To what extent does HV protection mitigate LV faults?

It all depends on the numbers.. 

The general principle is that a fault (of negligible impedance) on the LV will result in a certain size of fault current (depending on the characteristics of the supply and the length/size of conductors feeding the fault) - and that fault current will result in a corresponding overcurrent on the HV side - the size depending on the ratio of the windings (e.g. a 1000A fault current on the secondary of a 2:1 transformer would see 500A flow on the primary side). So it's then just a matter of seeing how the HV protective devices respond to that magnitude of current.

   - Andy.

Presumably it's more nuanced than this? Otherwise why would an LV switchboard have an ACB

why would an LV switchboard have an ACB?

Generally only when the PSSC on the LV side is high enough, that nothing less than an ACB will reliably break it. It is unusual in the UK - some bosky 'death or glory' fuses on the LV side are more common. These never blow unless the fault is at the level that would destroy something, but that is what they do.

That fault current in turn, is set by the transformer, and its internal impedances, and to some extent also by the impedance in the HV external loop. However, by the miracle of transformation, ohms on the HV side have far less effect on the PSSC than any similar resistance in the LV loop ;-) 

Any ACB on the HV side is normally only protecting against gross faults within the transformer itself, and perhaps a dead short on its output somewhere before the LV ADS. 

It might provide some coverage for slightly lower current LV faults, i.e. big short circuits but further downstream within the LV network, but it is not sensible to rely on it.
The problem is that without a lot of info about the transformer, the ACB set-up and the various loop impedances for that specific installation, there is no general rule. 

Mike.

why would an LV switchboard have an ACB?

Generally only when the PSSC on the LV side is high enough, that nothing less than an ACB will reliably break it. It is unusual in the UK - some bosky 'death or glory' fuses on the LV side are more common. These never blow unless the fault is at the level that would destroy something, but that is what they do.

That fault current in turn, is set by the transformer, and its internal impedances, and to some extent also by the impedance in the HV external loop. However, by the miracle of transformation, ohms on the HV side have far less effect on the PSSC than any similar resistance in the LV loop ;-) 

Any ACB on the HV side is normally only protecting against gross faults within the transformer itself, and perhaps a dead short on its output somewhere before the LV ADS. 

It might provide some coverage for slightly lower current LV faults, i.e. big short circuits but further downstream within the LV network, but it is not sensible to rely on it.
The problem is that without a lot of info about the transformer, the ACB set-up and the various loop impedances for that specific installation, there is no general rule. 

Mike.