I watched the YouTube video by John Ward about EV chargers and earthing.
I saw a 2nd video by the same person in connection with earthing of outbuildings.
See
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I happen to have a 3 core 10mm2 SWA run between the house integral garage and the metal wood shed and metal framed greenhouse.
This has yet to be connected up at both ends.
I was going to go down the TT earth rod route but John Ward in his video is saying that you can export the earth from the house to the outbuilding provided it is 10mm2 or higher.
He also seems to imply that exporting the earth is a better and safer option than using an Earth rod.
Would Adam Wadsworth, John Rumm et. al. like to comment?
Think about what happens if you short line to earth at the consumer unit.
What you want is for the fault current to blow the supply fuse - which might be 100A (but need a higher current to blow rapidly). That depends on having a low enough loop impedance from the transformer, through the live circuit, to the short, then around the earth/neutral circuit[1] back to the transformer, such that the current that can flow is enough to blow that fuse in a short enough time. If your earth impedance is too high, the fuse won't blow. On TN-S and TN-C-S loop impedance should be a fraction of an ohm.
Downstream of the CU the same applies, although there may be a low-rated MCB in the circuit instead. So you might have say a 32A type B MCB, which is rated to trip at 5*32A = 160A. You need a low enough earth impedance to carry 160A. Going to 10mm2 will reduce the impedance and increase the fault current, meaning the MCB is more likely to trip.
If you're on TT with earth rods the earth impedance depends on the state of the ground, the weather etc, but can be tens of ohms. That means a very low fault current (2A say) can flow that won't trip the MCB. This is why on a TT system you'd need an RCD to actively detect that small fault current and trip where the MCB won't notice.
So a 10mm2 earth conductor to the house is preferable to an earth rod, but both are helped by having an RCD at the outbuilding end. An RCD can't be the only means of protection, however, which is why the earthing matters.
I think.
Theo
[1] exact configuration depends on whether TN-S or TN-C-S
If you get into the situation of needing to export the house's equipotential zone to the outbuilding (usually when you have TN-C-S earthing in the house), then the earth connection needs to be large enough to function as a main bonding conductor, hence the 10mm requirement.
Exported earth when appropriate is usually less costly and less complex. There is also less need for ongoing routine testing. So its often a good choice.
However there are cases where it can be a less ideal choice. JW covered the case where it gets too expensive, but there is also the case of a building like a greenhouse where it is in effect impossible to maintain a proper equipotential zone, since there is ready access to a local earth (i.e. you are likely standing on it!) that you can't bond to your equipotential zone.
You then have the risk that under fault conditions the exported TN-C-S earth could be exporting a dangerous potential to the greenhouse.
I think the first question is what earthing system does your house have?
My earthing system at the meter cupboard is a TN-C-S system.
the wood shed is made of corrugated metal panels and the greenhouse is aluminium framework with glazing. So I would be looking to earth bond these in any case.
I have already got both an earth rod, a reel of 10mm single core earth wire and of course the already buried 3 core 10mm2 SWA cable. The length of this SWA is 20m.
The plan was to connect this to a 32A MCB in the house consumer unit and have all RCBO sub-consumer unit in the wood store to supply both the wood store and adjacent greenhouse.
Both are sited on a 100 mm sub base of MoT1, 50 mm of sharp sand and 50 mm x 600 mm x 600 mm paving slabs.
(all my greenhouse plants are in pots and there is an automatic watering system which is fed by 25 mm MDPE from the house so no metal pipe.
In that EV vehicle video, there was a discussion about putting in a earth rod and connecting that to the MET.
If it is permitted, I have no personal objection to putting an earth rod & exporting the MET earth for the eventuality that:
"that under fault conditions the exported TN-C-S earth could be exporting a dangerous potential to the greenhouse."
putting an earth grid over 20 m2 of my garden does not appeal to me!
The relatively high earth loop impedance means that any fault current is limited by that impedance. So if the earth loop impedance is 10 ohms the maximum earth fault current is 24(ish) amps. Which a 32 A MCB isn't going to worry too much about...
The RCD isn't detecting the fault current as such it's detecting an imbalance in the currents flowing in the L & N wires through it. The assumption is the imbalance current is going somewhere it shouldn't, like directly to earth be that an exported one or earth rod or through someone to earth.
Isn't there some reg that says any socket likely to have an appliance connected that is used outdoors needs to be RCD protected?
