When to use your own earth rods

[cojmh]

This is a whole other can of worms. If you have a TN-C-S supply you are not supposed to "export" the earth connection to a separate outbuilding because of the risks involved in a broken neutral. Under most circumstances it should have its own earth rod. However if you have a TT supply you can. This may be a better answer anyway if you are planning a battery installation as it covers the island mode situation as well.

Regulation 551.4.3.2.1 of BS 7671 states that, in TN systems, generators operating as a switched alternative to the public supply cannot continue to rely on the distributor’s means of earthing.

Best to look up chapter and verse on the IET web site e.g. for a start https://electrical.theiet.org/wiring-ma ... e-systems/.

You will see your earth rod is then also connected to the incoming grid earth (which is what I think I was also told by a man from WPD, now NG). So not a true TT supply in that case and it also runs the risk of your neighbours' fault currents going through your rod. Don't know if you are then allowed to export the earth to an outbuildng. If not then you can't have the extra earth wire you mention and should interrupt the sheath connections on the SWA too.

You will also see that EV charging point is shown as also requiring an earth rod of its own. But some EVCPs e.g. the zappi have better earth fault detection and don't require this IIRC.

Total minefield IMO.

From what you are saying, and I need to do more research - thanks for the link:

The solution would appear to be to forget the grid Earth and go to my own earth rods which are kept independent from the grid earth.

I guess this also means that I need to have my consumer units checked to make sure they will work properly with earth rods instead of grid earth.

As you say it seems to be a mine field

[Countrypaul]

As part of the works I am installing an underground conduit to an outbuilding which is about 20M away from the consumer unit (potential place for more solar in the future and a reasonable workshop). The conduit will contain 4 cables - possibly 5.

• 6mm2 armoured Twin and Earth

• 2.5mm2 armoured Twin and Earth

• 2 x CAT6A ethernet cables

Should I run an additional earth only cable in this conduit to place another earth rod near the out building? If so what size would people recommend?

Thanks

Running Ethernet cables close and parallel to Power cables is likely to be troublesome due to the introduction of signals into the ethernet from the changes in the power cables. Normal rules say leave 8 inches between the power cables and ethernet cables and better still 12 inches.

If you are only putting in the one conduit then consider using fibre rather than CatX for ethernet as with such a short distance you might get away with a 25m patch cable (around £20) plus two fibre to Cat6 converters (around £25 ea) depending on what other kit you have.

[cojmh]

Running Ethernet cables close and parallel to Power cables is likely to be troublesome due to the introduction of signals into the ethernet from the changes in the power cables. Normal rules say leave 8 inches between the power cables and ethernet cables and better still 12 inches.

If you are only putting in the one conduit then consider using fibre rather than CatX for ethernet as with such a short distance you might get away with a 25m patch cable (around £20) plus two fibre to Cat6 converters (around £25 ea) depending on what other kit you have.

Thanks for the thoughts.

Unfortunately I am only able to get one conduit in .... the Cat6A cables are shielded but not sure if that is enough.

the intention was to use one of the CAT6A cables to extend CT clamps - so that any remotely sited inverters can have a CT clamp that sits on the main incoming grid connection. Or at least this was how myEnergi suggested that it was done (up to 100m).

I might try the other cable as a data connection just to see if there is a performance drop .... for my own interest.

Given the effort I might also drop in a fibre cable as you suggest because all of the effort is in the ground work and I don't want to be doing this again!

so thanks for the suggestion

[sharpener]

From what you are saying, and I need to do more research - thanks for the link:

The solution would appear to be to forget the grid Earth and go to my own earth rods which are kept independent from the grid earth.

I guess this also means that I need to have my consumer units checked to make sure they will work properly with earth rods instead of grid earth.

As you say it seems to be a mine field

Yes, converting to a true TT supply avoids a lot of potential problems (sorry!) esp importing neighbours' fault currents but as you imply you may need to alter the protective devices in your CU in order to comply.

Also there is a further wrinkle - if you have an SPD you need to swap the N and E connections so the common terminal goes to the supply N not E. This is bc otherwise a L-E fault inside the SPD will probably not blow a TT incoming supply fuse to clear the fault as the earth rod will likely be too high an impedance.

the intention was to use one of the CAT6A cables to extend CT clamps - so that any remotely sited inverters can have a CT clamp that sits on the main incoming grid connection. Or at least this was how myEnergi suggested that it was done (up to 100m).

