounded and resistance grounded systems often require not
only to be monitored but to be interrupted as well. The ground
fault has to be sensed and power has to be removed ASAP (often
in milli seconds). Trip levels for the GFCIs (Ground Fault Circuit
Interrupters) vary from application to application. Personnel
protection is considered to be at 6mA in the U.S. Equipment
protection can be found anywhere from 10mA up to multiple
Amps. Industrial branch or load protection is often be set 5Amps.
Service entrance protection is most likely set to trip at levels
A1
120 V AC
k
A2
L
11
in the multiple hundreds of Amps. Below is a wiring schematic
outlining the connections between a typical Bender RCM420, a
shunt trip breaker and a three phase load.
A1, A2 = External power supply; K,L = Connection to the CT;
11,12 = alarm contacts will close and apply 120VAC to the shunt
trip in case of an increased set point. The shunt trip breaker
will the interrupt power to the load. Please note: The shunt trip
breaker will have to be manually reset after a trip.
RCM 420
12
Bender
RCM420
in failsave
mode
14
Supply side 480 / 277 V AC
L1
L2
L3
Motor, 3 p h
L1
T1
L2
T2
L3
T3
transformer ,
passiv
NGR
shunt trip breaker
GND
Schematic 06: The RCM420 connected to a shunt trip in a resistance grounded system
8
Grounded Systems
GF interrupting - Contactor
The wiring schematic on page 8 employed a shunt trip breaker
for interrupting purposes. A contactor in combination with the
Bender RCM420 can accomplish the same task. Please review the
schematic below:
A1
120 VAC
k
A2
L
11
A1, A2 = External power supply; K,L = Connection to the CT;
11,14 = alarm contacts will open and remove power (120VAC)
from the contactor holding coil in case of an increased set point.
The contactor will drop out and interrupt power to the load.
Please note:
The contactor can be automatically reset after the fault has been
cleared.
RCM 420
12
Bender
RCM420
GF relay
14
Supply side 480 / 277 V AC
L1
L2
L3
Motor, 3 p h
L1
T1
L2
T2
L3
T3
contactor
NGR
transformer ,
passiv
GND
Schematic 07: The RCM420 connected to a contactor in a resistance grounded system
9
Grounded Systems
Supply side 480 / 277 V AC
L1
L2
L3
Other issues with VFDs:
nThe VFD often incorporates built in EMI filters. They provide
a leakage path to ground and add to the overall system
leakage.
nThe drive uses a multiple kHZ carrier frequency. The carrier
frequency can cross
the gap between insulation and ground and add to the
inherent leakage.
n
Harmonic content
n
Transient voltage spikes
The solution: Grounded systems with VFDs have to be protected
by means of “active” current transformers with built in filter technology. The Bender RCMA470 in combination with the active CT
employs a double coil system which enables the unit to measure
accurate AC, DC and mixed AC/DC currents from 0 to 700Hz. Its
trip settings range from 6mA up to 3A. It can therefore be used
for personnel and branch/motor protection.
circuit breaker
L1
Variable Frequency Drive
EMI filter
Rectifier
DC link
Inverter
L2
diode
Neutral Ground (NG)
Jumper
T1
T2
L3
Bender RCMA relay (1)
with active transformer
Motor, 3 p h
Possible
AC fault
60 Hz
Capacitive
leakage
through
EMI filters
SCR
Internal
DC fault
T3
GND
AC systems with VFDs
The 60 cycle GFRs have limitations when the circuitry involves
VFDs (Variable Frequency Drives). Tests have shown that the
typical GFR cannot keep the adjusted trip point when the system
frequency changes to values below 60 cycles. Even worse, a
total failure can be expected at frequencies below 12 cycles. A
variable frequency drive converts the incoming AC internally into
DC, which will then be modulated again into a variable cycle AC
leading to the load. Internal VFD - DC grounds can not be detected with conventional GFR technology. The common “passive”
CT needs alternating currents to detect a ground fault, therefore
DC currents will go unnoticed. Some drives might be equipped
with their own internal scheme to detect ground faults which
will eventually trip in the high Ampere range. Early warning or
personnel protection cannot be guaranteed in this case.
Variable
cycle
AC faults
Schematic 08: The grounded AC system with a variable frequency drive (VFD) and RCMA technology
10
(1) Relays: RCMA 420 (Motor protection)
RCMA 421/426 series (Personnel
Grounded Systems
DC systems
Bender RCMAs monitor DC and mixed AC/DC systems. The
unique measuring principle can be used for protection if the DC
system is grounded as shown below. In this case the negative
pole of the DC power supply or the battery is tied into a chassis
or the building ground. The active CT would be placed around
both, the negative and positive conductor leading to the load.
