Should I make my ASHP a TT island?

[HughF]

Simple question really, ASHP on rubber feet, should I make it a TT island or export the earth from the house? It's 20m away.

 

The risk being that it's metal cased, and in the event of a PEN fault (we're on TNCS, overhead), the casing will rise to 1/2 the phase voltage. The same issue applies to to EVs where the incoming earth is bonded to the chassis, hence why charge points have an earth rod, or a PEN failure detection circuit.