Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
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
Boiler switch ON
External controls calling

Measurement:

Measure voltage across connection L2 and the connection marked N in main terminal block.
Note:
If no external controls are fitted a link must
be fitted between connections L1 and L2.