A DC leakage current will bypass the CT through ground. Its
circuit breaker
transformer ,
active
Motor, D C
I(L+)=10 A
L+
+
I(L-)=9 A
V
A1
F2
A2
V
V(L-G)=0 V
-
Bender RCMA relay (1)
with active transformer
T2
F1
V(L+G)=24 V
L-
Faul t
(isolation)
T1
GND
Supply side 24 V DC
magnitude will be relayed to the RCMA device and an alarm will
be triggered. The calculation is based on the same principles that
were discussed in previous chapters. Remember, it is important
to note that this technology works on AC, DC and mixed AC/DC
circuits. For example: A single RCMA relay can protect a DC and
an AC system at the same time. Imagine a DC control circuit and
a 120VAC power wire going through the same CT. A standard
GFR could only monitor the AC line. The RCMA will protect both.
IF=1 A
Schematic 09: 24V DC grounded system with RCMA technology and single fault
(1) Relays: RCMA 420 (Motor protection)
RCMA 421/426 series (Personnel
11
Grounded Systems
Locating ground faults with portable equipment
Normally, it is not too difficult to locate a ground fault in a
grounded system. As described before, the ground fault current
is usually in the high amp range and will force the over current
device or the ground fault relay to trip the faulty circuit. It can
get tricky when there is only one ground fault relay installed at
the power source, protecting a multitude of circuits down the
road. A typical situation is the roof mounted air conditioner in
an industrial plant. The AC unit faults to ground and trips out the
main service entrance, because there was no branch protection
provided. It will not be a simple task to find the culprit beneath
50 similar AC units on the roof. One solution to the problem is
provided below:
Supply side 480 / 277 V AC
L1
L2
L3
circuit breaker
CB open!
Locating faults in disconnected systems (Offline search)
A typical means of checking for a ground fault in a disconnected
system is the megger. A technician will connect a “meggering
device” between the motor leads and ground (chassis) and inject
a high voltage (normally 500V) into the motor circuit.
The ground fault is indicated if the “megger current” finds a
break through path to ground. Note: The megger only works on
disconnected systems. Make sure that the test object is voltage
free before applying the megger.
transformer ,
passiv
Motor, 3 p h
L1
T1
L2
T2
L3
T3
Fault 1
(isolation)
GND
500 V
Neutral Ground (NG)
Jumper
Megger current
Schematic 10: Meggering a load
12
Grounded Systems
Locating ground faults with a current clamp
Locating faults in life systems (Online search)
The current transformer based ground fault relay is basically
nothing else but a very sensitive ammeter. The typical clamp-on
type ammeter is used to measure load currents by simply clamping around a single conductor. The same ammeter will read
zero if the technician clamps around all conductors (Including
the neutral, if there is one present) in a healthy system. This is the
zero sequence principle we described earlier in the Grounded AC
system 60 cycles section on page 8. A healthy system will reveal
zero current, but a ground fault current will show up immediately on the clamp. Please note: Do not incorporate the ground
conductor when you put the clamp around the power wires. The
ground fault will be chased by clamping around the conductors
coming from the power supply first. From there we will work
on the branches. From there we will start clamping around the
individual loads. This is often described as “hunting down” the
problem.
A typical clamp designed for measuring load currents will do the
job if the ground fault exceeds a couple amps. A more sensitive
clamp has to be chosen if the ground fault has a magnitude
below 1 Amp. Below is the picture of a clamp which is actually
capable of detecting faults below 10mA. Here comes the tricky
part. Please be advised that using a clamp on a bolted fault is
pretty much useless. The over current devices or GFI breakers will
have tripped long before the handheld clamp can be employed.
The methods described above will only make sense at ground
faults below breaker trip level. A disadvantage of measuring at
lower amp levels will be the charging current of the system or
the already present inherent system leakage. A rule of thumb in
electrical installations is 1A leakage per 1000KVA. It will depend
on the experience of the electrician to determine if he is chasing
a charging current or a low level ground fault.
A simple example for explaining inherent system leakage: The
motor consists of windings, wire and insulation enclosed by a
metal frame. A conductor separated from ground via insulation
acts as a small capacitor. A piece of insulation between the conductor and the frame also has a certain resistance. The capacitance is usually extremely small and the resistance is in general
in the Meg Ohm range. Nevertheless, the combined values
can add up. The larger a power system is the larger the overall
natural leakage will be. Imagine a motor in an industrial plant
which leaks a minor fraction of current into ground (e.g. 1mA =
1/1000 of an Amp). That does not sound like very much and the
decision is made to employ a ground fault relay with a 10mA set
point. The GFR trips immediately because it was overlooked that
there are 15 similar motors connected to this branch. 15 x 1 mA
= 15 mA leakage already present in the system. This does not
even account for the capacitive leakage of cabling or for drive
components with their built-in EMI filter circuitry.
hot
neutral
Illustration 01: Measuring (1)
(1) Device: RCT 3283
Accessory:Large clamp RCT 9131
13
Grounded Systems
Locating ground faults with fixed equipment
Online system
Assume that there is a 208VAC three phase system with 120 sub
branches installed in a facility. The supervisor wants to know at
all times if the system is in good shape or if there is a problem. A
problem has to be indicated immediately, the source has to be
identified and fixed. The zero sequence technology described in
earlier chapters can help here as well. A fixed installed ground
fault scanning system can be employed to monitor unlimited
numbers of branches 24/7. In this case, a CT will be placed around
every single main and sub branch. The CTs will be connected to
12 channel evaluators which themselves are wired into a central
processing and display unit. The ground fault will flow from the
source into the faulty branch, into the faulty load and from the
ground back to the source.
