MV feeders - ANSI 51 pick up ?

[Mongao]

Hi folks,

Please, what is the usual practice for the ANSI 51 pick up for 4.16kV industrial (radial) feeders ?

I have read people saying from 10% to 600% of the ampacity, a bit confusing,

Thanks a lot,

Mongao

 

[davidbeach]

More than the maximum load current, less than the minimum fault current; and coordinated with the upstream devices. There is no numeric rule of thumb that will always meet those conditions; that's why we have Protection Engineers practicing the "Art and Science of Protection". Not just a science.

 

[leur2011]

Nice reminder davidbeach.

Mongao,
Another issues you have to look at is that it will not be easy to coordinate fuse at the primary of your transformer with the MV feeder circuit breaker.

Common practice for the pick-up setting of the 51 phase overcurrent relay for radial feeder should be approximately 1.3 times the full load current of the 4.16kV MV feeder.

 

[Parchie]

@leur2011,
Correct. It's not easy but it's done more often than not (fuse protection on primary and circuit breaker on the MV side. You just have to know the published fuse characteristics.

 

[Mongao]

Hi all,

Thanks for helping !

Page 16 of this presentation

http://www.hv-eng.com/OvercurrentProtectionAnnex.pdf

refers to NEC 240-100, that mentions that ANSI 51 can be anywhere between 100 and 600% ! It seems too high; but the PDF states that "the intent is to provide protection for overcurrent, not overload". I have used between 20% and 70% in my project, I hope it's OK,

What do you guys think ?

Thanks a lot, I really appreciate it !

 

[davidbeach]

Why would you set at less than 100%?

 

[Mongao]

I should have written between 120% and 170%, sorry,