And of course there is the problem of the exported earth riseing above real earth under fault condtions. That 160 A guaranteed 32 A MCB trip current will produce 80 V across 0.5 ohms. That's enough for "quite a tingle". OK 0.5 ohms equates to 250+ m of 10mm^2 (assuming perfect connections) but you get the picture.
So it sounds like with your greenhouse, you should be able to include all its extraneous conductive parts into the bonding should you want.
There is a slight risk still from the frame of the greenhouse itself though since someone standing outside of it, has access to an independent earth and the exported TN-C-S earth which I presume is accessible without the need of a tool.
(personally I would use the TN-C-S earth to protect the submain as far as the greenhouse (and other metal buildings), but isolate it there and make those TT).
If you add an earth rod to a PME earth, then it just becomes another of the multiple earth connections.
I was left a little bemused by the video in that it concluded there was no ideal solution to a TN-C-S setup except for the for the impractical 20m2 gr id only a least bad setup namely to export the earth. I have an interest in this question as I am hoping to take delivery of a PHEV at the end of the month so an EV charger is on the agenda. The plan is to house the charger i n the garage which at present is connected to the supply by 10mm2 T&E using the CPC as the exported earth inside the garage is a CU with an RCD for a main switch. The garage is not attached to the house but is only 1.2m away and the supply is run between buildings in a conduit attached to a beam abo ve a gateway. It did not seem worthwhile using SWA and burying for such a s mall distance since the longest portion of the supply cable runs under the floor of the house to the CU.
I take it from the video that to have a charger I will now have to run a se parate 10mm2 earth single back to the main CU. So what do I do in the garag e leave the earthing arrangement for all the other circuits as is and use t his 10mm2 bond exclusively for the charger or should I bond all the garage circuits to this earth?
I watched another video by an electrician fitting a charger to the outside wall of a house, he could easily have exported the earth with the CU mounte d on the same wall as the charger with the supply passing from the CU outsi de through a wall perpendicular to the fixing wall. He bonded the SWA at th e consumer end but not at the charger and used a separate earth rod for the charger which was some metres from the charger but did not look like 5m to me and was a matter of inches from the house wall. According to the JW vid eo that was not the best solution. Confused yes I am!
Look at videos on the Zappi charger either by Artisan Electrics, or eFixx, that doesn't need an earth rod (maybe it does still need 10mm bonding? I don't know)
I plan to change the 32A RCBO in the main CU to a 32A MCB
As for the Sub CU, I am going to put in RCBOs with functional earth which as I understand it gives a bit more protection against a missing neutral or a missing earth?
There is a concrete post and wood panel fence on two sides, then there is 10m2 of slabbing on eon e side.
On the 4th side there is 90 mm wide slabbing before meeting the lawn.
As I understood from John Ward, the exported earth has a lower impedance than an earth rod....
I seem to recall reading somewhere ages ago that connecting a PME earth and a TT earth together was not a good idea and that one should choose between one or the other.
It does - it allows it to trip on more fault conditions than just a straight current imbalance.
90mm (just under a brick's width)?
Either way, it sounds like your setup is somewhat more substantial than many a greenhouse sat directly on damp soil. So the risk of having the exposed PME earth is going to be vanishingly small (and in reality there are plenty of exported PME earths in garden sockets up and down the land that don't typically kill any one. (or at least not due to the earthing!))
It does (usually), although with TT you rely on the RCDs to handle earth faults, so you don't actually need a particularly low earth impedance for the system to work.
Well PME has multiple earth connections all along the LV side of the distribution network (its one of the ways they try to reduce the chances of there ever being an interruption in the PME conductor). So adding additional ones does not combine TT and PME - it just makes the PME network slightly bigger.
(there is an argument that if your property did get a PEN disconnection along with a lump of the neighbourhood, you may not want your earth spike serving the whole neighbourhood!)
That is a good point, I have on the external walls of the main house 7 separate external IP67 rated sockets.... 4 are singles and 3 are doubles.
I was careful to put the garden taps below the external sockets amd these 7 sockets feed back to the garage to switched FCus which in turn then connect to a ring main supplied by a 32A RCBO. The idea being two fold, I could isolate faulty sockets and if a fault occurred, no other internal electrics were affected.
Which is what I was hopign a RCBO with a functional earth might help with.... by tripping off in the case of a broken neutral/PEN
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