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 two different instantaneous pickups you can set. 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.