Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect connector X21 from the PCB

Measurement:

Test 1:
Measure the resistance between the brown and blue wires on X21 connector.
Resistance should be greater than 1M Ohm.
Test 2:
Measure the resistance between the brown wire on X21 connector
to a suitable earth point.
Resistance should be approximately 0.9M Ohms or greater.
Test 3:
Measure the resistance between the blue wire on X21 connector
to a suitable earth point.
Resistance should be greater than 1M Ohm.