Fault Rating of Single Phase Breaker in a 3 phase Switchboard.

[healyx]

Hi All,

I'd be interested in some opinions on this:

Should a single phase circuit breaker in a 3 phase switchboard have a fault rating >= the maximum 3 phase fault at the board or the maximum single phase fault at the board. The single phase fault (phase to neutral or phase to ground) is usually lower because the impedance of the neutral/ground return path.
I originally thought, you would only need to rate it to the lower fault level, but then considered the possiblity that a three phase fault could still occur if a shared fault involving adjacent breakers at the load side terminals was to occur. Pretty unlikely I guess, but technically possible. I'm now thinking the single phase brekaer should be rated to the 3 phase level.

For example, say at a 3 phase board the max. 3 phase fault level is 8kA, but the max. 1 phase fault is 5.5kA. SHould you use a 6kA rated single phase breaker on this board? or should you use a 10kA rated breaker? I think the latter.

Obviously this doesn't apply to a single phase switchboard, the maximum single phase fault is all you need to rate devices to.

 

[musaUKPN]

In your situation the 1-phase breaker should be rated for 3-phase fault level. I understand that the intake supply is 3-phase. A 3-phase fault downstream of the switchboard will produce higher current than 1-phase to ground (as per you figures).

musa UKPN