Set Appliance State:
Mains Power Off
Set Multimeter to DC Volts 
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Measure the DC voltage between the left hand terminal of the 24v external controls thermostat and a suitable earth.
Expect approximately 24 Vdc.
Expect approximately 24 Vdc.
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 connector x100.gif)
Isolate as per TOPs P4
and refit as necessary