TT or PME garage

[Buzz1980]

Hi all

Question regarding exporting PME earthing system to outbuilding/garage. The garage has no water/gas/oil supplies but has a concrete floor. What is the recommended method that most would use in this case. Take the PME out to the garage or isolate PME and then TT the building. Just had me thinking with the concrete floor can hold moisture in the event of a fault? Is there a regulation quoted for exporting PME to outbuildings ? Thanks in advance

 

[ferg]

Not exporting PME but extending equipotential zone.. Etc Etc

For me use the DNO earth

 

[Pretty Mouth]

Garage with no extraneous parts, unlikely users will be bare foot, floor probably dry. I'd say the risk from an open PEN is low. TT'ing it will eliminate that risk, but will replace it with another - namely relying on an RCD for fault protection. Personally I too would use the DNO earth.

 

[davesparks]

Take the PME out to the garage or isolate PME and then TT the building.

You are not taking the PME anywhere, that stays on the DNO distribution network and doesn't form any part of the customers installation. You can use the earth supplied from a PME distribution network.

Just had me thinking with the concrete floor can hold moisture in the event of a fault?

A concrete floor will hold moisture in the event of a plumbing fault and water spilling all over it. But I fail to see how you think it can hold moisture in the event of an electrical fault?

 

[pc1966]

Related article here:

Exporting A PME Supply To Another Building

In this article the technical team at Stroma Certification offers expert guidance on exporting a PME supply to another building. One of the most common queries to the Stroma help desk concerns running

professional-electrician.com

But as already said, the title should be something like "Exporting power from a PME supplied installation" as there in no PME beyond the DNO point.

 

[Buzz1980]

Thanks for all replies, and correcting me ?. With regards to the article above I came out with the below info
If the remote building has a concrete earth floor, or conductive floor, or the building is of metal construction or metal outside skin, the shock risk is increased. It’s therefore recommended that the remote building has a TT earthing system.
Now going on this I’m thinking that the earth concrete they are talking about is where there is earth tapes below the concrete and is installed before concrete is poured ? , but can’t see this being the case in a garage. Regarding the concrete floor my thinking was/is and correct me if wrong. The concrete well most concretes hold moisture and if this being connected to the soil below then is part of a different zone, so on certain days/weather the moisture content in concrete could get to levels which in my thinking could cause a issue if lost neutral, but I could be totally wrong in my thinking ?

 

[Wilko]

most concretes hold moisture and if this being connected to the soil below then is part of a different zone, so on certain days/weather the moisture content in concrete could get to levels which in my thinking could cause a issue if lost neutral, but I could be totally wrong in my thinking ?

Hi - my take is with PME if the N is lost there will be a safety risk at both the house and the garage. Sometimes the risk is greater and a different Special Area design from Section 7 may be needed.

 

[pc1966]

In the house you typically won't have such a risks as (if properly installed) all "extraneous conductive parts" that could be introducing true Earth potential are bonded to the PME-derived earth, and usually the building would have carpet, etc, reducing the risk of touching metalwork and finding something sufficiently conductive to Earth to be a risk.

In the garage/outbuilding it would depend on what use it was put to and the details of its construction as to how conductive the concrete could be, and how much risk there is of contact to that and the PME-derived earth. If it is used as a workshop with folk using power tools while lying on the ground under cars, etc, then you do have something potentially risky.

If there were steel work, rebar in the floor, etc, you could bond to, and you were able to run 10mm cable for that (due to high fault currents if you succeed in grounding an open PME-derived earth locally) then under fault conditions even if you can't keep the PME' potential down, you will probably raise the potential of the building to some degree and so you might have a safe situation as the touch potential is low enough.

Another option (perhaps of dubious certainty) is the likes of the Matt:E box for EV chargers to isolate the supply and CPC in the event of out-of-range supply volts implying a PME fault.

As Pretty Mouth points out going TT is trading one risk (PME fault causing volts to Earth) against the other risks (RCD fault ignoring leakage, earth rod going bad). You could manage that risk by using a reputable RCD brand, and possibly add a SPD to reduce the risk of surge damage to the RCD, and periodically testing it, etc.

 

[radiohead]

Nobody EVER asks, "I am installing external garden lighting and power, should I 'export' the PME"?
What is it with outbuildings that reaches the parts other installations don't reach?

 

[Buzz1980]

Hi thanks for the replies lads

I am thinking I will just use the PME earthing system and won’t bother TTing the garage. I will fit all plastic enclosures and use plastic conduit for install.Regarding the socket in the Garage more than likely just going to be a fridge freezer plugged into and won’t be used for work shop. If any thing will be used for plugging in for cutting grass and possibly unplugging to power wash car on drive which I can’t imagine they will be doing this in there bare feet. I would think this situation is in most garages through the UK ??

 

[pc1966]

Most garages are not used as maintenance garages, let alone car storage. They are junk stores and space for freezer, etc! So really the risk is negligible.

Most modern power tools like lawn mowers are double insulated anyway, so they don't have any risk beyond chopped cable and the RCD should deal with that even if the MCB is not tripped by it shorting out by the cutting blade.

 

[radiohead]

Hi thanks for the replies lads

I am thinking I will just use the PME earthing system and won’t bother TTing the garage. I will fit all plastic enclosures and use plastic conduit for install.Regarding the socket in the Garage more than likely just going to be a fridge freezer plugged into and won’t be used for work shop. If any thing will be used for plugging in for cutting grass and possibly unplugging to power wash car on drive which I can’t imagine they will be doing this in there bare feet. I would think this situation is in most garages through the UK ??

