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]

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.

Because I have no idea how deep the structural steelwork is. It is only a car port, not a major building, so any depth to the structure might only be 0.5m or less! So no idea if it would remain low R all year so might not meet regulation 542.2.4

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.

Regulation 411.5.3 Table 41.5
Note 2 in relation to RCD of 30mA and 100mA where the limits are 1667 and 500 ohms respectively states that "A value exceeding 200 ohms may not be stable"
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So yes if there was talk of a 300mA S-type RCD it would need to be lower (167 ohm or less, but earlier it was a 100mA S-type discussed), and it is always best to be as low as practical, but my point was if you discount all other fortuitous earthing due to buried metalwork that you have little or no information about, a known deep earth rod that meets the minimum for the RCD and is low enough to be considered stable stable would guarantee it meets the disconnection requirements.

 

[davesparks]

Because I have no idea how deep the structural steelwork is. It is only a car port, not a major building, so any depth to the structure might only be 0.5m or less! So no idea if it would remain low R all year so might not meet regulation 542.2.4


Regulation 411.5.3 Table 41.5
Note 2 in relation to RCD of 30mA and 100mA where the limits are 1667 and 500 ohms respectively states that "A value exceeding 200 ohms may not be stable"
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So yes if there was talk of a 300mA S-type RCD it would need to be lower (167 ohm or less, but earlier it was a 100mA S-type discussed), and it is always best to be as low as practical, but my point was if you discount all other fortuitous earthing due to buried metalwork that you have little or no information about, a known deep earth rod that meets the minimum for the RCD and is low enough to be considered stable stable would guarantee it meets the disconnection requirements.

Exactly, a value exceeding 200 ohms may not be stable, that does not in any way set maximum value of 200 ohms. Treating that as an upper limit, or even target value, is ridiculous, that's a very poor earth connection in my opinion.

Plus of course we don't know which brand of protective device will be used so we don't know the maximum specified resistance to earth for the devices.
A good example would be schneider RCBO's which specify a maximum resistance of 100ohms (I think that has more to do with them being French than a technical reason) so your 200ohms would not comply with manufacturers instructions.

 

[marcuswareham]

Plus of course we don't know which brand of protective device will be used so we don't know the maximum specified resistance to earth for the devices.
A good example would be schneider RCBO's which specify a maximum resistance of 100ohms (I think that has more to do with them being French than a technical reason) so your 200ohms would not comply with manufacturers instructions.

I do believe you and this to be true, however on an engineering viewpoint (not regulations). This does seem like an odd thing for an RCD manufacture to request as it is detecting a difference between L & N, if earth is there or not shouldn't bother the RCD? strange?
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I do believe you and this to be true, however on an engineering viewpoint (not regulations). This does seem like an odd thing for an RCD manufacture to request as it is detecting a difference between L & N, if earth is there or not shouldn't bother the RCD? strange?

Just read the datasheet for the BG RCD I will use for the car port, the word "earth" is mentioned once in it, and the words "maximum" & "resistance" are not in it at all. So I guess a need for 1667ohm earth rod resistance to insure touch voltages are under 50v for the short time before RCD trips (max disconnection time: 300 ms at I∆n & 40 ms at 5I∆n)

With me getting 40-50 ohms of a 4ft rod, I think I will be well in if I was too used extendable one

 

[pc1966]

This does seem like an odd thing for an RCD manufacture to request as it is detecting a difference between L & N, if earth is there or not shouldn't bother the RCD? strange?

The RCD looks at stray current to trip, but for it to trip quickly/reliably in the event of a fault it has to go somewhere to earth. Ideally not though someone.

If the installation's earth impedance is too high then touchable metalwork can become "live" to voltages that are dangerous to humans. Remember this is about the incoming RCD that could be 100mA (or less commonly for domestic 300mA or more) so they might not trip because of you getting a shock at a dangerous current!

If the earth rod impedance is low enough, and in this context that means 50V at trip current, then the risk to personnel from "earthed" metalwork that has a fault to live is acceptable low.

That upper safety value of 50V is what leads to the 1667/500/167 ohm limit for 30/100/300mA RCDs, but on top of that the rod impedance must be low enough all-year, and that is harder to be sure due to frost or drought, hence the IET adding the extra bit about not being happy with rods above 200 ohms, and the above discussion about going for rod(s) deeper than 4 feet.

 

[marcuswareham]

The RCD looks at stray current to trip, but for it to trip quickly/reliably in the event of a fault it has to go somewhere to earth. Ideally not though someone.

If the installation's earth impedance is too high then touchable metalwork can become "live" to voltages that are dangerous to humans. Remember this is about the incoming RCD that could be 100mA (or less commonly for domestic 300mA or more) so they might not trip because of you getting a shock at a dangerous current!

If the earth rod impedance is low enough, and in this context that means 50V at trip current, then the risk to personnel from "earthed" metalwork that has a fault to live is acceptable low.

That upper safety value of 50V is what leads to the 1667/500/167 ohm limit for 30/100/300mA RCDs, but on top of that the rod impedance must be low enough all-year, and that is harder to be sure due to frost or drought, hence the IET adding the extra bit about not being happy with rods above 200 ohms, and the above discussion about going for rod(s) deeper than 4 feet.

Got ya, so the recommendation for Schneider RCD is about keeping touch voltages down

yes i understand about rod resistance when ground dries up and freezes etc


on a tangent if you were to use an earth rod to measure voltage between PEN conductor and true earth such like mentioned in the electric vehicle charging bit of the regs (722.41.4.1 (iii)) then i guess the resistance of said rod could be higher then if it was being used at TT rod, as it hasn't got to sink any current just provide a reference to ture earth

 

[pc1966]

on a tangent if you were to use an earth rod to measure voltage between PEN conductor and true earth such like mentioned in the electric vehicle charging bit of the regs (722.41.4.1 (iii)) then i guess the resistance of said rod could be higher then if it was being used at TT rod, as it hasn't got to sink any current just provide a reference to ture earth

In one sense yes, you are only looking to see the potential difference between the "suppler earth" and true Earth, and test current could be very low. However, most chargers that use this method internally do periodic testing of the earth rod resistance in case of a fault (e.g. cable to rod breaks off, etc) and will isolate if the rod is too high an impedance.

