Special Notes On Ground (Earth) Fault Current Transformers; Commissioning: Ground Fault Protection - Non-Directional [Ansi 50X] - Eaton EMR-3MP0 Installation, Operation And Maintenance Manual

4.9.2

Special Notes on Ground (Earth) Fault Current Transformers

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.
Note that the ground fault current settings 50X are based on the ground CT rated
primary current (In), not on FLA or the phase CT ratio. For example, a pickup setting of
0.10⋅In 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 50X. 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.
4.9.3 Commissioning: Ground Fault Protection – Non-Directional
[ANSI 50X]
Please test the non-directional ground overcurrent using the procedure for non-directional
phase overcurrent protection,
directional [ANSI
EMR-3MP0
╚═▷ "Commissioning: Overcurrent Protection, Non-
50P]".
4.9 50X – DEFT Measured Ground Fault Protection
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4 Protective Elements
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