need to be raised. A combination of both blocking and raising of the overcurrent settings may be adopted. The CLP
over
current settings in this case must be chosen with regard to the motor starting characteristic.
This may be useful where instantaneous earth fault protection needs to be applied to the motor. During motor
start-up conditions, it is likely that incorrect operation of the earth fault element would occur due to asymmetric
CT saturation. This is due to the high level of starting current causing saturation of one or more of the line CTs
feeding the overcurrent/earth fault protection. The resultant transient imbalance in the secondary line current
quantities is therefore detected by the residually connected earth fault element. For this reason, it is normal to
either apply a nominal time delay to the element, or to use a series stabilising resistor.
The CLP logic may be used to allow reduced operating times or current settings to be applied to the earth fault
element under normal running conditions. These settings could then be raised prior to motor starting, by means of
the logic.
6.3.3 CLP FOR SWITCH ONTO FAULT CONDITIONS
In some feeder applications, fast tripping may be required if a fault is already present on the feeder when it is
ener
gised. S
uch faults may be due to a fault condition not having been removed from the feeder, or due to
earthing clamps having been left on following maintenance. In either case, it is desirable to clear the fault
condition quickly, rather than waiting for the time delay imposed by IDMT overcurrent protection.
The CLP logic can cater for this situation. Selected overcurrent/earth fault stages could be set to instantaneous
operation for a defined period following circuit breaker closure (typically 200 ms). Therefore, instantaneous fault
clearance would be achieved for a switch onto fault (SOTF) condition.
Chapter 6 - Current Protection Functions P14x
112 P14xEd1-TM-EN-1
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