PME carport

[marcuswareham]

Hello

Hope everyone if well with this cover-19 stuff going on

Quick question

I would like to put a power supply to a metal framed car port, the metal frame is buried into the ground so definitely introduces earth potential, the ground inside the carport is tarmac laid on top of soil

The power supply is coming from P.M.E house, I think the best option is to make the carport TT and install RCD protection, However I cannot achieve sufficient separation between PME bonded gas pipe and water pipes in the ground and the new earth electrode.

If I export the PME earth and bonding the metal structure then surely this is creating a large shock risk to anyone standing on the wet grass next to the car port and in contact with the metal structure if there was a PEN conductor fault.



Looking at 18th Edition Amendment 1 section on EV charging

722.411.4.1
(iv) Protection against electric shock in a single-phase installation is provided by a device which electrically disconnects the vehicle from the live conductors of supply and from the protective earth in accordance with Regulation 543.3.3.101(ii) within 5 s in the event of the utilisation voltage at the charging point, between the line and neutral conductors being grater than 253 C rms or less than 207 V rms. The device shall provide isolation and be selected in accordance with Table 537.4. Equivalent means of functionality could be included within the charging equipment. Closing or resetting of the device shall be possible only if the voltage between line and neutral conductors is in the range 207 to 253 V rms.



Maybe I could use a device like this but not for EV charging but instead to protect car port from open PEN

Although I cannot find anywhere to buy such a device

Many thanks

Marcus

 

[pc1966]

All this has also got me thinking about my outside lights, I have 2 class I each side of my front door at about shoulder height. That is probably not a good idea being PME hahaha

To be fair PME faults are not very common, and the real risk is for stuff that people are likely to be in good contact with while also outside. Electric cars being washed while on charge are an obvious risk, metal caravans on nice wet grass, boats on marinas being fed power while engines off, etc.

I guess most outdoor power or gardening tools these days are double insulated so no earth to worry about.

Thought there was a school fence that gave some kids shocks (thankfully none fatal) in the news not that long ago, so it is not unknown!
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I have a memera 2000 split C/U. However I do have RCD protection on all circuits, I have a sub C/U for electric shower containing RCD, and down stream RCDs for lighting circuits.

Is the sub-CU fed from the main one, or split meter tails?

If fed from the main one, is the main switch one that could simply be replaced with an RCD?

 

[marcuswareham]

To be fair PME faults are not very common, and the real risk is for stuff that people are likely to be in good contact with while also outside. Electric cars being washed while on charge are an obvious risk, metal caravans on nice wet grass, boats on marinas being fed power while engines off, etc.

I guess most outdoor power or gardening tools these days are double insulated so no earth to worry about.

Thought there was a school fence that gave some kids shocks (thankfully none fatal) in the news not that long ago, so it is not unknown!
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Is the sub-CU fed from the main one, or split meter tails?

If fed from the main one, is the main switch one that could simply be replaced with an RCD?

Yeah I hear what you are saying about limited risk, I guess the easiest option is to move the lights higher up, so the risk is even less, unfortunately I cannot do that, but I will not lose sleep over it just yet.

Maybe there needs to be a device like the matt-e thing which offers a little more protection by measuring the voltage but which fits in a single gang backbox, for use for outside sockets with low demand etc feed from F.C.Us

The sub C/U is feed from a 40A MCB from non RCD side of the main C/U, so yes replacing main switch with a RCD would be an option although I am limied as you cannot get the MEM breakers for my C/U anymore, doh

 

[pc1966]

The sub C/U is feed from a 40A MCB from non RCD side of the main C/U, so yes replacing main switch with a RCD would be an option although I am limied as you cannot get the MEM breakers for my C/U anymore, doh

Ah, that sucks A quick search found this 2nd hand one, the earth tail is cut short but it might just be worth a try if you have a MFT to check the trip characteristics, etc, on some simple test bed before deciding if it could be fitted:

MEM RCD 100 Amp 100ma Double Pole 100a Time Delay RCCB MEMERA 2000 A100MET for sale online | eBay

Find many great new & used options and get the best deals for MEM RCD 100 Amp 100ma Double Pole 100a Time Delay RCCB MEMERA 2000 A100MET at the best online prices at eBay! Free delivery for many products!

www.ebay.co.uk

Failing that the obvious solution is a new CU but that opens a whole can of part P worms...
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Or you could fit a small metal enclosure with a separate RCD in it. I think I have seen isolator switch & SPD combinations so why not RCD?

Yes, it is separating the meter tails and without an isolator switch you are in to the dodgy business of getting the DNO to pull the fuse (if it did not "fall out" accidentally...) but it is another way that avoids a new CU for now.

 

[marcuswareham]

Ah, that sucks A quick search found this 2nd hand one, the earth tail is cut short but it might just be worth a try if you have a MFT to check the trip characteristics, etc, on some simple test bed before deciding if it could be fitted:

MEM RCD 100 Amp 100ma Double Pole 100a Time Delay RCCB MEMERA 2000 A100MET for sale online | eBay

Find many great new & used options and get the best deals for MEM RCD 100 Amp 100ma Double Pole 100a Time Delay RCCB MEMERA 2000 A100MET at the best online prices at eBay! Free delivery for many products!

www.ebay.co.uk

Failing that the obvious solution is a new CU but that opens a whole can of part P worms...

