Current in TN-C-S main earth conductor

[Charliecambs]

Hi I am a retired degree qualified electrical and electronics engineer. I help out at my daughters exotic animal rescue/sanctuary.
Apologies for long post .

Unfortunately most of the animal enclosures require heating and at the moment use (expensive) electrical heating. We have been looking at power use with a clamp on ac ammeter ( and I’m looking at a better data logging option so we can evaluate solar etc.

Site is large (think farm size) and rural but has single phase supply and TN-C-S / pme with satisfactory fixed wiring report. Quite a few long SWA runs to out buildings each using TT with rods.

While looking at currents on main incoming tails and clamping line ,neutral and main earth there is a net current of the order of 17 amps with something like 55 amps line current. This seems odd to me and I even did the same check on my home installation where reading was zero ( 20A range ) as I’d expect.
It’s quite tricky to monitor readings due to number of loads switching on thermostats but I do intend to check line - neutral and neutral - earth voltages next time I’m there.

any comments.

 

[ericmark]

Yes it would ring alarm bells with me as well. In the 80's I was working in Algeria with a Turkish electrician, who stated earth and neutral are the same, and would use the same wire, he normally worked in Holland, in this country only the DNO are allowed to do that, but who has been working there before you, people do some daft tricks, you would not believe how many plumbers I have seem using green-yellow wires as line on central heating.

Clearly with the TT it would trip the RCD, but I remember working in a concrete works where under ground cables had been clearly damaged, so it is a get the insulation tester out and do some testing. Do remember they use DC so what the clamp on meter shows may be due to capacitive or inductive leaking, but not 17 amp.

 

[Charliecambs]

it’s been like this since we have been there but obviously any work I do I do check the additions for isolation/ continuity and rcd trip times.
think next time I’m there ( it’s 80 miles from my home) I’ll take my clamp meter and other testers ( I have old Robin continuity/isolation , red tester and loop/ psc testers.
As our switch fuse/isolator and first distribution boxes are other side of wall to incomers I’d expect no or very near zero voltage difference neutral earth there wouldn’t I.
think I need to take a few more readings and update.

our biggest heating loads maybe 6kWfan heaters are within 20 meters of the incomers.

and other than a cafe ( not currently in use as site closed this month) remaining loads on remote swa tend to be tubular heaters of 100w or less.

 

[JohnW2]

Site is large (think farm size) and rural but has single phase supply and TN-C-S / pme with satisfactory fixed wiring report. Quite a few long SWA runs to out buildings each using TT with rods.

OK

While looking at currents on main incoming tails and clamping line ,neutral and main earth there is a net current of the order of 17 amps with something like 55 amps line current. This seems odd to me and I even did the same check on my home installation where reading was zero ( 20A range ) as I’d expect.

It does sound very odd. I presume that you are talking about putting the clamp around all three (L,N & E)? ... but what do you mean by the 'main earth', given that you have a TN-C-S installation (hence N and 'E' will be joined at the origin of the installation)? What reading do you get if you clamp only the L & N ?

Kind Regards, John

 

[SUNRAY]

It does sound very odd. I presume that you are talking about putting the clamp around all three (L,N & E)? ... but what do you mean by the 'main earth', given that you have a TN-C-S installation (hence N and 'E' will be joined at the origin of the installation)? What reading do you get if you clamp only the L & N ?

Kind Regards, John

The only conclusion I come up with is there is current heading down the other earths within the system, that'll be the TT rods and water bonds etc etc

BUT 17A is a king lot

 

[JohnW2]

The only conclusion I come up with is there is current heading down the other earths within the system, that'll be the TT rods and water bonds etc etc

BUT 17A is a king lot

Yes, that's the most obvious conclusion but, as you say, 17A is an awful lot to be going down TT rods (implying the rods have an Re of only about 13.5Ω - hence some of my questions.

Kind Regards, John

 

[bernardgreen]

I would be inclined to

Quite a few long SWA runs to out buildings each using TT with rods.

If the armour of these SWA runs is "earthed" at both ends then there is a connection from the incoming Neutral to the TT rods.

If there is a significant voltage drop along the Neutral then the Neutral at the cut out will be at a significant potential above true ground potential.

