protection doubts_ metrosils

[bejoyengineer]

hello
Why is metrosils used across high impedance differential relay and not for other relays as other relays too could develop high voltage across the relay at time of fault?

 

[rcwilson]

The surge arrestor/ voltage limiter across the high impedance relay limits the voltage applied to the relay by the CT carrying fault current. Since it is a "high impedance" circuit the voltage ob teh coil can get very high. (V= high Z x high current = Very high voltage).

On other relay types, the current element is very low impedance so the IR drop across the coil is within reasonable levels during a fault.

Note that most high impedance differential circuits use contacts of the 86 lockout tripping relay to short out the high impedance relay coil to minimize any overheating or damage. The metorsil surge arrestor also protects those contacts from flashing over and preventing relay operation.