Ground Fault Relay Requirement

[powerjunx]

I would like to know the minimum transformer rating which requires a "back up ground fault relay, (51N)"?

I've seen different installation, but more obvious i'd
notice 2MVA haven't such relay as compared to 5MVA transformer and above.

I hope, you guys out there would further elaborate this matter. Please include Standard/Code about these requirement, if any.




Thanks,

Bil

 

[davidbeach]

I'm not aware of any Standard or Code that would require that protection. Think of it as insurance - what is your transformer worth, what would it cost you if the transformer were damaged, and how much of that damage could you prevent if you added that protection? Like so many protection questions, the answer is "It Depends."

 

[rbulsara]

Agree with davidbeach.

Bil: your observation is accurate. We regulary specify transformers of 2500/3300 kva ratig.

When we have a vaccum circuit breaker on the primary side and a long feeder is feeding a transfromer (or transformers, think of a campus distribution), we would have a 51N relay in it. This is more to detect ground faults on the feeder cable than one in the transformer, and also it is easily implemented as you have to have 51 phase overcurrent relays at any rate.

If there is only a fusible switch interrupter, of course, there will not be any relay.

 

[powerjunx]

Well, thanks dave!

We do have the same awareness, It's a matter of a Cost! - the Insurance of the equipment.


rbulsara: "If there is only a fusible switch interrupter, of course, there will not be any relay."

Do you mean if this interrupter is present, a ground fault relay is not recommended? We have this kind of scheme, but i can't find any substantial documents to support it. Can you cite a certain "standard" to convey your statement?

Again, i ask..
What is the minimum transformer rating (and protection scheme, i think) which requires a "back up ground fault relay, (51N)"?




Thanks, guys.

 

[rbulsara]

Billybry:

Not sure why I wandered here on this thread after so long. But to answer your last question, there is no standard that "require" any type of protection. At end of the day protection schemes beyond normal overcurrent protection (which is minimum for safety) are mere insurances against limiting the damage. Its upto the owner to determine how much insurance to buy based on his/her risk tolerance.

Best you can go by is what are the accepted practices in general and by consulting experienced engineers. I would say, for transformers rated less than 5000kVA, you will not find GFR.

Without going in much detail, it would suffice to say that most practices have been accepted overtime, becuase they have been found adequate in most instances.