When to use your own earth rods

[Stinsy]

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.

My memory isn't 100% on the exact regs. However the story is told about a ham radio enthusiast who lived at a TN-C-S house and whose earth rod (that he used for radio purposes) ended up burning his house down because a PEN-fault caused it to become the return path for his entire neighbourhood!

I too bulk at the concept of a contactor being in an earthing system. And I also don't like the idea of engineering a system that is sometimes TN and sometimes TT. But people with "backup generators" do have contactors that switch between the supplier's earth and the rod depending if the property is on-or-off grid.

Just converting the whole thing to TT seems simpler. However (as mentioned above) you need to make sure you are actually TT and not accidentally TN via a neighbour's bonding to a common water/gas pipe or because your rod is too close to something bonded to the supplier's earth...

[Stinsy]

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

Metal gas/water pipes complicate things to do with earth rods for a whole load of reasons. Doing away with them get's rid of a whole load of problems. And simply knowing for certain where they are is a big step in the right direction!

If I were you, I'd convert the whole installation to TT. Just make sure to do as good a job as you can with the rod. It isn't difficult to use a coupler to double the length and an SDS drill makes short work of whacking it in (pro-tip use a sacrificial coupler for the whacking in). You can even put a 2nd rod in howevermany metres away from the first. Most sparks consider a Ze of anything below 100Ω to be "acceptable" and <60Ω to be good (and remember that if the ground is frozen or dried out the impedance will increase). You also need to bear in mind that your overcurrent protective devices will nolonger provide circuit protection so you'll need a 100mA time-delay RCD covering the whole installation in addition to the 30mA RCD on each circuit.

[Marcus]

Oh dear - i appear to have been left behind by this topic

Re: mechanical protection for the earth wire:-
i was quoting thd regs, but was thinking of a bit of tough pipe/conduit, but it really depends on applying common sense to the situation : if the rod cover is close to the wall and the wire is likely to remain clearly visible to anyone working there (e.g. concrete and little vegetation) then it's unlikely someone will damage it

If it's a longer run in a flowerbed that could get grown over, then you ought assume soneone might hit it with a brushcutter or spade and provide robust protection.

Also i may have been wrong with the 10mm2 minimum : if it's buried or might get buried then 16mm2 minimum without protection.

Yes earthing is a can of worms even before you factor in the peculiarities of a particular installation.

[cojmh]

Metal gas/water pipes complicate things to do with earth rods for a whole load of reasons. Doing away with them get's rid of a whole load of problems. And simply knowing for certain where they are is a big step in the right direction!

If I were you, I'd convert the whole installation to TT. Just make sure to do as good a job as you can with the rod. It isn't difficult to use a coupler to double the length and an SDS drill makes short work of whacking it in (pro-tip use a sacrificial coupler for the whacking in). You can even put a 2nd rod in howevermany metres away from the first. Most sparks consider a Ze of anything below 100Ω to be "acceptable" and <60Ω to be good (and remember that if the ground is frozen or dried out the impedance will increase). You also need to bear in mind that your overcurrent protective devices will nolonger provide circuit protection so you'll need a 100mA time-delay RCD covering the whole installation in addition to the 30mA RCD on each circuit.

Thank you .... so my plan is:

1. I will install an earth rod - Double length if I can - in the walkway down the side of my house. It will be about 1M to 2M away from the new National Grid meter box and I will run a 16mm2 earth cable from the rod in the ground and up the plastic sleave that the main incoming grid connection come through. The in ground box I am going to use is this one

2. Inside the meter box I will install an earth bar (I think that is what it is called) so that the house earth can be connected to the earth rod. The bar I intend to use is this one

3. Inside the house I will have the start point of the conduit that will run to the out building inside the conduit will be:

• 6mm2 armoured Twin and Earth - Connected to the main consumer unit

• 2.5mm armoured Twin and Earth - Connected to the Island mode consumer unit

• 2x CAT6A cable - Unconnected for the time being

• 16mm2 Earth cable - Connected to the earth bar in the house - will see if I can find armoured

• Fibre cables - armoured - Unconnected for the time being

4. At the Outbuilding I will drop another earth rod - double if I can - and install in the same enclosure.

5. Get the RCDs in the house consumer units checked and replaced if necessary


Hopefully this will provide as much protection as I can do reasonably and when the rods are put into the ground I know they are well away from anything else.

Thoughts are welcome as always.

[cojmh]

Oh dear - i appear to have been left behind by this topic

Re: mechanical protection for the earth wire:-
i was quoting thd regs, but was thinking of a bit of tough pipe/conduit, but it really depends on applying common sense to the situation : if the rod cover is close to the wall and the wire is likely to remain clearly visible to anyone working there (e.g. concrete and little vegetation) then it's unlikely someone will damage it

If it's a longer run in a flowerbed that could get grown over, then you ought assume soneone might hit it with a brushcutter or spade and provide robust protection.

Also i may have been wrong with the 10mm2 minimum : if it's buried or might get buried then 16mm2 minimum without protection.

Yes earthing is a can of worms even before you factor in the peculiarities of a particular installation.

I think for the earth rod nearest the house I will not worry too much about armoured cable - unless I get some for the main run to the out building.

With it's proximity to the incoming grid connection people will be wary of digging there (this is for after I have sold the house in the future) and in my case the walkway is going to be slabbed and sealed .... so I am not too worried about armoured cable.

The rod at the out building will follow the edge of the garden and is mostly under paving .... but I might go armoured if I can.

Thanks for the thoughts.

[Stinsy]

Metal gas/water pipes complicate things to do with earth rods for a whole load of reasons. Doing away with them get's rid of a whole load of problems. And simply knowing for certain where they are is a big step in the right direction!

