TNCS Outbuilding earthing arrangement

[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.