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]

On 25/07/2022 19:21, rick wrote:
> 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.

ok

> 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 ?

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
> achieved if you use for example the standard TLC 3/8" (9mm) rod which is
> only 1.2m long ?

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

> It has no threaded section so you can't join them together like you can
> with 5/8" rod

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.

> Or is 4' (1.2m) long enough for domestic install ?

IME yup, but YMMV

> 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 decision list is:

http://wiki.diyfaq.org.uk/index.php/TT_Earthing#Connecting_to_your_earth

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

(assuming that your Ze is 1 ohm or greater)

> The TT circuit it is protecting is 6mm2 32A (via SWA underground cable)
> RCD protected
> What is the correct size earthing conductor to use ?

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.

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

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.


http://wiki.diyfaq.org.uk/index.php/Taking_electricity_outside#Exporting_an_Equipotential_Zone

http://wiki.diyfaq.org.uk/index.php?title=Earthing_and_Bonding#Bonding_practice_and_technical_data

--
Cheers,

John.

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[rick]

On 25/07/2022 20:17, John Rumm wrote:

>
> 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.
>

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:
https://www.dropbox.com/s/vzspjva7cm8nxml/Working%20Sketch.pdf?dl=0

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]

On 26/07/2022 11:52, John Rumm wrote:

>
> 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

>
>

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]

On 26/07/2022 13:04, rick wrote:
> On 26/07/2022 11:52, John Rumm wrote:
>
>>
>> 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
>
>>
>>
> 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

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.

> many people having problems with EV charging.

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)

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

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.

> 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.

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

[ARW]

On 26/07/2022 10:00, rick wrote:

> On 25/07/2022 20:17, John Rumm wrote:
>
>>
>> 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.
>>
>
> 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.

Nice picture and very helpful.

> 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:
> https://www.dropbox.com/s/vzspjva7cm8nxml/Working%20Sketch.pdf?dl=0
>
> Welcome your input.
>

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

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[rick]

On 25/07/2022 20:17, John Rumm wrote:

> 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.

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]

On 27/07/2022 11:12, Brian Gaff wrote:
> Surely it needs to be low impedance as possible and hence as short as you
> can and impervious to being damaged.
> Brian
>

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

[John Rumm]

On 27/07/2022 11:12, Brian Gaff wrote:

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

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.