# Ncees 137, 50/51 relay operation



## vang83 (Oct 12, 2013)

With a multiple pickup current of 3.6 shouldn't the instantaneous unit trip @ 0.02s? What I am confused about is when does the instantaneous unti (50) trip? Any help or information regarding a 50/51 relay is apprecianted.

Thank You

Vang83


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## iahim (Oct 12, 2013)

Very good question! My guess is that the specified pickup is for the 51 function only. The instantaneous relay would have a different pickup setting. Topically you have a lower pickup for the 51, with a time delay coordinated with the downstream relay, and a much higher pickup for the 50, so it doesn't reach into the zone of protection of the downstream relay. Otherwise you can't have coordination.


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## vang83 (Oct 12, 2013)

I'm also assuming that there's different settings for the 50 relay. Can any one else confirm this?

Thanks


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## iwire (Oct 13, 2013)

I am confused now...since the Relay operation is .9 sec...then the on graph Time dial is =2

but if you use If/Ipu = 3.6 on graph it you get TD setting is 2 too

Relay 50 is instantaneous protection...


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## daw4888 (Oct 14, 2013)

It never says to use the 50 function. Since it specifies a operation of 0.9 seconds its clear to see that it wont be the instantaneous unit, it has to be the time unit.


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## vang83 (Oct 14, 2013)

I still don't know how a 50/51 relay works. Is there really a different pick up current setting for the 50 relay? Anyone here with experience of the 50/51 relay? I've taken the PE exam once before, and the 50/51 relay does come up. And like always the NCEES questions are never clear about what they're asking.

I can't repeat the question asked on the PE exam, but I will say that only someone who is familiar with a 50/51 relay will be able to answer that question. Please help..


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## vang83 (Oct 14, 2013)

Also as a tip to anyone taking the PE for the first time, the PE exam is composed of about 50% calculations and 50% word questions. I was able to answer all the calculation problems in about an hour in both AM and PM. Spent the next 3 hours flipping through my notes for answers I couldn't find. This time around not only am I studying how to do the problem, but how to apply the problem as well.


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## iahim (Oct 14, 2013)

Thanks vang83 for the tip! So the real exam is nothing like the sample NCEES exam?


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## vang83 (Oct 14, 2013)

iahim said:


> Thanks vang83 for the tip! So the real exam is nothing like the sample NCEES exam?




The real exam is alot like the NCEES practice exam? Just as confusing too. Think about how you felt the first time you took the practice test. You must have sat there just trying to figure out what they were asking, one wrong assumption and you got the question wrong. All the possible wrong answers will be on the test, so triple check your work. The questions that I wasn't prepared for were the word questions, such as "how does this relay operate"?

The difference is, there were less calculated problems and more application types of problems. Then again, I've been told the PE exam changes all the time.


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## power62 (Oct 14, 2013)

iwire and daw4888 are correct.

50 unit is Instanteneous - no intentional time delay. Typically have higher setting value i.e. severe faults.

51 time delay unit - when you want intentional time delay for a particular value of fault current. Time Dial settings is way for you to set the relay (it gives you multiple possible time delays for a particular fault current or vice versa).

Hope this helps.


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## daw4888 (Oct 15, 2013)

vang83 said:


> I still don't know how a 50/51 relay works. Is there really a different pick up current setting for the 50 relay? Anyone here with experience of the 50/51 relay? I've taken the PE exam once before, and the 50/51 relay does come up. And like always the NCEES questions are never clear about what they're asking.
> 
> I can't repeat the question asked on the PE exam, but I will say that only someone who is familiar with a 50/51 relay will be able to answer that question. Please help..




Most relays these days are just self contained computers basically. For distribution a common relay is the SEL351S-6. You can program them to do about anything you want. So yes, a 50/51 are contained in the same relay. You can set different pickups for both.

Depending on the needs of the user, there are can two different instantaneous pickups. One is used as a quick trip, or fuse saving operation. Most times this is set to the same pickup as the 51, and allow the ability to have fuse savings (it opens quicker than a fuse can burn, which allows you to let transient faults clear). Then you set a high instantaneous, which is set near or above the max fault current you can have out on the line (normally where a fuse wont help you anyway), and will trip instantaneously and not reclose to help protect the transformer.


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## daw4888 (Oct 15, 2013)

daw4888 said:


> vang83 said:
> 
> 
> > I still don't know how a 50/51 relay works. Is there really a different pick up current setting for the 50 relay? Anyone here with experience of the 50/51 relay? I've taken the PE exam once before, and the 50/51 relay does come up. And like always the NCEES questions are never clear about what they're asking.
> ...


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