Taking cables from a house to a garage

[ianmacd]

With TNCS, if you loose a neutral then everything will stop working implying you have no power but you will have 230V potential at the Line connector and also at the neutral connector.Because your neutral and earth are linked all your CPC's will be at a higher potential. Since you have just exported the earth from your MET your class I accessories and bonded metalwork outside will now also be at a 230V potential waiting for someone to touch it.

'What about the RCD' I hear you cry. Well that will not trip as there will be no imbalance since the Earth and neutral are linked before the RCD so there is no imbalance. MCB will trip but a long time after you are already crispy.

 

[Dambo]

With TNCS, if you loose a neutral then everything will stop working implying you have no power but you will have 230V potential at the Line connector and also at the neutral connector.Because your neutral and earth are linked all your CPC's will be at a higher potential. Since you have just exported the earth from your MET your class I accessories and bonded metalwork outside will now also be at a 230V potential waiting for someone to touch it.'What about the RCD' I hear you cry. Well that will not trip as there will be no imbalance since the Earth and neutral are linked before the RCD so there is no imbalance. MCB will trip but a long time after you are already crispy.

Thanks Ian. 2 quick questions:

1) If you lose the neutral why will the neutral connector still have a 230V potential?

2) If in a TNCS it is dangerous to take the E from the MET to a garage because, in the case of a fault, the N and E will be lost at the same time, how can it be safe to have it in the house? Or anywhere for that matter?

 

[ianmacd]

1) If the network looses a neutral outside your property

2) It would seem other countries that run PME networks often put a stake in at the property so in this situation your CPC's are still at earth potential. Aparently we (in the UK) do not do that possibly due to cost).

 

[PBC_1966]

So its just our implementation of PME thats crap then? That sounds much more sensible. Does the 'exported' earth have to be of sufficient size to carry the fault current at the head?

The earth run to the outbuilding is sized the same as for any other branch circuit, based upon the circuit rating. For feeders up to about 30A that will mean that the grounding conductor is the same size as the circuit conductors; for larger feeders it may be smaller, e.g. for a 40A feed using #8 conductors (approx. 8.4 sq. mm) the grounding conductor must be a minimum of #10 (approx. 5.3 sq. mm).

The grounding electrode conductor is sized according the supply cable size. Minimum size is #8 for service conductors up to #2 (approx. 34 sq. mm). A 200A service using 3/0 conductors (85 sq. mm.) would need a GEC of #4 (21 sq. mm.) minimum, and so on. (All assuming Cu. conductors, Al. is different.)

Outbuildings can also be fed without a separate grounding conductor, instead extending the TN-C-S connection to the outbuilding by bonding the neutral to the distribution panel frame and the local earth rod, effectively making the feed to the outbuilding the same as the main service to a building. It's only permitted if no other direct metallic paths exist between the buildings which would result in significant parallel neutral currents.

I believe that this latter method is not now permitted for new installations, although it's still listed as acceptable in the 2002 NEC.

P.S. Just to complete the picture, the main bonds to water pipes etc. are also sized according to service conductor size, and are generally the same minimum sizes as for the GEC.

 

[Lurch]

The earth run to the outbuilding is sized the same as for any other branch circuit, based upon the circuit rating. For feeders up to about 30A that will mean that the grounding conductor is the same size as the circuit conductors; for larger feeders it may be smaller, e.g. for a 40A feed using #8 conductors (approx. 8.4 sq. mm) the grounding conductor must be a minimum of #10 (approx. 5.3 sq. mm). The grounding electrode conductor is sized according the supply cable size. Minimum size is #8 for service conductors up to #2 (approx. 34 sq. mm). A 200A service using 3/0 conductors (85 sq. mm.) would need a GEC of #4 (21 sq. mm.) minimum, and so on. (All assuming Cu. conductors, Al. is different.)

Outbuildings can also be fed without a separate grounding conductor, instead extending the TN-C-S connection to the outbuilding by bonding the neutral to the distribution panel frame and the local earth rod, effectively making the feed to the outbuilding the same as the main service to a building. It's only permitted if no other direct metallic paths exist between the buildings which would result in significant parallel neutral currents.

I believe that this latter method is not now permitted for new installations, although it's still listed as acceptable in the 2002 NEC.

Is that not in American?

 

[PBC_1966]

Is that not in American?

Yes, in response to Ian's query about how the issue of PME and outbuildings is handled elsewhere.

