Set Appliance State:

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

Measurement:

Measure the voltage at the grey wire and the neutral on the main terminal block.