The CTs in this path will recognize the current and alert the
respective evaluator. The central processing unit will display the
amount of fault current and the fault location. Remote alarms
can be sound and even the alert via the internet is no problem
nowadays. Fixed ground fault location systems are tremendously
beneficial when down- time is not an option. Please note that the
CTs are measuring the full amount of ground fault current and
are designed to detect leaks in an early stage. Fast developing
ground faults will most likely trip the over current device before
the RCMS will locate the fault. The RCMS in a solidly grounded
system will perform at its best when the fault level stays below
the branch breaker trip levels. Above these values, the branch
protection will kick in.
Illustration 2: Application of RCMS460-D multi-branch system
14
Resistance Grounded
Basic power systems - Resistance Grounded
Resistance grounded systems are created if the neutral is tied
to ground via NGR (Neutral Grounding Resistor). Typically these
NGRs can be found in three phase Y configured mining power
systems. Industrial plants employ these systems on occasion
as well. A main advantage of the resistance grounded system
is that the resistor limits the amount of current available to the
fault. The operation may be continued until the electrician locates the fault and fixes the problem.
The ground fault
A ground fault current in a resistance grounded system will be
limited in its magnitude. This is the main difference when compared to the solidly grounded system we encountered before.
The ground fault magnitude depends on the system voltage, the
resistance of the ground fault causing part and the resistance
of the current limiting resistor. Typical limits are 5, 10, 15 or 20
Amps. The resistor should be rated for continuous use under full
load if there is no protection device on this circuit.
(3)
IF =
IF
RGF
V3G
+ RGR + RNG
= Fault Current
V3G = Voltage between faulted phase and ground
RGF = Resistance value at shorted point
RGR = Resistance of ground path
RNG = Resistance of neutral ground bonding jumper
(Please note: The resistances were rando
omly chosen)
IF =
277 V
= 5A
0.1Ω + 0.2Ω + 55 Ω
The calculation shows that a theoretical fault current will not be
as devastating as in a solidly grounded system if a dead short
occurs. The maximum fault current will be limited, therefore vital
machinery can be kept running until the process is finished. Of
course, problems should be fixed ASAP.
The resistance grounded system also allows for selective ground
fault tripping which is achieved through adjustable time delays
on multiple relays.
L3
L1
T1
L2
T2
L3
T3
NG path
Resistance = 55 Ohm
NGR
Fault current path via ground
Resist ance assumed = 0.2 Ohm
)
L2
Motor, 3 p h
n
t
ul atio
Fa sol
(i
L1
circuit breaker
GND
GF= i.e. Dead short
Resistance=0.1 Ohm
Supply side 480 / 277 V AC
Schematic 11:The resistance grounded system
15
Resistance Grounded
Resistance grounded AC systems
GFGC Ground Fault Ground Check
The resistance grounded system could employ a typical 60 cycle
ground fault relay to ensure electrical safety. However, some resistance grounded systems in modern power applications require
more than just the ground fault part. A perfect ground connection has to be established to ensure that dangerous frame
voltages will be clamped effectively to ground. A combination
GFGC (ground fault - ground check) monitor will solve the issue.
This relay will measure the residual current in the respective circuit or branch of the system by means of a CT. For that purpose,
all active conductors (phases + neutral) are to be passed through
the CT (F1).
Supply side 480 / 277 V AC
circuit breaker
If the ground fault current (GF1.1) exceeds the response value,
the “Alarm Ground Fault” LED lights and the alarm relay switches.
The alarm contact can be delayed by a selectable time. The alarm
remains stored until the RESET button is pressed. The GC relay
function will monitor the impedance of the grounding conductor. For that purpose, the relay superimposes a voltage of 12V
between the grounding conductor and the ground check wire.