Ahh thats the puppy, yes I have MFT and I spend alot of my working life testing RCDs so thats fine :- D

Yes I have before now though it might be time for a new C/U, I will wait until I need to do other additions etc, then kill all the birds with one stone

Just had another thought on the outside lights, Another workaround could be to use B22 12v bulbs in the fitting and supply the fittings via a 12v transformer, providing electrical isolation and extra low voltage, so no need for the earth bond, and no risk of touch voltages, maybe :- S

 

[pc1966]

If there is any simple way to TT supply the whole house and car port then all of the PME risk problems go away in one step.

I would try to ascertain if the service pipe work is bonded to the PME of neighbours first as that would negate any TT benefits. Some gas pipes now have insulated sections for this reason, and often the gas main is plastic even if the pipework in to the home is metal, so you might be OK there. Otherwise the trouble of sorting gas pipes is way too much and unlikely to be done during lock-down as not essential work!

 

[marcuswareham]

If there is any simple way to TT supply the whole house and car port then all of the PME risk problems go away in one step.

I would try to ascertain if the service pipe work is bonded to the PME of neighbours first as that would negate any TT benefits. Some gas pipes now have insulated sections for this reason, and often the gas main is plastic even if the pipework in to the home is metal, so you might be OK there. Otherwise the trouble of sorting gas pipes is way too much and unlikely to be done during lock-down as not essential work!

Yes I agree,

I am not on mains gas nor is any of the village, so the metal gas pipe runs to a shared (with neighbours) gas bottle storage which is bonded to all services supplied from it, Although that said bottled gas is so expensive I actually don't use it (haven't done for 2 years) as I have economy 7 and a log burner works out cheaper to heat the house with logs and the water at night time.

I do like having the gas option there as a back-up but in reality I could probably do away with it, meaning no metal service pipes entering the house.

It would still need it to be capped correctly by a gas safe person, and like you say with lock-down. However I am in no rush it can wait till after, whenever that may be

 

[pc1966]

It might be worth asking a gas safe person if they could install an isolation section so you can keep the gas but not the bonding.

If you search for "Insulation inserts in metallic gas service pipes to consumers' premises" you should find an IET article on this sort of thing (pasting links to the IET site seem to get borked on this forum).

 

[marcuswareham]

MyEnergi Zappi would be the product for just this.

EV Charger | Low Cost Home Electric Car Charging

Get a fully installed zappi home EV charger, featuring low cost energy tariff integration and solar charging, all proudly made in Britain.

myenergi.com

Sorry I missed your replay,

Just had a look at this product it would appear to me that this device instead of monitoring the voltage. It uses a sensing coil round the earth conductor and once it detects a few 10s of mA it disconnects line, N and PE , correct?

Thanks Marcus

 

[Lister1987]

Sorry I missed your replay,

Just had a look at this product it would appear to me that this device instead of monitoring the voltage. It uses a sensing coil round the earth conductor and once it detects a few 10s of mA it disconnects line, N and PE , correct?

Thanks Marcus

It has inbuilt RCD protection and uses the current clamp to monitor overall load on incoming supply and lowers/increases its power consumption during time of high/low demand, if I'm remembering correctly it monitors current instead of voltage and being the only one (apparently) to work on this manner it will isolate correctly in the case of a broken neutral, whereas others won't

Of course rereading your post your just after sockets, not an EV Charger, my bad.

 

[marcuswareham]

If there is any simple way to TT supply the whole house and car port then all of the PME risk problems go away in one step.

I would try to ascertain if the service pipe work is bonded to the PME of neighbours first as that would negate any TT benefits. Some gas pipes now have insulated sections for this reason, and often the gas main is plastic even if the pipework in to the home is metal, so you might be OK there. Otherwise the trouble of sorting gas pipes is way too much and unlikely to be done during lock-down as not essential work!

Looking at the gas bottle store, It will be very easy to disconnect earth links between gas pipes to the other house and the separation between the pipes is around 1.2m. the gas bottles themself cannot easily touch the others and they are also connected via rubber hoses. Will this be enough separation in the ground, I could do a resistance test between the gas pipes for the other houses.

If I do convert to TT, what tests would I need to do in addition, I am thinking measure Ze, earth rod impedance, test all RCD's disconnection times and that it? , and all of the bonding already in place for PME will be oversized but that's no problem

It looks as if I should be able to get earth rod resistance down to around 15-20ohm using 2 rods and the gas pipe

Also I do minor works certs for the work I do on my house, would a minor works certs for changing to TT do, and if so the format of a minor work cert doesn't lend its self to this, so what documentation could be better instead

 

[davesparks]

Looking at the gas bottle store, It will be very easy to disconnect earth links between gas pipes to the other house and the separation between the pipes is around 1.2m. the gas bottles themself cannot easily touch the others and they are also connected via rubber hoses. Will this be enough separation in the ground, I could do a resistance test between the gas pipes for the other houses.

