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 think you're looking at the wrong end of the right problem.

    You're correct that we need to limit touch potential. We need to know that in the event of a fault the potential will not exceed 50V under fault conditions save for ADS timeframe when not in a special location.

    Bonding is a tool to achieve this. But that's a design problem and the sufficiency of the bonding is a question that is answered by either the tabulated approach or calculation. The tests are to ensure that the bonding is working and to highlight if there's any problems. But the testing alone won't tell you if it meets the requirements.

    Disconnection during testing is required to prevent parallel paths.

    There's also a problem with extraneous conductive parts that a fault might come from outside the installation. Unlike fault currents of internal origin where we know the fault current, you can only speculate what an external fault current might be from e.g. a PEN fault. So in the case of TN-C-S, the CSA of the earthing conductor and main bonding is dictated by tabular form in the regs - which for most electicians is absolutely adequate. 

  • We need to know that in the event of a fault the potential will not exceed 50V under fault conditions save for ADS timeframe when not in a special location.

    Although the 50V is only for supplementary bonding - BS 7671 has no requirement at all for what main bonding has to achieve - just a c.s.a. - nothing limits bond length (and so impedance) or indeed specifies the current it needs to cope with. Basically chunk in 10mm² (or whatever's tabulated) and hope for the best.

       - Andy.

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  • We need to know that in the event of a fault the potential will not exceed 50V under fault conditions save for ADS timeframe when not in a special location.

    Although the 50V is only for supplementary bonding - BS 7671 has no requirement at all for what main bonding has to achieve - just a c.s.a. - nothing limits bond length (and so impedance) or indeed specifies the current it needs to cope with. Basically chunk in 10mm² (or whatever's tabulated) and hope for the best.

       - Andy.

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