EMR-5000
IM02602012E
Ground Fault Protection
Elements:
Ground Fault Protection
Functional Description
This CT has a large primary window through which all three-phase conductors can pass. The most common ground
fault CTs have a ratio of 50:5 or 50:1.
The protective device is recognized to UL 1053, Ground Fault Protective Device standard. This may eliminate the need
for a separate ground fault protector in many applications that formerly required one.
Note that the ground fault current settings 50R[x] and 50X [x] are based on the ground CT rated
primary current (In),
not on FLA or the phase CT ratio. For example, a pick-up setting of 0.10 gives a trip or alarm for an actual ground
leakage current of 5 A on the primary side of the sensor with a 50:5 CT.
This function is only useful for a grounded power system. The ground return is normally made from the neutral of the
secondary wire winding of the supply power transformer. Resistance grounding is acceptable as long as the resulting
fault current is at a level the protective device can be set to detect.
The ground CT, which provides sensitive protection for high-resistance ground faults, may saturate for a robust heavy-
current ground fault in a solidly-grounded system. Minimize the saturation problem by minimizing the burden. Use the
shortest and heaviest leads possible between the ground CT and the relay. The relay itself has very low burden,
usually much lower than the connecting wiring. Calculate the current magnitude that saturates the ground CT.
Consider the CT secondary voltage capability and the total burden of the CT secondary winding itself, the connecting
wires, and the relay. Make sure this saturation current is well above the minimum sensitivity of the
phase
IOC function
and/or the motor fuses.
A residual connection – the wired summation of the phase CT circuits through the ground CT input – requires a much
higher ground fault time setting to avoid false tripping. Thus, sensitivity is not nearly as good as with a separate flux-
canceling CT.
If the relay is installed where a residual connection is used, XCT should be set to the same value as CT Pri. The User
must then set the ground fault trip level at a high value to avoid nuisance tripping from CT ratio errors, third harmonic
and certain higher harmonics, or other measurement errors producing false residual currents. Monitor the metered
ground current during various loading conditions to ensure a good margin between these error currents and the ground
fault trip current setting 50R [x]. Also, watch out for phase CTs that saturate during motor starting. The saturation
produces a large residual current and a ground fault trip. This may be a problem if the CTs have a low voltage
capability (e.g.: C5 or C10), have long wiring runs, or are otherwise heavily burdened.
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