TNCS Outbuilding earthing arrangement

[swifters]

I have a large shed (affectionately known as the warehouse) that is fed by a 16amp rcbo. The main earthing arrangement at the distribution boards is tncs and now a more experienced electrician than myself has stated that the earth in the 3 core 2.5mm armoured cable should be connected to a separate earth rod and make its own tt earthing arrangement rather than be fed back to the main supply, stating parallel paths are the reason. Is he right? Could anyone point to it the new regs book?

many thanks, Jon

 

[Midwest]

Have you read the pdf in post 29?

 

[Midwest]

Guidance Note 8, gives more...........guidance. Page 66 (in green version).

 

[Monkeyblaine]

Definitely not haha. Will do now. Thanks a lot.

 

[Monkeyblaine]

Thanks guys. Four questions.
1. Is it saying that if your making the outbuilding a TT system and you're supplying said outbuilding with an armoured cable, then don't earth the armour at all as so not to cross PME earth from dwelling with the TT earth of outbuilding. So SWA Armout is not earthed at all??

2. When keeping outbuilding within the PME EZ and the outbuilding has extraneous parts, is it saying to run a 10mm earth seperate to the supply cable of the outbuilding to connect a designated 10mm earth from extraneous parts in OB to MET of dwelling?? Or can you just bond parts with a 10mm earth to the MET of the outbuilding which is ultimately fed from dwelling.

3. I've been told that the dwelling should made into a TT as the sockets within could be used to plug in an extension lead which means you could potentially come into contact with something far away from the outhouse which could be extraneous but surely there's no difference from an outside socket attached to the dwelling itself?? Confusing!!!

4. What about if the dwelling has a TNS or TT earthing arrangement? Why is PME more dangerous.

 

[davesparks]

The armour of an swa cable must be connected to the earthing system of the installation it is fed from.

The main bond and cpc can be the same conductor as long as it meets the requirements of both.

The type of earthing system is irrelevant, if extraneous parts in an outbuilding need bonding then it needs to be a connection which meets the requirements of the origin or else a seperate earthing system needs to be installed.
PME gets a lot more press because it requires much larger bonding than any other system.

 

[telectrix]

Thanks guys. Four questions.
1. Is it saying that if your making the outbuilding a TT system and you're supplying said outbuilding with an armoured cable, then don't earth the armour at all as so not to cross PME earth from dwelling with the TT earth of outbuilding. So SWA Armout is not earthed at all?? if TT'ing the outhouse you just earth the SWA armour at the supply end

2. When keeping outbuilding within the PME EZ and the outbuilding has extraneous parts, is it saying to run a 10mm earth seperate to the supply cable of the outbuilding to connect a designated 10mm earth from extraneous parts in OB to MET of dwelling?? Or can you just bond parts with a 10mm earth to the MET of the outbuilding which is ultimately fed from dwelling. a pme earthing system needs 10mm bonding due to the high fault currents in bonding conductors are liable. if a 10mm 3 core SWA is used, then it will comply

3. I've been told that the dwelling should made into a TT as the sockets within could be used to plug in an extension lead which means you could potentially come into contact with something far away from the outhouse which could be extraneous but surely there's no difference from an outside socket attached to the dwelling itself?? Confusing!!!

4. What about if the dwelling has a TNS or TT earthing arrangement? Why is PME more dangerous.

comments in red.

 

[Monkeyblaine]

Thanks for comments but it's thrown a few more questions up. Haha. If you Earth the armour at supply end are you saying to not connect armour into gland at outhouse end as surely If its a metal CU then it will be all connected to TT side as well. And for the second comment your saying a 10mm 3c Can use one of the cores for the CPC of the submain plus the adequate bonding requirements for extraneous parts of the outhouse?

 

[Monkeyblaine]

I hope they're not stupid questions.

 

[Midwest]

Thanks for comments but it's thrown a few more questions up. Haha. If you Earth the armour at supply end are you saying to not connect armour into gland at outhouse end as surely If its a metal CU then it will be all connected to TT side as well. And for the second comment your saying a 10mm 3c Can use one of the cores for the CPC of the submain plus the adequate bonding requirements for extraneous parts of the outhouse?

In answer to your first question; as the other guys have said, earth the swa at supply end only. As to the reference on the gland at the outbuilding end, you could use an insulated CU. Arrgh you say, what about A3. I believe you could still fit an insulated CU after 1st Jan in these circumstances. I cannot point you to any reg or informative sheet, but if you watch this vid below, you'll see that they are advising that an outbuilding/garage not connected to domestic property, does not need to comply with 421.1.201. Another ill-conceived part to this reg. Alternatively, someone posted elsewhere about a brass gland for swa, that doesn't pass through the earth connection.

https://www.youtube.com/watch?v=nJvLT7uwO58&feature=youtu.be This vid talks about 421.1.201. The bit about above is toward the end.

Your second question has been already answered. Just make sure you have the correct size bonding in the main property.

In answer to your question in another post, this vid explains the different types of supply in UK and mentions dropped pen conductor;
https://www.youtube.com/watch?v=AWxeb2MI37c

 

[Deleted member 26818]

There is no Regulation requiring out buildings to be TT.
The main reason for considering making an out building TT is simply cost.
Is it cheaper to run a 10mm bonding conductor from the house MET to the out building, or to install an earth rod at the out building?

 

[Monkeyblaine]

Thanks so much again, but I'm still struggling with the bonding. So I've run a 10mm 3c from main PME installation in house, then in outhouse run a 10mm earth bond from extraneous water pipe to the MET of the outhouse submain CU, which in turn is connected to the 10mm CPC core of SWA, which terminated back to house. No seperate 10mm from pipe in outhouse to main dwelling?? Is this correct for the scenario of extending PME EZ to outhouse via 3c SWA. SORRY FOR BEING DIFFICULT. I struggle with simple things sometime. Just want to be correct. Thanks for your patience.

 

[Deleted member 26818]

Yep sounds good, except for the terminology.
There is only one MET in an installation.
Anything else is just an earth connection or terminal.
Remember the SWA armour needs earthing, in your case, at both ends.

 

[DPG]

Or EMT. Daz

 

[Monkeyblaine]

Ok thanks a lot, I maybe over thinking it. . In your previous message you say is it cheaper to run a 10mm bonding conductor from MET to outhouse or put in an earth rod??? Are you talking about the 10mm core of an SWA here when talking about the bonding conductor.

 

[Deleted member 26818]

Yes.
Is it cheaper to run a 3 core 10mm SWA, or 2 core SWA and an earth rod?