Outbuilding Earthing

[i_am_fubar]

3 Sparkies, 3 opinions. Any chance on a consensus?

How should the earthing be handled when running power to my workshop?

• I have a TN-C-S earthing system in my dwelling.

• I have a brick built, poured slab, external workshop aprox 20m from the CU. with no extraneous conductors.

• 60A fused isolator already installed adjacent to meter in the dwelling as my source.

• Steel CU with RCBOs on all circuits within the workshop as my destination.

• Cable will be run as 10mm SWA (core number depending on answers), pulled through buried pipe.

I'm not going to give the suggestions I've already received, but would be interested to know what answer I should expect according to regulations as they currently stand. Or, what other information would be needed to make the decision.

Cheers,
Fubar.

 

[ban-all-sheds]

Three opinions?

I can't see how there can be more than two - export the TN-C-S earth or make the building TT.

Or was the third opinion use the supply earth and also install a rod for the outbuilding?

FWIW I'd tend to the latter - I've never seen the sense in discarding a perfectly good supply earth unless you really have to.

Although depending on what you think is the tail, and what the dog, the size of the cable affects what you do, or vice-versa.

 

[mwatsonxx]

There are no special requirements, the armouring of the swa is your main earth therefore the swa can be 2 core.

The isolator seems oversized, if you really need a 60A, you could be over loading your house supply and you should consult your local dno. To find your dno go to the national grid website.

Edit, Also consider installing phone/data cabling, easier to do it now in conjunction with the other works

 

[bernardgreen]

the armouring of the swa is your main earth therefore the swa can be 2 core.

If exporting the Earth then use three core, best not to trust the armouring to carry the Earth to the outbuilding

The SWA is steel. Steel rusts, SWA cable buried relies on there being no damage to the sheath, if there is any ingress of moisture to the steel armouring via damaged sheath then the steel will corrode. The "earth" from a TN-C-S supply is at the potential of the Neutral which is not always at local ground potential. A few volts potential difference between ground water and the armouring can accelerate the corrosion.

 

[mwatsonxx]

best not to trust the armouring to carry the Earth to the outbuilding

It's an approved, tried and tested installation method that has withstood the test of time.

 

[bernardgreen]

method that has withstood the test of time.

How long was that time ?

The "Earth" to an equipment building "dis-appeared". Continuity from end to end of the armouring had become resistive. A new cable was installed and the faulty cable abandoned. ( the site was also converted to TT earthing ). The failed cable could not have been more than about 20 years old, Testing on the abandoned showed there was low impedance continuity between the armouring and ground. Damage when installed was given as the cause of the cable's failure .

 

[bernardgreen]

Interesting reading in this that has just arrived in my In_Box

https://www.voltimum.co.uk/articles...dium=email&utm_campaign=18_01_16_VoltiNews_OW

 

[i_am_fubar]

Cheers for the replies guys. To answer the first question:
Option 1 : 2 core with local TT
Option 2 : 2 core with earth exported on armour
Option 3 : 3 core with earth exported on 3rd core, armour only terminated to ground at supply.

As far as all combinations go, I also see:
Option 4 : Export on 3rd core AND have stake
Option 5 : Export on armour AND have stake
Option 6 : Export on armour AND 3rd core AND have stake
Option 7 : 3 core with earth exported on 3rd core, Armour only terminated to ground at supply.

I also had one person suggest:
Option 8 : 4 core, one for earth and one as ground, connected to both ends to the shield and to source end.

At this point, I think my preference would be to go for option 3, but yea, so many opinions on it :/

 

[JohnD]

It's an approved, tried and tested installation method that has withstood the test of time.

I've seen steel-armoured distribution cables where the steel has rusted so much it has broken apart and hangs in limp segments around the core.

Tested by time, and failed.

 

[mwatsonxx]

How long was that time ?

100 plus years, if there was a realistic danger then most buildings would require new supply cables, but there isn't and they don't.

However, there is now a massive problem due to the use of concentric cabling using AL or CU conductors, that can easily corrode due to damaged outer sheaths. But in a typical uk gov approved rip off, the cure and cost of that danger has been dumped onto the consumer by the bonding regulations. Leaving the dno's laughing all the way to the bank

 

[i_am_fubar]

Ok, so from this, I'm running on the basis that if I do export the earth, it will be via a conductor as opposed to armour.

 

[mwatsonxx]

Ok, so from this, I'm running on the basis that if I do export the earth, it will be via a conductor as opposed to armour.

The only difference is cost.

Why do you need a 60A supply? Are you starting a waccy baccy farm?

 

[Notch7]

Out of interest, if the earth is exported to the outbuilding using 1 of the conductors of a 3 core SWA cable, should the armour of the cable be terminated at the supply end, but not connected at the outbuilding end?

Im thinking otherwise the earth has 2 paths, or does that not matter.

If it is not connected at the outbuilding end how would the SWA cable be terminated? I presume it has to be terminated to a metal box and normally with a suitable gland.

 

[bernardgreen]

Arnour terminated at the source end. Terminated into a non conductive transition box (*) at the shed end, conductors and inner sheath passing through the box uncut.

 

[EFLImpudence]

Out of interest, if the earth is exported to the outbuilding using 1 of the conductors of a 3 core SWA cable, should the armour of the cable be terminated at the supply end, but not connected at the outbuilding end?

It should be connected at the supply end and mayas well be connected at the other end.

Im thinking otherwise the earth has 2 paths, or does that not matter.

No.



I am having a 20m. extension built on the back of the house. Should the sockets at the far end be TT'd?