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]

These folk make them, product list at bottom of the fairly crappy web site:

Installation Manuals

Installation Manual v1.10 Oct 2021Models: SP-EVCP-T (Cxx) SP-EVCP-M (Cxx)SP-EVCP-R (Cxx) SP-EVCP-B (Cxx)IP-EVCP-T (Cxx) IP-EVCP-M (Cxx)IP-EVCP-R (Cxx) IP-EVCP-B (Cxx)Download SP-EVCP Installation Manual May 21Models: SP-EVCP-T(Cxx) SP-EVCP-M(Cxx)SP-EVCP-R(Cxx) SP-EVCP- B(Cxx) SP-EVCP-...

matt-e.co.uk

I mean, WTF do they think you want to see the logo filling whole page and have to scroll down to see the useful stuff?

Manual for non-RCBO version:

https://matt-e.co.uk/wp-content/uploads/2019/02/SP-EVCP-T.pdf

If the pipes are close/accessible though (as I guess your TT rod issue suggests?) then under such faults you might still have step-voltage risk.

Edited to add: A quick search shows you can get them from CEF and Rapid for around the £100+VAT mark and with RCBO for about £30 more.

 

[Lister1987]

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

 

[buzzlightyear]

I will not require to be put on a TT.

 

[Midwest]

Just a thought, what is the power supply for? Would it be for Class II apparatus?

 

[marcuswareham]

These folk make them, product list at bottom of the fairly crappy web site:

Installation Manuals

Installation Manual v1.10 Oct 2021Models: SP-EVCP-T (Cxx) SP-EVCP-M (Cxx)SP-EVCP-R (Cxx) SP-EVCP-B (Cxx)IP-EVCP-T (Cxx) IP-EVCP-M (Cxx)IP-EVCP-R (Cxx) IP-EVCP-B (Cxx)Download SP-EVCP Installation Manual May 21Models: SP-EVCP-T(Cxx) SP-EVCP-M(Cxx)SP-EVCP-R(Cxx) SP-EVCP- B(Cxx) SP-EVCP-...

matt-e.co.uk

I mean, WTF do they think you want to see the logo filling whole page and have to scroll down to see the useful stuff?

Manual for non-RCBO version:

https://matt-e.co.uk/wp-content/uploads/2019/02/SP-EVCP-T.pdf

If the pipes are close/accessible though (as I guess your TT rod issue suggests?) then under such faults you might still have step-voltage risk.

Edited to add: A quick search shows you can get them from CEF and Rapid for around the £100+VAT mark and with RCBO for about £30 more.

Yeah nice idea, I have seen a matt-e video on youtube although not in this much detail and have never seen anywhere to purchase it.

How would this fit in with the reg, as it is not being used for a EV charging point

The TT idea the earth rod would only be able to go a maximum of 1m away from bonded service pipes (and the DNO incoming cable). There has been some digging for other reasons which is why the location of these is known. I guess an insulation test between PME earth and the earth rod would indicate what PME fault voltages could be transferred to the TT?

Thanks



Thanks

 

[marcuswareham]

Just a thought, what is the power supply for? Would it be for Class II apparatus?

A mix, but definitely some Class I appliances, one being a mig welder (although the welding current is separated by a transformer). The chassis of the welder and all other metalwork is bonded to the CPC

 

[pc1966]

Yeah nice idea, I have seen a matt-e video on youtube although not in this much detail and have never seen anywhere to purchase it.

How would this fit in with the reg, as it is not being used for a EV charging point

I'm not sure. All it is doing is disconnecting everything (L, N & E) in the event of voltage going out of range (which is usually the symptom of a broken PME circuit). So in that sense it would revert your car port to the previous unconnected situation.

The TT idea the earth rod would only be able to go a maximum of 1m away from bonded service pipes (and the DNO incoming cable). There has been some digging for other reasons which is why the location of these is known. I guess an insulation test between PME earth and the earth rod would indicate what PME fault voltages could be transferred to the TT?

The separation of the earth rods (for example the 10m for caravans) is not so much about the resistance between them but more about the TT rod being at a low enough potential if the PME 'rod' (cable, pipes, whatever) becomes live and the ground around it shows a dropping potential with distance. In the caravan case that TT rod is connected to the caravans that could be some distance away, so the area around the caravan is close to true Earth and that is why the TT rod's separation from PME (and possible potential rise) matters.

