Set Appliance State:

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

Measurement:

Measure Mains Voltage across the room thermostat terminal marked SWITCH L (Red wire) and the neutral PERMANENT N (Blue wire) on the main terminal block.