Also I do minor works certs for the work I do on my house, would a minor works certs for changing to TT do, and if so the format of a minor work cert doesn't lend its self to this, so what documentation could be better instead

Don't disconnect the bonding between the gas pipes, you'll have two different earth potentials within close proximity of each other.
Like it or not you are stuck with those bonds being in place and these various connections to the PME earth.

No a minor works certificate isn't appropriate for changing the earthing arrangements of the entire installation in my opinion, a full EIC should be issued with all circuits fully tested.
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It looks as if I should be able to get earth rod resistance down to around 15-20ohm using 2 rods and the gas pipe

Can you clarify what you mean by two rods, how long are they and how far apart?
You cannot use a gas pipe as an earth electrode, so don't factor that in to any calculation. Of course the problem with that is you can't stop fault current flowing through the gas pipe whatever the regulations say, and if it is in good contact with the ground it will have a low Ra of its own whatever you do.

 

[marcuswareham]

Don't disconnect the bonding between the gas pipes, you'll have two different earth potentials within close proximity of each other.
Like it or not you are stuck with those bonds being in place and these various connections to the PME earth.

No a minor works certificate isn't appropriate for changing the earthing arrangements of the entire installation in my opinion, a full EIC should be issued with all circuits fully tested.
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Can you clarify what you mean by two rods, how long are they and how far apart?
You cannot use a gas pipe as an earth electrode, so don't factor that in to any calculation. Of course the problem with that is you can't stop fault current flowing through the gas pipe whatever the regulations say, and if it is in good contact with the ground it will have a low Ra of its own whatever you do.

yes i hear what you are saying about the gas pipes, even though they are connected to different bottles and go to different houses as they are in the same store you could touch both at the same time

yes i would agree a minor works cert is not enough, a full EIC is the one, i have done one of those before when I installed my shower (no i am not part p registered but i did it the correct channels and notifed, they used 3rd party to cert i was down as installer and tester but under supervision)

I have an area i could get more than 2 rods in, i have tested one in the ground now and i am getting around 45ohm mark, its 4ft long and they can be between 1.5m - 3m apart they are in a small area behind the house and as far as i know at least 5m away from any buried metallic services and other buildings

its says in my book (iet earthing & bonding) that the gas pipe can be used as an electrode I haven't checked though the wiring regs although
the gas pipe without any connection to PME anywhere is 54ohms so the total would be earth resistance of 16ohm


i understand what you are saying about the gas pipes, and just wondering what would happen in this instance if there was already one of the houses on PME and the other was TT how would you get round the gas pipe bonding as i own the bit of land my bottles sit on and the other house own the bit of land there's sit on, you couldn't force the other house onto TT or PME etc

 

[davesparks]

I have an area i could get more than 2 rods in, i have tested one in the ground now and i am getting around 45ohm mark, its 4ft long and they can be between 1.5m - 3m apart they are in a small area behind the house and as far as i know at least 5m away from any buried metallic services and other buildings

4' long is no good, you need to extend them by at least another 4', or more if its possible. A 4' rod will suffer quite badly with seasonal variations as the ground dries out or freezes, any reading you take now will be in reasonably favourable ground conditions.

The rods should be spaced by a minimum of the length of the longest rod to reduce the interaction between them.

 

[pc1966]

I would agree with davesparks that having the gas un-bonded at the bottles is a very bad idea. I think the only sane way to isolate the gas supply electrically would be to has a gas-safe engineer install a proper isolation joint or to replace some of the buried pipe with plastic so there are no conductive areas within touching distance that could be differing potentials.

Also I suspect (but don't know) that the gas regulations might prohibit the removal of bonding in that area due to the risk of a spark igniting gas, etc, when changing supplies.

As already said, you really are much better with a deeper rod than two shallow ones. In your area there might not be much danger of freezing down to 3-4 feet but drying out is a very real risk, both greatly increase soil resistance.

You can get rod joints to allow one rod to be driven in then another attached and the pair driven in further (with a hammer on to a driving stud, or SDS drill attachment, etc). You can also get a 1m SDS drill bit to get the top section of ground bored out if you want to use a single rod of 2m or more length, allows you to have it partially in so not up a stepladder hammering it!
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While you might get additional grounding via service pipes and the car port legs (the original point of this!) you really ought to have at least one rod that can be identified as your "means of earthing" at your main earth terminal, and it only need be < 200 ohms. But you want reliability so a deep rod is far better, though if you want two rods for extra security that is fine, but to get the greatest benefit they should be spaced by their depth (or more) so they are not sharing much of each other's region of influence in the ground.

 

[davesparks]

While you might get additional grounding via service pipes and the car port legs (the original point of this!) you really ought to have at least one rod that can be identified as your "means of earthing" at your main earth terminal, and it only need be < 200 ohms.

Why do you say they 'ought to have' a rod in addition to buried structural steelwork? There's nothing in the regulations to support this statement.
Why does it only need to be below 200 ohms? There's no regulation which says that and it makes no technical sense to use 200 as an upper limit.