A termination device between both conductors at the other end
of the cable has to be installed. By evaluating the voltage drop at
this termination device, the GC part recognizes series resistance
faults (cable high-resistance or open) (F3) or cross resistance faults
(short circuit) (F2) of the cable. The alarm relay trips immediately if
one of the above occurs.
transformer ,
passiv
Motor, 3 p h
(F1)
L1
L2
L3
L1
T1
L2
T2
L3
T3
Bender GFGC Relay (1)
Ground Fault Ground Chec k
NGR
Ground pilot wire
GF 1.1
(isolation)
GND
Ground check
terminator
device
Fault 2
(break
and GF)
Fault 3
(Ground broken)
Schematic 12: The resistance grounded system with GFGC ground check relay
16
(1) Device: RC 48C
Resistance Grounded
Resistance grounded AC systems
Ground Fault - NGR Monitor
The resistance grounded system will limit potential fault currents
by means of the NGR (Neutral Grounding Resistor). A typical CT
(F1) based ground fault relay can be used to sense the ground
fault current. What would happen if the current limiting resistor
is destroyed or looses one of its connections (F2)? The answer
is simple. The resistance would become infinite and the former
resistance grounded system would become floating.
As learned before, the typical GF relay is not effective on floating
systems and is therefore now useless. A common practice is to
employ a potential transformer across the neutral grounding resistor. The potential transformer will indicate a voltage rise across
the transformer caused by a ground fault current. However, the
Supply side 480 / 277 V AC
circuit breaker
voltage rise will occur disregarding if the resistor is online or not.
Therefore, the potential transformer makes sense as a ground
fault protective device, but does not provide an integrity check
for the resistor.
Please review the schematic below. The Bender RC48N series will
provide a solution. If the GF current (GF1.1) exceeds the response
value, the “Alarm Ground Fault” LED lights and the alarm relay
switches. The NGR part monitors the resistance of the neutral
grounding resistor, connections through the transformer, and
the connections to ground (F2). It also monitors the voltage drop
on the neutral grounding resistor via a coupling device. An alarm
is indicated when the ground-fault current or the neutral ground
voltage exceed the set point. The RC 48N can be used to backup
or evaluate the potential transformer.
transformer,
passiv
Motor, 3 p h
L1
L2
L3
T1
L2
T2
L3
T3
GND
Bender GF NGR
Ground fault relay (1)
NGR monitor
NGR
Fault 2
(connection to
NGR broken)
1 n)
1. io
F at
G sul
n
(I
(F1)
L1
NGR coupler
Schematic 13: The resistance grounded system with GF NGR ground fault relay NGR monitor
(1) Device: RC 48N
17
Resistance Grounded
Locating ground faults
The resistance grounded system is subject to the same techniques as described in the grounded system. The only difference is
that the possible fault current is limited to 5, 10 or 20 Amps. The
same zero sequence CT based technologies applies.
>> PLEASE REVIEW THE PAGES 13 & 14 FAULT LOCATION IN
GROUNDED SYSTEMS <<
The RC48C is built into the substation shown above. It
will trip out a contactor if one of the following conditions should occur:
a) Ground fault
b) Ground wire resistance increased
c) Ground wire cross fault
18
Ungrounded Systems
Basic power systems - Ungrounded (Floating)
Floating systems are derived from a power source where there
is virtually no connection to ground. 480VAC delta configured
transformers are a typical supply for a floating system. Some
deltas in the mining industry can be found in hoists. 480VAC
deltas are also in wide spread use to supply 1000 Amp - 2000
Amp main feeder circuits in general industrial applications.
Floating systems are often used in areas where a sudden shut
down must not occur. Examples are Intensive care units (ICUs) in
hospitals, signal circuits, and emergency backup systems.
The ground fault
The magnitude of a ground fault current in an ungrounded
system is very small. It depends on the system voltage, the resistance of the ground fault causing part and the system capacitances.
I F = Fault Current
V3G = Voltage between faulted phase and ground
RGF
V3G
+ RGR + RNG
I F = Fault Current
Fault Current
VI 3FG = Voltage
between faulted phase and ground
V
=
Voltage
between
faulted
phase
value at
shorted
pointand ground
RGF
3G = Resistance
(Please note: The resistance were randomly chosen)
(Please note: The resistance were randomly chosen)
IF =
L1
L2
L3
circuit breaker
Motor, 3 p h
L1
T1
L2
T2
L3
T3
Ground Fault
i.e. Dead short
Supply side 480V AC
277 V
= 0.00027mA = 0.27 mA
0.1 Ω + 0.2 Ω + 1 MΩ
GND
Leakage
capacitance
RNG
IF =
Resistanceof
value
at shorted
RRGR
ground
path point
GF == Resistance
Resistance of
of neutral
groundground
path bonding jumper [in this case: MΩ t
= Resistance of ground path
RRNG
GR == Resistance
Resistance
of
neutral
ground
bonding
=
jumper
[in this case: MΩ
= Resistance of neutral ground bonding jumper [in thisRcase:
M
Ω
through
air]
(Please note: The resistance were
randomly
chosen)
NG
RGF = Resistance value at shorted point
RGR
Example: If a grounded object with low resistance touches a
live conductor, the resulting current flow will be negligible. The
ground fault loop will be incomplete because the return path
to the source is missing. It is important to note that the systems
capacitance will provide some paths for capacitive currents to
flow. The resulting current is also known as charging current.
