Equipotential bonding and voltage

Question

I have some questions about protective equipotential bonding. main protective bonding . I understand the principle, that with no potential difference no current will flow I'm wondering about volt drop. You will get volt drop if you have a flow of current. So if you have two extraneous CPs and you are in between them. One has 230V on it, and the bonding raises the potential of the other, Ext CP to 230V so no difference 
 Now if an insulating section was put in between one of the Ext parts as in the example 600&Omega; I am wondering what would be the outcome. This is a standing voltage, and current would not flow between the equal potentials ? (If there was no other path.) But would it flow between the 230v and 70v example. Would you actually get this volt drop? Picture might say it better.

mapj1 · Accepted Answer

Well, in the situation as drawn, it is not clear why the upper section would be at 70V, if the lower is 230V, We must assume that not shown is a touch over a couple of hundred ohms to external ground to get that 170V dropped over the 600 ohms (the 200R and 600R dividing the 230V between them in the ratios 200/800 and 600/800 )

If there is no path to ground, then it all floats at about 230V wrt terra-firma, as there is no current flowing, and no voltage difference to drive that current. An equipotential zone, to use the definition. 
 However, assuming there is such a path, off sheet somewhere, then as you show, the chap downstairs gets no shock but the one upstairs does. Adding a sup bond where you show it means neither of them get a shock, but maybe now someone outside does instead, depending on where that path to ground is. 
 Mike.

gkenyon · Answer

JohnE said: insulating section was put in between one of the Ext parts as in the example 600Ω 
 I know this is only an example, but we'd not really class 600 &Omega; as "insulating" especially when the human body resistance is typically double this or more. 
 But, it's a good value for an example, as it might let sufficient current pass to do harm

AMK · Answer

I concur that this situation warrants consideration. Although, Would it not be detected during an R2 trailing lead test? One would certainly inspect the pipes leading into a bathroom to ascertain whether they exhibit extraneous conductive properties. 
 Andrew

mapj1 · Answer

Realise also that the MET comes up to meet the live when fault current flows into it, much as the line voltage drops under load . Although in TN type systems the effect is small, in TT systems, and indeed on any external parts when the current flows into the ground rather than via copper, then the voltages at places that you think of as grounded are elevated during fault conditions. 
 600 ohms is quite a short interruption, in plumbing perhaps one plastic joint in a heating system with a conductive corrosion inhibitor, but quite credible, and in the awkward range of impedance. 
 M.

AJJewsbury · Answer

I understand the principle, that with no potential difference no current will flow 
 
 Not entirely. In practice there will always be non-zero resistance and usually some current flowing so some voltage differences. Sometimes the currents can be very substantial (hence the large c.s.a.s needed for some bonding conductors). Working with actual resistances rather than theoretical perfect zero or infinity ones often makes things easier to understand. 
 Where a exposed part has become live it would usually have been Earthed, so a significant current will usually be flowing back along the c.p.c. - creating a measurable voltage difference between the part and the MET. If someone then touches the faulty part and something bonded to the MET they will have a voltage difference across them - and some current will flow - the amount depending on the voltage difference and the person's body resistance (often assumed to be in the 1k&Omega; region). If your 600&Omega; is in series with the person, then it'll reduce the current flow and part of the voltage difference will appear across that resistance instead of across the person. (Which is partly why we tend to bond things on entry to an installation or location, and aren't that bothered about what happens to them inside). If however the 600&Omega; were between the MET and a part that had it's own means of Earthing (e.g. a gas or water pipe coming out of the ground) then you'd get a perhaps significant voltage difference across the 600&Omega; which could then pose a shock hazard of itself. 
 - Andy.

gkenyon · Answer

JohnE said: and had a resistance of say 600Ω.mainly dues to the water in the pipe work. 
 This is precisely why the requirements change in BS 7671:2018, for a different approach, being the condition that no local bathroom bonding need be provided, so long as: 
 (iv) The conditions for ADS are met for all circuits in the location. 
 (v) all final circuits in the location have additional (10 mA or 30 mA) RCD protection; and 
 (v) any metal pipework and other extraneous metal entering the location (not IN the location) is connected back to ME. 
 It's also why there's very good advice, that, where supplementary local equipotential bonding is required for a Section 701 location, the the supplementary local equipotential bonding is provided to pipework as it enters the location, so that if pipework is changed in the location (e.g. with section of plastic pipe) the extraneous-conductive-parts of the location are still bonded according to 701.415.2, and the effect of the 600 &Omega; pipe section is inconsequential.

AJJewsbury · Answer

Is this what you are describing? 
 
 For the 2nd case - yes exactly that.Similarly where the insert is downstream of the main bond but before some other location that may requires bonding (e.g. a bathroom), where faults from elsewhere in the installation provide the 'other potential' rather than true Earth. 
 As an aside I did some experiments with plastic fittings in copper pipe a while ago - when filled with my local (fairly soft) water, I was getting readings in the region of 10k&Omega; for a simple straight joint, and higher for Tee fittings. There's likely to be a lot of variation with water chemistries (basic hardness to anti-corrosion additives in central heating circuits) - but like your 600&Omega; it can often fall between two stools of being too high to provide positive bonding (e.g. <<1&Omega;) and too low to provide isolation (e.g. >23k&Omega;). 
 - Andy.

mapj1 · Answer

what to do? 
 Slightly flippantly, not to lie awake at night worrying ! It is unavoidable that there will be fault voltages somewhere, but 
 1) Realise that if the system is well designed, all these dangerous voltages are only present between the instant the fault develops, and the time it takes the ADS in the form of an RCD/ RCBO or whatever to trip the supply off. When that is working properly, that duration is a small fraction of the half a heartbeat period or so that a shock needs to last to have a sporting chance of killing someone.. 
 2) What is the chance of any-one even being there and touching both things during that short dangerous interval anyway ? 
 3) Most accidental contact shocks are not good connections to large areas of wet skin with one contact either side of the heart, so the resistance presented is higher due to dry skin and smaller contact areas, and at the same time, the current path is probably not hand to hand. 
 what to do? 
 Minimize the risk in places where folk are wet and not fully dressed such as bathrooms and swimming pools, and use bonding to move the risk out to other places that are less risky. 
 And, do the periodic tests of the RCD function please. 
 Mike.

gkenyon · Answer

JohnE said: So would 'A' would be what you described? 
 With the advice to bond just outside, A is in effect what I was describing. B would also require a bond to the cpc of the equipment (if it were supplementary local equipotential bonding). Otherwise, B might as well be C. C is the situation requiring RCD protection (the "exception" if you will).

gkenyon · Answer

600 &Omega; is very low - it would perhaps be equivalent to << 10 mm of plastic pipe with tap water, or 30 mm of central heating pipe, based on the resistances in this IEE WIring Matters article: www.voltimum.co.uk/.../201207138449.pdf

Equipotential bonding and voltage

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