Equipotential bonding and voltage

600 Ω 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

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).

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.

Thank you .  Yes thats the exact problem, what to do?

gkenyon said:

(v) any metal pipework and other extraneous metal entering the location (not IN the location) is connected back to ME.

It's seems the regs are satisfied if extraneous CPs have an insulating section on entry to a building, or are effectively connected to the MET on entry to the building.

Which make its sound like as long as I confirm that, my duty is done.
But I wonder about these EXT CP as they travel through the building. 

On a periodic inspection. I R2 wonder lead pipe work in bathrooms. This is where I get into doubt.
I know the pipe work satisfies the regs on entry into the building, but does not in a location.

Because everything is generally hidden and tiled over, we only have out meter readings to go on.
So I get a 600Ω reading on pipework back to the MET or a high sup bond reading in the location.

What to do?? Not worry and let the RCD take care.

But  we not satisfying note 6  701.415.2 as we are no longer effectively connected to the MET?

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 10kΩ 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 600Ω it can often fall between two stools of being too high to provide positive bonding (e.g. <<1Ω) and too low to provide isolation (e.g. >23kΩ).

   - Andy.

Is this what you are describing?

(Sorry I need visuals to get a better idea)

Thank you 
So would 'A' would be what you described?

Hypothetically if there was no RCD, Sup bonding would be required in this situation

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 Ω pipe section is inconsequential.

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 1kΩ region). If your 600Ω 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 600Ω 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 600Ω which could then pose a shock hazard of itself.

   - Andy.