Overshoot on removal of current?

[davidbeach]

I'm looking at a request for quote on for a protective relay and running into a term I'm not familiar with. The request comes from Australia, and I don't speak Oz. The requirement is:

Overshoot on removal of 20 times setting current - Not greater than 0.05 sec.

Is that a drop out time? Is it when an inverse element is timing but hasn't tripped yet and the current is removed? Or is it something else all together?

Thanks.

 

[ScottyUK]

Hi David,

I don't speak Oz either, but it sounds like a reference to the days of mechanical induction disc relays. An induction relay with 20x setting current would be moving fairly quickly: on removal of the current the disk would have a tendency to overtravel in the forward direction (toward tripping) before it slowed and reversed back toward its quiescent state. Is this a replacement for a very old relay, or a spec issued by someone who has borrowed it from yesteryear and not updated it?


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Sometimes I only open my mouth to swap feet...

 

[davidbeach]

Thanks Scotty. If that's what it is, it is certainly a strange requirement for a numeric relay since the relay responds to the removal of the current within a bit over 20ms at 60Hz or 25ms at 50Hz (one cycle plus 1/4 cycle processing time). 50ms would be a very long time.

 

[ScottyUK]

I occasionally see stange things like this when a standard written years ago by people long since retired is quoted by people today who don't fully understand the spec they are quoting. I agree, it seems a very strange thing to find in a spec for a numeric relay.

If you find out what this really refers to, could you post the meaning here so I can understand what they mean too? Thanks.


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Sometimes I only open my mouth to swap feet...

 

[rbulsara]

I was also thinking that this would have to do with days of electromechanical relays. But I thought someone more familiar with electromechanical terminology would chime in.

 

[rbulsara]

david:

A google search for "overshoot electromechanical relay" gives sevral hits. Here is one of them

http://www.markenrich.com/appnote.html

I hope these can answer your question.

 

[jghrist]

I suspect that relay time overshoot rather than voltage overshoot, as described by ScottyUK, is the reference. In Considerations In The Application of Modern Ovecurrent Relays, by Walt Elmore, Westinghouse Electric Corp., overshoot of CO relays is given as 0.01 to 0.04 sec. My copy of this reference is undated, but you can see how long I squirrel away stuff if the CO relay is considered Modern.