Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Main On/Off Control in the On position
CH Temperature Control to Maximum
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

If a fault code is displayed press boiler 'Reset' button.
Measure Mains Voltage across connection 3 (L) and connection 5 (N) on boiler connector block.