Set Appliance State:
Set Multimeter to AC Volts 
CH Demand
Selector switch in CH & HW mode
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
Measure the voltage between the blue wire and the outermost orange wire (as shown in the picture) at the mains terminal block.
Isolate as per TOPs P4
and refit as necessary