Switchboard Short Circuit Rating

[sibeen]

I'm doing a study on a relatively old building and have run across something that I'm not sure how to handle.

The building has a main switchboard which has a design kA rating of 63 kA for 1 second. The main ACB circuit breakers have a Icu / Ics / Icw rating of 75 kA. The board is fed by three emergency generators, each of 2250 kVA at 400 volts, 50 hZ.

Te generators have a subtransient reactance of 12.6%, transient reactance of 17.6% and a zero sequence reactance of 3.16%. The feed from the generators to the switchboard is approximately 30 metres. It is slightly more complex than that as the third generator has lower subtransient and zero sequence impedances.

When I do a calculation I find that the symmetrical RMS short circuit current for a three phase fault is approximately 78 kA, which is above the circuit breaker and board rating. This current drops very rapidly, so that after approximately 1 cycle is is down around 55 kA.

When I do the calculation for the single phase short circuit current (L - E)I get a figure of approximately 88 kA. Again this current falling off rapidly.

I'm not a switchboard person, so my question basically is, what should the fault current rating of the switchboard and the circuit breakers be in this situation?

 

[sibeen]

healyx, thanks for the response.

My question over the L-N or L-E fault currents being the highest wasn't so much about the generators per se. I've known for a fairly long time that for generator systems it is quite common for this to be the case. It was more a question on whether there was buried, somewhere in the IEC land standards, a difference to be handled between three phase and single phase fault. I was 99% sure there wasn't, but wanted to cross it off my list.

A letter was sent to the client today laying out the issue, and hopefully this will lead to another job being raised. I only discovered this whilst performing an earthing / ground fault investigation on the building, and whilst I have a duty of care to inform the client, and perhaps raise future work, I wasn't going to go into to much depth on the solution side until they come up with some filthy lucre.