More Earthing Clarification!

[Karin]

as usual, further reading brings up more questions than answers but i feel like (hope!) different pieces are starting to click together!

a) is one reason we are cautious with using TN-C-S earthing outdoors due to touch voltages that can occur between MET/installation earth and terrafirma/true earth?

b) ..that is caused in normal service due to the fact that the DNO PE conductor is also a live conductor and subject to current and therefore voltage fluctuations?

c) ..that wouldn't be a problem in a TN-S system as the PE conductor only sees current under fault conditions?

thanks as always!

 

[hightower]

I thought it was to do with the worry of a break in the neutral along the supply, making the earth side of things rise to 230v, meaning if you touch a metal casing (which is at 230v due to broken neutral) and are stood on grass bare foot, some of that is going to go through you. At least in the house (equipotential zone) anything with an earth potential will rise to a similar level through bonding, but outside isn't going to work like that.

I'm not saying I'm right, this was just my understanding - I'm here for the ride with you.

 

[Karin]

cheers HT, that's certainly true with a broken PEN. i'm currently swimming around (drowning ) in a pool of touch/step voltages and potential differences. trying to research and learn means i have lots of bits of information i need to relate into an understanding, so i may well be applying the wrong bits to the wrong places. but then hopefully someone can at least turn me in the direction of shore.

good to have you aboard man

 

[happysteve]

@hightower has got it right. It's to do with the risk of losing the neutral. In this circumstance, a single fault (loss of neutral) can easily kill someone for the reasons he's stated, and that's an unacceptable risk. It's particularly bad as an RCD - which you might usually think of as your last line of defence (additional protection) will not save you. It's one of the few things that's in statutory regulations, in this case The ESQCR - see article 9 on page 7 (it's also in the Regs for various special installations and locations).

 

[Karin]

thanks steve, i will check out the ESQCR reference as a point of interest, but maybe the question was phrased badly. i was really focusing on touch voltages under normal service, not fault conditions. am i shooting up the wrong tree here or can you point me in the direction i need to be looking?

 

[Richard Burns]

These are some ideas that I have come up with, they may not be rigorous but hopefully explain some of the issues you are asking about. I may not have the information totally correct but it should be in the right range.
I hope the picture comes out clearly enough to read!

 

[Karin]

perfect Richard! excellent diagram and explanations so thanks a lot for that. that has clarified a lot of what i have been reading and now i feel like i'm understanding it. thanks!

 

[rolyberkin]

I thought it was to do with the worry of a break in the neutral along the supply, making the earth side of things rise to 230v, meaning if you touch a metal casing (which is at 230v due to broken neutral) and are stood on grass bare foot, some of that is going to go through you. At least in the house (equipotential zone) anything with an earth potential will rise to a similar level through bonding, but outside isn't going to work like that.

I'm not saying I'm right, this was just my understanding - I'm here for the ride with you.

What I can't get my head around is if there is a concern with a loss of the DNO neutral why isn't it standard practice to give each house or installation a locally provided Earth rod?

 

[Richard Burns]

What I can't get my head around is if there is a concern with a loss of the DNO neutral why isn't it standard practice to give each house or installation a locally provided Earth rod?

Probably because the loss of a supply neutral is something that the DNO must ensure is as unlikely as possible (by the ESQCR regs) and if the DNO were to supply earth rods then they would be responsible for their maintenance and this would probably be prohibitively expensive and inconvenient to manage for a very low risk requirement.
It is easier for the DNO to take measures outside a customers property to ensure there is no loss of neutral, such as the PME connections made to the supply cables.