Set Appliance State:

Mains Power Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
CH Demand
Use Insulation Mat as required
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

With the Fan running measure the Voltage across the Yellow wire (connection marked L1) on PCB connector PL2 and the Blue wire connection marked MAINS (N) on the main terminal block.