Are you certain that the observed current is from the equipment installed at the site ?  I consider it possible that you are measuring diverted neutral current from the suppliers network.

A telecommunications site suggests steel masts or towers that are in excellent contact with the general mass of earth. Therefore if the suppliers combined neutral and earth is a few volts away from earth, then a significant current may flow from the suppliers PEN conductor, to true earth via your bonding connections to true earth.

There might even be an open circuit PEN conductor somewhere upstream resulting in your bonding connections being the mains means of earthing part of the suppliers network.

Yeah, sounds like more like diverted neutral currents rather than equipment leakage. What is the current on each of the conductors attached to the MET? For example if it's 16A on the EC, 16A on the bonding to a metal water pipe and nothing to the DB earth, then it's not your equipment, and its more likely another site with a broken PEN is effectively using the water pipe and then your water bonding and EC as their N return path.

How is the site supplied? Are there other installations nearby?

Agree with broadgage, current on the main earthing conductor of a TN supply is not a measure of leakage current.

This appears to be a TN-C-S (PME) supply arrangement, and could well be diverted neutral current. TN-S systems with extraneous-conductive-parts (or exposed-conductive-parts in contact with the ground, or in contact with metalwork in contact with the ground) can also exhibit a similar phenomenon - diverted earth currents (which could be diverted leakage currents from other installations, or diverted earth fault currents).

Leakage current can be compared with the current in this earthing conductor by putting the clamp around the live conductors (only), or on comparing leakage currents on protective conductors that are not connected downstream to anything else that is earthed or bonded (summed together).

Diverted neutral currents (and diverted Earth currents, as well as protective conductor currents) can be hazardous. See Appendix D of the 9th Ed (2022) of Guidance Note 3 Inspection & Testing.

15 A is not insignificant ... but as others have said, may not be unusual for this particular installation, depending on the surrounding PME system. If you are in doubt about the safety of the supply, check with the DNO.

The diverted neutral thing can be quite scary -

the mechanism is described in more detail here

https://www.tangle-tamers.com/tncs/TNCS-DN-1-RD-V1.6.pdf

and for a video of a more worrying effect, of the kind of thing that can happen with diverted current in the plumbing,  about 1minute into this video.
https://www.youtube.com/watch?app=desktop&v=Oi7XJImO-_o

Mike

That’s more than enough to boil a kettle.

That’s more than enough to boil a kettle.

Well 16a at 230V would boil a kettle or two, but of course there is not 230V behind it - if one was very brave and interrupted that earth path (and this is a 'thought experiment' only do not try it) then the open circuit voltage would almost certainly not be 230V, unless there is an open circuit in the street main, but more like a few volts to perhaps 10V or so.

What is happening is that there is a few hundred amps RMS flowing in the street neutral, and that has a resistance of some tens of milliohms, and adding another few tens to hundreds of milliohms in parallel takes a measurable percentage of it, in the usual  resistors in parallel sort of way. That can still be enough current, even at a few volts, to cause some serious heating if connections are a bit rough.

The whole thing only becomes a really serious hazard when the official neutral path is faulty and most or all of the neutral current is sent via the plumbing etc. That is the situation when anyone plumbing and getting themselves in the current path, perhaps when replacing the stopcock, may receive a lethal shock,

The other scary part of course is that removing the building main fuse(s) and switching off all loads has little to no effect, as the current is coming in from elsewhere.

Mike

There is a considerable degree of misunderstanding going on here, why do you think this is DC leakage?. Because there is DC operated equipment with rectifiers etc does not mean that there is any Earth leakage! I agree with the others, this is probably NORMAL PME diverted neutral current due to the site having a better Earth than anywhere else. You also need to be careful that you choose the correct Ammeter, you need one that only measures the DC component, which Hall effect ones are unlikely to do, and ideally you need to measure the spectrum of the leakage current as your equipment may well have mains filtering which may well pass much more 50Hz to Earth than consumer type equipment. As above, measure each of the MET cables separately to assess what is actually happening.

I'd be surprised if there were hundreds of amps flowing in the street neutral - I'd expect the phases to roughly balance out along the street and the neutral to carry just the residual imbalance, unless by unhappy chance every third household along the street decided to  boil a kettle.

So for a service cable PEN fault, I'd expect one house's full N current to be diverted along pipes into a few neighbour's bonding cables, while for a street cable open PEN I'd expect just the residual N of the further half of the street to appear.

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

An interesting question would be what to do for the arcing gas pipe connector shown in that video (apart from calling 105)? Disconnect the gas bond at the MET (using goggles and gloves and treating everything as live)? Something else? Nothing?

Thanks for the feedback, broadgage .

Not certain at all to be fully honest with you, my initial thought was earth leakage due to the extents of DC equipment that's being used. I agree that idea of diverted neutral current may be more plausible as this isn't a common issue across all of our sites with similar equipment installations. It is however a semi-regular occurrence which would point towards DNC given its a major issue for DNO's. 

This installation is beside and serving equipment on a  HV pylon site therefore we would assume good earthing on the pylon side. As part of these works the supply for the site will be converted to a TT system in line with G78 regulations for LV infrastructure on HV pylons which I understand would remove any DNC issues associated with TN-C-S system ?

Thanks for the feedback, wallywombat 

As per my response to broadgage , This installation is in the middle of a field and serving equipment on a HV pylon site, the nearest connect would be a farm c200m away.