If I were you, I'd convert the whole installation to TT. Just make sure to do as good a job as you can with the rod. It isn't difficult to use a coupler to double the length and an SDS drill makes short work of whacking it in (pro-tip use a sacrificial coupler for the whacking in). You can even put a 2nd rod in howevermany metres away from the first. Most sparks consider a Ze of anything below 100Ω to be "acceptable" and <60Ω to be good (and remember that if the ground is frozen or dried out the impedance will increase). You also need to bear in mind that your overcurrent protective devices will nolonger provide circuit protection so you'll need a 100mA time-delay RCD covering the whole installation in addition to the 30mA RCD on each circuit.

Thank you .... so my plan is:

1. I will install an earth rod - Double length if I can - in the walkway down the side of my house. It will be about 1M to 2M away from the new National Grid meter box and I will run a 16mm2 earth cable from the rod in the ground and up the plastic sleave that the main incoming grid connection come through. The in ground box I am going to use is this one

2. Inside the meter box I will install an earth bar (I think that is what it is called) so that the house earth can be connected to the earth rod. The bar I intend to use is this one

3. Inside the house I will have the start point of the conduit that will run to the out building inside the conduit will be:

• 6mm2 armoured Twin and Earth - Connected to the main consumer unit

• 2.5mm armoured Twin and Earth - Connected to the Island mode consumer unit

• 2x CAT6A cable - Unconnected for the time being

• 16mm2 Earth cable - Connected to the earth bar in the house - will see if I can find armoured

• Fibre cables - armoured - Unconnected for the time being

4. At the Outbuilding I will drop another earth rod - double if I can - and install in the same enclosure.

5. Get the RCDs in the house consumer units checked and replaced if necessary


Hopefully this will provide as much protection as I can do reasonably and when the rods are put into the ground I know they are well away from anything else.

Thoughts are welcome as always.

Sounds like a good plan. A few points:

• "Armoured" and "Twin and Earth" (T+E) are different things.

• I'd go for armored Cat5e or Cat6 cable

• I've never seen armoured earth cable. Just whack some unarmoured earth cable in twin-wall duct.

• What is the fibre for? You can reliably send 1000mbps through 300m of cat5e. Do you need 10gbps?

• You'll need a clamp for each earth rod.

• Don't wallop the actual earth rod, get a sacrificial coupler, screw that on and wallop that.

• You should have the RCDs in your house tested, they're probably fine though, the same ones are used for TN and TT systems. Worths swapping any "split" boards for all-RCBO while you're messing about though. However important point is that you'll need an additional "type s" RCD. You can swap out the main switch on your CU(s) for one of these. (Hope this makes sense)

[cojmh]

Sounds like a good plan. A few points:

• "Armoured" and "Twin and Earth" (T+E) are different things.

• I'd go for armored Cat5e or Cat6 cable

• I've never seen armoured earth cable. Just whack some unarmoured earth cable in twin-wall duct.

• What is the fibre for? You can reliably send 1000mbps through 300m of cat5e. Do you need 10gbps?

• You'll need a clamp for each earth rod.

• Don't wallop the actual earth rod, get a sacrificial coupler, screw that on and wallop that.

• You should have the RCDs in your house tested, they're probably fine though, the same ones are used for TN and TT systems. Worths swapping any "split" boards for all-RCBO while you're messing about though. However important point is that you'll need an additional "type s" RCD. You can swap out the main switch on your CU(s) for one of these. (Hope this makes sense)

Thanks for the points.

1. I see the confusion on T+E and armoured .... what I am actually getting is 3 Core like this one this one and this one. Apologies for the terminology mixup .... but I intend to use it like Twin and Earth.

2. I already have the Cat6A cable and was going to reuse what I already have.

3. OK ... I will stop looking for armoured earth cable. Will need to see what I can fit through the conduit protection wise.

4. Cat6A is also rated to 10gbps ..... the fibre is because I already have it and it is easier to put it in now while I am doing it rather than cursing not doing it a few years down the line. If I ever want to add a network connection this will not be susceptible to the power lines that are running in close proximity as pointed out by CountryPaul.

5 & 6 - Thank you for the advice that will save me time and heartache!

7. I need to digest this and do some research as I don't fully understand the nuances of the different things.

Cheers.

[John_S]

3. OK ... I will stop looking for armoured earth cable. Will need to see what I can fit through the conduit protection wise.

You may or may not be aware that armoured single core cable only comes with aluminium armour. Otherwise there is a magnetic field when current flows which messes things up.

[AGT]

16mm2 earth conductor is sufficiently robust that it didn’t need mechanical protection so can be direct buried, clipped etc

[cojmh]

• You should have the RCDs in your house tested, they're probably fine though, the same ones are used for TN and TT systems. Worths swapping any "split" boards for all-RCBO while you're messing about though. However important point is that you'll need an additional "type s" RCD. You can swap out the main switch on your CU(s) for one of these. (Hope this makes sense)

So I have been doing quite a bit of digging on this and I am beginning to understand what I might need .... but a couple of things are still confusing me.

1. There are various types of RCBO and I think it is type B that I want as more and more devices use inverter drives etc. and this is why type A is not necessarily the best fit for the job (according to a youtube video from what seemed like a knowledgeable person). However I keep finding Type B RCBO's that then say they are Type A RCBOs with a type B curve and I have to admit this has somewhat confused the matter for me. Can you have a type A RCBO with a type B curve and is this different from a type B RCBO?

2. I don't understand the "type S" RCD .... I can only seem to find info on Type AC, A, F, B and B+ ..... so could you explain a little more what the type S is as I might be looking for the wrong thing? I also am not sure why this is necessary as my (rather basic) understanding is that each RCBO effectively has and RCD in it .... so why is an additional one needed?

Thanks for any help and I apologise if I am completely missing the point.