I found the harvi does this quite well, I have ~30m of 4-pair phone cable to a harvi and a wireless link from there about 10m across to the zappi on the garage. And then a different link back from the zappi to my wifi. (You are not supposed to use a wireless link as part of a G100 current limiting scheme though). The harvi didn't register small currents properly (we are looking for only 100W of export here) but fortunately I had 12V d.c. available from the rainwater harvesting system control wiring on the other pairs. Alas I had to rectify it as I had aready used polarity reversal to signal low tank level from garage to the house!

[Stinsy]

Pros and cons!

Generally a supplier provided TN-C-S PME earthing arrangement is preferred if available. The primary benefit of this is that you’re guaranteed a really low earth loop impedance. This results in reliable activation of the overcurrent protective device in the event of a fault. It is the overcurrent protective device that is the means of circuit protection. RCDs are not for “circuit protection” for as they aren’t reliable enough, they instead provide “additional protection”.

You cannot expect an earth rod to have an earth loop impedance low enough for a fault to trigger the overcurrent protective device. But RCDs don’t need a really great earth loop impedance to operate so 100mA time-delay RCDs are used in addition to standard 30mA RCDs to provide redundancy and mitigate the inherent unreliability of RCDs. However earth rods are tricky things. If the ground is frozen or dried out you might not have an impedance low enough to operate the RCD leaving you with no protection at all even if it tests fine while the spark is there.

On the other hand TN-C-S earthing systems are susceptible to PEN-faults. This is fine if you are inside your house and protected by the equipotential zone, or outside using double-insulated garden tools. However if you’re using a granny cable to charge your EV then the bodywork will be live in the event of a PEN-fault!

Thanks for the information - I think I understand most of it!

Would a solution (and best of both worlds) be that you connect grid earth to the earth rod(s) there by each has a backup .... if the ground is frozen etc. then the low impedance of the grid earth will cover you .... if the grid has a PEN fault then the earth rod will cover you?

Or am I talking rubbish?

You have opened a royal can of worms!

Unfortunately what you suggest is (or was) expressly prohibited.

Australia is the only country that has an earthing arrangement almost identical to our TN-C-S. There every house is required to have its own earth rod in parallel to the suppliers earth. The fear in the UK is that your earth rod could become the return path for your entire neighbourhood causing an overload of your earthing/bonding conductors and burn your house down (guidance note 8 has muddied the waters considerably on this prohibition with some horrible ambiguous wording).

Lots of people with on/off grid installations (with ICE generators being more common than batteries) have contactors that switch between the supplier’s earth and an earth rod when running on off-grid. However, if you disconnect from the supplier’s earth and use your own rod (either permanently or only when running off-grid) then you need to consider that your earth system will be bonded to your water and gas pipes. And your neighbour also bonds their supplier’s earth to the same water and gas pipe. Possibly defeating all the effort you’ve gone to.

JP has done some interesting yoooochoooob videos on this topic (or adjacent topics) I’ll try to find them later.

[Stinsy]

Here you go:

[cojmh]

Yes, converting to a true TT supply avoids a lot of potential problems (sorry!) esp importing neighbours' fault currents but as you imply you may need to alter the protective devices in your CU in order to comply.

I think this is the route I will have to go down - it is not much effort to do now as the ground is all up and open. This will allow me to get the infrastructure in place as cheaply and easily as possible.

Also there is a further wrinkle - if you have an SPD you need to swap the N and E connections so the common terminal goes to the supply N not E. This is bc otherwise a L-E fault inside the SPD will probably not blow a TT incoming supply fuse to clear the fault as the earth rod will likely be too high an impedance.

I do have an SPD device - it is external the consumer unit (as I have more than one consumer unit) so this should be easy to swap over

I found the harvi does this quite well, I have ~30m of 4-pair phone cable to a harvi and a wireless link from there about 10m across to the zappi on the garage. And then a different link back from the zappi to my wifi. (You are not supposed to use a wireless link as part of a G100 current limiting scheme though). The harvi didn't register small currents properly (we are looking for only 100W of export here) but fortunately I had 12V d.c. available from the rainwater harvesting system control wiring on the other pairs. Alas I had to rectify it as I had aready used polarity reversal to signal low tank level from garage to the house!

