Earthing arrangement and single to 3-phase upgrade

[DieselShunter]

Newbie here - please go easy on me, and I hope I'm posting in the right forum!

Existing arrangement as follows - single phase TNC-S supply provided in a remote building. This building has a substantial 3-phase company supply, but my supply is tapped off the incommer, and through a a single company fuse (80A) and single phase meter to my isolator. This is a 63A 3-phase Glasgow switch fuse. Output side of that then runs about 175mtrs on a 4-core 25mm SWA to the building in question. Although the SWA that was used is 4-core, only 2 were effectively used, with only one (63A) fuse ever fitted in the Glasgow switch at the supply end. There's also a separate 25mm earth run all the way alongside that SWA. At my building there was a 4-pole isolator with a single phase CU from that.

My building is now being replaced with a larger, steel contructed, building, and the load requirements dictate we need a lot more power, and also plan to run some 3-phase machine tools there in the future.

Intention therefore is to get the DNO to upgrade the supply to 80A 3-phase at the remote building and connect onward using the existing isolator and same 25mm SWA supply cable, livening up the two hitherto unused conductors in it. New building will have a nice shiny 3-phase DB.

Finally to my questions:

Can I, or indeed should I, be putting a couple of earth rods into the ground at the new building, and adding that at the MET where the bond to the building, 25mm earth, and SWA armour come together?

I'm paranoid about loss of neutral continuity, so to safeguard against that can the supply neutral be rebonded to the MET at my end, thus extending the TNC-S to the entry point of my building?

I suspect I'm about to be shot down in flames for that last question! Look forward to your responses.

 

[Deservices]

I'd say the earth is fine at 25mm also if your earth the steel armour at the main board and the new board this will add addition earth.

 

[Billythekid]

Hi and welcome to the forum.

What is the estimated maximum demand for the new building? You've said some three phase equipment for future use? 63A seems a bit small tbh (not knowing anything about the install, this is a bit of a guess).

Look up PEN conductor in the bgb, and see the other posts on the forum regarding "exporting pme".

Hope this helps

Billy

 

[Deservices]

The steel armour on 4 core 25mm is equivalent of 34mill copper earth.so you would have 59 mm worth of earth.

 

[malcolmsanford]

Well your set up sounds like you have 4 core 25mm SWA and then a 25mm CPC.

Firstly this new building is steel so will need to be bonded as will any metallic services ie water that is coming into the building. If you do as you say intend to extend the equipotential zone from your main building I would personally be rethinking the current set up.

Is there a good reason why you are not using the SWA for your CPC. Unless there was an environmental issue ie corrosive due to moisture or chemical I would use my SWA for that.

I would then convert my 25mm into a bonding conductor tie that back to the buildings MET and then at your new building connect that into a Earth Marshaling Terminal ( EMT) and you can then bond your building and any services to this point.

As for what you propose with your neutral that is a big no no. Both the Regs and more important than them, the ESQCR-2002 strictly forbid the use of a TN-C system within an installation.

 

[DieselShunter]

Thanks for the rapid responses.

The separate 25mm CPC is already there, whoever installed it originally obviously felt it necessary. It was certainly my intention to also utilise the armour to supplement it, but I was always lead to believe that the armour on an SWA cable was not considered good enough on its own to provide a CPC - that down to the integrity of connections (how tight's yer banjo?) and risk of damage/corrosion rather than the effective CSA.

The max anticipated load would be just under 50A per phase. The 3-phase additions would be light duty machine tools, small lathes and stand drills etc.

Point taken on the export of the PME Malcolm, I suspected that was the case. But I'm curious to understand what risk it represents and why?

Is there anything else that can be done to safeguard against the loss of neutral continuity? So is there some kind of protective switch device that can trip the supply if the P-N voltage on any phase at the DB rises above a safe value?

And nobody commented on the earth rods? Is that unnecessary, or inappropriate?

 

[malcolmsanford]

View attachment SWA-pvc-xple cpc comparision.pdf

Here a table that will give you the SWA equivalent, there are quite a few you can google, but they are all roughly the same and as you can see 25mm SWA armour is easily comparable.

There is nothing really stopping you making the out building a TT system if you so wish. The only problems with that is that is start to become expensive. If you chose an upfront 100/300mA then any fault on any circuit would trip it, as you would be unlikely to get a Zs low enough on your system to operate a MCB. So for a commercial/industrial system that is far from ideal.

If you decide that to stop the whole installation tripping you would use RCBOs on individual circuits then you would have huge costs, so IMO the best system would be to extend the equipotentional zone.

What is this worry about a Neutral. The ESQCR-2002 is very specific about the issue of a TNC-S system and that the DNO if they supply one must afford it every possible means of safety to ensure we don't get the floating neutral scenario. So they are really very safe. As for losing a neutral in your installation.

You could fit a Phase failure relay, they will detect the over/under voltage on the system and if you lost your neutral they would trip out. Again this is an expensive road to go down, but you may find that most modern 3 phase equipment will have these fitted as standard.

 

[DieselShunter]

Malcolm, Thanks. I guess I'm being really paranoid about the floating neutral thing - I'll stop worrying....

 

[Knobhead]

DS if the foundation slab hasn’t been cast yet, I would pay to bond to the slabs ReBar to form an Ufer earth.

 

[Engineer54]

With a PME/TNC-S supply it would in fact be prudent to have a good local additional earthing electrode point(s). It will in effect, be earthing the supplies neutral as part of the ''Protective Multiple Earthing'' system. The use of a Ufer earth, or the use of steel columns if they have been deep driven (or even a combination of both) make excellent earthing electrodes. Connect the additional earthing point(s) to your buildings EMT

With electrodes providing very low Ra levels, at the point of utilisation, would reduce significantly any voltage raise in the case of a lost neutral upstream, ...not that they'll be in all honesty, much chance of that!! lol!!

 

[La Poste]

Why is it such a big no no to re-bond the Neutral to the MET of this new building?

What dangers would this introduce?

Thanks.

 

[La Poste]

I've thought about it and the reason is because the Neutral voltage within this new installation could be well above Earth potential (especially when under load) and so re-bonding an Earth in this new building could be a dangerous thing to do as your Earth will be at the same voltage as your Neutral and hence the bonded metalwork within this building will also rise to the same potential as the Neutral, not a very safe state of affairs.

Is that correct?