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

 

[Moses]

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

Its nothing to do with parallel paths!!!!

The reason is that as Main Earth on a TN-C-S is derived from the Main N at the supply head, if you loose the the N at the head for any reason, all earthed equipment could become live and your RCD's, RCBO's have no neutral/earth to work correctly, leaving a very dangerous situation.

Thats why a TT is recommended, I don't think you will find any reg nos.

 

[telectrix]

if you have extraneous parts in the shed, these would require a 10mm bonding conductor back to the house MET. also, no harm in adding a rod at the shed and keeping the TNC-S.

 

[Deleted member 9648]

Its nothing to do with parallel paths!!!!

The reason is that as Main Earth on a TN-C-S is derived from the Main N at the supply head, if you loose the the N at the head for any reason, all earthed equipment could become live and your RCD's, RCBO's have no neutral/earth to work correctly, leaving a very dangerous situation.

Thats why a TT is recommended, I don't think you will find any reg nos.

Funny how they always pick on outbuildings though....never outside lights/sockets etc etc.

 

[ample current]

there is no reason why you cant connect the out building to the MET in the house, as long as as your Zs meets requirements and as Tel says, if there is extraneous parts in the shed then these would need to be connected to the MET also with a reading below 0.05 ohms, only then would you have to TT and divorce the earth from the house.

 

[swifters]

Thats what I thought. The trouble is my multi tester trips the rcbo everytime I do a Zs test.

 

[telectrix]

so measure R1+ R2 and Ze , then calculate Zs.

 

[swifters]

Hadn't thought of that. Will do.

 

[JUD]

Thats what I thought. The trouble is my multi tester trips the rcbo everytime I do a Zs test.

Which tester is it. Does it not have a No-Trip setting??

 

[swifters]

Its a Di Log 9083p I think. It is meant to be non tripping but always does.

 

[Guest55]

............Is he right?

Not by a million miles lol.

 

[Engineer54]

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

Obviously not as experienced as you first thought, ...is he!! lol!!

 

[Engineer54]

Thats why a TT is recommended, I don't think you will find any reg nos.

That's why a TT is Recommended by Who?? ...Scaremongers??
It's no more dangerous than the PME system you have in the main house.

 

[Sav]

That's why a TT is Recommended by Who?? ...Scaremongers??
It's no more dangerous than the PME system you have in the main house.

I agree with Engineer54,
I wired a small office in a rear garden (approx 30 meters from main house ), 16mm armoured from isolator inside the house, then to office. I just extended the equipotential zone.
No need for TT

Sav

 

[swifters]

Many thanks everyone, I did think he may be a bit off the mark but I will calculate Zs just to be sure.

Right, got a reading of 0.5ohms for R1+R2 and 0.09 for Ze so even with adjustment for temperature table 41.3 states 1.44ohms or 1.15ohms adjusted for a 16amp RCBO so all good right?

 

[JUD]

Many thanks everyone, I did think he may be a bit off the mark but I will calculate Zs just to be sure.

Right, got a reading of 0.5ohms for R1+R2 and 0.09 for Ze so even with adjustment for temperature table 41.3 states 1.44ohms or 1.15ohms adjusted for a 16amp RCBO so all good right?

Is the RCBO a type B?

If so that's for a 32A. For 16A it's 2.87Ω (2.3Ω adjusted) so looks like you're well under.

 

[telectrix]

probably got a type c rcbo. in which case it is 1.44 (1.15 corrected)

 

[swifters]

I'm not sure if it's a type b or c. It's not as apparent as the MCB's. It's nsbs16 - something... They are Wylex RCBO's.

 

[JUD]

I'm not sure if it's a type b or c. It's not as apparent as the MCB's. It's nsbs16 - something... They are Wylex RCBO's.

Have a look at the pic at the bit in the circle...

This is a 16A type B. If it's type C it will say C16.

The bit underneath says NHXBS1B16 so that should give you a clue also

 

[swifters]

Yes, it's a B type. I was being blind.

 

[pencilpusher]

I have exported the equipotential zone to outbuildings before with no problems and mostly without having to bond any extraneous parts, because there have been none. Nice low ZS readings. No problem. (unless "Rodman" sees this! He thinks it is a big no no for some reason)

 

[Moses]

I have exported the equipotential zone to outbuildings before with no problems and mostly without having to bond any extraneous parts, because there have been none. Nice low ZS readings. No problem. (unless "Rodman" sees this! He thinks it is a big no no for some reason)

read post#2

 

[pencilpusher]

I have read all the posts! What about the house TNCS then? The world is all if's and but's!

 

[Moses]

I have read all the posts! What about the house TNCS then? The world is all if's and but's!

run a 2 core SWA to the hut, an RCd cu in the hut with TT supply from the SWA, and an earth rod at the hut end. SWA earthed at the supply end but the SWA earth insulated at the load end.

 

[pencilpusher]

^^^^^^^ What about the TNCS in a dwelling with no outside hut?

 

[rich.250]

Where do all these rumours come from.........?
Its all written in black and white (with green covers)
Exporting TNCS or PME is fine!! along as any extraneous conductive part is connected to the MET.
How many times have i heard the you can't do this rumours only to look in the regs to prove them wrong haha

 

[pencilpusher]

I have seen this on here many times. There seems to be an obsession about exporting the equipotential zone in outbuildings. Nothing wrong with it and its fine according to the Regs. Period.

 

[rich.250]

I can see there point about loosing the neutral.... as I've seen it a few times!!
But if it was that much of a concern TNCS supplies would be banned and it shouldn't occur with PME

 

[pencilpusher]

Here is the main thread about this very subject previously. Same old rumours.

http://www.electriciansforums.net/e...me-outbuildings.html?highlight=exporting+tncs

 

[Moses]

I have seen this on here many times. There seems to be an obsession about exporting the equipotential zone in outbuildings. Nothing wrong with it and its fine according to the Regs. Period.