Never assume that it is safe to touch a bare conductor in a floating system. A capacitive jolt will be extremely hazardous. The
example below reflects resistive values only. The capacitance,
though important, was not added to the equation.
(4)
Schematic 14: The ungrounded system with ground fault path
19
Ungrounded Systems
Insulation Monitoring Device IMD
Ungrounded systems will not produce the amount of fault
current needed to trip a common GFR. The IMD is the device of
choice for the protection of floating systems. IMDs come in two
styles:
a) Passive devices
b) Active devices.
Passive IMD (Forbidden in IEC-standard)
The most known passive device is most likely the three light
bulb system in industrial 480V delta systems. Three lamps are
Supply side 480V AC
T1
L2
T2
L3
T3
GND
Leakage
capacitance
L1
V1=277 V
L3
Motor, 3 p h
V =277 V
3
L2
circuit breaker
V2=277 V
L1
connected on their secondary side together and from there to
ground (Star or Y configuration). Each lamp is then connected
to the respective phases L1, L2 and L3. In a healthy system, all
three lights will burn with the same intensity. In case of a ground
fault, the faulted phase will assume a value close to ground
potential. The respective light will dim, while the other two will
brighten up. The light bulb system often does not offer additional trip indicators for remote alarms. It also needs to experience
a serious fault condition before people are becoming aware that
something is going wrong. Even worse, symmetrical ground
faults (A balanced fault on all three phases) will not be detected.
Schematic 15: Three light bulb system under no fault condition
L2
L1
T1
L2
T2
L3
T3
GND
V3=0 V
V1=480 V
L3
Motor, 3 p h
V =480 V
2
L1
circuit breaker
Ground Fault
i.e. Dead short
Supply side 480V AC
Schematic 16: Three light bulb system under single fault condition
20
Ungrounded Systems
Insulation Monitoring Device IMD
Passive IMDs
Ungrounded DC systems are often equipped with a passive
measuring device. The “balance” method is the most commonly
known method to protect a floating DC bus. Two voltmeters
are connected between the positive, negative lead and ground.
The voltmeter acts like a voltage divider. Each voltmeter should
indicate about 125V in a 250V system. A ground fault will clamp
one of the voltmeters closer to ground and it will indicate a value
=> Review the UG 140P series
L2
L2
250 V load
V2G=125 V
+
V1G=125 V
V2N=125 V
-
N
125 V load
+
V12=250 V
V1N=125 V
L1
L1
Supply side:
Battery system
125/250 V DC
closer to zero. The other voltmeter will see an increase and move
up closer to 250V. The magnitude of this shift will depend on
the magnitude of the ground fault. As with all passive systems,
balanced ground faults will not be detected with this technology. These devices also do not provide early warning or trending
capabilities.
circuit breaker
Schematic 17: 125 / 250 V DC system voltage measurement s no fault condition
L2
250 V load
125 V load
L2
Ground Fault
i.e. Dead short
V2G=0 V
+
V1G=250 V
N
V2N=125 V
-
Supply side:
Battery system
125/250 V DC
+
V12=250 V
V1N=125 V
L1
L1
circuit breaker
Schematic 18: 125 / 250 V DC system voltage measurement s single fault condition
circuit breaker
250 V load
V1G=125 V
125 V load
L2
V2G=125 V
+
L2
V12=250 V
V2N=125 V
N
-
Supply side:
Battery system
125/250 V DC
+
n
ow
ak d eg s
br e w o l
r n)
n ce n t
st a s o at e
esi Ohm er w t u at i o
d
=r
i
GF 1000 e un u l t s
t o . cab l ed f a
i . e l an c
( Ba
V1N=125 V
L1
L1
RF2
RF1
Schematic 19: 125 / 250 V DC system voltage measurement s double fault condition
21
Ungrounded Systems
Insulation Monitoring Device IMD
Active IMDs
The active IMD is considered to be an online megger. It will be
connected via pilot wires between the system and ground. A
constant measuring signal will be sent from the IMD into the
power wires. It will spread out evenly into the secondary side of
the supply transformer and the attached loads. If this signal finds
a break through path to ground, it will take this path of least
resistance and return to the monitor. The IMDs internal circuitry
will process the signal and trip a set of indicators when the set
point is increased. IMDs measure in Ohms (Resistance) and not
in Amps (Current). A ground fault will be indicated as “insulation
breakdown”.
Great insulation = healthy system = multiple kilo ohms or
mega ohms
Low insulation = ground fault = less than one kilo ohm or
low ohm range
L2
T2
L3
T3
V1G=480 V
L3
Motor, 3 p h
T1
V3G=0 V
L2
circuit breaker
L1
V2G=480 V
L1
Example: A monitor in a 480V delta system would be set to trip
at 480 x 100 = 48 kilo Ohms. Please be advised that this figure
cannot be used for all situations. Example: A customer has
meggered a motor and figures that his system must be at around
1 Meg Ohm insulation. The insulation monitor keeps alarming
and indicates lower levels than previously assumed. The answer
is simple: It was forgotten to take into consideration that there
are 10 of these motors connected to the same system. The IMD
will measure and indicate the OVERALL resistance of the system.
