Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
Turn Hot Water Tap On
Selector switch to HOT WATER position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage across the brown wire (live) and blue wire (neutral) on the fan.