Earth connection via SWA

[addjunkie]

in some cases the reinforcing grid can be used instead of an earth rod we all know that the steel p[illars on the building are well in to the ground and serve the same purpose but its well accepted that you have visible earth rod that can be tested . watch any tester and see whats on the report if there isn't one

So I guess the old round light switches and cotton wrapped twisted two core is a no no these days then! This is what I found in a shed up the road, when I was asked to weld something for her. Oh and it was full of straw. I didnt by the way.

 

[bigegg]

outbuilding supplies and earthing explained https://www.voltimum.co.uk/articles/supply-detached-outbuilding-technical

Very useful - thanks!

My understanding is:
I have a PME/TNC-S earthing arrangement at the house.
Table 54.8 says that i have to have a copper equivalent CSA equal to the PEN conductor on my incoming supply (25mm², I think) if i run the earth back to the house DB.
If i use both the SWA, AND the earth core, then they BOTH have to be this size. So i can't use the earth core as a return earth.

My SWA armour has an actual CSA of 39mm²
The copper equivalent CSA of the steel armour is
41/115 x 39 = 13.9mm², so it is NOT big enough by the regs.
So i'll have to disconnect/insulate the armour (and earth core) at the garage end of the SWA, and install a suitable earthing rod to earth the garage.

Thanks for all the info.
I have a much better understanding of section 54 of the regs, now.

 

[Bornfree]

Iam about to do a similar job. I want a 40amp supply to my garage it's joined to my bungalow so the cable won't be outside.i was going to use 10mm twin&earth running from a 40a MCB in the CU to a small garage 3way CU. But the bungalow is TT earthed so do I need another earth rod for the garage CU and if I do need a rod do I connect the earth in the 10mm cable at both ends. The CU in the bungalow is a dual RCD unit with twin 30ma RCDs

 

[brightspark]

you can run in 10mm twin and earth to a 3way db as its rcd protected at source and is 30ma protected no need for another earth rod and you can use the earth in the flat twin and earth to the garage db

 

[bigegg]

Iam about to do a similar job. I want a 40amp supply to my garage it's joined to my bungalow so the cable won't be outside.i was going to use 10mm twin&earth running from a 40a MCB in the CU to a small garage 3way CU. But the bungalow is TT earthed so do I need another earth rod for the garage CU and if I do need a rod do I connect the earth in the 10mm cable at both ends. The CU in the bungalow is a dual RCD unit with twin 30ma RCDs

I think i know this (please bear in mind i'm a final year student/apprentice, not fully qualified).

As long as your t+e does not go through insulation or fastened to other cables, then 10mm cable is OK for 40A.
You won't need a separate earth (rod).
It might also be cheaper/easier to install the new db in the bungalow, and run the smaller cables for individual circuits.

 

[jimbo84]

Am I correct in thinking that if I run, say, 10m of 3 core 6mm <edit:10mm> SWA to my shed DB(20A welder, 32A ring, 6A lighting MCBs) from the house DB (63A MCB), (actual load is likely to be <40A total - if i have the oven on at the same time as the plasma cutter, and compressor) then:

If I use the armour as an additional earth path, then I don't *have* to make it a TT install, as long as the Zs is within limits?

If you're on PME extra care has to be taken banging in earth rods and TT'ing things, your earth rod can become the earth for multiple properties via the bonding in the event of an open PEN conductor. There's also minimum distances your earth rod needs to be from other PME supplies/the DNO's earthing and buried metallic services, I think it's something like 2 or 3m, this isn't in BS7671 but is covered in the DNOs regs I believe, and is to prevent the voltage (relative to the ground) of your earth rod/exposed conductive parts in your installation rising.

2 core armoured sizes in the normal sizes you'd use in a domestic environment the SWA gives more than an adequate CSA to use it as the earth conductor. Not the case where extraneous conductive parts are involved.

If your still learning have a look for John Ward on youtube and check out Sparkyninja, both very good resources.

 

[EddyP]

Well everyday is a school day, just learnt my out buildings aren't earthed properly. We're on a PME supply, when we moved in the out buildings had a bit of 6mm T&E on a catenary from an MCB in the board. I changed this to an underground 10mm SWA and replaced the board in the outbuildings and rewired them, looks like I best drill a hole through the floor and get an earth rod in and disconnect the earth to the house.
This is quite a useful link too, https://professional-electrician.com/technical/stroma-certification-supply-chain/

Thanks guys.

 

[jimbo84]

Well everyday is a school day, just learnt my out buildings aren't earthed properly. We're on a PME supply, when we moved in the out buildings had a bit of 6mm T&E on a catenary from an MCB in the board. I changed this to an underground 10mm SWA and replaced the board in the outbuildings and rewired them, looks like I best drill a hole through the floor and get an earth rod in and disconnect the earth to the house.
This is quite a useful link too, https://professional-electrician.com/technical/stroma-certification-supply-chain/

Thanks guys.

