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Testing if supplementary bonding is required.

MrJack96
I have read some bits stating we can measure between the two metallic parts in question and referring to the formula 


50/ia will give you the resistance needed to keep touch voltage to less than 50v 


my questions can we do the same test to determine if the part has a high enough resistance to not require bonding and what sort of figures we should look for?
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    Certainly can. You have to pick an maximum acceptable body current - often 10mA is chosen from purely a avoiding death by electric shock point of view (but some designers might prefer lower figures especially for wet areas or where the result of a non-fatal shock might still be fatal - e.g. falling from a height).


    The maximum voltage is likely to be around full mains voltage - say 230V - so it's just them Ohm's law for a resistance to keep the current low enough - so say 230V/0.01A = 23k Ohms. Some subtract a likely body resistance from that (say 1kΩ) but not bothering errs on the side of safety.


    Similarly if you picked 1mA and thought you might have 250V rather than 230V to worry about, you'd end up with 250kΩ.


       - Andy.
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    So is the 230v to err on the side of safety? As the voltage is unlikely to be that high on a well earthed system.
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    For a TT system, the installation's earthing system could well be close to 230V above true earth nominal during a L-PE fault - likewise on a PME system during a broken CNE event. For a non-PME TN system it shouldn't get a lot above half of 230V, but if you want (for simplicity) to pick a single number for most cases 230V is probably a reasonable one.


       - Andy.
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    I get it so it’s just allowing for the worst possible case scenario.
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    Am I right in saying in both cases (broken PEN)the installation should be safe only considering all bonding requirements have been met? The only issue is outside the Equipitential zone? That’s why in some cases it’s not a good idea to bond outside taps due to the fact if the Met was at 230v within the installation the tap would then be at the same potential meaning the folk who ends up touching this will be in a dangerous scenario?
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    MrJack96:

    Am I right in saying in both cases (broken PEN)the installation should be safe only considering all bonding requirements have been met? The only issue is outside the Equipitential zone? That’s why in some cases it’s not a good idea to bond outside taps due to the fact if the Met was at 230v within the installation the tap would then be at the same potential meaning the folk who ends up touching this will be in a dangerous scenario?


    Indeed - the risk from an outside tap with a TT system is small as disconnection should occur reasonably rapidly - certainly within 1s under current regs - but for a PME system where a broken PEN condition might exist for days or weeks, an insulating insert in the pipework feeding an outside tap is often recommended (or better still these days, entirely plastic pipework). The same can apply for Class I (earthed) appliances outdoors - e.g. outside lights and for the same reason the vast majority of electric garden tools are Class II (double insulated).


       - Andy.


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    Actually what we should assume for the "worse case"  scenarios are not as clear cut as you imagine - This recent example on this forum  demonstrates what can happen with a defective RCD, that is not even on the property where folk are getting the shocks. This one is seriously complicated by  having a lower electrode impedance at the fault than at the substation, and is an example of this kind of thing that the regs do not consider as  it is 'unlikely' . But occasionally the unexpected happens.


    It is also faults like this that make me very careful in descriptions to distinguish terra-firma earth voltage  (the voltage with your feet in muddy stuff outdoors) from "earth" that comes via a wire, as supplied via a CPC and the DNO neutral.


    Mean while I'd agree, anything with no wiring to it and reading more than a few tens of k ohms to terra-firma is floating, and need not be bonded, as it cannot sensibly carry enough current to give you a bad shock.

    Personally I'd not cut it as fine as a limit of 23k, as the quality of the connections to things like bits of  pipe set in  concrete floors and so forth are a humidity (and voltage) dependant thing, and you may be measuring on a dry day. But even if you make the "action limit" value  say 50K, then you still do not need to bond shelf brackets and toothbrush holders and all the other things that connect to the wall or floor with a few rawlplugs.

    Which is good, cos we do not, and if we did, we would get laughed out. (shadows of the 15th edition, one of the least followed versions of the rgs..)

    Mike
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    AJJewsbury:
    MrJack96:

    Am I right in saying in both cases (broken PEN)the installation should be safe only considering all bonding requirements have been met? The only issue is outside the Equipitential zone? That’s why in some cases it’s not a good idea to bond outside taps due to the fact if the Met was at 230v within the installation the tap would then be at the same potential meaning the folk who ends up touching this will be in a dangerous scenario?


    Indeed - the risk from an outside tap with a TT system is small as disconnection should occur reasonably rapidly - certainly within 1s under current regs - but for a PME system where a broken PEN condition might exist for days or weeks, an insulating insert in the pipework feeding an outside tap is often recommended (or better still these days, entirely plastic pipework). The same can apply for Class I (earthed) appliances outdoors - e.g. outside lights and for the same reason the vast majority of electric garden tools are Class II (double insulated).


       - Andy.




    I would add that we no longer refer to an equipotential zone for electrical safety. This is chiefly because in some installations, i.e. those with plastic service installation pipes, and external plastic service pipes, there may well simply be no such thing - a house say with no gas service, plastic external water service, plastic internal piping, and no structural steelwork, would certainly find it difficult to have a useful equipotential zone.


    It's also true that in very large installations, even if an equipotential zone is formed, there will be voltage differences between points on the earthing system, which can be transferred from one place to another on screens of communications and control cables, for example - there are ways of managing that very easily, but it's just not good practice to consider the term "equipotential" for the two distinct ends of a screen or armour of a cable over 1 km long, for example !


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    We still use the word equipotential - e.g. in reg 411.3.1.2 - although I agree it's not a good description - being more of a zone of reduced or perhaps minimised potential differences rather a zone of truly equal potentials.


    I'd also suggest that the equipotential zone or protective zone or whatever you want to call it isn't just made up of extraneous-conductive-parts and bonding conductors, but also exposed-conductive-parts and circuit protective conductors (they have to be connected back to the MET too) and we tend to have plenty of them still even with all plastic plumbing.


      - Andy.
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    AJJewsbury:

    We still use the word equipotential - e.g. in reg 411.3.1.2 - although I agree it's not a good description - being more of a zone of reduced or perhaps minimised potential differences rather a zone of truly equal potentials.


    Well, we are not theoretical physicists, so some tolerance is allowed. Even then, it's relative potential rather than absolute. +/- 25 V?


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