Here we are dealing with 10 parallel resistances of 1 Meg Ohm
each. The overall resistance will drop to less than 100000 Ohms
in this case.
Bender
IMD relay (1)
AMP measuring mode
Schematic 20: The ungrounded AC system with Bender
(1) Single phase AC systems
22
Single phase AC, DC or mixed systems Three phase AC systems
Three phase AC, DC or mixed systems
Voltages above 650VDC, 793VAC
=>
=>
=>
=>
=>
Review the IR420 series
Review the IR425 series
Review the IRDH275 and IRDH375 series
Review the IRDH275 and IRDH375 series
Review the AGH couplers
GF (=i.e.
Death short)
Supply side 480V AC
A power system’s overall resistance depends on the number of
loads, the type of insulation used, the age of the installation, the
environmental conditions etc. A typical question when it comes
to floating deltas is always: “Where should my set or trip level
be?” The typical “ball park” figure for industrial applications is
100 Ohms per volt.
GND
Ungrounded Systems
Active IMDs
An active IMD is the preferred choice for ungrounded DC
systems as well. As in floating AC systems, a DC IMD will be
connected via pilot wires between the live (or dead) system and
ground. A constant measuring signal will be sent from the IMD
into the power wires, from there it spreads out evenly into the
Supply side:
Battery system
125/250 V DC
circuit breaker
GF= resist ance breakdown
to 1000 Ohms on two legs
i.e. cable under water
(Balanced fault situation)
V1G=125 V
-
L2
V =125 V
2G
V2N=125 V
+
L2
250 V load
V12=250 V
V1N=125 V
+
N
125 V load
L1
L1
secondary side of the supply (E.G. battery) and the attached
loads. If this signal finds a break through path to ground, it will
take this path of least resistance and return to the monitor. The
IMDs internal circuitry will process the signal and trip a set of
indicators when the set point is increased.
Bender
IMD relay (1)
AMP measuring mode
Schematic 21: 125 / 250 V DC system with Bender IMD technology
IRDH 275
IRDH 375
IRDH 575
Illustration 05: Relays different styles
(1) Device: >> review previous page <<
23
Ungrounded Systems
Ungrounded systems - Locating ground faults with portable
equipment
Locating faults in ungrounded deltas is different from doing so in
a grounded Y. A typical leakage current clamp will not perform.
We learned in earlier chapters that even a dead short to ground
will not create hazardous currents in floating systems. If there
is no GF current flow then there will be nothing available to
be picked up. The solution is to send a low level artificial signal
via pulse generator into the faulted system. The signal will be
impressed between the power wires and ground. Naturally, it
will follow the ground fault path into ground and return to the
pulse generator. This signal can be traced with a hand held probe
which appears to be similar to the current probe discussed in
previous chapters. Nevertheless, it has to be emphasized that
this is a specially designed clamp which will trace the pulse and
not ground fault currents. Both the clamp and pulse generator
will work in unison. One without the other will be useless in a
floating system.
EDS 3091PG Ground fault location ki t
for AC/DC systems below 120V
300V
EDS 3090PG Ground fault location
kit for AC/DC systems above 120V
300V
Illustration 06: Ground fault location kit
Illustration 11: Application of Ground fault location kit
24
Ungrounded Systems
Ground fault location in a 480V delta fed system
A manufacturing plant encountered a decrease in insulation
value. The system broke down from 45Kilo Ohms to less than 5Kilo
Illustration 08: Location I
The PGH pulse generator is connected via pilot wires. The
injected signal can be seen on the hand held EDS165 evaluator
device. There are 12 branches in this panel. Each one has to be
checked for the pulse. That takes approx 30 seconds each. We
locate the pulse in branch 2F7.
Illustration 10: Location III
The evaluator shows 15 mA going through these water cooled
transformers into ground. The transformer specs and the local
electricians verify that this is a normal situation. But there are still
10 mA vanishing beyong this point. Let’s continue the hunt.
Ohms. Shutting off breakers did not reveal any improvement.
The EDS3090 fault locator was chosen for the task since we were
dealing with a 480V 3Ph system. (2000A main bus, 42 branches)
Illustration 09: Location II
Branch 2F7 leads us further into the production area. The pulse
is still strong. It is amazing how accurate the device is, considering the fact that we are only sending 25 mA into the system. We
have now arrived at a sub panel leading to various machines and
water cooled transformers.
Illustration 11: Location IV
The culprit is found. The remaining 10 mA led us to this transformer which was well hidden inside a metal bending machine.
Obviously there is a need for a replacement part.
