Set Appliance State:
Mains Power Off
Set Multimeter to AC Voltage 
CH Demand
CH/HW switch in CH position.
Ensure connections on the base of PCB 27 do NOT come into contact with boiler during test as shorting will occur.
Ensure connections on the base of PCB 27 do NOT come into contact with boiler during test as shorting will occur.
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Measure mains voltage across the connection marked RS 2 (white/black wire) on the PCB 27 and the neutral (blue) on the mains supply connector block.
Isolate as per TOPs P4
and refit as necessary