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.

like the non existent earth electrodes, there are the non-existent insulating joints in the gas network to guard against that.

The problem in all cases is legacy - about half the houses in the UK are pre-war, but it is not an even mix - you get whole estates with lead water pipes and black iron gas mains, and former TNS with the jacket rotted off the PILC, and then whole streets of new houses all with plastic utilities and TNC-s

Even so, if each of the 50- 70 houses per phase on a substation managed a 100 to 150 ohm electrode (the ground is stony round here, if you have London clay, halve that), then we'd be looking at  total electrode resistance of about an ohm. Depending where the break is, there is risk that the substation earth voltage gets pulled more off true terra-firma potential than the broken off section of the network if the latter has a lower electrode resistance.

Mike

like the non existent earth electrodes, there are the non-existent insulating joints in the gas network to guard against that.

The problem in all cases is legacy - about half the houses in the UK are pre-war, but it is not an even mix - you get whole estates with lead water pipes and black iron gas mains, and former TNS with the jacket rotted off the PILC, and then whole streets of new houses all with plastic utilities and TNC-s

Even so, if each of the 50- 70 houses per phase on a substation managed a 100 to 150 ohm electrode (the ground is stony round here, if you have London clay, halve that), then we'd be looking at  total electrode resistance of about an ohm. Depending where the break is, there is risk that the substation earth voltage gets pulled more off true terra-firma potential than the broken off section of the network if the latter has a lower electrode resistance.

Mike

mapj1 said:

Even so, if each of the 50- 70 houses per phase on a substation managed a 100 to 150 ohm electrode (the ground is stony round here, if you have London clay, halve that), then we'd be looking at  total electrode resistance of about an ohm.

Hence in an ideal world we should be getting newbuilds to install parameter tape electrodes - down in Greece nearly everything is 3 phase TT. The standard of work is at first glance horrific for me, but then the TT is done so well with Type S RCCB's at the incommer that it's very difficult for a fault to become a thermal hazard - even if the electrician was completely incompetent.

mapj1 said:

Depending where the break is, there is risk that the substation earth voltage gets pulled more off true terra-firma potential than the broken off section of the network if the latter has a lower electrode resistance.

Should note that the substation's earth should be sized to accomodate a fault on the HV side. So it's often huge. However in rural locations I've seen many examples of a 6.6kV pole mounted transformer feeding a single property with the earthing electrode at the property itself providing protection to the pole mounted transformer - my family are originally from cumbria where in fairness to ENW and their predecessors the ground is 5cm of soil followed by rock. An L-E fault on the HV side of that transformer would be very bad news, even though the properties typically don't have any extranous conductive parts except maybe a propane tank.

Round here (SSE) where the transformer is a small pole pig to a few houses or a farm, the HV earth to the transformer core and steels, goes down the pole with the transformer on it, and the next wooden pole along has the earth electrode for the LV/neutral side of things, so they are more then 10m apart, and the LV electrode at least can measure 10 plus  ohms. I don't know, but I'd expect the HV earth to be similar resistance given the construction. Ground mounted substations on housing estates do rather better and have buried mesh and all sorts to get sub-ohm and combined HV  LV earth.  I can well imagine the local soil will have a significant effect ;)

PNB can be a bit prickly and there one does see shared HV-LV earths  on single user supplies.

Interesting to know what happens elsewhere.

Mike

Should note that the substation's earth should be sized to accomodate a fault on the HV side.

Indeed - but the HV side is often not a simple TN or TT arrangement, but impedance earthed (with N not distributed) which yields surprisingly modest earth fault currents - sometimes only a few tens of amps - with disconnection then done by a HV device similar to an RCD (but typically directly monitors current returning to the star point rather than having to infer it from the difference between live currents). Where  a HV/LV transformer doesn't have metallic earth connection back to the source (e.g. if there's any section of wooden pole overhead line anywhere upstream), the LV earth tends to be separated from HV earth ("hot" vs "cold" sites in old school terminology, but driven by calculation these days I gather, if with usually similar results).

   - Andy.

AJJewsbury said:

impedance earthed (with N not distributed) which yields surprisingly modest earth fault currents

Good point. They use Z wound transformers, the symbol for which looks like a very troubling political emblem.