Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Disconnect the inline connector to the safety fuse harness from the main boiler harness
Measurement:
Measure the resistance between the two black wires at the disconnected connector as shown.
Expect closed circuit.
Expect closed circuit.
 connector safety fuse harness.gif)
 inner,covers,removed,resize.gif)
Isolate as per TOPs P4
and refit as necessary