I don't know if they are smart enough to also isolate if the rod resistance was very low (i.e. implying someone has managed to earth it to the PME structure)!

 

[marcuswareham]

In one sense yes, you are only looking to see the potential difference between the "suppler earth" and true Earth, and test current could be very low. However, most chargers that use this method internally do periodic testing of the earth rod resistance in case of a fault (e.g. cable to rod breaks off, etc) and will isolate if the rod is too high an impedance.

I don't know if they are smart enough to also isolate if the rod resistance was very low (i.e. implying someone has managed to earth it to the PME structure)!

Ahh yes good point about being smart enough to also isolate if rod was somehow connected to PME.

Such a device could be achieved by using a voltage sensing relay and contractor. so the voltage sensing relay will make the contractor disconnect if the voltage between PME earth and the earth rod is greater than what you set it to (50v). Would this be a legitimate thing todo, as you are using already existing products and getting the benefits of a low impedance PME earth but not having the risk of open pen while outside? like the carport?

 

[davesparks]

I do believe you and this to be true, however on an engineering viewpoint (not regulations). This does seem like an odd thing for an RCD manufacture to request as it is detecting a difference between L & N, if earth is there or not shouldn't bother the RCD? strange?

It depends on what type of RCD it is, an RCBO such as the schneider one I mentioned uses a functional earth connection whereas most RCCBs do not.

 

[marcuswareham]

It depends on what type of RCD it is, an RCBO such as the schneider one I mentioned uses a functional earth connection whereas most RCCBs do not.

Ok I see, yes like the MEM ones in my C/U also have funtional earth connections

 

[marcuswareham]

Another question on TT for carport

If there was to be a N to E fault in the carport the RCD might not trip due to the higher impedance of the earth rod, this would prevent the RCD tripping in a L - E fault as the fault can now return via the natural with less impedance. I also means that while the N to E fault exists your earth is connected to PME (the whole point of TT is to avoid that)

What is the best way to mitigate against this?

Thanks

 

[pc1966]

In most cases you design a system to be safe for a single point of failure.

So in the case of your TT car port, for example, you install it with adequate insulation and measures (such as grommets or cable glands) to avoid insulation damage, then you test it to make sure the insulation is all good to 500V, etc, before it is energised. Once energised you then test the RCD so you know it will trip as intended.

If you are really worried and/or have genuine reason to care about multiple faults there is no simple and reasonable approach for it, best case would be to go for the split 55-0-55 supply used for building sites, etc, so even under fault cases of exposed live parts it is still considered safe to touch under most dry-ish conditions.

The only other approach I know of are the 'IT' supplies used in hospitals there the supply is transformer isolated but not grounded as it has a continuous monitor of insulation resistance. But I don't think that is permitted generally for use and it is very expensive!

 

[marcuswareham]

In most cases you design a system to be safe for a single point of failure.

So in the case of your TT car port, for example, you install it with adequate insulation and measures (such as grommets or cable glands) to avoid insulation damage, then you test it to make sure the insulation is all good to 500V, etc, before it is energised. Once energised you then test the RCD so you know it will trip as intended.

If you are really worried and/or have genuine reason to care about multiple faults there is no simple and reasonable approach for it, best case would be to go for the split 55-0-55 supply used for building sites, etc, so even under fault cases of exposed live parts it is still considered safe to touch under most dry-ish conditions.

The only other approach I know of are the 'IT' supplies used in hospitals there the supply is transformer isolated but not grounded as it has a continuous monitor of insulation resistance. But I don't think that is permitted generally for use and it is very expensive!

Fair enough yes I understand about cable protection and doing full insulation test before it is commissioned.

Just wondered if the fault was introduced at a later point from a damaged bit of flex on an appliance etc.

I am not worried about it was just a hypothetical question as to how to mitigate under such circumstances.

And yes like you mention with a site transformer with 55v each side of the transformer mid point is some mitigation in an environment likely to cause damage to cables

 

[pc1966]

The general assumption is that faults should be rare, so a double-fault very unlikely.

I'm sure you are aware of PAT testing, that is really the point behind it: to inspect and test things to identify faults and fix (or discard) them before they become an accident.

 

[marcuswareham]

The general assumption is that faults should be rare, so a double-fault very unlikely.

I'm sure you are aware of PAT testing, that is really the point behind it: to inspect and test things to identify faults and fix (or discard) them before they become an accident.

I am all too aware of PAT testing and yes I hear what your saying, was just wondered if there was a way round it that I couldn't work out

I guess in the house on a TN-C-S supply if you had such a fault (say from a mouse in the loft chewing some cable (yes the mouse shouldn't be there and the cable should be protected if they are there)) then even if the RCD was not to work as you have such high earth fault currents then the MCB could work as a back up

 

[Midwest]

Another question on TT for carport

If there was to be a N to E fault in the carport the RCD might not trip due to the higher impedance of the earth rod, this would prevent the RCD tripping in a L - E fault as the fault can now return via the natural with less impedance. I also means that while the N to E fault exists your earth is connected to PME (the whole point of TT is to avoid that)

What is the best way to mitigate against this?

Thanks

The resistance would have to be in excess of 1667 ohms to prevent a 30mA rcd from operating (reg 411.5.3 and final circuit not exceeding 32A. Or is that not what your asking?