25
Ungrounded Systems
Ungrounded systems - Locating ground faults with fixed
equipment
The ground fault location system can be installed as a fixed
installation if 24/7 monitoring should be required. A complete
ground fault location - detection scheme for a floating system
incorporates
IMD Insulation Monitoring device (Incorporated into the
IRDH 575 IMD)
EDS Evaluators
Function: Information (Fault) gathering from the CTs and
indicating the faulty branch
CTs Current Transformers
Function:Specially designed for sensing the trace pulse and
sending information back to the evaluator.
FTC and COM Gateway
Function:Detects the fault and alarms when the set point has
been reached. IMDs were discussed on pages 23 & 24
Function:Designed for Interfacing via Modbus TCP & RTU,
Ethernet and Profibus
Pulse generator (Incorporated into the IRDH 575 IMD)
The Bender EDS ground fault location - detection system is an
excellent tool for the maintenance personnel in a large facility with extensive wiring. Faults will be located automatically
during normal business operation. No shutdown required. No
handheld tracing and/or accessing panels required. The beauty
of this system lies in its non invasive operation. If the case study
on page 25 would have been equipped with a fixed GF location
system, the faulty branch and its connected machine would have
been identified during the first 120 seconds after the first alarm
emerged. The ungrounded system is only safe for its user as long
as the occurring faults are immediately traced down and eliminated. If that is not the case, then the second ground will follow
sooner or later and short circuit the system.
Function:Sends the trace pulse into the power wires once a
fault is detected. (Please review page 25 for more
background information on the test pulse issue)
Central Control Unit (Incorporated into the IRDH 575 IMD)
Function:Gathers the information from the evaluators and
displays alarms.
Illustration 12: Application of IRDH575 on a sea going vessel
26
Ungrounded Systems
Ungrounded systems - Locating ground faults with fixed equipment
IRDH575 in combination with EDS460
An insulation monitor is connected to three series EDS460 evaluators. This is a small sub system capable of evaluating 36 circuits.
Illustration 14: Application of EDS460
EDS490 Ground Fault Location System
This system is intertied via COM functions
with additional insulation monitors in adjacent rooms and can be extended without
limitation. Information from the intertied
devices is gathered and relayed to a central
processor for user interfacing and display.
Illustration 14: Application of EDS490
27
Offline Monitoring
The Off-line monitor is basically a fixed installed control relay,
which duplicates the function of a technician who “highpots” or
“meggers” a load to check for ground faults or insulation break
downs. These loads can be dewatering pumps, fire pumps,
motors or any other electrically fed piece of equipment which
does not operate on a continuous basis. In many installations,
even continuously running equipment will be shut down in
regular intervals to perform a high voltage line to ground test.
The megger task involves disconnecting the load from the power
system. Then the high voltage device or so called megger will be
connected to the motor leads and ground. The voltage will be
superimposed onto the wires and windings for a brief moment
Supply side 480 / 277 V AC
L1
L2
L3
disconnect
fuse
L1
during which the technician looks out for a break through indication. The complete task will take up a considerable amount of
time. The job sites are often in remote areas, wiring and disconnecting tasks have to be performed and safety regulations have
to be fulfilled. Amazingly enough, it is widely unknown that this
manual task can be fully automated by using a low cost relay.
Offline IMD’s (Insulation Monitoring Devices) superimpose a
DC measuring signal to the system being monitored in various
applications across the nation.
The relay will alarm if the superimposed signal finds a break
through path to ground. The available Bender offline IMD’s
are shown in table 5.1. The table also indicates the measuring
voltage (max.) of the individual IMD which will be superimposed
between the power wires and ground.
motor starter
open!
Motor, 3 p h
NO
L3
T2
T3
GND
T1
NO
L2
Water
intrusion
Offline monitoring with Bender IR ... devices
Offline monitoring addresses the monitoring of idle consumers,
to avoid activation in the presence of a ground fault.
NO
NC
Bender
Offline
monitor
Neutral Ground (NG)
Jumper
closed!
IT = Test current
Schematic 22: O ff line monitor under fault condition
28
Offline Monitoring
Offline IMD’s
Standards for Offline monitors
One of them is the American standard, ASTM F1134-88, which
describes the monitoring of idle consumers in shipboard applications. In this standard the max. measuring voltage allowed is
24V DC. The IEC standard, IEC61557-8, (Title: “Insulation Monitoring Devices for ungrounded AC Systems, for ungrounded
AC systems with connected DC circuits and for stand alone DC
systems”) describes the IMD’s in general, limiting the measuring
voltage to 120V (peak value). The diagram on page 28 shows an
example for off-line IMD Bender Series IR420-D6 and IR425-D6.
This IMD among others are connected to the system being
monitored via a pilot wire. An auxiliary contact will be used to
initialize the IREH relay when the motor goes offline (1).
Offline IMDs often pay themselves off during their first year of
operation. A typical 100HP pump motor whose seals have failed
can easily be put back into operation by repairing the seals and
drying out the stator windings. This will work only if an offline
monitor prevents a “wet start-up”. Rewinding the stator instead
will cost the operator thousands of dollars and unnecessary
down time.