Was the problem with the Harvi (which I also have - but not installed yet) that you were running 12V DC in one of the other twisted pairs in the same cable?

[cojmh]

You have opened a royal can of worms!

This is unfortunately quite common with me ..... lots of cans of worms!

Unfortunately what you suggest is (or was) expressly prohibited.

I shall avoid that then ..... it was just a thought.

Australia is the only country that has an earthing arrangement almost identical to our TN-C-S. There every house is required to have its own earth rod in parallel to the suppliers earth. The fear in the UK is that your earth rod could become the return path for your entire neighbourhood causing an overload of your earthing/bonding conductors and burn your house down (guidance note 8 has muddied the waters considerably on this prohibition with some horrible ambiguous wording).

Lots of people with on/off grid installations (with ICE generators being more common than batteries) have contactors that switch between the supplier’s earth and an earth rod when running on off-grid. However, if you disconnect from the supplier’s earth and use your own rod (either permanently or only when running off-grid) then you need to consider that your earth system will be bonded to your water and gas pipes. And your neighbour also bonds their supplier’s earth to the same water and gas pipe. Possibly defeating all the effort you’ve gone to.

As part of this mammoth rearrange we are moving the incoming gas connection and I believe the new pipe will be plastic and we are going to be replacing some of the incoming water main too as it would appear the previous builders (seemingly a bunch of cowboys) have encased the current copper water main in the concrete foundations. I believe the new pipe will be MDPE.

So hopefully this will mean that I am isolated from any potential neighbours through the pipework.

JP has done some interesting yoooochoooob videos on this topic (or adjacent topics) I’ll try to find them later.

Thank you

[sharpener]

Would a solution (and best of both worlds) be that you connect grid earth to the earth rod(s) there by each has a backup .... if the ground is frozen etc. then the low impedance of the grid earth will cover you .... if the grid has a PEN fault then the earth rod will cover you?

Or am I talking rubbish?

You have opened a royal can of worms!

Unfortunately what you suggest is (or was) expressly prohibited.

[/quote]

I was brought up to think it was a bad idea if not actually prohibited, but it is unavoidable if you follow the guidance in Fig. 3 of the IET article from my link upthread.

I have that arrangement and was actually told by WPD some years ago I was allowed to disconnect their supply earth, but I decided not to do it for reasons that now escape me. It would have been better from the pov of the outbuildings but there was some disadvantage I cannot now remember.

Maybe it was the requirement to protect all TT final circuits with an RCD, as the original PV and (dedicated and labelled) freezer socket are by design on a mini-CU with only a main switch to avoid nuisance tripping if we are away. There was a supply surge which had taken out a few things including the PV inverter, which was fixed under warranty but I lost several weeks' generation in high summer.

Personally I would not want in any event to rely on a contactor guaranteeing earth continuity. The N-E bond relay is perhaps not so critical.

[sharpener]

I found the harvi does this quite well, I have ~30m of 4-pair phone cable to a harvi and a wireless link from there about 10m across to the zappi on the garage. And then a different link back from the zappi to my wifi. (You are not supposed to use a wireless link as part of a G100 current limiting scheme though). The harvi didn't register small currents properly (we are looking for only 100W of export here) but fortunately I had 12V d.c. available from the rainwater harvesting system control wiring on the other pairs. Alas I had to rectify it as I had aready used polarity reversal to signal low tank level from garage to the house!

Was the problem with the Harvi (which I also have - but not installed yet) that you were running 12V DC in one of the other twisted pairs in the same cable?

I don't think so. From reading around the myenergi forum I think my conclusion was that at sub 1A current levels it took too long to harvest sufficient power from the available CT voltage to track the consumption fast enough. Might have been the sheer length of the CT cable I suppose but that was within published limits.

I originally planned to have the harvi next to the CU which is at the diagonally opposite corner of the (stone) house from the garage but the comms were not good enough. Was lucky that there was the rainwater wiring already going to the garage and doubly lucky I could tap off the 12V or it would have suddenly become a much bigger job - which happens! Also lucky as I had used two pairs commoned to give less voltage drop to the solenoid valve in the house but it was not in fact necessary as it works reliably at 9V(!).