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.

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.

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.

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.

That'll be an FI then. 

Yup.

The test like an RCD - by which I mean that is L&N tails both go through the clamp meter is the more interesting - that is only a test of stuff inside the installation, and anything more than perhaps 0.1% of the main circuit breaker rating needs a very good explanation, The current in the bonding, or in pipes etc, may be  larger, and when it is, that is then due to things outside.  

The regs may imply PENs never break, but they have been known to do so anyway ;-)

Mike.

Mike and John, many thanks for your contributions. It all varies, but perhaps 80% of L going down N and 20% down the earthing conductor.

House insurance renewed yesterday. :-)