protection doubts_Switch into fault

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hello
1.How operation of bus bar protection is avoided in case of open CT?

2. Why switch in to fault?
3. If a fault occurs while fuse failure is detected, should the relay give a trip command?

Thanks
 

rcwilson

Senior Member
Location
Redmond, WA
1. If you have an open CT circuit the bus bar protection will trip if there is any load in the circuit the CT is monitoring. Why would we not want it to trip?

2. Switch-on-to-fault protection is usually applied in distance relays that need a voltage signal to calculate the circuit impedance to determine the fault location. Prior to breaker closure, there may not be a voltage signal available to feed the relay. The switch on to fault logic speeds up the tripping. Utility engineers told me the main purpose was to quickly trip if a line crew left the ground leads on a circuit. Some relays can use the same logic to trip quickly if a three-phase fault occurs as soon as the breaker closes. The trick is to distinguish between normal line charging and load inrush current and fault current on breaker closure.

3. Fault during Fuse Failure - The answer takes some engineering decisions on your part. If you have distance relaying, the relay senses zero line impedance if any current is flowing. (Z= V/I = 0/I = 0 ohms). It will trip. Most engineers would not trip on fuse failure and would rely on back up overcurrent protection to trip if a fault occurred. If it is a 51V or 51C voltage controlled/restrained overcurrent, it may trip on normal load current on fuse failure. One method is to inhibit tripping on fuse failure unless a fault detector, (a high set instantaneous overcurrent element) indicates a high current into a fault.
 
Hello
Thanks for the replies..
But as you had asked -Why should the breaker not trip?? Usually with the bus differential relays ,we have the bus wire supervision relays connected for which I have understood that it's used for avoiding trip if CT is open and will send an alarm to the maintenance department ,who will further come and sort out the problem ?? is this true?
 
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