Set Appliance State:
Mains Power Off
Set Multimeter to AC Volts 
CH temperature selector to 0 position
DHW temperature selector to 0 position
Lower control box cover
DHW temperature selector to 0 position
Lower control box cover
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Allow appliance to self check and return to standby mode.
Measure voltage between connection 1 (white lead) on the 20 connection connector that is towards the centre of the bottom edge of the PCB and the main earth point on the appliance.
Expected voltage approximately 27 volts AC.
Measure voltage between connection 1 (white lead) on the 20 connection connector that is towards the centre of the bottom edge of the PCB and the main earth point on the appliance.
Expected voltage approximately 27 volts AC.
Isolate as per TOPs P4
and refit as necessary