I don’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 kΩ. Based on this assessment, I made the decision to install main equipotential bonding. 

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

Hi Mike, I use a Megger MFT instrument set to the ohms measurement. would it be considered sufficiently accurate for this test? 

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.

I don’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.

I think you're looking at the wrong end of the right problem.

You're correct that we need to limit touch potential. We need to know that in the event of a fault the potential will not exceed 50V under fault conditions save for ADS timeframe when not in a special location.

Bonding is a tool to achieve this. But that's a design problem and the sufficiency of the bonding is a question that is answered by either the tabulated approach or calculation. The tests are to ensure that the bonding is working and to highlight if there's any problems. But the testing alone won't tell you if it meets the requirements.

Disconnection during testing is required to prevent parallel paths.

There's also a problem with extraneous conductive parts that a fault might come from outside the installation. Unlike fault currents of internal origin where we know the fault current, you can only speculate what an external fault current might be from e.g. a PEN fault. So in the case of TN-C-S, the CSA of the earthing conductor and main bonding is dictated by tabular form in the regs - which for most electicians is absolutely adequate. 

It's more an issue if you've got either supplementary bonding - which must be disconnected to avoid parallel paths - or if you've got e.g. a class I appliance mounted on the extraneous conductive part such as e.g. a gas boiler is connected to the gas pipes and any copper water pipes.

You might think an external metal staircase doesn't have parallel paths. But then some external metal staircases have galvanised conduit running on them to feed an outside light.

Sometimes it might not even be practical to split up the parallel paths. One problem with conduit as a CPC is that dismantling for testing isn't as trivial as a wire into a MET. So you've got to work with what you've got and find the most reasonable way to be assured of the integrity of the bonding.

OK there are two shock scenarios we wish to avoid, 

1) a fault in our kit that raises the local metalwork to a nasty voltage relative to something bringing in a low impedance true terra-firma earth from outside.

Drain pipes,  building steel and fences and things.

2) a fault on the neighbours kit bringing a nasty voltage inside via some shared service (water main or similar) and being dangerous relatve to our own metalwork at near true earth voltage.

Both can be mitigated by bonding the CPCs of our local kit to the bits of metal that leave the zone.

But the peak current that may flow in fault is quite different.

I'd ague that if the metal conduit already connects to the metal stairs, then there is not a lot of point in an additional bond.  So there is a danger band of reistances - less than 22k - a dangerous current could flow, and more than perhaps an ohm, where the connection is not good enough to keep things below  50V for a large but credible earth current. The CPC of the water heater type situation, probably makes it fairly well bonded already and therefor unlikely to be at a dangerous voltage, unless there is so much fault current that the CPC is at risk of failure.

There is a very important infuence of the external impedance to ground, that we do not generally know. I have been told of cases where earth bonding has caught fire due to diverted neutral current, and much to the surprise of the victims pulling the company fuses did not stop it. At some very high level of current there is a danger  point when the big no-no of a fuse in the CPC suddenly looks attractive again.

The regs do not really distinguish between a shared service pipe that may have a sub-ohm connection to the substation star point, thanks to all the neighbours bonding, and really could take a large fraction of the street neutral current, and the random pipe in the ground that is more like an adventitious earth electrode, where the current is inherently limited by the resistance of the mud that surrounds it. They are however really quite distinct cases with different risks.

Mike

mapj1 said:

have been told of cases where earth bonding has caught fire due to diverted neutral

It is something we should be mindful of but try as I might, I cannot find any official documentary evidence of such occurrence. The LV network in ROI is almost entirely TNCS but none of my contacts in various authorities can point to a single incident of either fire or shock caused by loss of PEN. 
NI has also an extensive use of PME but similarly, HSE cannot point to an incident that caused injury. I am, however, aware of several loss of PEN incidents, one of which caused extensive damage to electronic kit in a large pharmacy.

Of course, the lack of incidents may be largely due to network integrity and effective bonding practice.

mapj1 said:

So there is a danger band of reistances - less than 22k - a dangerous current could flow, and more than perhaps an ohm, where the connection is not good enough to keep things below  50V for a large but credible earth current.

I think I'm following the 22k magic number - yes treatment in Chapter 6 GN8. 22k+1k[human body impedance - although hopefully restance or else it won't really count] will ensure that <10mA flows when fed from 230V therefore below threshold for let-go.

mapj1 said:

The regs do not really distinguish between a shared service pipe that may have a sub-ohm connection to the substation star point, thanks to all the neighbours bonding, and really could take a large fraction of the street neutral current, and the random pipe in the ground that is more like an adventitious earth electrode, where the current is inherently limited by the resistance of the mud that surrounds it. They are however really quite distinct cases with different risks.

This is why 411.4.2 recommends an additional earth electrode for TN installations - yet many installations don't have this. It's of communal benefit because whilst in theory a diverted neutral could be >100A, if everyone in the street had an additional earth electrode then a large number divided by a large number is a relatively small number.

Unfortunatly this is often not followed. Even new builds often neglect to provide a earth electrode - which is a shame because it would be peferable - and easier - if the builders installed a parameter tape electrode as they do in southern Europe. And hence we see diverted neutrals of >10A going via the water and gas services. This ultimately causes damage to the water and gas pipework along with introducing metal oxides into potable water - hence the utilites are so keen to replace pipework with plastic which compounds the problem onto the fewer properites with legacy metal supply pipes.