In some applications, particularly relating to earth fault protection resistors and metrosils may be necessary to limit the current or voltage in the earth circuit.
A stabalising resistor, or stab resistor, can often be found in motor earth fault circuits using a residual or holmgreen type connection to a protection relay. They are included to help eliminate spurious tripping which can result through the inherent mismatch of phase CTs creating a spill current during a motor start.
Some of the motor protection relays compensate for the need of a stab resistor by dynamically increasing the EF trip threshold during a motor start, and more often new CTs are less prone to contribute to spill currents due to increased uniformity in their manufacture.
Metrosils (nonlinear resistors) may also be required in restricted earth fault circuits to help limit the voltage rising in the secondary circuits during a fault which would ultimately cause damage to the protection equipment. The size or rating of a metrosil can depend upon the EF level and trip time, the system voltage, the available fault current.
P&B can provide can supply and help end users calculate the correct resistance for their earth fault application.
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