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Ground Fault and protection scheme

Ground Fault and protection scheme

Ground Fault and protection scheme

(OP)
When coordinating  the main feeder ground fault protection with downstream fuses, does the kind of protection scheme matters?
If it is Residual connection or core balance or calculated?
My feeder ground detection is calculated based on Ia+Ib+IC value, do I need to coordinate the ground curve with fuses downstream?
 

RE: Ground Fault and protection scheme

The answer is it depends.

If you do, your clearing times will be much greater.
If you don't, you risk having a ground fault beyond the fuses, and clearing it with your main.

Which is a bigger problem on your system?

RE: Ground Fault and protection scheme

(OP)
Cranky108,
What do you mean by "If you do, your clearing times will be much greater"?

I was always under impression that when coordination study is done, phase current protection curves will be coordinated with downstream fuses or relays, does this mean that ground curves also need to be coordinated with downstream fuses and relays?
Ground curves have lower setting, how this could be achieved?

RE: Ground Fault and protection scheme

Welcome the the he!! of coordinating fuses with anything else.

RE: Ground Fault and protection scheme

Full coordination of ground fault element with downstream devices without GF is almost impossible, except for very small feeder compared to the main.

Although, GF has definite time delay, which helps eliminate most nuisance trips of the main. History and experience has shown that it is generally acceptable (and hence the codes are the way they are, in the USA).

Having said that, one should consider providing multiple levels of GFP when discrimination is very critical.

Rafiq Bulsara
http://www.srengineersct.com

RE: Ground Fault and protection scheme

Some of us like to set the ground curves lower than the phase curves. And thus faster operation for a ground fault than a phase fault.

Provided your possible zero sequence unbalance allows it.

RE: Ground Fault and protection scheme

Residual vs. core balance is determined by what GF system you are using.  Generally, low voltage system utilizes individual CTs and current is summed to determine if a GF has occurred or not.  I interpret that a "core balance" system is a GF system employing the use of only 1 CT encompassing all phase and neutral (if used) conductors.  Now as far as coordination, fuses are used for overload and short circuit protection, not ground fault.  Therefore, you need only to coordinate GF protection (i.e. you want downstream devices timed lower to take out the GF first before taking out the main(s).  But as Rbulsara says, you need to identify if you even have any downstream devices in order to have anything to coordinate with.

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