Relative risks of TN-C-S and TT

[ban-all-sheds]

AIUI touch voltage is the voltage wrt E that an earthed exposed-conductive-part will rise to before the protective device operates.

50V is considered the maximum safe voltage, and if you take the simplistic view of the operation of a 30mA RCD, that's where the figure of 1667Ω comes from.

 

[JohnW2]

That's also my understanding but, as I said, it does not for me correspond to a particularly useful indicator of anything - since (within an adequately constituted equipotential zone) what actually matters the the PD between two simultaneously-touchable things - and, as I just wrote (in the presence of an L-CPC fault), if the earth impedance is lower, that PD will be higher, although the 'touch voltage' (as you have defined) will actually be lower. Would you be happier to 'experience' that (than the alternative with a high earth impedance) "because the 'touch voltage' was lower" (but the "touchable PD" higher)? Am I missing something?

 

[bernardgreen]

Am I missing something?

The boundary between the equipotential zone and the "outside" extranious world. It is when crossing that boundary that the hazard is greatest. Standing on wet grass and touching the outside tap that is bonded into the equipotential zone is just one example of crossing the boundary

 

[bernardgreen]

This bird is one hell of an equipotential zone.........click here

 

[John D v2.0]

This bird is one hell of an equipotential zone.........click here

Nice, I've sometimes pointed out that a chain mail suit would be a very safe personal equipotential zone, seems it's not practical in that form.
I was a bit puzzled by "this is an unbelievably dangerous job", if it's that dangerous surely they would have a duty to do it another way.
, the job seemed to combine the danger of close helicopter operations with the danger of climbing. I don't feel like the obvious voltage issue would cause a significant danger.

 

[JohnW2]

The boundary between the equipotential zone and the "outside" extranious world. It is when crossing that boundary that the hazard is greatest. Standing on wet grass and touching the outside tap that is bonded into the equipotential zone is just one example of crossing the boundary

Indeed - but that's obviously not a 'something that I missed', since I acknowledged that a couple of posts ago ....

True - but if the building is properly constituted as an equipotential zone (i.e. all appropriate bonding is in place), that will not have any consequences (other than for a person 'half in and half out' of the building!).

Can you think of anything else that I 'missed'?

Kind Regards, John

 

[JohnW2]

Standing on wet grass and touching the outside tap that is bonded into the equipotential zone is just one example of crossing the boundary

No-one can deny the reality of that risk, or the similar example I often mention of a person standing on wet grass and reaching through the door of an outhouse to turn on a (metalclad) light switch just inside the door just before they enter.

However, when it comes to 'risk assessment', those risks are unbelievably small. The major potential risk arises with a TN-C-S supply that 'loses its neutral' - but that, in itself is extremely rare (something that most people will probably never encounter in a lifetime), and unlikely to persist for long. The amount of time anyone spends touching an outside tap or outhouse lightswitch is also extremely small - a tiny tiny proportion of their life. Hence, if one multiplies those two probabilities (sorry EFLI!) together, the chances of anyone touching an outside tap or outhouse light switch at the very when time there is a lost neutral is, as they say, 'vanishingly small' - I suspect less than the average chance of being struck by lightning, and considerably lower than very many risks that we accept as 'a fact of life'.

Kind Regards, John

 

[EFLImpudence]

The boundary between the equipotential zone and the "outside" extranious world.

Isn't it only the electrical equipment and bonded parts which constitute 'the equipotential zone'?
Surely the 'outside extraneous world' encroaches inside where there is, for example, flagstones on mud or damp walls of certain construction.

It is when crossing that boundary that the hazard is greatest.

Where is that boundary in the above examples?

Standing on wet grass and touching the outside tap that is bonded into the equipotential zone is just one example of crossing the boundary

It is.
Why does the equipotential zone not extend to the outside tap area - or shed?
How can a brick wall be a defining limit of an equipotential zone?
Would the kitchen tap and living room switch be another example in such a building as above?


Have you installed suitable flooring below your outside tap?
Why is plastic pipe not demanded for outside taps?
Indeed, for all plumbing. It seems a much simpler solution than messing about with a lot of electrical 'safety' measures.

 

[JohnW2]

Why does the equipotential zone not extend to the outside tap area - or shed?

It would if one effectively bonded the 'ground' in the vicinity of a tap or shed - but we don't do that, so it isn't. An equipotential zone is a zone that has been made equipotential - it is not necessarily bordered by walls of a building and, indeed, does not necessarily have to be (partially or at all) within a building - in the presence of livestock, one might constitute, say, an outdoor paddock as an equipotential zone.