In that case you need to ensure that you Zdb at the hut complies with the max for the earthing system that you are exporting.

are you going to calculate the distribution cable R1+R2, and hence calculate the Zdb, as well as the cable size for the expected load? not many do.

or as most people, just design for the expected load and then install the cable, and then take the Zdb measurement afterwards? and hope that it is within the regs?

 

[pencilpusher]

In that case you need to ensure that you Zdb at the hut complies with the max for the earthing system that you are exporting.

Absolutely and I always do full testing on the outbuilding CU and record Zdb on the test sheet. I don't think that point was ever disputed.

EDIT: They may do, I don't. It's a fair point. I do Zdb + (R1+R2) and then do my ZS tests from the outbuilding. Job done.

 

[Moses]

Absolutely and I always do full testing on the outbuilding CU and record Zdb on the test sheet. I don't think that point was ever disputed.

My point is if the hut is 'too Far',
and you have not done any R1+R2 calculations for the cable,
that you have designed for the expected load,
then how do you know that the Zdb you are going to test after the installation is going to meet the regs?

 

[pencilpusher]

My point is if the hut is 'too Far',
and you have not done any R1+R2 calculations for the cable,
that you have designed for the expected load,
then how do you know that the Zdb you are going to test after the installation is going to meet the regs?

Agreed. However, that is a seperate point from not exporting the equipotential zone at all!

 

[rich.250]

In that case you need to ensure that you Zdb at the hut complies with the max for the earthing system that you are exporting.

are you going to calculate the distribution cable R1+R2, and hence calculate the Zdb, as well as the cable size for the expected load? not many do.

or as most people, just design for the expected load and then install the cable, and then take the Zdb measurement afterwards? and hope that it is within the regs?

As they say, a little knowledge is dangerous LOL

they look at max CCC at best and say "it be ite" LOL

 

[Moses]

Agreed. However, that is a seperate point from not exporting the equipotential zone at all!

No it isn't, if you are exporting the EZ, then you must meet the Ze max requirements for the whole system. Other wise, the touch voltages will be above 50V, because of increased Ze.

You have no worries of any sort if the hut is converted to TT.

 

[pencilpusher]

No it isn't, if you are exporting the EZ, then you must meet the Ze max requirements for the whole system. Other wise, the touch voltages will be above 50V, because of increased Ze.

You have no worries of any sort if the hut is converted to TT.

You did not make that point initially though. Your point was a loss of the neutral. This is a secondary discussion. If your ZS does not meet the regs then of course you will have to TT it or take the house nearer to the outbuilding! I do my designing properly and do what is required to meet the regs TBH. BTW, your ZE is at the origin of the supply, not the secondary CU.

 

[Moses]

You did not make that point initially though. Your point was a loss of the neutral. This is a secondary discussion. If your ZS does not meet the regs then of course you will have to TT it or take the house neare the outbuilding! I do my designing properly and don't have that problem TBH.

I'm sure you do your designs properly.

but the fact remains, that most of us never calculate the expected Zdb, when we calculate the cable for a load at the end of the garden.

We just go ahead and calculate the cable according to the load, and install and then we test to see if the Zdb meets the system.

Touch voltages are one problem.
Loss of a N at the head is the second.

Loss of N is the more serious, as it leaves all appliances's metallic bodies live,
If this happens at the end of the garden, wet and damp, its a lot more dangerous than in the house.

 

[swifters]

run a 2 core SWA to the hut, an RCd cu in the hut with TT supply from the SWA, and an earth rod at the hut end. SWA earthed at the supply end but the SWA earth insulated at the load end.

What if a gardener/person puts a fork/spade through the cable? Or the cable just detoriates with age? Or there is some other accident that damages the cable.

 

[rich.250]

Why would your Zdb need to meet the max Ze reading??
Its a sub main.....

 

[pencilpusher]

I cannot answer for others and their designing. Before I buy 80 meteres of SWA for their job, I want to know it satisfies the Regs so I don't waste a lot of money getting it wrong!

If the neutral is lost, it is lost. I would not be happy touching any metal part, wet or not! I must also advise people not to use an extension lead plugged in the house (TNCS) and taken all the way to the bottom of the garden either then! We all know they do this too. Common sense prevails here me thinks.

 

[swifters]

What is the Zdb?

 

[pencilpusher]

^^^^^^ Ze (at origin) + (R1+R2) of SWA submain (for example). There is only one ZE, so if you have a secondary CU in a garage or the like, it is the R1+R2 of the sub main added to the orginal Ze at the primary CU.

 

[Moses]

Why would your Zdb need to meet the max Ze reading??
Its a sub main.....

If you are splitting from a Henley then you do.

If from a CB then you need to stay within the Zs for the CB.

 

[rich.250]

If you are splitting from a Henley then you do.

If from a CB then you need to stay within the Zs for the CB.

Thats what i was thinking!! thanks

Never though of coming from henley as suppliers would have a fit over 3 metres LOL

- - - Updated - - -

What is the Zdb?

Zs at sub board effectively or calc above

 

[Monkeyblaine]

Sorry to be stupid here but I'm pretty new to this. Why does the bonding conductor have to be a seperate 10mm all the way back to the original installation MET? If you add a rod to the outbuilding and make it a TT system can you not just add a 10mm bond from extraneous parts, to the TT MET in the outbuilding?? More to the point I'm confused as why when you have an original PME system and your supplying an outbuilding, why do I read that the outbuilding should be made into a TT system. Can someone please explain the dangers in lay mans terms and why this doesn't apply if the original installlation is a TT SYSTEM.

 

[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.