Extraneous conductive part test

The extraneous conductive part test

Sorry to bring this up, but it is something I can never get to grips with.  

I understand the thinking behind it, (I think) proving that a conductive part is not able to introduce an earth potential, (generally)
That could be hazardous, if a fault appeared on another accessible conductive part, and a person was in contact with both.

The formula Rcp > Uo / Ia or I∆n   (I've left out the resistance of the body..)

And with  I∆n we can choose our value depending on risk factors 0.5mA - 10mA  - 30mA  The level of resistance 

between the two parts increasing as the mA decrease.

It's the actual  test procedure. No publication really seems to get into the details with it

GN8 says -  The measured resistance between the conductive part concerned and the main earthing terminal (MET) of the installation (in Ω)

Then put that resistance through the above formula, If you are above it can be considered extraneous, and would not need protective bonding.

CPS helpline says disconnect the earthing conductor and test from that to the part in question

NIC pocket guides says disconnection of parallel paths my be needed. - very non committal.



So my question is.  Do we remove the earthing conductor, or bonding conductors. when carrying out these tests?



Say we want 22kΩ resistance between the two

If its greater than 22 kΩ when protective conductors are connected, surely disconnecting will not decrease the 22kΩ

If we get greater than 22kΩ when disconnected - it could be possible to reduce that when re connecting protective conductors ? , to a level that would require bonding?
Not a direct connection, to the part, but a step like increase., an induced earth for want of a better term.

In my mind it makes sense to have all the protective conductors connected to test.  
Then we can see, how the installation is day to day.

But I suspect my thinking is wrong.
Thanks

Parents
  • I don’t remove anything.

    I'd be slightly more cautious. Say you had a copper gas pipe feeding a boiler - the boiler c.p.c. would likely provide a very low impedance to the MET - so a test between the pipe and MET would then conclude that it was very much an extraneous-conductive-part and need main bonding even though the gas might come out of the ground in MDPE plastic pipe and the copper pipe have no more chance of introducing a potential into the installation than a rubber duck. The point being that what you really want to test is whether the part can introduce a potential into the installation(*) rather than just following whatever potential was already within the installation(*).

    Ideally you'd test the gas pipe before it was connected to the boiler, but was already connected to its supply - but often that's not exactly practical. Disconnecting the boiler c.p.c. might be an option, but there might be other parallel paths (e.g. from a heated towel rail) back to the MET. Another option is to disconnect the entire installation from its means of earthing - but again there could be parallel paths (e.g. via other extraneous-conductive-parts). Generally I suspect it's a case of taking every situation on its own merit, and looking for ways of eliminating misleading parallel paths e.g. by taking advantage of any useful features - e.g. plastic piping - to isolate the part under test from the MET. If all else fails, the safe policy is to bond anyway (there's no loss of safety if the part is already conductive to the MET).

    * of course, read 'location' for 'installation' where you're evaluating parts in terms of supplementary bonding (e.g. within bathrooms).

      - Andy.

  • Disconnecting the boiler c.p.c. might be an option, but there might be other parallel paths (e.g. from a heated towel rail) back to the MET. Another option is to disconnect the entire installation from its means of earthing - but again there could be parallel paths (e.g. via other extraneous-conductive-parts).

    One of my relatives had a water leak last christmas which tripped an RCD on their split board.

    I went and cleaned all the water - the short was on an appliance but I wanted to be sure everything was okay and like an idiot I tested IR on each circuit to well and truly open pandora's box.

    Turned out there was an N-CPC fault on a ring final which also served the boiler. It was inside the wall so I assume the plasterer did it. What recklessly dangerous step had the electrician taken to prevent? Disconnected the TN-S earth from the MET. So how did the flood trip the RCD, and why were I getting Zs readings of around 3ohms [not acceptable but not open circuit either]? The entire installation was earthed via the 1.5mm CPC in the boilers flex, down the gas pipe, through the gas meter and down the street.

    Parallel paths can mask all kinds of issues if we’re not careful.

Reply
  • Disconnecting the boiler c.p.c. might be an option, but there might be other parallel paths (e.g. from a heated towel rail) back to the MET. Another option is to disconnect the entire installation from its means of earthing - but again there could be parallel paths (e.g. via other extraneous-conductive-parts).

    One of my relatives had a water leak last christmas which tripped an RCD on their split board.

    I went and cleaned all the water - the short was on an appliance but I wanted to be sure everything was okay and like an idiot I tested IR on each circuit to well and truly open pandora's box.

    Turned out there was an N-CPC fault on a ring final which also served the boiler. It was inside the wall so I assume the plasterer did it. What recklessly dangerous step had the electrician taken to prevent? Disconnected the TN-S earth from the MET. So how did the flood trip the RCD, and why were I getting Zs readings of around 3ohms [not acceptable but not open circuit either]? The entire installation was earthed via the 1.5mm CPC in the boilers flex, down the gas pipe, through the gas meter and down the street.

    Parallel paths can mask all kinds of issues if we’re not careful.

Children
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