Boiler Fault Finder

Mains voltage and polarity tests

Mains voltage and polarity tests:

This test will ascertain the correct polarity between L and N although, some boilers will operate unaffected by reversed polarity.

  • Connect boiler to power supply via 3 pin plug top or inserting 3 amp fuse into switch
  • Ensure boiler is on but with all controls off, so there is no demand
  • Set multimeter to correct voltage scale on V AC
  • Firstly put the black lead from multi meter to the E terminal block on the boiler and secondly the red lead from multi meter to the N terminal block on the boiler – this should read 0V AC, but up to 15V AC is ok

Note: Should a reading be greater than 15V AC, this indicates a fault is present within the house wiring, but the polarity at the fused switch should first be checked, prior to calling an (NICEIC) approved electrician.

mains voltage polarity tests