Thanks for taking time to answer my qeestions.
I'm having trouble actually describing what Im considering.

basically its 701.415. 2 and being effectively connected to the met.
If plastic plumbing was inserted into pipe work, and had a resistance of say 600Ω.mainly dues to the water in the pipe work.

What would happen to any touch voltages.  And what possible dangers it could create.
I was wondering if just the person bridging the two voltages would create a situation where current might move and receive a shock.

If you had some connection to the mass of earth via your feet, that would cause current to move 
But that would be the case for both.
Basically could an high resistance section add more shock risk.

I understand high resistance in CPCs will cause longer disconnection times.
Im just wondering about high resistance in bonding and possible negative effects
Thank you

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

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.

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.

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.

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

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

Is this what you are describing?

(Sorry I need visuals to get a better idea)

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?

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.

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

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.