Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Remove the live, switch live (SWL), neutral, and earth wires from the outgoing side of the boiler terminal strip and make safe
Refit any blown fuses and reset any tripped circuit breakers
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

With the wiring to the appliance isolated does
the fuse still blow / circuit breaker still trip?