Signalling

Question 1

What is a ‘PIT’? Give an example application.

Answer 1

Permissive Inter-trip. A trip from a remote relay conditional upon something the local relay has to check first. A distance scheme might send a PIT to trip in fast zone 2 time.

Question 2

What’s the philosophy of echo logic?

Answer 2

“my CB is already open so I can’t detect faults or generate PITs. If you send me a PIT from your zone 2, I’ll bounce it back so you know you’re feeding the fault”.

Question 3

What’s a DIT? Give an example application.

Answer 3

Direct inter-trip; philosophy “I will trip as quickly as possible”. PITs used more often; say a transformer-ended feeder where the HV breaker is local and the LV breaker is at the final substation. In the event of a fault, the local site with the transformer and protection relays will send a DIT to the remote CB to trip immediately.

Question 4

What’s a SIT?

Answer 4

Serial Inter-trip - like two duplicated DITs that can be wired either in series (thus requiring both to come through before tripping) or parallel (either or).

Question 5

What are some rough times for DITs, PITs and BITs?

Answer 5

DIT ~ 30ms
PIT ~ 20ms
BIT ~ 15ms

Question 6

What are mirror bits called on GE, SEL and MiCom relays? What type of signal are they?

Answer 6

SEL = Mirrored Bits
GE = Direct Bits
MiCOM = InterMiCOM
All RS232 serial

Question 7

What is the purpose of mirrored bits and how do they work?

Answer 7

They monitor the status of the transmitting relay and repeatedly sends these to the remote relay. As soon as something happens, a bit is asserted.