Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
APS is located at rear of boiler, top left
Attach red crocodile clip to blue wire
Attach black crocodile clip to a suitable earth
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage between the blue wire and a suitable earth.
Mains voltage expected after fan has run for a few seconds.
Note 1:
The design of switches may vary from appliance to appliance.
Note 2:
Crocodile clip should always be connected to the blue wire irrespective of which spade it is connected to.