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RCBO's and IR testing

Jaymack
I have had a problem with 5 off Wylex RCBO's. These were bought by a houseowner and fitted by him as there was no RCD protection for some circuits requiring such. These Wylex RCBO's have a warning hidden on the body that connections should be disconnected for Insulation Testing ..........too late the man cried!. For expediency and availability, I have redesigned the arrangement to have MCB's with a common RCBO as a M.S..

I have been in contact with Wylex technical help, who say that they should have been OK to withstand 250V IR testing as in this case, but to return them to the dealer. Anyone else have had similar problems?


Jaymack .
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    If insulation resistance testing used in periodic I+T was a commercial product, I would fear for it. The modern installation is full of electronic equipment and where there is global 30mA RCD protection, I simply wouldn’t bother. Sure you might need it in fault diagnosis but it just isn’t worth the effort in a standard periodic inspection where every circuit has RCBO protection. There are far more important things to attend to. I have a HSE document somewhere in which they refer to 30mA RCDs as effective insulation monitors.
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    I have a HSE document somewhere in which they refer to 30mA RCDs as effective insulation monitors.




    Not wishing to blow my own piano but I said that years ago. Am I due any royalties?


    Z.

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    I have great respect and admiration for you Zoom but there is a tad more heft in HSE utterances!
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    A 30mA RCD will trip at a resistance of around 10kOhm, whereas IR testing will also detect faults needing to be investigated in the 10-1000kOhm range. Also, an RCD won't detect deteriorating L-N insulation. So periodic IR testing could be considered as picking up on deteriorating conditions before circuits starts randomly tripping or things start overheating.
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    Yep ins res test whether RCD or not. If practical a Live to Live is a good idea but if not practical then a Live(s) to E test as a minimum on a periodic. Initial testing should always include all though. If the chappy testing is also the installer then not a problem
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    I think that there is a serious problem lying hidden here. I do not like disconnecting anything for a periodic inspection, and there is a good case that anything that fails is unsuitable for further service anyway, because how do you know that it does not have an insulation fault inside which could cause a fire? The whole point of a 500 V test is that if two connections could spark between them the test will fail, and if passed the normal mains voltage will not form an arc. Perhaps we are told we need AFDDs because we cannot carry out an insulation test at 500 V? It is fairly easy to design electronics which will not be damaged by a range of mains connected voltages, from several kV transients to the insulation tester. The fact that it seems that these products cannot withstand 500V without damage says that they are not satisfactory or safe for use in the real mains system! It may well be that some items may show low IR test readings (the humble neon indicator for example) but that is something we can all cope with perfectly well. Total failure is completely unacceptable, particularly if that failure is not completely fire proof and safe. There is an IEC document on mains transient capability although I have forgotten what it is called or the number. Clearly the manufacturing standard is faulty in its specification, which should include both 500V DC test and transient resistance.



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    davezawadi:

    I think that there is a serious problem lying hidden here. I do not like disconnecting anything for a periodic inspection ...



    Quite right too - 651.2 "Periodic inspection shall be carried out without dismantling, or with partial dismantling, as required."


    The more you dismantle, the further away you get from the installation as is.


    Any comments on my point about about testing circuits with SRCDs or USBs which have to be disconnected?
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    Chris Pearson:




    davezawadi:

    I think that there is a serious problem lying hidden here. I do not like disconnecting anything for a periodic inspection ...



    Quite right too - 651.2 "Periodic inspection shall be carried out without dismantling, or with partial dismantling, as required."


    The more you dismantle, the further away you get from the installation as is.


    Any comments on my point about about testing circuits with SRCDs or USBs which have to be disconnected?

     




    There are no two ways about it. If you wish to test insulation resistance of fixed wiring at 500 Volts then all loads and every other item that may affect the results or that may be damaged by the testing has to be disconnected. Years ago this just meant turning off fixed appliances by the double pole switch, removing lamps and unplugging appliances at sockets. Nowadays it is more complicated than just a forgotten door bell transformer still connected to a lighting circuit in the loft.


    Z.

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    You have to remove all the plugs and lamps to do the test. Otherwise you are not testing the fixed wiring installation at all, you are testing appliances. That has always been the case and is nothing new. The new part are the allegedly mains powered things added to the fixed wiring preventing it being tested properly. Why anyone would buy one of those "USB" sockets is beyond me. USB is the data not the 5V supply, so why call them USB sockets. Crazy. The 5v chargers I have are easily unplugged for testing. The point of the insulation test is being lost as I said, it is to check for gaps of less than 1-2mm between live conductors, or lives to Earth. Do you PAT test at 500V? I do and found a faulty kettle only last week. Worked fine on 250V test, but still dangerous in my view, particularly as the cordless connector Earth connection was also intermittent. It could be a life saved, so PROPER testing is important.
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    Zoomup:




    Chris Pearson:




    davezawadi:

    I think that there is a serious problem lying hidden here. I do not like disconnecting anything for a periodic inspection ...



    Quite right too - 651.2 "Periodic inspection shall be carried out without dismantling, or with partial dismantling, as required."


    The more you dismantle, the further away you get from the installation as is.


    Any comments on my point about about testing circuits with SRCDs or USBs which have to be disconnected?

     




    There are no two ways about it. If you wish to test insulation resistance of fixed wiring at 500 Volts then all loads and every other item that may affect the results or that may be damaged by the testing has to be disconnected. Years ago this just meant turning off fixed appliances by the double pole switch, removing lamps and unplugging appliances at sockets. Nowadays it is more complicated than just a forgotten door bell transformer still connected to a lighting circuit in the loft.



    Sparks: Madam, could you please confirm that we have unplugged everything?


    Householder: Yes we have.


    Sparks gets a reading of 100 kOhm.


    Sparks: there seems to be something which is still connected. Do you have ... [long list]


    Householder: Oh yes, I forgot the aerial - reception is useless here.


    ...


    So for a whole board test, lives to earth - better make sure that the main switch really is off! ?

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