The CPC and hence the armour of the SWA at the cut out will be at a significant potential above the other end of the SWA armour which is connected to true ground via the TT earth rods.

 

[JohnW2]

If the armour of these SWA runs is "earthed" at both ends then there is a connection from the incoming Neutral to the TT rods. ... If there is a significant voltage drop along the Neutral then the Neutral at the cut out will be at a significant potential above true ground potential. ... The CPC and hence the armour of the SWA at the cut out will be at a significant potential above the other end of the SWA armour which is connected to true ground via the TT earth rods.

All true, but as both Sunray and myself have said, 17A through TT rods would be 'going some'!

Kind Regards, John

 

[Charliecambs]

Apologies by main earth I mean the earth conductor run from the DNO incoming cable termination through the wall to the distribution board. So yes clamping l-n-e. I had daughter do this earlier and reading was 14 amps . I seem to recall that for example neutral only would be say 50amps line would be 50 plus 14 so 64.

I hope to “map” the various cables and fuses/ rods etc as it’s usfull for fault finding and so on. As art of that I’ll take readings and due to changing loads might have one meter at source end and use the walkietalkies to get simultaneous numbers in order to assess volt drops. I should be able to get current readings from line/neutral/earth.

 

[Charliecambs]

its funny how things like this bug one particularly when unable to do any measurements!

what if neutral and cpc were transposed in one of the non rcd feeds to a sub panel. At the remote point the feed cpc is not connected ( but say is actually neutral from source) and the cpc has become load return ( pseudo neutral?) to source. Load current would balance at the remote Rcd? But at source be flowing in the cpc to DNO earth point?

I hope to be up there in the next week or so for other jobs So expect I’ll be spending some time taking readings and scratching my head!

 

[JohnD]

Earth leakage is very dangerous to quadrupeds, so you need to get it resolved.

Try turning off circuits one at a time.

 

[SUNRAY]

its funny how things like this bug one particularly when unable to do any measurements!

what if neutral and cpc were transposed in one of the non rcd feeds to a sub panel. At the remote point the feed cpc is not connected ( but say is actually neutral from source) and the cpc has become load return ( pseudo neutral?) to source. Load current would balance at the remote Rcd? But at source be flowing in the cpc to DNO earth point?

I hope to be up there in the next week or so for other jobs So expect I’ll be spending some time taking readings and scratching my head!

That would still show as balanced if you test L, N & E. Example:
15A in use across L & N at main board and 10A at sub board but returning via DNO earth

 

[JohnW2]

Apologies by main earth I mean the earth conductor run from the DNO incoming cable termination through the wall to the distribution board. So yes clamping l-n-e. I had daughter do this earlier and reading was 14 amps . I seem to recall that for example neutral only would be say 50amps line would be 50 plus 14 so 64.

Thanks for clarifying. As has been said, the only apparent explanation is that a very substantial amount of current (17A or 14A) is getting back to the DNO's transformer by some route (presumably via 'earth') other than through their neutral (CNE) conductor.

However, as I've said, that seems like an awful lot of current to be getting through TT rods. Are there any other possible low impedance paths to earth around of which you are aware?

As has also been said, even small leakages to hearth can be a hazard to livestock, and the figures you're talking about are far from small. This definitely needs to be investigated.

Kind Regards, John

 

[JohnW2]

what if neutral and cpc were transposed in one of the non rcd feeds to a sub panel. At the remote point the feed cpc is not connected ( but say is actually neutral from source) and the cpc has become load return ( pseudo neutral?) to source. Load current would balance at the remote Rcd? But at source be flowing in the cpc to DNO earth point?

As has been said, that would not result in what you are apparently observing. Indeed, unless you want Mr Kirchoff to start turning in his grave, the results you are apparently seeing must mean that a substantial part of the current in the L must be getting back to the transformer by some route other than the DNO's neutral conductor.

Kind Regards, John

 

[ericmark]

I wonder if nothing to do with power being used on the premises, I remember a case where the owner of a house was a radio ham, and he had earth rods for his hobby, connected to the DNO earth, and a fault with the PEN caused excessive current to run, in his case until the earth wire joining the two earths melted.

It needs to be TN or TT you can't mix the two.