Extraneous conductive part test

Question

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 &Omega;) 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 22k&Omega; resistance between the two 
 If its greater than 22 k&Omega; when protective conductors are connected , surely disconnecting will not decrease the 22k&Omega; 
 If we get greater than 22k&Omega; 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

AMK · Accepted Answer

I don&rsquo;t remove anything. In a specific scenario, I encountered a metallic staircase within an installation where it was possible to come into contact with the staircase while simultaneously touching an exposed part of the installation. To assess the situation, I conducted a test using an R2 wandering lead between the MET (croc lead the Main Earth Terminal) and the staircase. No disconnections were carried out during the test, and the measured resistance was less than 22 k&Omega;. Based on this assessment, I made the decision to install main equipotential bonding.

AJJewsbury · Answer

I don&rsquo;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.

mapj1 · Answer

I'd agree - assuming that this is all about limiting shock currents to 10mA or less, when someone holding a semi-live thing that is faulty then touches the mystery object, even for a very wet (low resistance) human, it is most unlikely that if someone is getting a shock, they will have time to run round and disconnect the bonding to the gas main or whatever to lower the shock current a bit and then reconnect themselves ! It has to be measured with the wiring in the normal operational state whatever that is. 
 The assumption is that the semi-live faulty thing is at or near mid-rail voltage as for a perfect low impedance fault and mains supply with equal resistance in L and E cores, then the exposed case of the thing is at half mains voltage, until something trips. 
 Note also that the battery powered meter lead test can give results that are a bit at odds when compared to a 'zs' style mains derived test- things like concrete floors that are slightly humid have a non-linear resistance becoming more conductive at higher voltages than ohms law constant resistance would predict. 
 Mike

mapj1 · Answer

I'd not be too worried unless the reading was really borderline - the whole thing is a bit 'measure with micrometer, mark in chalk , cut with axe' anyway as you are getting a snapshot reading of some leakage resistances that may depend on when the floor was last washed, recent weather or if the heating has been on yet this year 
 I'm just cautioning that a battery powered measurement made with a few volts DC may not read the same as an AC mains test, generally the DC test reads higher, even on the same day and with the same conditions. But the fact we are looking for 22k ohms, or 10mA rather than the RCD limit of 30mA means we have a bit of safety margin in term of how bad the largest unmitigated shock might be. 
 Mike.

Extraneous conductive part test

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