Set Appliance State:

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

Measurement:

Test 1.
Measure for resistance across the brown & blue wires of connector X3.
Expected resistance greater than 1 MOhm.
Test 2.
Measure for resistance across the brown wire of connector X3 and a suitable earth connection.
Expected resistance greater than 1 MOhm.