You cannot use a plastic DB in a domestic install, unless you enclose it in a fire containing cupboard.

 

[Buzz1980]

I would have thought that the consumer unit in the garage would not have to be metal if there was no chance of fires spreading out of there. As this garage is split by the side house foot path and has concrete floor and brick walls, only would inside the garage is the wooden roof joists and wooden door. But is there a limit that has to be from the main building ?

 

[pc1966]

I doubt you get plastic DB easily in the UK now, and it would be simpler to cover a metal garage CU with a plastic or wooden box if needed. But in any case I don't see that as an issue because contact with the DB is usually rare and they are painted so little opportunity for a good connection to touch.

As a "don't try this at home girls & boys" experiment I took a damp dish cloth and tried a 500V test to earth on my Wylex DB, only got < 1M when it was in contact with the edges of cover, etc.
[automerge]1587473701[/automerge]
Something like this:

https://www.NoLinkingToThis/p/british-general-5-module-3-way-populated-garage-consumer-unit/1926g

OK it comes with 32A for the ring which might be too much depending on your feed to it, but if you have a 40A C-curve MCB and adequate cable it would give to enough for two fridge freezers and any gardening tools to be fed, etc.
[automerge]1587474255[/automerge]
Has your main house CU got spare slots for a feed?
Are they all on RCD already?
How do you plan on cabling it?

 

[Strima]

In a domestic dwelling the DB must be made from a non-combustible material. This isn't about fire stopping, it's about the DB not providing fuel for a fire.

 

[Buzz1980]

They have already had a 6mm t&e supplied to the garage which is coiled up inside garage comes along high level in plastic conduit about 600mm high level then enters the garage. The cable is coiled at the consumer unit. They have a 16th edition board which has RCD side so can put it on that side on a 32amp MCB then take 6amp for lighting and 16amp for socket circuit which is the plan. There are still plastic consumer units on the market so I can’t find info where I couldn’t use a plastic consumer unit inside the garage. The load that is going to be pulling through is nothing compared to the current for a consumer unit inside a house. Plus is this any different than me putting a switch fused spur instead of consumer unit. Put on 20amp MCB RCD side have socket come off supply side then lighting come of the fused side, fusing down to 3amp still doing the same amount of connections than I would do with the plastic consumer unit but just inside a fused spur ??but suppose if the regs say can’t use plastic consumer units in domestics then prob best to stick to this ?

 

[pc1966]

Have you been able to measure the Ze (PFC) at the home CU?

How long is the 6mm cable run?

How well protected is it along the way from accidental damage (both outside and in the route to the home CU)? I.e. is it obvious in conduit so nobody would drill it, etc, or is at least 50mm behind any walls?

The reasons for asking is about selectivity between the home circuit and the garage. If you get one of the inexpensive garage CU (like the British General one from Screwfix or numerous others) they often have a 30mA RCD, so you don't need to take it from a RCD side at the supply unless the feed cable needs protection. With SWA that is fine, for T&E it would need to be sensibly routed so very unlikely to be punctured.

Advantage of doing so is a fault in the garage won't trip the home circuits which is an enormous advantage! Two RCDs in series won't be selective unless the upstream one is both a higher trip current and a time-delay sort (e.g. 100mA S-type).

You are right they probably don't need a 32A ring out there, in fact 16A is enough but would definitely trip on a fuse blowing in any plug. Even 20A B-curve will generally go faster than a 13A fuse!

And that comes to the second aspect of selectivity, so you ideally only trip the MCB in the garage on a fault and not also tripping the feed in the home CU (with a loss of lights to the garage as well). This is hard to do effectively with MCB as you don't get any real control over the instantaneous magnetic trip part, so your best hope is to make the upstream breaker as high an instant trip point as you can get away with, and the downstream ones as low as reasonable for the loads involved.

For example, with a TN-S supply having a max Ze of 0.8 ohms you can use only type B MCB up to 40A (or type C to 20A) if relying on a non-RCD feed. While 40A is generally too much for 6mm cable that is OK for short circuit protection if the downstream load is limited, and so if the garage CU had 6A MCB for lights and 16A or 20A sockets you would get some selectivity. But if the Ze is lower you could push up the feed MCB to have a better chance of selective operation.

 

[Buzz1980]

Hi pc1966

The customer has said that the cables were routed down a boxed section in a bedroom then under floor too consumer unit. From the garage it enters the eaves is clipped around then goes under 1st level floor. So I have taking that the other info from the customer that the cable is routed down the boxed cable section then under floor to consumer unit, this was pulled in when they upgraded upstairs. It is obvious it’s in conduit as it comes out the soffit and shoots across to garage into a ip rated enclosure. The run is about 20metres long long and can’t see how it could be damaged. So I could prob put this on a MCB and RCD the garage side. I’m more inclined now just to make it a radial circuit on a 32amp MCB on RCD side with the 6mm t&e as hardly any load on the circuit anyway ?

 

[pc1966]

If you are looking at 16A/20A max on the garage side then you can do a radial in 2.5mm and it is fine.

The Screwfix BG example has a 32A MCB fitted but you can get a 20A one for £4 from Screwfix anyway so hardly an expensive change!