Main Earth Conductor Current (Bridge structure TN-C-S)

700mA for a substantial installation doesn't feel too bad to me - at first (and of course ill-informed guess) I'd start by looking at the light fittings - small amounts of damp ingress can easily reduce IR readings and Class I LED drivers can have have a.c. leakage due to filters.

BS 7671 is happy with high leakage currents provided the c.p.c.s. are "high integrity" - which is generally satisfied by a single ≥10mm² conductor, or  ≥4mm² conductor with some mechanical protection, or duplicated conductors if smaller than that. Even using extraneous-conductive-parts as c.p.c.s is permitted - provided reasonable precautions are taken against their removal - and I think most would agree that it's not likely that structural parts of a bridge are likely to be removed (at least not while its electrical installation remains in operation).

Broken PEN conductors are pretty rare and when they do happen are often caused by fallen overhead lines or damaged underground cables - neither of which sound particularly likely if you're running off your own HV transformer within the structure.

As for the multicore SWA, I'd hope the armour was the c.p.c. so it shouldn't be easy to "cross" N & PE by misidentifying cores. Perhaps if a core was used as an additional c.p.c. it could be crossed or bridged to the N core by mistake, but still the armour should remain as the primary c.p.c. and be able to keep exposed-conductive-parts at a reasonably low potential even if the N was broken/disconnected. Even if the worst did happen and the bridge was acting as N for some circuits, I suspect it would have more than adequate c.s.a. to cope.

I'm no civil engineer, but I'd be very surprised if the bridge itself didn't act as a decent earth electrode. I know steel bridges have expansion joints and the like often filled with rubber compounds, but I've be very surprised if presumably several thousand tonnes of steel wasn't ultimately in pretty good contact with the equally massive concrete foundations, which were in pretty intimate contact with the ground below. A long lead test to an independent earth reference (say a screwdriver stuck into a grass verge) might provide some reassurance on that score. In any event simple continuity between the bridge structure and the MET/Tx Earth would provide adequate protection from local potential differences - and a large number of small connections can often provide just as good results, often with better resilience, than one large connection.

   - Andy.

It sounds depressingly typical of a system that is not really maintained, so much as managed on a 'fire brigade' basis. That said there are loads of set up like this, and not normally piles of dead bodies, so its not worth too much lost sleep, though your description of the 'lead' sparks is a little disconcerting, and may be a reaction to an otherwise overwhelming  task.

So do what you can, sensibly,  without hurting yourself physically or professionally,  and be clear about where your responsibility ends..

A "Christmas tree" wiring architecture is often the only sensible one on a long thin structure, and as you say gives lots of ends, but it may be sensible to cut to the chase and just test at one at or near the far end, and assume the rest are better or verify R2 only with a wander lead and suitable spike on a stick - its a lot faster and you can concentrate on the ones that seem to have no earth at all. They presumably have live, or they would not come on..

It may not be wonderful, but if the lights are solidly earthed via cable trays and the structure of the bridge, the presence or absence of a regulation green and yellow  CPC is not so important - indeed if it was conduit, you'd not be  surprised if there was not CPC core as such.
IR -  if those low resistance sections are > 100m lengths of  Pyro / MICC then nothing to see, they tend to absorb damp and while you can tease it out with a blow lamp, it is scarcely worth it.  It rather depends on the length of run and no of fittings served as to what to expect - a 1meg pass/ fail limit is a very blunt tool, and the same installation but measured  in two  halves would probably pass.

If it was more like a few ohms I's be wondering about an NE short so you are in luck if not one of those. Label what you can identify, and leave marks or stickers on fittings you have tested. 

The odd voltages may or may not be real - are the 110 circuits center earthed or floating? do the funny voltages drop to nothing if a test lamp is connected ?

Having 2 or more phases in one box is fine, but if it is a concern, then some '400V' labels (not required since several back issues of the regs but if you think there is scope for confusion) may be added.

700mA of combined leakage - you do not say the rating  of the supply or what fraction that is of the total load, again not great, needs to be reported, but maybe not able to be identified or fixed in this session.

Its not great, but it does not sound like a killer.,,, The report will have lots of 'further work is recommended' I fear.

Mike

One more thought...

1. A reading as high as 730 mili amps - Taken with a clamp meter on the main earth supply from the HV/LV Transformer to MET. 

Is the transformer supplied by underground HV cables? If so there's a possibility that it's a shared LV & HV earth and the bridge is acting as an additional earth electrode for the HV system ... so some of that current could be due to leakage on the HV side! (or HV leakage could be out of phase and cancelling LV leakage...) Earthing systems can have curious things happen to them at times ... generally you're better estimating leakage by clamping all the live conductors together rather than clamping the earthing conductor.

  - Andy.

AJJewsbury said:

700mA for a substantial installation doesn't feel too bad to me

What would people think was acceptable for an ordinary single-phase domestic installation please?

This is an interesting question, and under some non-fault conditions in a PME street could be several amps actually but usually isn't. It may well have almost  nothing to do with the installation itself, and everything to do with what is outside it.

The problem is that the neutral and earthed services gas, water usually, maybe in some cases cast iron drains as well, are (usually) pretty solidly bonded at each property,  The street neutral will carry the out of balance current between the 3 phases, and that could well be many tens of amps and in a few cases a hundred or more - but that neutral is in effect in parallel with all that piping, which may be a comparable or even lower impedance than the street main itself so the current shares in the ratio of the conductivity of the 2 paths.

