Reset time in ANSI/IEC TCC curves

[pwrtran]

Some modern relays have a "Reset Time" for ANSI or IEEE and IEC inverse-time overcurrent curves. I understand the reset time is to emulate the disk type relays for the disk to return its zero position. My question is how does the reset time in the modern relays affect the TCC curves so as to the coordination?

 

[davidbeach]

If you aren't doing reclosing there is not impact on coordination.

On the other hand, every relay has, and has always had, a reset time. It may be less than a cycle, or it could be seconds, but there is a reset time none the less.

 

[pwrtran]

Thanks David, can you please explain what if there is a reclosing?

 

[davidbeach]

If you have an instantaneous reset relay downstream of a relay with an inverse time reset characteristic you can get miscoordination following a trip and high speed reclose because the downstream relay will take the full amount of time to trip again (starts fully reset) while the upstream EM relay starts out only partially reset and therefore trips faster than expected. Many other combinations and considerations. Fuses have their own "reset" characteristics (cooling) that have to be considered.

The second and subsequent trips in a reclosing sequence require reset be considered unless the only devices picking up for the fault are all instantaneous reset. You can find papers on fuse/recloser coordination that provide considerable detail. But if you don't coordinate reset as well as tripping characteristics you can find the "wrong" device clearing the fault.

 

[dpc]

When reclosing with induction-disk relays, the breaker may reclose into the fault before the disk has had a chance to fully unwind (reset). This will, in effect, have the relay operating on a faster time dial that it is set for. This has to be taken into account when doing coordination.

David Castor

http://www.cvoes.com

 

[pwrtran]

I also found there are quite confusing terminologies used: reset time, resetting time, reset delay, reset after reclose and AR reset time. Is there a good way to distinguish the reset time in between the trip sequence and the operation reset time?