When to use your own earth rods

[cojmh]

I have to admit that having your own earth rod(s) instead of through the grid is a subject that really confuses me.

However as I am in the position that my concrete floor in the place I have consumer units etc. is currently dug up I am wondering if I should install my own earthing points?

The main reason that I am thinking about this is that my setup and future expansion would be capable of running whilst the grid is down and in the that situation is it sensible to to be relying on a grid earth - which may or may not be working in a grid down situation?

What do people think?

I am not an electrician - but generally able to grasp most concepts so laymans terms would be helpful to me.

Thanks

[AGT]

The rod connection needs to be accessible for testing purposes so best to take it outside and connect it with a 16mm2 earth conductor to the DNO earth

Rods can be screwed together to go deeper and linked together, but you want to avoid any spheres of influence for example a 2 m rod with you planning another 2 m rod should be 4m apart minimum.

Structural steel can be used with structural engineer sign off too?

[Marcus]

Well i wouldn't install earth rods 'indoors' under the floor anyway, if that's what you're thinking:-

1. The rod head and it's connection ought to be accessible for inspection/testing purposes.

2. The ground inside the foundations tends to be drier than outside so you are less likely to get a good (low) resistance.

If you're wanting to run a wire for the earth rod out through the wall during building works then yes, now's the time. 10mm2 min unless you provide mechanical protection. You might want to leave plenty of extra wire at the rod end as one rod isn't always enough and there's no point in putting two rods in the same place, so you may need to daisy chain them.

Edit: agt beat me to it so there's duplicate info.

[Krill]

I had an earth rod installed a good 15 metres from the consumer unit. The distance shouldn't be a problem but if the floor is up, we'll...

For context I've seen it in action once in 18 months, and to make the most of it, have a hybrid inverter. I made a mistake there, all separate units so the PV doesn't generate...

[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!

[AE-NMidlands]

having read and tried to understand all(?) the posts about the earths and other isolators needed if you are going to run off-grid when the mains has failed, I can't remember whether it is necessary to keep your earth isolated from the grid? i.e. not allow your earth to be linked to the incoming mains "earth" during a fault and when you are running in island mode.
I can understand the need to not feed power back to the mains when people might be working on it, especially as the grid "neutral" and earth are linked in places...

[cojmh]

Thanks for the quick response ....

A bit more info would probably help.

Part of the garage conversion is a small "plant" room inside the house which will house all of the house services and as such the floor will be plain concrete and accessible so that is why I was considering installing one of the in floor housings. But I take the point that the ground is potentially drier and hence not necessarily a good place for an earthing conductor.

That said - it will be just as easy to install a rod into the pathway next to the house as that is all dug up at the moment too.

A couple more questions if I may?

• Marcus - when you say mechanical protection do you mean armoured cable? Or some other form of protection?

• Is it OK to connect the earth rods to the grid earth? - I was under the impression it was either/or not both connected to each other?

• Can I run an earth cable out from the electric meter box down to a rod in the pathway then install a standard earth bar in the meter box to connect the earth rod to the house and the grid earth?

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

[cojmh]

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?

[cojmh]

having read and tried to understand all(?) the posts about the earths and other isolators needed if you are going to run off-grid when the mains has failed, I can't remember whether it is necessary to keep your earth isolated from the grid? i.e. not allow your earth to be linked to the incoming mains "earth" during a fault and when you are running in island mode.
I can understand the need to not feed power back to the mains when people might be working on it, especially as the grid "neutral" and earth are linked in places...

I am the same I think in that I have tried to understand previous threads and to be honest had more questions ....

I had heard something similar hence the idea of my own earth .....

[sharpener]

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

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.