Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect connector X1 from the PCB

Measurement:

Test 1:
Measure the resistance between the Brown and Black wires.
Resistance should be greater than 1M Ohm.
Test 2:
Measure the resistance between the Grey and Black wires.
Resistance should be greater than 1M Ohm.
Test 3:
Measure the resistance between each of the three wires to a suitable earth point.
Resistance should be greater than 1M Ohm on each wire.