Technical Data
Effective: May 1998
Page
18
TD.44A.01.T.E
Cutler-Hammer
Ground Fault
When the sensing circuit detects a
fault, the green “normal” light will turn
off and the red “ground fault” light will
turn on. The ground current ammeter
will indicate the total ground current.
To use the pulser, turn the system con-
trol switch to “pulse.” The pulser con-
tactor will cycle on and off as control-
led by the recycle timer relay. Use the
clamp-on ammeter to locate the
faulted feeder. Open the feeder and
clear the fault. If the reset control
switch is in the “manual” position,
turn it to “reset” to reset the sensing
circuit. (If reset control is in “auto,” it
will reset itself.) When ready to restore
service to the load, close the feeder.
Return the system control to “normal.”
Zone Selective Interlocking
By definition, a selectively coordinated
system is one where by adjusting trip
unit pickup and time delay settings, the
circuit breaker closest to the fault trips
first. The upstream breaker serves two
functions:
1. Back-up protection to the down-
stream breaker and
2. Protection of the conductors
between the upstream and down-
stream breakers. These elements
are provided for on Digitrip trip
units.
For faults that occur on the conductors
between the upstream and down-
stream breakers it is ideally desirable
for the upstream breaker to trip with
no time delay. This is the feature pro-
vided by zone selective interlocking.
Digitrip trip units may be specified to
utilize this option.
Zone selective interlocking is a com-
munication signal between trip units
applied on upstream and downstream
breakers. Each trip unit must be
applied as if zone selective interlocking
were not employed, and set for selec-
tive coordination.
Application note:
It may not be possi-
ble to precisely locate faults within a
conduit. The ground current may
divide into many components,
depending on the number of cables
per phase, number of conduits per
feeder, and the number and resistance
of each ground point along the con-
duits. The resulting currents may be
too small to allow detection, or may
take a path that the ammeter cannot
trace. An important note to keep in
mind is that while the pulser can
greatly aid in locating a fault, there
may be certain conditions under which
the pulses cannot be readily traced,
and other test procedures (meg-ohm,
high-potential, etc.) may be needed.
Sequence of Operation
Normal
Green “normal” light on.
Red “ground fault” light off.
White “pulse” light off.
System control switch in “normal”
position.
Reset control switch in either “auto” or
“manual.”
Test
Turn and hold the system control
switch in the “test” position. Phase B
will be grounded via the test resistor.
The ground current will activate the
sensing circuit, causing the green
“normal” light to turn off and the red
“ground fault” light to turn on. The
pulser will be activated as well. The
white “pulse” light will turn on and off
as the pulser contactor closes and
opens. The ground current ammeter
will display the total ground current,
including the incremental pulse cur-
rent. When ready, return the system
control switch to “normal.” The pulser
will stop. If the reset control is in the
“manual” position, turn it to “reset” to
reset the fault sensing circuit. The red
“ground fault” light will turn off, and
the green “normal” light will turn on.
Test mode is not available if the sys-
tem is detecting a ground. The sensing
circuit will disable the test circuit.
During fault conditions, each trip unit
that senses the fault sends a restrain-
ing signal to all upstream trip units.
This restraining signal results in caus-
ing the upstream trip to continue tim-
ing as it is set. In the absence of a
restraining signal, the trip unit trips the
associated breaker with no intentional
time delay, minimizing damage to the
fault point. This restraining signal is a
very low level. To minimize the poten-
tial for induced noise, and to provide a
low impedance interface between trip
units, special twisted pair conductors
are utilized for interconnection. For this
reason, zone selective interlocking
must be specified.
Ground fault and short delay pickup on
Digitrip Trip Units may be specified
with zone selective interlocking. Since
most system faults start as arcing
ground faults, zone selective interlock-
ing on ground fault pickup only is
usually adequate. Zone selective inter-
locking on short delay pickup may be
utilized where no ground fault protec-
tion is provided.
Zone selective interlocking may be
applied as a type of bus differential
protection. It must be recognized, how-
ever, that one must accept the mini-
mum pickup of the trip unit for
sensitivity.
It must also be recognized that not all
systems may be equipped with zone
selective Interlocking. Systems contain-
ing multiple sources, or where the direc-
tion of power flow varies, require special
considerations, or may not be suitable
for this feature. Digitrip zone interlocking
has been tested with up to three levels
with up to 20 trip units per level.
Magnum DS
Metal-Enclosed
Low-Voltage Switchgear
Application
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