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

 

[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.