Set Appliance State:

Mains Power Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
CH Demand
Gain access to PCB (use Insulation Mat as required)
Heating switch ON
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage across terminal 3 of Control Box and the mains neutral.