Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Gain access to PCB (use Insulation Mat as required)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Using socket & see confirm the polarity
is correct at the mains terminal block.
Note:
If socket and see is unavailable use
multimeter - refer to TOPs for guidance.