Earth Rod

[rick]

2 parts to an earth rod Q.

On advice on another forum - I need to install an Earth rod when a new TT circuit is installed.

What size earth rod do I need ? Looking at some of the the information I have it seems to be that an 8' rod is needed ? ..... how is this achieved if you use for example the standard TLC 3/8" (9mm) rod which is only 1.2m long ? It has no threaded section so you can't join them together like you can with 5/8" rod Or is 4' (1.2m) long enough for domestic install ?

The 2nd part is where every post I have found gives a different answer - the protective earth cable from the rod (in its inspection box) will be

6491X single cored green/yellow PVC insulated, and carried in flexible conduit to the termination point about 2m in total. So at no time is it in contact with the ground though for about 1m the flexible conduit is buried, as it routes to the termination point. I see cable size stated from 2.5 to 16 mm2 The TT circuit it is protecting is 6mm2 32A (via SWA underground cable) RCD protected What is the correct size earthing conductor to use ?

Or as some time in future there may be a need to export PME ... should I use 10mm ? as that is minimum.

[John Rumm]

ok

It will largely depend on your local soil conditions. In these parts (Essex clay soil) 4' will get you down into permanently damp soil and a Ze of under 10 ohms typically.

In different soil you may need longer.

..... how is this

You can use additional parallel electrodes if you space them out enough so that they are not in each others resistance zones.

Indeed, so if you need more than 1.2m, the opt for the thicker rod. (you may have trouble driving the thinner rod to adequate depth without it bending if you could extended it anyway.

IME yup, but YMMV

The decision list is:

So that sounds like buried, protected from corrosion, but no mechanical protection, so 16mm^2

(assuming that your Ze is 1 ohm or greater)

You don't strictly speaking need a dedicated earth conductor. You would normally just have 2 core SWA, with the armour connected at the source end to the supply earth (to ensure the cable has fault protection), and then the armour would be isolated at the far end (plastic gland), and the TT Earth connected to the installation earth instead.

Depends on if you need to export the equipotential zone as well. If there is no access to anything local that could be at earth potential, then you may not. If there is local access to an independent earth (or extraneous metalwork that would normally need to be bonded) then your CPC will need to meet the requirements of a main bonding conductor as well and that would mean 10mm^2 minimum.

[rick]

Hi John .... there is an existing RCD protected 40A TT cct to a garage (only really uses a couple florescent lights)... underground cable armours earthed and house end but not at garage. The consumer unit in garage picks up local earth via earth rod.

I'm adding a new 32A TT circuit from the garage out to decking for a Hot Tub. Was advised not to export earth from garage, but to install new earth rod for the new TT circuit (hence my Q on earth rods)

A sketch of what I am doing is attached here:

Welcome your input.

[John Rumm]

So the supply is TN-C-S/PME, but you are not exporting that at all by the looks of it? (one common approach would be to use that to earth the armour of the first submain as far as the garage).

It looks like you have one 30mA trip RCD at the head end of the garage feed, and are relying on that for all subsequent protection. While this will work, it does leave scope for any fault to create nuisance trips with a wide effect since you have no discrimination. So damp in a JB at the hot tub, will trip the RCD, and lose lighting in the garage for example. This may not matter at the moment - but the significance would depend on what the garage is used for. (e.g. if used as a workshop, or to power a freezer, then it would be more of an issue).

Not completely clear from the diagram why there is a separate switch fuse feeding a TT garage supply? You could for example have the switch fuse sized appropriately to give fault and over current protection to the submain, and export the PME earth with it to as far as the garage. No need for a RCD at the head end then. Isolate the PME earth at that end, and then do TT at the garage CU. That could then be either a split load or all RCBO CU so that you have discrimination between lighting and power in the garage as well as the feed to the hot tub.

(again, only you can decide how much this matters in practice, but limiting the effects of faults is generally "good practice". Physical layout also plays a part - where is the easiest place to walk to if you need to reset something?).

[rick]

The house supply is currently not PME ... it's on a temporary supply (TT) It will become PME next month.

I was advised that exporting PME circuits outside is an issue .. and many people having problems with EV charging.

However it could always be changed to use PME if required, but was told its best to have external circuits sty as TT.

On RCD - If I do get nuisance trips its as easy to walk to garage to reset as to house, so don't think that will be an issue.

[John Rumm]

Exporting PME does have its issues... especially in cases where you have access to an independent earth at the far end - say an outbuilding that also has a water supply in metal pipework or a greenhouse with exposed soil. Then you would need to either make the outbuilding part of the house equipotential zone. So in those cases making it TT is usually simpler, sice doing the former can range from difficult to impossible.

In cases where you use the PME earth on the armour of the submain - that is not really exporting the earth. It is not available to touch at any point, and it will only be used to protect the submain, not the installation.

Yup that is an added new complication. Many of those are made TT, and often need a different type of RCD. (although modern fixed chargers tend to include many of the required protections anyway)

Yup, in which case if it were my install, I would stick a type S 100mA trip RCD at the switchfuse end - that will give earth fault protection for the both submains inside the required disconnection times. However the time delay built into the type S RCD will allow it to discriminate with downstream RCDs.

You could then have a split load CU in the garage, with separate RCBOs for power and lighting, and also the hot tub.

(The head end RCD will do earth fault protection for the second submain as well if you wanted the RCD/MCB for the tub to be at the far end)

That way you have submain protection at each stage in case you manage to get a spade through one. Then separate shock protection with discrimination at both tub, and garage circuits.

Fair enough. So stick the RCD/RCBO for the tub in the garage CU.

[ARW]

Nice picture and very helpful.

Adding a new earth rod to the hot tub circuit that is isolated from the garage earth seems to be of little benefit.

I believe John has pretty well covered everything else.

[Brian Gaff]

Surely it needs to be low impedance as possible and hence as short as you can and impervious to being damaged. Brian

[Tim Streater]

Earth Rod? Surprised someone didn't do that years ago.

[rick]

That is how garage is currently supplied The installation getting its earth from a local earth rode. The comments were that I should not extend the earth from the garage .. leave that all 'as is' and install the new 32A circuit as its own standalone TT system.

Again only connect armour wires at one end to the earth (which comes from a new earth rod) not to earth of garage installation. There is no other metal pipes or services in the area. The thought was that I have armour wires insulated at Garage end, and grounded at Hot Tub end to local earth rode ..... that allows me to get new earth rod at least 5m away from Garage Earth rod...... as eart

[rick]

More rod in contact with ground the lower the impedance I would assume.

[John Rumm]

Compared to the typical impedance of the earth itself, the cable connecting it usually makes a negligible difference. (i.e. if the rod is

10 ohms, a few milli-ohms extra from the wire is not going to matter.