Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
On/Off Switch in ON position
Mode Selector Switch to Constant
Heating Thermostat to MAX
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure Mains voltage across connection 1 (L White) and N (Blue).