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A single-phase (line to neutral) fault will be approximately half that
of the three-phase fault (across all lines). This affects the fault
current ratings of individual devices for the distribution board.
For example, if a distribution board has a three-phase 10kA
prospective fault current, then the single-phase devices need
to be selected to have a minimum fault current rating of 5kA,
assuming that the line to neutral prospective fault current is equal
to or higher than the prospective earth fault current.
434.5.1 states that the short circuit capacity of devices in the
distribution board shall not be less than the prospective fault
current where the device is installed.
A lower breaking capacity is permitted if another device on the
supply side has the necessary breaking capacity. In this case the
supply side device provides back up protection for the load side
device. Manufacturer’s data should be sought to obtain the level
of back up protection provided.
Using back up protection can produce a more cost effective
installation with perhaps the incomer to a TP&N board being
a 250A MCCB. 10kA outgoing devices could then be installed
where there is a 20kA fault level at that distribution board, subject
to manufacturer’s data.