Question on Plug Setting Multiplier in Protective Relays

This is not a good candidate for an anonymous question. Sorry.

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Please specify the manufacturer and part number of the equipment in question.

Dear Mr PWSlack,

Rephrasing my question, PSM = ratio of fault current to the pick up current. Here why the fault current is included in the formula of plug setting multiplier, as we have already assumed the fault current to be the maximum load current expected.

BTW, there is no relay, I've just started studying the basics of overcurrent protection.

Click on the link i provided, open the first website in the list and your question will be answered.

I always thought research was a part of the study process, time have changed.

Dear Mr. Tony,

That was a good reference, but still I have some questions. Basically PSM of any value greater than 1 is fault current and the relay should operate for PSM of value above 1. So why don't consider always plug setting multiplier as 1 and then calculate the actual time of the operation from the Time/PSM graph.

Why should one find the fault level of the apparatus to be protected? My assertion is that finding the fault level and calculating the operating time of the relay at that fault level might help in relay co-ordination to set the upstream relays. Is this right?

That seems painfully evident.

The basic current of a traditional relay was 1 amp or 5 amp. Being a wound-coil device, other special values of basic current were possible. Operating curves are the same, being in multiples of the basic current which was given on the relay rating plate.

In converting a primary actual fault current to a relay setting one must include CT ratio and basic relay current.

If you're a consultant posing as a protection engineer then you're in over your head, if this is an interview question then be glad that you didn't get the job.