Setting of 49/50 & 50/51 relays 1

[SELEC]

I niticed on IEEE buff book-P298 that overload relays- 49/50 should be set at 100% cable capacity and induction relays should be set at125% cable capacity. Is this the common practice here in North America?

 

[mpparent]

If it's in the IEEE, then chances are it's a best practice,...so it doesn't matter where it originates from.

Mike

 

[SELEC]

What about eletronic solid state relay 50/51? Does 1.25 factor stil apply? I assumed solid state relays should act faster than traditional induction relays so 1.25 sholud still apply.

 

[mpparent]

The 1.25 factor has nothing to do with "speed" as far as I know. The 1.25 factor is a margin for pickup.

Mike

 

[SELEC]

But how should we set 50/51 for SR750 or SEL 587? 1.25 or 1?

 

[davidbeach]

How you should set it depends on what you are trying to accomplish. The answer to all protection questions is "It depends." There are no fixed rules that govern in all cases. I could imagine a set of TCCs and give one answer, someone else could come up with something else entirely different, and we could both be right because we are looking at different systems. You are looking at yet another system. Nobody here can give you an absolute answer to your question. You really need to be able to sit down with someone in a situation where you can explain the system you are working on, what you have done, and discuss the specifics of this problem. The PE or P.Eng. you are working under, being able to see the work done, and the whole picture, would be in a much better position to answer these types of questions than we are here, not seeing the whole picture.

If you can't get that assistance at work, perhaps an applicaiton engineer from one of the relay manufacturers can provide some assistance. The manufacturer's application engineer might also be able to suggest classes or seminars that help you out. One to consider, (full disclosure, I do have a vested interest here) is the Basler Relay Application School held each June; 5 1/2 days of relaying from soup to nuts. More information on that can be found at

http://www.basler.com.

I know that at least one of the relay manufacturers you reference also has classes that might be useful, check their websites.

 

[cflatters]

The modern electronic microprocessor based relays have a closer pick-up than induction disc types (which are typically 1.25 - 1.3).

Clearly the objective is to adequately protect the asset, be that a cable, transformer or switchgear.