Now, a large CPC current that changes with the house load, or goes off altogether if the CU is isolated, is not the same sort of thing at all and should direct attention at things like local EMC filters,  and if more than perhaps a few tens of mA, then also could be  neutral earth faults in the installation.

It is worth comparing the CPC current, with a clamp measure of L & N tails measurement - they may well be quite different, and it allows currents originating inside and outside to be distinguished.

Problems inside the installation should only be tens of milliamps at most.

Mike.

If there is a private HV/LV Tx on site isn't it much more likely to be a TNS earthing system. if it is a short run of TNC where T & N are the same bit of cable, I'd imagine this is a very short run to the LV switch gear and not subject to being dug up or something. I don't know what you've got though. 

Quote, "My main recommendation is that the bridge requires a robust bonding system, and a back up TT earth system which is tested regularly due to general public pathway on the bridge (Being metal) contractors and employee's traversing the internal bare metal structure daily."

My Thoughts: The Bridge is a substantial TT system...............it also sounds like a substantial metal structure, where the metalwork of the bridge will provide a better electrical path than any green and yellow cable an electrician could run. What would you achieve by additional bonding (Assuming the bridge has a bond of some sort at the moment to LV earth - even if this is via substantial associated metal work clamped to the structure at switch gear etc. Plus many, many earthed circuits to back boxes, lights etc providing alternate paths to the Tx. 

Credit to JP for this thought - you mentioned a bare metal copper conductor on a tray - perhaps a zinc coated tray (Its more likely galv though) could have some detrimental  electrical interactions.  Zinc + copper and voltage circulating network currents.  

Neutral earth faults in older installations is very common, so the earth leakage you are measuring is probably only a fraction of what's actually there - plenty will run to ground I'd imagine through the bridge metalwork into the earth/soil and via other paths back to the Tx.

IRs - on long runs of  individual cables become a increasingly unreliable  indicator of cable condition the longer the cable gets

IRs on large or very large groups of cables even more so

I'd probably be much more concerned with the continuity of earth between all bridge metal bits and maybe between big metal bits and the terra firma where a man might stand and lean against the bridge perhaps - but even then because current divides along the path of resistance back to the Tx, I'd imagine that with giant meatal work paths available - all the fault current would flow that way, and not chose to flow to terra firma through some chaps increased, dressed, high resistance earth path. <50V anyway. 

Zs of individual circuits would be my primary concern. 

Perhaps - if you're able - to cast an eye to the longest circuits and see if volt drop might be an issue. the real measured R2 value in reality might be zero though, due to the bridge. The 5 second L-N fault path would be more of a consideration - but's probably out of scope of an inspection and test.

Esus said:

I am seriously concerned with the possibility of the PEN conductor being lost,

I think the plethora of dire warnings about loss of PEN in EVSE installations have driven a sense of unnecessary paranoia about the subject in other situations. I concur that it would be the duty of an inspector to consider each on its own merits, but from a general perspective 114.1 offers the fundamental consideration that where ESQCR applies, the connection of the supply neutral with earth can be regarded as permanent. As I read it, that means the integrity of the supply neutral can be considered fully in place.

It begs the question why it was thought necessary to flap about loss of PEN at all, but there you go! Maybe dire consequences were envisaged for EV charging should the DNO fail in their duty to prevent loss of neutral and thus huge expense was incurred in the implementation of measures to mitigate the risk, hence the requirements of 722.411.4.1. If there really was serious concern, then you would imagine that the absence of those control measures would also have been serious. Clearly, there are some who think it is only worthy of a recommendation.

 Best Practice Guide 4 version 7.2 gives the absence a Code 3.

( I would have posted the actual statement but it looks like the bods have been messing around with things and I can no longer use my snipping tool to post). 

mapj1 said:

This is an interesting question, and under some non-fault conditions in a PME street could be several amps actually but usually isn't. It may well have almost  nothing to do with the installation itself, and everything to do with what is outside it.

Thanks, Mike. That'll be an FI then.

It begs the question why it was thought necessary to flap about loss of PEN at all

It's not new to EVs - similar concerns have been there for PME with caravans and boats for a very long time (probably predating the ESQCR). Whereas other metalwork outdoors (e.g. street lights) have been happily on PME with nothing more than the occasional additional electrode thrown in. I suspect much of the apparent inconsistency comes for the wider situation rather than the electrical installation per se - i.e. how likely are wet/bare feet. how often will someone actually grab hold of the thing, is the ground likely to be covered with vaguely insulating tarmac and so on.

the connection of the supply neutral with earth can be regarded as permanent

All depends on your definition of "permanent" - I suspect the dictionary has it closer to "indefinitely" rather than "forever" - i.e. it could well come to an end someday, we just don't know when yet.

   - Andy.

AJJewsbury said:

All depends on your definition of "permanent" - I suspect the dictionary has it closer to "indefinitely" rather than "forever" - i.e. it could well come to an end someday, we just don't know when yet.

A bit like the Universe perhaps! 

I think the issue of open PEN has been well and truly covered on the forum, whether EV, caravan, boats, PFS. Refreshing to now include steel bridges! 
I was merely pointing out that if the DNO provides a PEN suitable for the installation to connect its exposed and extraneous conductive parts to, why would there be any need to question its integrity?