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Cause of RCD not tripping with test switch

Alan B

I have a strange fault where an RCD operates as expected when tested on the bench but when installed the test switch does not cause it to trip.

I have asked this on another forum but not really managed to get an explanation of what could cause it and why.

I have 3 RCD's giving the same fault, they trip satisfactorily when tested with an MFT when installed but test switch does not cause a trip with the downstream MCB's on or off.

My thoughts so far.

Unlikely to be the RCD as swapping has no impact.

MFT test creates fault current to earth, test switch creates fault current from live to neutral prior to sensor coil, how much current flows??

Insulation tests on installation gives about 1.04m ohm live conductors to earth at 250V on two circuits, 0.84 for installation at 240v this gives a leakage current of 285mA. Why doesn't this trigger the RCD, or do I have the decimal point in the wrong place?

The above is leakage current at 240V, but problem exists with CB's of, therefore presumably very limited voltage across circuit and current flowing.

Others have said its likely to be a neural earth fault. I can see the logic that the line is disconnected by the CB's and therefore it must be related to the neutral plus I could see that if there was a fault current in the neutral side it would balance the test switch current, preventing a trip. But what would drive the current as the potential difference between Neutral and earth is close to zero, and why would testing with a meter give a different result.

I thought about the RCD being bypassed in someway but I couldn't see any unexpected electrical connections.

I have considered DC leakage current, but would you get DC leakage with everything switched off? Maybe batteries feeding back through the charging circuit? There are led lights,  one TV and quite a few gadgets plugged in, but very limited compared to most, no car chargers or computers.

The house was rewired about 20 years ago with black / red PVC cable that looks reasonable. There is quite a bit of dodgy DIY, with fixed lights on flex, plugged in in to sockets, lots of extension leads and adaptors; unplugging these will be  a good starting point.

One strange thing I did find was that the house has a TT earth but when the main earth cable is disconnected at both ends I get an earth impedance of 0.3 ohms from the cable. Presumably a hidden connection to steel frame of the semi detached Cornish house, maybe with next door on TNC-S. Water pipes are bonded but isolated on entry to the house.

As some background this is on an old Wylex WSTM 100/2, 100mA type S rcd.

I realise I am going to have to disconnect circuits to break the problem down but would very much appreciate any thoughts on possible causes and theory behind the cause. I have a horrible feeling it's gong to come down to cumulative effect of multiple issues which are going to take considerable time and money to sort out. I won't be able gaurantee making 1 or 2 fixes will solve the problem and the customer has a genuine lack of money.

Any guidance on possible faults to look for and why they would cause the symptoms would be much appreciated, I am not a fan of just trying things without understanding the background science.

On a slightly different tack, I am considering failure of test button test as a C2 that needs resolving, am I over coding it and would a C3 be more appropriate? I am certainly not that comfortable with it as the owner has no way of knowing if the RCD is working and presumably there is one or more faults in the house.

Parents
  • 0

    Thanks al for your thoughts

    I agree I was mixing up my decimal places, not thinking straight at all when I wrote the post.

    I have 3 RCD's to work with (original and 2 replacements as I blamed the RCD's to start with)

    The devices do date back to 80's/90's. The RCD in question is the main incomer switch to the CU, further 30mA RCD in CU to protect socket circuits, this means that there is domestic lighting and cooker protected by a 100mA time delayed RCD. Ideally should be replaced but that would involve a considerable cost which is going to be challenging for the customer and I would prefer to avoid unless really necessary.

    I have been holding the test button for 45 seconds when installed and they trip more or less instantly with a bench test

    I don't think it's the test button, 3 devices same issue and all 3 work on bench test connected to 13a socket (unless we are looking at cumulative errors) but I will test the resistance to double check.

    On Andy's explanation, I think the explanation assumes test resistor is connected live out to CPC. In this case the resistor is connected live out to neutral in. All I can think of is that there is some pre existing leakage current neutral to earth which then balances out the current introduced on the live side. I would assume most RCD's work like this as most don't have an earth fly lead?

    My KT65DL will not run an RCD test unless it sees reasonable voltages and correct polarity on the test leads

    The downstream 30mA RCD works correctly after replacement. Presumably this means I have less than 30mA imbalance on the socket circuits and they are completely isolated from the 100mA RCD when the 30mA RCD is turned off. Therefore my issue is likely to be in the lighting or cooker circuits.

    One thing that comes to mind is that to my knowledge insulation tests with a meter are DC, maybe I have much less impedance with AC due to capacitors or capacitance.

    Looks like this is going to come down to lots of testing as nothing is obvious.

    I think I am going to do the following 

       As a sanity check, test installed with line and neutral disconnected on the load side.

      Use clamp meter to test imbalances on supply cables with one circuit energised at a time

      As above with the neutrals disconnected, along with trying the test button and insulation tests again 

  • 0 in reply to Alan B

    My device of the same make and model disconnects in under one second when I press the test button. Please see my post above. Were your replacement devices new or from fleabay? I have bought old faulty devices from fleabay in the past. They may test o.k. by tester but not work on test button.

    Z.

  • 0 in reply to Alan B

    If appliances work correctly in the house and there is no appreciable Voltage drop when they are on, that then confirms the supply side of things is O.K.

    Your R.C.D. does not need an earth to trip off via test button, as the test circuit  is connected L to N. Any internal house wiring problems will not affect the test button operation of the R.C.D. unless it is faulty, which I suspect it is.

    I believe that you are overthinking this problem.

    A new R.C.D. installed in the tails will solve the problem.

    Z.

  • 0 in reply to Zoomup

    I am using second hand parts from one of the specialised suppliers, no new parts available.

    The reason I was going down the path of replacing the RCD is that I believe I am allowed to make a like for like replacement, even if it doesn't comply with current regulations. But if I use an external RCD is it still considered a like for like replacement?

    Is my interpretation of the regs correct?

Reply
  • 0 in reply to Zoomup

    I am using second hand parts from one of the specialised suppliers, no new parts available.

    The reason I was going down the path of replacing the RCD is that I believe I am allowed to make a like for like replacement, even if it doesn't comply with current regulations. But if I use an external RCD is it still considered a like for like replacement?

    Is my interpretation of the regs correct?

Children
  • 0 in reply to Alan B

    I would install a new 100mA S type R.C.D. in the tails. Your customer will appreciate the fast remedial action. I have been in your position in the past when I was younger, and learned my lesson then. The more you delay and fuss about, the more the customer will become suspicious and nervous and lack confidence in you. You can't really rely upon the condition of old pre-used parts.

    Z.

  • 0 in reply to Zoomup

    OK decided to go with your advice and ordered the parts, fingers crossed it solves the problem.

  • 0 in reply to Alan B

    Very good luck Alan. Let us know how you get on please.

    Z.

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