Thanks for the feedback and advice with testing, gkenyon .

I plan to escalate the issue to the DNO for advice on how to proceed and I anticipate they will suggest the TT earthing solution which is ultimately what we would be looking to do in any case. I'll make sure to share the outcome with all involved in this thread. 

well it is possible that it is your own load contributing, and if there is an open PEN then the supply voltage may be about go off piste, so turning off whatever can safely be is a good idea.

Turning off the gas tap - perhaps with gloved hands, is another good idea. Then go outside to wait for the DNO who can pull something at the substation that will quieten things down.

Err OK perhaps 'hundreds of amps' is a bit hyperbolic on my side, so my apologies for that, but 16A diversion is in the range that may well  be not an actual fault. It is all about the relative resistance of the mains and the parallel path, and that is not always that favorable, especially when street piping is quite chunky.  And a neutral  imbalance current of perhaps a hundred amps or so is credible at certain times at least on a larger TX (half megwatt and up )and will of course depend  on local use patterns time of day etc.

M

If you're using conventional clamp meter, then I doubt it's capable of registering d.c. components anyway as the clamp mechanism usually forms a single-turn transformer, which won't pass d.c..

The other thought, as it's all mounted on a HV pylon, is that what you're seeing is a.c. leakage from the HV lines - using the LV Earth as an additional  electrode as it were (and one that's to some extent remote from the line of the pylons so less susceptible to a rise in earth potential due to the HV system).

I wouldn't expect parallel N currents to reach 16A without an open PEN. But I could be wrong!

A lot of rural LV systems are single-phase - so there might not be any cancelling. The picture might suggest a 3-phase head (with at least one phase unused) which may or may not mean a 3-phase LV system (a lot of the newer work uses 3-phase cables, in anticipation of future upgrades, but still connected to a single-phase transformer for the present). Plus a lot of rual loads can be quite lumpy - e.g. from storage heating or farms at milking time.

   - Andy.

Generally I would say flick the main switch off and see if there’s still current on the main earth conductor, but I doubt that’s appropriate on this occasion.

well that is good news. You had not mentioned it was on the side of a pylon and G78 applied - we may have got there rather sooner if you had. Still glad it is resolved - every day is a school day an all that.

Separation of HV and LV earth can be a mixed blessing, and over time opinion on best practice has ebbed and flowed a bit. It used to be that if the electrodes were less than an ohm to terra-firma then the site was considered 'cold' (rise of earth while clearing largest credible HV side fault acceptable) then combine, and if not, then keep separate and consider the HV side to be 'hot' (rise of earth while clearing largest credible HV side fault gives rise to a dangerous voltage).
This lead to most pole pig transformers in the sticks being 'hot' and most large substations with big earth arrays or mats rather than one or two rods being considered 'cold'.  The newer and more scientific approach factors in the HV supply characteristics and needs about half a side of maths, to express the earth voltage rise, and then conclude more or less the same except in a few corner cases that probably could have been wired up either way...

Mike

This lead to most pole pig transformers in the sticks being 'hot' and most large substations with big earth arrays or mats rather than one or two rods being considered 'cold'. 

Also the method of cabling made a difference - buried cables had an earthed armour which could connect things to a wider earthing system - whereas overheads on wooden poles provided no such facility. Pylons - having an earth wire strung along the top are more like buried cables in that respect. It had to be continuous of course - buried cables with a short length of overheads in the middle of the run miles away might still mean a hot site.

    - Andy.

Hi Ryan, I am having a similar problem but in an industrial plant with leakges going upto 13.5A when some motor loads are starting, I have been trying to get possible reasons for this and if there are any standards that we can base our earth leakage results on for an industrial plant. IEC 60479-1 has a graph that shows human response to electrical shock at varying currents and time. But that doesn't give a clear guidance on what would be deemed acceptable in an industrial setup

13A may sound like a lot, but what is it as a percentage of the connected load ? - a percent or two is probably credible, though less would be nice. The other thing to bear in mind is what voltage would this rise to if the main CPC were ever interrupted  (please do not do this as a live test... ) - as that is the deadly fault condition. If the answer is more than about 50V then any interruption of the earthing is a single fault to danger, rather than the usual second or third fault to danger  so works that might interrupt this path, like plumbing or moving of metal supports for trunking, need more careful consideration, and earth wiring and termination methods need to be that much more robust.

Mike.

I have asked the client for PQA for now because the earth resistance seems to be very normal.

Can the high leakage current be pinned to the nature of loads i.e inductive loads

Can the high leakage current be pinned to the nature of loads i.e inductive loads

Yes and no. Inductive loads as such shouldn't cause protective conductor currents, but many kinds of loads do naturally have relatively high leakage. Likely culprits these days are Class I items with electronic PSUs - as the EMI filters inevitably leak some current to PE. In industrial environments that might include some quite large motors with variable speed or inverter drives. More traditionally you might look for things with less than perfect insulation - e.g. mineral insulated heating elements (e.g. immersion heater or old radiant cooker elements) or even MICC cabling if there's lots of it (especially if the insulation has got damp) and the really old school classic would be the electrode water heater.

   - Andy.