Would an RCD trip if supply polarity is reversed?

Discussion in 'Electricians' Talk' started by Frustrated homeowner, Dec 7, 2015.

  1. I've been having a few problems with electricians I've recently used and I'm not sure who to trust. It's a long story for some of it so i'll try and explain quickly.

    I got an electrician to change my electrical board to a new modern one. He done some other work at the same time as a previous 'electrician' I used turned out to be a handyman and left quite the mess. He gave me a test certificate and I was very happy with the work. He came back out a few months later as the RCD started tripping. He said it was bad wiring in the loft. I recently couldn't get a hold of him when one of my lights stopped working so had to get someone else out. They spent 90 minutes trying to find the problem and eventually said it was reverse polarity from the meter to the new board. He said this is very dangerous and would stop the RCD working properly. I contacted the previous electrician as I was very angry that he left a dangerous job. He is adamant that he tested the job properly and it couldn't have been reverse polarity since the RCD tripped after he changed the board.

    I am not sure who to believe. Can anyone shed any light on this or advise me at all?
     
  2. roymondo

    roymondo Member

    With changing the fuse board you got a ' signed your life away' certificate with test result pages, in there, is a tick box for correct polarity. To get a tick for correct polarity on sockets a live polarity test is very simple, just plugin tester and thats it . That would also have shown up reversed polarity at the fuse board. Do you remember the electrician going around all the sockets with a tester ?

    Even if there was crossed polarity at the fuseboard it would not make any difference to the operation of an RCD it would not stop it doing its job. So the electrician who said it would stop the RCD working properly when it was actually tripping /working sounds suspect. Are your previously tripping lighting circuit and other circuits still in exactly the same setup as the certificate ? I say this because the tripping lighting circuit could be rectified by taking off the RCD !
    RCD can be frustrating !
     
  3. MGW

    MGW Screwfix Select

    I have made errors myself. Had a whole load of sockets 90% had line one side and a few had line the other side. But as soon as I plugged in the tester the error was shown and corrected. As to danger swapped polarity means fuse/MCB/switch is monitoring neutral instead of line, in houses before 2008 that could be very bad in that an earth fault would not auto remove supply. But RCD will still work so since 2008 the danger is quite small. RCD will still trip auto disconnecting supply.

    Reverse polarity will not stop a light working. So I would read "He said this is very dangerous and would stop the RCD working properly" to really mean "I want all your future work even if I need to lie to get it" which would kind of put me off him.

    Oh and yes RCD will work with reversed supply.
     
  4. Bazza-spark

    Bazza-spark Screwfix Select

    Not quite so MGW. The RCD will only trip with an imbalance between phase and neutral, ie an earth fault.

    OP The RCD will not operate under short circuit conditions and if the install is reverse polarity neither will the circuit protective device. This means excessively high currents can flow until either the main fuse blows or the fault clears. Very high risk of fire through cables overheating etc.

    Kind regards
     
  5. peter palmer

    peter palmer Screwfix Select

    Bazza-spark is correct, no chance of a RCD tripping if its a L-N fault. I found out the hard way years ago when I tailed one of those stupid square D boards in backwards, all MCBs off and then I snipped into a cable for an alarm spur or something, made a bit of a mess of my snips.

    Offending board

    [​IMG]

    Why on earth would they be backwards to about 99% of the other boards out there.
     
  6. Not quite so Bazza Spark. The answer to the question "will the RCD still function normally with reversed polarity?" is yes, it will.
    The circuit protective device will also operate completely as normal except that the neutral will be disconnected leaving the line "live" which is obviously potentially dangerous (however, if you were a knowledgeable sort you would test for dead before poking your fingers anywhere).
    I'm not sure where your last two alarmist sentences came from though.
     
  7. Bazza-spark

    Bazza-spark Screwfix Select

    /
    I did say the RCD would operate.

    Yes, you are correct about the short circuit. Must remember to read the post and not to reply when eating dinner lol. Sorry OP
     
  8. Bazza-spark

    Bazza-spark Screwfix Select

    Actually it depends where the short is. If the short is before the MCB no it will not operate and my first post is correct. A screw through a cable in the wall will be before the circuit potective device so will not cause it to trip. My first post was correct under those conditions. You are talking about overload on a circuit.

    Kind regards
     
  9. peter palmer

    peter palmer Screwfix Select

    How will the MCB still operate as normal. The blue will be the conductor at 240v and will be connected straight to the "neutral bar. The brown would still be connected through the MCB but effectively be the neutral conductor. I'll agree that if its a brown to blue short the MCB will still trip but if its a blue to earth fault then the only protection would be the main incoming fuse.
     

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