Short circuit rating of the power junction box for 415V application

[krisys]

DG set terminal box is not adequate to terminate the outgoing cables. Hence, we are using the Junction Box to facilitate the termination of extra cables.

Does this junction box need to have any short circuit rating?
The DG Set will also be synchronized with grid thus the resultant short circuit current of the system might be as high as 65kA. When the DG set is supplying power in the Island Mode, the short circuit current will be about 15kA.

Your valuable input and sharing of experience would be helpful.

 

[Marmite]

You don't give any clue as to the voltage,and junction box is such a generic term it could mean anything. Can you provide further details? Is this a phase segregated junction box?


Regards
Marmite

 

[krisys]

Marmite (Electrical),

This is an Emergency DG Set. The rated terminal voltage is 415V and output power is 800kW.
Even though the DG set is required for emergency, we take the load trail of this DG Set once in a week. We run the DG for about an hour in synchronization with mains supply and slowly applying the load upto 90% of the DG set capacity.

This way we do not use the load bank, as well as save some energy!

 

[cuky2000]

Junction boxes is an static component part of the raceway system no rated for continuous current or short circuit rating.

Check UL, CSA, NEC or other standards including your local code

 

[FreddyNurk]

Whilst I understand that using the network to load a backup generator is often appealing (at least in terms of not requiring a load bank, and not affecting plant operation in order to switch to the generator), I can't help but suspect that if there's not enough room in the terminal box for connection of the appropriate sized cables, then its also possible that the set breaker and other components may not be rated for 65kA either.

Is it a rental set where there's no room to change or modify the connection points? I'd not be keen on providing enough room in a box for termination of two cable sizes in a junction box or similar.

I'd expect that for what I suspect is multiple cables per phase from both generator and grid connection, some common connection arrangement (most likely busbar on standoff insulators) would be needed, which would then be subject to similar risks of short circuit as what the generator terminal box would, and to me, would need to be appropriately constructed to stop cable movement or similar in a fault situation. I'd thus expect it'd look surprisingly similar to a switchboard cubicle, with similar requirements.

What size of cables are you dealing with? Standards will also come into play depending on location.