In your case it is slightly different as the risk is the potential difference between the car port metalwork and the immediate surrounding area which as you say includes the pipework and so any possible PME effect.

You say the car port metal work is already grounded, have you tried making a bonding resistance check between it and, say, your house PME earth to get an idea of how well grounded it is?

It might serve as your TT rod, or you might consider digging a small hole in the tarmac at a far for a dedicated rod. As others have advised, if you can hand-dig down by around 1m (say using a fence post style of tool) then you can be sure of no cables/pipes befor driving a rod in and then back-filling it.

 

[marcuswareham]

I'm not sure. All it is doing is disconnecting everything (L, N & E) in the event of voltage going out of range (which is usually the symptom of a broken PME circuit). So in that sense it would revert your car port to the previous unconnected situation.

Yes I agree there could still be a situation of a broken PEN but the voltage remaining within that range, depending on the balance of loads on the other phases in the street. Strange how the regs say it is ok for EV charging, I guess the fact PME should have multiple rods along the supply and that it will detect a broken PEN in most cases it is enough mitigation.

I had also not thought about your comment on returning the carport to the current situation, i.e without a power outlet in it. So basically there could be a shock risk from the carport regardless due to the fact that the steel posts are in the ground in close proximity to the service pipes. Doh !

You say the car port metal work is already grounded, have you tried making a bonding resistance check between it and, say, your house PME earth to get an idea of how well grounded it is?

I have not done as you say to the PME MET, I have done an earth loop impedance test on it using a flying lead from the C/U, there are 5 posts and most are around the 150ohm range.

If I do manage to get a rod in far enough away (what separation would you suggest in this instance), would you bond the carport metal posts to the carport TT MET or leave un-bonded. I think this would bring the PME risk onto the TT CPC, but it could also reduce the touch voltage of the carport posts in a PEN fail

Thanks Marcus

 

[pc1966]

Are the posts all bonded together? I'm guessing they would be bolted to metal beams, etc, but that may or may not be giving a good resistance between them. Either way as you are seeing < 200 ohms then it would normally be considered good enough for a TT earth rod.

In fact having a set of posts spread out should mean that they are close in voltage to the surrounding soil, and that is all that really matters from a shock point of view. If your service pipes are not exposed outside then I guess there is a low risk of seeing enough step-potential for it to be dangerous (also given the relatively low risk of a PME fault).

Certainly you should bond the posts and any circuit earth in the garage together with 10mm or similar as the whole point is to achieve an equipotential zone. Should the whole zone be at, say, 100V up under fault conditions but your soil around it is within 25V or so of it then there is little real shock risk.

Off hand I don't know if you can use a structure as the TT earth (would have to look that up), but if you are sure there is nothing buried in the immediate vicinity of one of the posts then it would be safe to drill & drive a dedicated rod in even if you are really relying on the posts as the main earthing means. I would not worry about getting far away as, again, what matters is keeping the exposed metalwork voltage close to the surrounding soil.

Are they H-section beams or similar? Getting a good bond should be as simple as drilling and using a ring tag. If rusted and/or painted then a bolt serrated flange nut can be used first to clear it off and then some Vaseline or proper contact grease used to avoid galvanic corrosion afterwards.

 

[Wilko]

Hi - re the carport bonding to the house PME MET vs it’s own TT - there is always a risk introducing two systems if they can be simultaneously touched. If there’s an outside light on the house adjacent to the carport etc. If the carport is adjacent/attached to the house and it’s just an outlet planned (for a vacuum cleaner for example) I’m thinking I’d just extend the house installation earthing system in the usual way and probably not bother to bond the carport. Am I a bad person?

 

[Strima]

If you're getting 150 ohms from the steel work one rod isn't going to be much better.

 

[pc1966]

Some useful feedback there! Is there any risk for touching the two system simultaneously?

If you do go down the TT route then you don't need 10mm bonding, only 4mm or phase size (which ever is greater), and 542.2.2 allows foundation structures to be used, though 542.2.4 has the proviso they are deep enough so drying/freezing causing a rise in impedance is not a risk.

 

[marcuswareham]

Are the posts all bonded together? I'm guessing they would be bolted to metal beams, etc, but that may or may not be giving a good resistance between them. Either way as you are seeing < 200 ohms then it would normally be considered good enough for a TT earth rod.