Illustration 15: IR420-D6
(1) In applications with a grounded supply, the contactor needs
to interrupt all power lines including the neutral (if available).
29
external CT required
Notes
2 Alarms
LC-Display
Notes
1 kΩ...10MΩ
2 Alarms LC-Display
Response value
Notes
for ground fault
location
Notes
Type
Illustration
IRDH575
1 kΩ...10MΩ
Type
Response value
Illustration
IRDH375
Type
Illustration
IRDH275
1 kΩ...10MΩ
Type
Response value
W-, WR-, WS- series
RCM420
30...300 mA
Illustration
AC / DC & DC
W...A series
10mA
RCMA426
6mA
RCMA421
internal CT
30...500 mA
RCMA423
external CT required
30 mA ... 3 A
RCMA420
Mining Processing
Power generation
Mobile Gensets
Type
AC
solidly grounded
Ground fault
relay
Response value
Illustration
AC / DC & DC
Mining Pocessing
Power generation
Power distribution
Voltage
ungrounded
Power
Systems
Application
Insulation
Monitor
Measuring, monitoring &
protection device
30
RC48C
Ground contnuitly and shorted
ground monitoring via ground
check
IΔn 0.1...10 A
RC48N
grounding conductor for low and
high resistance and shorted
external CT required
IΔn 1...10 A
HW135
Power distribution Shaft power
supply up to 10 kV
AC
solidly grounded and resistance
grounded
Ground fault relay &
Ground continuity
monitor
Ground
continuity
monitor
Ground continuitly and shorted
ground monitoring via ground
check wire
50...1000 Ω
GM420
Trailing
AC & DC
Ungrounded or grounded
Ground Fault Relays
Selection Guide for
voltage and current monitoring of grounding resistor
IΔn 0.1...10 A
Trailing cables up to 5 kV
AC
solidly grounded and resistance grounded
Ground fault relay &
Ground continuity monitor CSA M421-00
AC & DC
Ungrounded or grounded
Offline
insulation
monitor
Index
A
Active IMD 10, 22, 23
AGH coupler 22
ASTM F1134-88 29
B
Backup system 19
Balance 21
Basic rule 7
Battery 23
Bypass 7
C
Carrier frequency 10
Center tap 5
Central Control Unit 26
Control relay 28
CT 4, 26
Current clamp 13
Current transformer 4, 7
D
DC systems 11
Delta configured 19
Donut 7
E
EDS460 27
EDS461 27
EDS490 27
EDS491 27
EMI filter 10
Evaluator 26
F
FCI 8
Filter technology 10
Fixed equipment 14, 26
Floating 4, 19
G
GC 16
GFCI 5
GFGC 16
GFI breaker 13
GFR 4
GF interrupting
- Contactor 9
- Shunt trip breaker 8
Ground fault 6, 15, 19
H
Healthy system 7
Highpots 28
High voltage 12
Hospital 19
Human body 6
Hunting down 13
I
ICU 19
IEC61557-8 29
IMD 4, 20, 26
- measuring voltages 29
- monitoring 28
Insulation breakdown 2, 22
IR420 22
IR425 22
IR470Y 22
IR475Y 22
IRDH275 22
IRDH375 22
IREH470 relay 29
J
Jumper 4
K
Kirchhoffs laws 7
L
Leakage 13
Life system 13
Light bulb system 20
Locating GF 12, 13, 14, 24
Low voltage 29
M
Meggers 28
Mining 29
Mining industry 19
Mixed AC/DC 11
Motor protection 10
N
NG 5
NGR Monitor 17
O
Off-line
Off line search 12
Online system 14
On line search 13
P
Passive 10
Passive IMD 20, 21
Personnel 10
Portable equipment 12, 24
Power systems 4
Protection 10
Public rest rooms 7
Pulse generator 26
R
RCMA 11
RCMS 14
RCM 420 7
RCM 420 7, 10
RCM 421 10
RCM 426 10
RCT 3283 13
RCT 9131 13
RC 48C 16
RC 48N 17
Resistance Grounded 15
S
Shipboard 29
Single phase transformer 5
Solidly grounded 5
T
Termination 16
Three phase 4, 5
V
VFD 7, 10
W
Wet area 7
Wet start-up 29
31
Bender Canada Inc.
5810 Ambler Drive • Mississauga, ON • Canada
Toll-Free: 800-243-2438 • Main: 905-602-9990
Fax: 905-602-9960 • E-mail: info@bender-ca.com
www.bender-ca.com
Pictures: Bender archives.
Bender Inc.
420 Eagleview Blvd. • Exton, PA 19341 • USA
Toll-Free: 800-356-4266 • Phone: 610-383-9200
Fax: 610-466-2071 • E-mail: info@bender.org
www.bender.org