It's not always a case of just banging in an earth rod, what length of rod or will just one be enough to get a stable reading, what's in the region of where you're knocking it in, is the ground good enough all year to provide a stable enough earth, are you going to measure the earth rod's resistance once installed. What RCD rating are you going to use for protection etc. there's a lot to consider. Knocking an earth rod through a gas main or a buried cable is far more risky than using the PME earth in some cases.

A lot of sparks fill their trousers about outbuildings on PME supplies but the same ones happily install outdoor sockets, class 1 garden lights, class 1 external wall lights etc to PME supplies no worries. Do you worry about your outside tap on a PME supply, open PEN conductor and turning on your outside tap to water the garden or wash your car could take you out just as easily.

 

[brightspark]

the lousiest job I had was up the dales in rocky ground had to bang 3 rods in and link together 1 meter apart to get the readings down below 200 ohms. another was put a 6 foot rod in and it was Russian roulette to miss drainage gas pipe and 3 phase mains

 

[brightspark]

the lousiest job I had was up the dales in rocky ground had to bang 3 rods in and link together 1 meter apart to get the readings down below 200 ohms. another was put a 6 foot rod in and it was Russian roulette to miss drainage gas pipe and 3 phase mains

I've actually seen so called electricians pouring water over the rod to get readings down in sandy soil

 

[Spark plug]

If you think earthing your shead is confusing, just wait until people are installing EV chargers all over the place and banging in earth rods willy nilly.

 

[bigegg]

If you think earthing your shead is confusing, just wait until people are installing EV chargers all over the place and banging in earth rods willy nilly.

I think thats changed now in the 18th edition.

 

[Spark plug]

I think thats changed now in the 18th edition.

They change things faster than you can keep track with them, as far as I’m aware if you can’t guarantee that the CPC and neutral remain separate all the way back to the transformer, then the charging point should be installed as a tt installation.

https://professional-electrician.com/technical/ev-charging-equipment-the-new-rules-explained/

 

[mtt.tr]

Rods are dead easy here routinely get 80ohms with a 3/8 rod

 

[bigegg]

They change things faster than you can keep track with them, as far as I’m aware if you can’t guarantee that the CPC and neutral remain separate all the way back to the transformer, then the charging point should be installed as a tt installation.

https://professional-electrician.com/technical/ev-charging-equipment-the-new-rules-explained/

i got it wrong - again(!)

Basically, amendment 1 of 18th ed, changed the regs to allow "alternate means of disconnection", allowing a new type of EV charger to be fitted - that don't need an Earth rod.
https://www.efixx.co.uk/NEWS/BS7671-18th-Edition-amendment-1

 

[mtt.tr]

i got it wrong - again(!)

Basically, amendment 1 of 18th ed, changed the regs to allow "alternate means of disconnection", allowing a new type of EV charger to be fitted - that don't need an Earth rod.
https://www.efixx.co.uk/NEWS/BS7671-18th-Edition-amendment-1

They realised that pme was mostly underground supply and hitting services was more likely.

Issue is only 2 brands have alternative means of disconnection

 

[brightspark]

Rods are dead easy here routinely get 80ohms with a 3/8 rod

the best I've ever had was 30 there was a rod already in and tested 220 sunk the new one 6 inch away and and 30 had to double check and my mate used his meter and was correct

 

[dobbslc]

You're up in the rocky north though!
I've got a reading of between 17-19 ohms in my house. I was thinking of banging another in for good measure!

 

[bigegg]

Very useful - thanks!

My understanding is:
I have a PME/TNC-S earthing arrangement at the house.
Table 54.8 says that i have to have a copper equivalent CSA equal to the PEN conductor on my incoming supply (25mm², I think) if i run the earth back to the house DB.
If i use both the SWA, AND the earth core, then they BOTH have to be this size. So i can't use the earth core as a return earth.

My SWA armour has an actual CSA of 39mm²
The copper equivalent CSA of the steel armour is
41/115 x 39 = 13.9mm², so it is NOT big enough by the regs.
So i'll have to disconnect/insulate the armour (and earth core) at the garage end of the SWA, and install a suitable earthing rod to earth the garage.

Thanks for all the info.
I have a much better understanding of section 54 of the regs, now.

It turns out I DON'T have a better understanding.

Table 54.8 says I need a copper equivalent CSA of 10mm for a PEN conductor size of up to 35mm2

Which means both the earth core AND the SWA armour are big enough.

 

[bigegg]

I'll find the book and show you

I found it...
BS7671 Table 54.1
16mm2 copper for a (BURIED + not protected from mechanical damage) conductor
it's also 16mm2 for a steel conductor (not the copper equivalent).

since the steel armour is 39mm2, I would still be OK if it was buried.

I should have pointed out, that the SWA will not be buried - it's clipped to a wall