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Odd changes in readings during insulation resistance testing

Former Community Member
Former Community Member
Still waiting for account to be passed so can’t reply directly. 

have updated the post with regards to questions. 


During insulation resistance testing I was getting very low readings whilst the cores were left terminated, removing them from the terminals caused the  readings to increase dramatically. I have never come across this before, does anyone have any suggestions of possible causes? 


Split load metal consumer unit - British General

Flat PVC cabling, modern cabling. masonry and stud walls

All MCBs off, 

RCD off. 

RCDs do not trip when installation is energised. 


Firstly, with Line and neutral bridged testing against earth. All cores left in the terminals. This gave a very low reading, which I tried to further narrow down by separating line and neutral and testing each down to earth. 


The cores left terminated in the top of the MCB, neutral bar and MET. MCB open, RCD open. 

@500V

Between line and earth ~0.3 MΩ

Between neutral and earth ~0.3 MΩ


@250

Between line and neutral ~0.07 MΩ (USB sockets still connected) 

This was done just as a control. 


Cores removed from the terminals. Including CPC removed from the MET. 

@500V

Between line and CPC >999 MΩ

Between neutral and CPC  >999 MΩ


@250

Between line and neutral ~0.07 MΩ (USB sockets still connected)

Proves there is an issue within the CU with earthing. 


I have not tested from the RCD’s terminals to the MET. 

However, I did test from the top of each MCB (under the same RCD) to MET which all gave the same low reading. But if they are open there should be no path back to the RCD. 


On the other RCD a lighting circuit gave the same results, extremely low when terminated then rose dramatically when removed from terminals including earth core from MET. 


From the results I can only assume something is wrong in the CU, but what could cause such readings? 

Regards
  • 0
    To be clear what you are measuring - with the L and N wires of the final circuit taken out of the terminals (but is the CPC still connected to the MET ?), you are testing from the bare L and N ends to the MET and seeing 999, or near open circuit.

    When you add the internals of the RCBO in parallel with the wires, you see more like 300k.

    IS that right?


    What does the RCBO output terminal measure to ground when there is no wire in it? There will be the self test electronics in the RCBO and that may be what your meter is seeing.


    As an aside, take care measuring from L to N with things like USB sockets and LED lights installed, I'd always strap them in parallel and measure (L+N) as one,  relative  to ground - after all we expect to see loads between L and N, and they may not appreciate 500VDC, but no proper load should have a DC path to earth.
  • 0
    Nail through cable that did not show up until backboxes were connected to earth.


    JP
  • Former Community Member
    0 Former Community Member
    That’s what I thought, or compression fault in back box. However,

    every other circuit gives the same 0.3MOhm reading. 


    Appreciate the input folks, it’s really left me stumped and falling short of redoing the entire CU, I don’t know what to do.
  • 0
    There are one or two things here which I do not understand.


    Cable disconnected - why is the IR at 500 V so much higher than it is at 250 V? If the cable is disconnected, how does that prove that there is an issue within the CU. It could only be so if the cable has been damaged where it enters the CU.


    If a cable has been removed, then there should be a high IR from the load side of an MCB to anything else when it is open. If not, it must be faulty.


    From the sound of it, this is a periodic I&T. Presumably everything was working previously.
  • Former Community Member
    0 Former Community Member
    Yes EICR as the client didn’t get a certificate for their extension when it was built. 


    Sorry that’s my terrible description I think. 


    During testing I left the sockets connected, but pulled them away from the wall. All work was carried out in the CU. 


    The 250V IR test was done across line and neutral which have almost a perfect short due to USB charging sockets. 


    I suspect issue in CU as all other circuits produce the same reading to earth when left terminated in MCBs and neutral bar. 


    Is it possible for IR test to get readings through open circuit breakers?
  • 0
    Gus Carrie:

    Yes EICR as the client didn’t get a certificate for their extension when it was built. 


    ...


    The 250V IR test was done across line and neutral which have almost a perfect short due to USB charging sockets. 


    So was this a typo?

    Cores removed from the terminals. Including earth cores removed from the MET. 

    @500V

    Between line and neutral >999 MΩ

    Between neutral and earth >999 MΩ


  • 0
    I am still not clear what you have been doing here. If your RCDs and MCBs are open, how can you do a whole board IR test?


    I think that you must have got in a muddle and that the installation is just fine.
  • Former Community Member
    0 Former Community Member
    Doing individual circuits, again original post not clear. Apologies.


    I tested other circuits from the out going side of (open) mcb and it also gives 0.3MOhm across another 5 circuits.
  • Former Community Member
    0 Former Community Member
    Indeed you are correct! A mistake on my part. I am actually testing the CPC not earth. 


    I will update the original post.
  • 0
    number of possible things to verify to lift the fog .


    1) Reading from RCD/ MCB output terminals to ground with no out going wires connected - There is  internal electronics in some rcds and rcbos, you may be stimulating it....

    Not all RCBOs break neutral, and L-N will be connected via loads - make sure L and N to the final cct are really not connected round the back.


    2) Reading from cpc to MET. Note that back boxes and maybe appliances with plumbing will connect cpc to building fabric, but maybe only loosely - via resistance of a washing machine hose or slightly damp brickwork.


    3) Exactly which resistances are voltage dependant?  This finds surge arrestors, neons to ground, gas discharges...


    Mike


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