Fault Finding Electrical Circuit: RCD Tripping

elec 1Welcome to another post from Sparkyhelp.

Today we will look at one of the most common faults you may be called to on a domestic or commercial installation. The RCD (earth leakage device) keeps on tripping.

 

This is a fairly common thing to happen, yet can keep you searching for hours, especially if it is an intermittent fault. The first thing is to find out when it tripped, in other words, did they turn anything on and that was when it tripped? You will find that in a lot of cases, the occupant plugged something into a socket outlet and when it tripped the RCD, they unplugged it and now the RCD won’t latch again when trying to turn it back on. They pay no attention to what they tried turning on as obviously that was not what caused it and they don’t tell you about it.

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First, let us assume that is what happened. Turn off all the circuit breakers associated with the RCD. Now reset the RCD and once that is on, turn on the circuit breakers one at a time. It is always good to wait a few seconds before turning the next one on. Everything stays on? Great, job done. It trips when you turn on one of the circuit breakers? Move to the next step.

If one circuit trips the RCD, repeat the procedure to reset the RCD but leave the faulty circuit turned off. If it is not clear what is supplied from this circuit breaker, move through the installation and determine which outlets have no power. Once you know that, you can proceed to unplug all devices on that circuit and then restore power to the circuit. The next step is to find the faulty device. Do this by testing each device, not by turning them back on!

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Now let us say that the RCD will not reset at all even when all the Circuit Breakers have been turned off. This is a little more complicated and what I generally do in this case is to isolate my main switch, test to make sure it is isolated and then remove the connections from the load side of the RCD. In other words, it has nothing connected to it apart from the supply. Once I have done that I will CLOSE the board first and then turn the main switch back on. Now I will reset the RCD and if it stays on, I will once again isolate the main switch, open the board and move to the next sequence of testing.

  • Ensure all devices are unplugged from the entire installation.
  • Test each circuit from the distribution board with your insulation tester.
  • Disconnect the faulty circuit.
  • Close the Board.
  • Restore power to the main switch.
  • Proceed to find what parts of the installation has no power.
  • Test all equipment on that circuit.
  • Remove the faulty equipment.
  • Isolate the Main Switch.
  • Re-connect the Circuit that was faulty.
  • Test the circuit again.
  • Close the board and restore power.

Where you come across an intermittent fault, it is not that easy to find and you will be relying on the readings you get when testing the insulation resistance. Look for the circuit that has the worst reading and start looking at that. Be warned, this could turn into a very long process to find the culprit!

Here is what you NEVER do: disconnect the earth wire from the equipment that causes it to trip. You will come across the old fridge or washing machine that works, but does trip the RCD every so often. This is telling you there is a fault. Do not disconnect the earth from it and use it. You are risking causing someone serious injury if not worse.

2 thoughts on “Fault Finding Electrical Circuit: RCD Tripping”

  1. Hello SparkyHelp
    I am electrically competent (HNC Electrical Eng) but have been asked to resolve a problem which I can’t.
    A friend of mine bought a portable plug-in RCD to give protection when using a Flymo grass cutter.
    He tells me that when first used it accelerated the 240 volt, single phase Flymo motor leaving it rotating at a much higher speed than normal.
    I suggested he tried other devices and he reported that an electric drill and vacuum cleaner both performed normally as did the Flymo if plugged directly into the mains socket.
    He lives too far away to investigate first hand.
    I can only speculate that there is some capacitive component in this particular RCD that has corrected the power factor for this motor.
    However, such a component must be significant and have no place in the workings of a regular RCD.
    Your comments would be most appreciated.
    Alan Hearne
    alan.hearne@yahoo.com

    1. Hi Alan
      This is very interesting. I have certainly not come across this before. In order to get an indication of what could have happened, I will need a bit more information though

      You mention that this happened when plugged in to the portable RCD. This brings me to the first question:- to be able to use this, he would have used an extension lead no doubt? Was the lead fully rolled out or was it coiled up? Reason for this question is that you are correct. There are no components in the portable RCD that would cause this to happen, but a coiled up extension lead could have an effect on the motor behaviour.

      Have a chat to him about this and if this was the case, get him to ensure the lead is fully ‘rolled out’ and then test it again. I would love to hear the result!

      Kind Regards

      Frans

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