Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
Turn Hot Water Tap On
ON/OFF Switch ON
Summer/Winter button set to Summer
CH Temperature selector to minimum
DHW Temperature selector to maximum
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across the Live connection (brown wire) and Neutral connection (blue wire) at the fan.