Set Appliance State:
Gas Off
Set Multimeter to AC Volts 
Isolate Mains Power to appliance as per TOPs P4
Gain access to mains terminal (Use Insulation Mat as required)
Gain access to mains terminal (Use Insulation Mat as required)
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
Using either a socket and see or multimeter re confirm the polarity on the incoming mains supply to the appliance.
Is the polarity correct?
Is the polarity correct?
Isolate as per TOPs P4
and refit as necessary