Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Boiler ON/OFF switch ON.
Measure on connector JP6 the voltage across the 2nd
brown lead (Live) and blue (Neutral) on JP4 on the PCB.