The posts are only joined via timber beams across the top of them, so they would all need bonding in that case, one of the posts is about 70cm away from the gas pipe which is bonded to PME and enters the house about 1.5m away above the soil.

I have tested a spare rod in the garden, about 1m away from potential PME bonded stuff in the ground. I get about 60ohms from it, doing an earth loop impedance test in the same was as conducted on the posts.

Hi - re the carport bonding to the house PME MET vs it’s own TT - there is always a risk introducing two systems if they can be simultaneously touched. If there’s an outside light on the house adjacent to the carport etc. If the carport is adjacent/attached to the house and it’s just an outlet planned (for a vacuum cleaner for example) I’m thinking I’d just extend the house installation earthing system in the usual way and probably not bother to bond the carport. Am I a bad person?

There is not much potential of touching outside lights on the house and the carport at the same time. (although should outside lights on PME supply really be class I, like you see everywhere, surely that is a very large shock risk from touching outside light while standing on wet garden grass etc).

Extending the PME installation might be the best way, although I know class I appliances will be used in the carport and possible in contact with soil/grass on a rainy day

So not sure whether it is better to go TT or use the Matt-e device

Thanks Marcus

 

[pc1966]

Why not convert your house to TT supply and then you can bond the car port beams, etc, all together with your main earth & service pipes, etc?

Of couse said service pipes might be bonded to neighbours PME...
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I guess it would cost you a 100mA type-S delayed RCD as incoming protection and an identifiable earth rod, but it should be a reasonable solution to this.

 

[pc1966]

These days many service pipes will be plastic in the street, so if you are unsure and can safely/easily manage it then you could disconnect your PME earth and check the impedance between that and the rest of your home's earth (i.e. including the service pipes). If it is 1 ohm or so then obviously they are shared/bonded at neighbours!

 

[davesparks]

I have tested a spare rod in the garden, about 1m away from potential PME bonded stuff in the ground. I get about 60ohms from it, doing an earth loop impedance test in the same was as conducted on the posts.

Is that a single 4' rod giving you 60 ohms? If it is then you should be able to get a pretty good Ra when you've gone up to 2x 8' rods.

Have you considered adding supplemental earth electrodes connected to the MET to gaurd against the risks?

 

[Ian1981]

Some useful feedback there! Is there any risk for touching the two system simultaneously?

If you do go down the TT route then you don't need 10mm bonding, only 4mm or phase size (which ever is greater), and 542.2.2 allows foundation structures to be used, though 542.2.4 has the proviso they are deep enough so drying/freezing causing a rise in impedance is not a risk.

TT requires a minimum of 6mm csa for a bonding conductor. No less that’s half the size required for the earthing conductor and no less than 6mm copper

 

[pc1966]

TT requires a minimum of 6mm csa for a bonding conductor. No less that’s half the size required for the earthing conductor and no less than 6mm copper

Yes, should have looked it up!
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Have you considered adding supplemental earth electrodes connected to the MET to guard against the risks?

With potentially a hundred of more amps to play with you would be hard pushed to keep the potential down. You might at least keep the potential gradient around the car port low enough which I guess is the critical thing.

In that case the earth between house and garage, etc, might need to consider the worst-case current that might flow, though if we are looking at something in the 10 ohm region then 6mm or more should be adequate.

 

[marcuswareham]

Why not convert your house to TT supply and then you can bond the car port beams, etc, all together with your main earth & service pipes, etc?

Of couse said service pipes might be bonded to neighbours PME...
[automerge]1586529768[/automerge]
I guess it would cost you a 100mA type-S delayed RCD as incoming protection and an identifiable earth rod, but it should be a reasonable solution to this.

I like this option as I am not the biggest fan of TN-C-S. However I am not Part-P I work setting up temporary power for events. I am not sure if changing the house supply to TT is notifiable work, or if you need to notify the DNO.

The water pipe is definitely plastic, although the gas pipe if definitely copper, this runs underground to gas bottles (out in the countryside), where the gas bottles are stored is shared with 2 other houses (presumed to also be PME)

I have considered adding supplemental earth electrodes connected to the MET to guard against the risks, however I am very limited on outside space to do as such unfortunately,

Thank you all very much for the replys. So I figure that the carport being TT and having all of the posts bonded is no worse than the carport not having power supply inside it, as posts are close to PME earth in the ground.