Calling all Sparkies! Earthing System Type

Gaijin · 5 Apr 2017 at 11:54

Hi guys,

To cut a long story short, i'm looking at getting electric gates for the driveway and I will be getting in an Electrician to do the works.

However, this will be the first time I will be working with an electrician here and as such they will be a relative unknown to me despite any recommendations I may get.

As such, I am trying to educate myself as much as possible so that i am: A.) Happy that the proposed work is competent and safe B.) I don't get ripped off and C.) I'm actually interested (nosey)

So first things first, can anyone please confirm the earthing system I have from the picture below? Am I correct in thinking it's a TN-C-S?

lemonkettaz · 5 Apr 2017 at 15:29

TN-C-S

Lumeycom · 5 Apr 2017 at 21:21

Tncs (often called PME).

Your spark should be aware of the potential issues exporting that earth type beyond the property when there is metalwork attached to the other end.

Also your meter tails are not double insulated and that cable going out from the bottom of the isolator/kmf looks awfully like 16mm twin - if it is your earth is likely undersized.

Beer_MonsterUK · 6 Apr 2017 at 00:17

Is this property in the UK? The RECI label indictates it is an Irish installation, which conform to different regulations. Similar yes, but different.

The concern about exporting a tncs system is that if the incoming neutral becomes broken then a dangerous voltage may arise on extraneous metalwork.

Gaijin · 6 Apr 2017 at 10:57

Thanks guys for your comments.

It is an Irish property but my understanding is - like you say Beermonster - that the systems a very similar. I won't be taking anything for granted regs wise - it's more the electrical theory that I want to understand more.

I've done a little more reading on what both of you have mentioned regarding exporting the earth. Obviously i'm concerned about safety as in essence, the supply to the gates is theoretically going to be run from a consumer unit in the garage via an underground SWA cable to the IP65 metal control box which would be about 40 metres away.

So with that in mind, and from the reading i've done this morning, do I need to ensure that a secondary earth is used - such as an earthing rod or similar? I've seen a few different opinions on electricians forums - some suggesting to treat external runs such as this as a separate TT installation and earth the box and it's contents locally using a rod.

If the electrician were to use an earthing rod, would it best be used in combination with the supply earth or on it's own?

And would it be best situated near the control box or nearer the consumer unit end?

I've seen a suggestion to connect the earth of the SWA to the earth at the consumer unit end, but not at the other end, and then earth the control box and it's contents near the box itself TT style - does that make sense?

Thanks again for your help guys.

Gaijin · 6 Apr 2017 at 10:59

Lumeycom said:
Also your meter tails are not double insulated and that cable going out from the bottom of the isolator/kmf looks awfully like 16mm twin - if it is your earth is likely undersized.

Thanks for highlighting this Lumeycom - I will point this out to him - What should be done about this in your opinion, and what are the current implications of this being how it is?

Beer_MonsterUK · 6 Apr 2017 at 11:30

As far as i can see there is no actual regulation, in BS7671, which prohibits the exporting of a TN system. The fear is that losing the supply neutral ( an extremely rare occurance) will create hazardous voltages on extraneous metalwork and not trip any RCD devices.

If you decide to TT the gates, then you run into the whole other issues of testing the earth electrode and RCD on a regular basis to check they are still offering the protection they were designed to do.

I've looked at a few forums and there doesn't seem to be a general concensus.

Adam_151 · 6 Apr 2017 at 19:42

Its a judgement call at the end of the day and factors will include:

How far away from the property are the gates
Is it concrete or just mud grass around the gates
Is there any other metalwork near by
How much metalwork is being connected to the earthing system, is it just a metal IP rated cabinet, or with the gate motors being 24v or whatever or does the whole gate end up being connected to earth

At the moment there is a bit of confusion over electric car chargers, the general thing is that they can't be connected to PME if the car is outside, however they do allow it in domestic only if the other alterantives are not reasonably practiable.

If you were to TT a supply to an EV / gates / whatever, generally it would take the form of armouring earthed to supply earth at the source, RCD at source. At the far end, don't connect the armouring and put an electrode local to the load. Suitable labeling at the DB of whats gone one.

An electrode in conjucntion with the PME earth is one thing thats suggested as a possibility for electric car chargers, but the resistance you need to make it actually usful to protect against a lost CNE conductor is much lower than one rod can achieve.... think metallic grid set into the concrete. Plus the earth to it is now a bonding conductor and must be sized accordingly. Its not practiable in your situation

Gaijin · 6 Apr 2017 at 23:36

I've literally been reading all day about this and essentially all i've read is arguments between electricians. Essentially the regs are ambiguous if not a bit contradictory. There seems to be two camps:

Use supply earth, yes loss of neutral before the premises is potentially very dangerous, but incredibly unlikely - enough so as to not worry about.
Set up outbuildings etc. as TT and use localised earth electrode to bring down the risk, but with other potential issues.

The nightmare scenario I have is that if the unlikely happened and the control panel box became live - and then my son touched it.

Adam_151 said:
Its a judgement call at the end of the day and factors will include:

How far away from the property are the gates
Is it concrete or just mud grass around the gates
Is there any other metalwork near by
How much metalwork is being connected to the earthing system, is it just a metal IP rated cabinet, or with the gate motors being 24v or whatever or does the whole gate end up being connected to earth

For clarity here's the answers to your questions:

How far away from the property are the gates? Approximately 40 metres SWA cable length from CU to proposed Control Panel
Is it concrete or just mud grass around the gates Where I've imagined the control box to be is in mud/grass, but right next to a kerb and tarmacadam
Is there any other metalwork near by? No, none really although i've flirted with the idea of running another SWA for garden lighting if a trench would be dug.
How much metalwork is being connected to the earthing system, is it just a metal IP rated cabinet, or with the gate motors being 24v or whatever or does the whole gate end up being connected to earth. The control panels i've been looking at (and the electrician may well change it) are metal IP65 painted in epoxy polyester paint (not sure if this provides any insulation). Gates would be 240V and the one i'm looking at has earth connections on the PCB so I imagine it would use the earth that the control panel uses.

Gaijin · 6 Apr 2017 at 23:40

Here's a crude diagram in MS paint:

If it's not clear, the control panel in my mind (again, electrician may see different) would sit halfway between the gatepost and the flowerbed you see just above the post box.

If it's not obvious, there is another (primary) consumer unit in the house near the back door.

Adam_151 · 7 Apr 2017 at 23:55

I'd be tempted, I think to TT the garage board I think and make everything in the garage TT, run the supplyto the lights from the same board (one thing we really must avoid, is to have metalwork connected to different earthing systems within reach of each other). If the whole gate ends up being earthed, it is rather a large expense of metalwork adjacent a public foot path likely to be touched. Therefore I'd probably consider the risks of PME earthing on it to be similar to an EV on charge in the event of a lost neutral (A study has shown this to be relativly low, but not so low that we should overlook it). We generally avoid putting EV chargers on a PME earth (there are other ways round it - but lets not digress too far)

I'd also recommend that you go to gate automation company rather an an electrical contractor for the actual gates, the risks of getting the actual gates and safety interlocks wrong are far greater than those presented by PME earthing, there have been fatalities where safety systems were not working correctly

Lumeycom · 8 Apr 2017 at 10:55

I'd agree with Adam 100%, in my opinion the Garage could do with being TT anyway so to covert the garage would be an ideal overall solution, it would also likely be a tidier solution