The fact that that, in certain circumstances (and in the absence of 'bonded ground'), the tap (or things in shed) and nearby ground can be at different potentials is proof that the 'nearby ground' is not part of the house's (or shed's) equipotential zone.

Why is plastic pipe not demanded for outside taps? Indeed, for all plumbing. It seems a much simpler solution than messing about with a lot of electrical 'safety' measures.

Indeed, if one felt that the risk was great enough to to warrant being addressed, then required (electrical) isolation of the outside tap from the indoor plumbing system would be the simplest way to achieve it. Similarly if the existence of extraneous-c-ps was felt to introduce risks great enough to warrant addressing, then a requirement for (electrical) isolation at the point of entry might make more sense than bonding (although probably impractical in the case of some extraneous-c-ps - like structural metal). However, in both those cases the magnitude of risk is so small that it is presumably not considered necessary to impose any regulatory requirements to reduce them even further (or 'eliminate' them).

Kind Regards, John

 

[EFLImpudence]

My questions were partly rhetorical.

It would if one effectively bonded the 'ground' in the vicinity of a tap or shed
- but we don't do that, so it isn't.

That's not true. The equipotential zone only includes, and protects against contact with two, items of equipment or bonded parts.
We do not 'bond the ground' inside a property; it is just not (usually) accessible because of floor boards but sometimes it is - and nothing is done about it.

An equipotential zone is a zone that has been made equipotential

It, though, only includes the electrical parts; not the ground or walls.
An outside tap and adjacent socket would be part of the equipotential zone. The ground below is a separate matter.

it is not necessarily bordered by walls of a building and, indeed, does not necessarily have to be (partially or at all) within a building - in the presence of livestock, one might constitute, say, an outdoor paddock as an equipotential zone.

Exactly.
It has to be protected form potential difference between different parts of the ground for animals; nothing would be done only for people.
This is not the same as an equipotential zone in a house.

The fact that that, in certain circumstances (and in the absence of 'bonded ground'), the tap (or things in shed) and nearby ground can be at different potentials is proof that the 'nearby ground' is not part of the house's (or shed's) equipotential zone.

Neither is the ground within a house; it is just not (usually) accessible.

This is my point about distance.
A shed may be nearer the installation bonding than are parts of the main building, so what is the difference?

 

[JohnW2]

That's not true. The equipotential zone only includes, and protects against contact with two, items of equipment or bonded parts.

That might be an electrician's view, but (in my opinion) it's not really correct ...

We do not 'bond the ground' inside a property; it is just not (usually) accessible because of floor boards but sometimes it is - and nothing is done about it.

An equipotential zone is an equipotential zone - i.e. the necessary steps have been taken to ensure that everything touchable within that zone is 'equipotential' (or, at least, as close to equipotential as can be achieved). It is very true that certain things (like walls and floors) are usually deemed not to require any attention (bonding), since they are regarded as electrically 'floating' (i.e. not having a low enough impedance path to anything else to be of any concern). However, we have recently discussed the fact that this may sometimes not be a reasonable assumption in relation to floors (and bernard often suggests the same about his walls) - but the fact that, in such circumstances, we very rarely attempt to bond such parts means that, strictly speaking, we are not creating a true equipotential zone (even though we probably should).

It, though, only includes the electrical parts; not the ground or walls.

See above.

An outside tap and adjacent socket would be part of the equipotential zone. The ground below is a separate matter.

One can define the boundaries of "the zone" however one likes. The normal convention is to restrict it to things within the building or (as your tap and socket) electrically connected to the MET inside of the building, but that doesn't have to be the case.

It has to be protected form potential difference between different parts of the ground for animals; nothing would be done only for people.

If "nothing is done" and the 'ground' in question (even if it is the floor within a building) has a sufficiently low impedance path to 'something' (be that 'true earth' or something else), then, strictly speaking, one does not have an equipotential zone.

This is my point about distance. A shed may be nearer the installation bonding than are parts of the main building, so what is the difference?

As I've said repeatedly, I think that 'distance' is a red herring. Whether within a house or in an an outbuilding distant from the house, if the floor has a sufficiently low impedance path to true earth, then it must be bonded in order to create an equipotential zone. The fact that we very rarely do it (in either house or outhouse), even when there is a relatively low impedance, means that we are not creating a true equipotential zone, not (necessarily) that there is no need to create such a zone.