 

[Lurch]

Ah, missed that bit!

 

[steptoe]

Is that not in American?

yes, but they do it differently,

I think we are the only country that uses PME and dont actually spike just before it enters the building,

almost making it TT, but with the added benefit of having a very low Ze.

 

[spinlondon]

With TNCS, if you loose a neutral then everything will stop working implying you have no power but you will have 230V potential at the Line connector and also at the neutral connector.Because your neutral and earth are linked all your CPC's will be at a higher potential. Since you have just exported the earth from your MET your class I accessories and bonded metalwork outside will now also be at a 230V potential waiting for someone to touch it.'What about the RCD' I hear you cry. Well that will not trip as there will be no imbalance since the Earth and neutral are linked before the RCD so there is no imbalance. MCB will trip but a long time after you are already crispy.

Not every thing will stop working, as the earth will then be used as the neutral return.

 

[ianmacd]

Not every thing will stop working, as the earth will then be used as the neutral return.

That rather depends on where the break is and the resistance back to the (resistiviicly) closest neutral point.

 

[steptoe]

Not every thing will stop working, as the earth will then be used as the neutral return.

BUT

in tncs, if you have no neutral, then you have no earth, except for the path via your bonds,

hence the fact that everything you have earthed may/will be at 230v potential,

and if you touch any metalwork you then become part of said circuit.

and all this defence of tncs usually comes from people that are against neon testers.....

 

[spinlondon]

BUTin tncs, if you have no neutral, then you have no earth, except for the path via your bonds,

hence the fact that everything you have earthed may/will be at 230v potential,

and if you touch any metalwork you then become part of said circuit.

and all this defence of tncs usually comes from people that are against neon testers.....

Not quite sure what your point is?

The bonded services will become the path to earth that the neutral will take.

The main danger arises when there is no bonded services, or the services are supplied in plastic. The installation earthing will become live, and a person may provide a path to earth. Which is why in other countries they rod at each installation.

How this relates to neon testers or affects the expotrting of a PME earth I don't know?

 

[steptoe]

Not quite sure what your point is?The bonded services will become the path to earth that the neutral will take.

The main danger arises when there is no bonded services, or the services are supplied in plastic. The installation earthing will become live, and a person may provide a path to earth. Which is why in other countries they rod at each installation.

How this relates to neon testers or affects the expotrting of a PME earth I don't know?

at the risk of being nailed for having a go at people who have taken a different path to being an electrician to me this I think is one of the very fine examples of having tried to explain it, and them not being bothered to try and investigate it further for themselves then I start to get fed up trying to explain the very basics of electrical installations and earthing systems yet again.

even a search on this forum would throw up quite a good variety of debates on this subject, never mind google.

bad day explode

 

[ianmacd]

How this relates to neon testers or affects the expotrting of a PME earth I don't know?

Neon's, not a clue. Exporting TNCS, then I guess either we didn't explain it well again or you are missing the point you made yourself about a person becoming the path to earth and onto neutral. How much more chances are there for that to happen when its exported?

 

[spinlondon]

I'm certain that we had this discussion in full back in May.

I seem to remember Steptoe stating that using a rod with a PME installation was attempted murder or some such nonsense.

I don't think that the problem is that you haven't explained it properly, more that you don;t understand it.

If you are in any way worried, just install a rod.

 

[PBC_1966]

How many other countries have PME networks?

It's the norm in the U.S., although unlike here the NEC requires an earth electrode bonded to the neutral at the service entrance to the building. It's also the norm to "export" the earth from that point to any other outbuildings, and to provide a further earth electrode at the outbuilding.

Ian,

I found this article while browsing which illustratres some of the points I outlined about American methods. Thought you might find it interesting:

http://www.iaei.org/magazine/?p=2616

 

[dave2]

Last year I called NIC helpline about this export PME problem.

They said that if there were no extraneous water or gas pipes etc. in the outbuilding then it was acceptable to export PME!!

dave

 

[ianmacd]

Surely they mea extraneous-metal-parts. And what if you plug in something that is class I?

 

[batty]

Last year I called NIC helpline about this export PME problem.They said that if there were no extraneous water or gas pipes etc. in the outbuilding then it was acceptable to export PME!!

dave

Its dno that don't like you exporting pme to out buildings and if you do it in edf area on a new build they will not connect you to supply.

 

[steptoe]

I really need to add that bit about NICEIC NOT making the rules into my sig,,,,,,,,