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 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...
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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.

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.

 

[pc1966]

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.

That would be my take on things, but worth checking.

If you only need a small amount of power then the simplest option is a TT supply fed from a 13A plug in the house. Not too expensive to put in a small garage CU for it, and no Part P to worry about. Ultimately the metalwork is no less safe than it currently is in respect to the risk from nearby PME faults.

 

[marcuswareham]

That would be my take on things, but worth checking.

If you only need a small amount of power then the simplest option is a TT supply fed from a 13A plug in the house. Not too expensive to put in a small garage CU for it, and no Part P to worry about. Ultimately the metalwork is no less safe than it currently is in respect to the risk from nearby PME faults.

Yes I was going to spur out of C/U on one of the socket rings and do a minor works cert for it.

I might put a new circuit in and notify building regs, but shall see.

Just can't seem to get a straight answer from the council's documentation about if changing from TN-C-S to TT would be notifiable

Yes think TT is the way to go, getting that earth rod as far away from the gas pipe as I can.

Thank you very much for your advice,

 

[Wilko]

Hi - for what it’s worth, my understanding is a change to the installation’s earthing arrangements is not a notifiable activity in England.

 

[pc1966]

I guess if your CU is already all-RCD then it is simply a matter of changing how the MET is connected to "earth"

 

[shaun1]

If the water pipe is plastic then would it help to change the gas pipe to plastic also (even just a small section where it enters the house?

 

[westward10]

You cannot have a plastic gas pipe entering a property, think about the risks.

 

[marcuswareham]

I guess if your CU is already all-RCD then it is simply a matter of changing how the MET is connected to "earth"

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.

They are all rated at 30mA and I have done disconnection time on all of them and they are within spec. If changing to TT would I not also need a 100mA RCD for the whole system. Also what to do with the DNO (TN-C-S) earth from the service head just terminate it?

 

[pc1966]

Yes, if you have any non-RCD circuits at all then you would need an overall RCD to meet disconnection times on them. To achieve discrimination with your 30mA ones it would have to be a higher current (e.g. 100mA) and a delay S-type.

As far as I understand it, you should not touch the DNO cut-out so if changing the cable I would simply put a terminal block or similar on the end removed from your MET to insulate it and label things so anyone in the future knows why it was disconnected.

 

[marcuswareham]

Yes, if you have any non-RCD circuits at all then you would need an overall RCD to meet disconnection times on them. To achieve discrimination with your 30mA ones it would have to be a higher current (e.g. 100mA) and a delay S-type.

As far as I understand it, you should not touch the DNO cut-out so if changing the cable I would simply put a terminal block or similar on the end removed from your MET to insulate it and label things so anyone in the future knows why it was disconnected.

Yes fair enough,


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

 

[Ian1981]

Yes fair enough,


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

Class 2 are non metallic.
opps 2 class 1 doh!!??‍