Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect connector X3 from the PCB

Measurement:

Test 1:
Measure the resistance between the brown and blue wires on X3 connector.
Resistance should be approximately 160 Ohms or greater.
Test 2:
Measure the resistance between the brown wire on X3 connector
to a suitable earth point.
Resistance should be greater than 1MOhm.
Test 3:
Measure the resistance between the blue wire on X3 connector
to a suitable earth point.
Resistance should be greater than 1MOhm.