In article , John Stumbles
writes

I've just been out to look at an ISAR whose PCB is blowing the fuse in
the mains FCU. Checked that it's not the inline filter or the switch:
I'm seeing about 3 ohms across L-N on the PCB/module connections with a
normal DMM (i.e. not a megger or anything stupid).

I've not seen an Isar PCB, but in this case, assuming it has low voltage
circuitry supplied by a conventional linear or switching power supply,
would disconnect it from the boiler and do the usual checks on the
primary side of that (i.e. check bridge rectifiers, chopping transistors
if the supply is a switching type, etc.) 3 ohms is the sort of reading
you'd get when a semiconductor component has decided to let the magic
smoke out.

If the pump is run from the boiler (i.e. it has pump overrun), check
it's not a failed pump.

--
..sigmonster on vacation