Kind Regards, John

 

[EFLImpudence]

AIUI touch voltage is the voltage wrt E that an earthed exposed-conductive-part will rise to before the protective device operates.

It may be the term also used for that example (not sure), but -

50V is considered the maximum safe voltage, and if you take the simplistic view of the operation of a 30mA RCD, that's where the figure of 1667Ω comes from.

Doesn't that only apply to contact between two parts - exposed-cps and/or extraneous-cps when both are live?
That is for supplementary bonding considerations in special locations. No consideration is generally given elsewhere to the actual value.

Were the contact between a live part and true earth the (touch?) voltage will be 230V, will it not?

 

[JohnW2]

Doesn't that only apply to contact between two parts - exposed-cps and/or extraneous-cps when both are live? ... That is for supplementary bonding considerations in special locations. No consideration is generally given elsewhere to the actual value. ... Were the contact between a live part and true earth the (touch?) voltage will be 230V, will it not?

That's why I am never sure exactly what people mean when they talk of 'touch voltage'.

BAS was talking about the potential difference between 'a part' and true earth, but that 'part' is not 'live' in the sense of being at 230V relative to true earth. He was talking about a situation in which an exposed-c-p became 'live' (as a result of a fault to L) but, assuming that CPCs were in place, that potential would be a lot less than 230V (wrt earth) with TN, but could approach 230V with TT. However, as I said, if there is an adequate equipotential zone, the fact that the potential relative to earth (whether one calls it 'touch voltage' or not) can be close to 230V (wrt earth) with TT should not present a hazard, since anything else simultaneously touchable should be at (almost) the same potential ('almost' because, as I said, there will be a tiny difference due to the VD in the CPC carrying the fault current).

Kind Regards, John

 

[EFLImpudence]

BAS was talking about the potential difference between 'a part' and true earth,

He did not say true earth.

but that 'part' is not 'live' in the sense of being at 230V relative to true earth.

Why not?

He was talking about a situation in which an exposed-c-p became 'live' (as a result of a fault to L) but, assuming that CPCs were in place, that potential would be a lot less than 230V (wrt earth) with TN,

Yes, but I did say true earth so why not 230V if the ground is not bonded?

but could approach 230V with TT.

Surely not as the ground is bonded.

However, as I said, if there is an adequate equipotential zone, the fact that the potential relative to earth (whether one calls it 'touch voltage' or not) can be close to 230V (wrt earth) with TT should not present a hazard, since anything else simultaneously touchable should be at (almost) the same potential ('almost' because, as I said, there will be a tiny difference due to the VD in the CPC carrying the fault current).

I presume by adequate equipotential zone you are including bonded ground but how was this achieved?

 

[JohnW2]

He did not say true earth.

He didn't. However, I assumed that's what he meant - but maybe I was wrong. However ....

Why not?

Because it is connected to true earth (at the transformer) through a path of (roughly) half the EFLI (at least with TN-C-S). All things being equal, with TN-C-S that would mean that the potential of the exposed-c-p would rise to about half of the supply voltage above true earth (i.e. nominally about 115V), not 230V.

Yes, but I did say true earth so why not 230V if the ground is not bonded?

Maybe I should not have written "a lot less" but, as above, 115V is appreciably less than 230V. My point was the contrast with TT, when the potential of the exposed-c-p will rise to something approaching '230V'.

Surely not as the ground is bonded.

What 'ground' - that in the vicinity of the earth rod? If so, it is 'bonded' by the Erathing Conductor and rod.

In any event, I don't really understand your comment. If one connects L (with an impedance back to tranny of a fraction of an ohm) via an exposed-c-p, CPC, MET and Earthing conductor to a rod which has a resistance/impedance of, say, 50Ω to true earth, then Messrs Kirchoff and/or Ohm will tell you that the potential of the exposed-c-p (wrt true earth) is going to be something like 98% of the full line voltage.

However, as I keep saying, all this talk about 'potentials above true earth' is only relevant (other to someone partially in, and partially outside, of the equipotential zone) if there is not an adequate equipotential zone, since, if there is an adequate equipotential zone, nothing simultaneously touchable will be at an appreciably different potential from that of any exposed-c-p.

I presume by adequate equipotential zone you are including bonded ground but how was this achieved?

I meant what I wrote - "adequate" (to achieve an acceptable equipotential zone). If the 'ground' (do you mean floor?) needs bonding to achieve that, then it should be bonded, but if (as in the most common situation within a building) it does not need bonding (since it is essentially 'floating'), then it doesn't.

Kind Regards, John