TT to PME?

[JohnW2]

The trouble is you will have to really know your stuff and be able to justify any deviations from the regulations that you have made if you are unlucky enough to be up in front of the beak should a problem occur !

Very true - although, as we all know, 'being up in front of the beak' is, despite all the talk about it, something which is going to occur only very rarely.

The more interesting question, effectively raised by Bernard, is how a (hopefully sensible) court would react if someone's defence was that they had complied with all the regulations, yet it was being argued (by the 'prosecution') that anyone who 'knew their stuff' should have realised that, even though 'compliant', what they had done was not really safe. If the defendent was a 'qualified electrician', the court might just buy that latter argument - we all know that "acting under orders" (cf "complying with regulations") is a defence which has been known to fail!

Kind Regards, John.

 

[EFLImpudence]

I presume, in that case, the prosecution would first have to prove (to the Judge or Jury) that the regulations were wrong.

 

[JohnW2]

I presume, in that case, the prosecution would first have to prove (to the Judge or Jury) that the regulations were wrong.

To do that explicitly would obviously be one course, but I don't think the only one. I imagine that the court could choose to largely ignore the regulations and simply test whether (on the basis of engineering principles, not regulations) a 'competent qualified electrician' would/should regard what had been done as safe.

We all know how/when the inadequacy of an "acting under orders" defence first became widely publicised. At that time, I doubt whether there was much time wasted on debating whether or not it was wrong for 'officialdom' (c.f. regulations) to order soldiers to 'go out and cold-bloodely kill a few thousand civilians'. Instead, the court will presumably have concentrated on whether or not the individuals on trial were guilty of doing something that was clearly 'wrong', regardless of the reason they did it.

Kind Regards, John.

 

[Risteard]

A proper "earth" which could pull the steel armour to a voltage above ground via a path from the neutral ( which regulations say should be treated as live conductor )

Firstly, the Regulations do not state that the neutral should be treated as a live conductor. They state that the neutral conductor is a live conductor, as are line conductors. Each are equally live conductors - not one moreso than the other.

Is it not also the case that the Regulations state that by convention a PEN conductor is not considered to be a live conductor?

 

[westie101]

To do that explicitly would obviously be one course, but I don't think the only one. I imagine that the court could choose to largely ignore the regulations and simply test whether (on the basis of engineering principles, not regulations) a 'competent qualified electrician' would/should regard what had been done as safe.

As stated earlier, every installation is unique, every problem arising is unique as the solution at one location cannot be guaranteed to work at another location.
Each fault is unique
etc.etc,

To hope that a set of guidelines can for a defence given the actual conditions is, sorry to say, rather naive.
Yes, follow the guidelines but be prepared to stop and think if they fit each situation and work it out for yourself, not forgetting to record what ypu do and why.

 

[EFLImpudence]

Do we have an example?

To repeat:
The proposition is that an electrician has made an installation compliant with BS 7671.
Yet this was deemed 'unsafe' and he should have known better.

By deduction, therefore, (that part of) BS 7671 is 'unsafe' - i.e. dangerous.

 

[SimonH2]

If a number of items are bonded and one of them is also "earthed" then electrically all those items are connected to "earth" but the regulations and electricians who stick to those regulations seem to deny those items are earthed insisting they are only bonded.

Do you have an example of that, because that's not what I see people saying.

"Bonding" is the act of connecting a number of items together so that they cannot be at different potentials - principally to avoid a shock risk to someone in contact with the two items.

"Earthing" is the act of connecting something to 'earth' so that it cannot be at a significantly different potential to 'earth' and/or to ensure that sufficient current will flow during fault conditions to activate a protective device to disconnect the supply.

Now, if you bond several items together, and also earth one (or more) of the items (as it usually the case), then the rest will be incidentally earthed but that does not change the purpose of the bonding.

Consider an extreme situation - a room which is inside a metal box (as some site offices etc are).

For a person inside the box, the presence or otherwise of an earth is irrelevant. All that matters is that all conductive parts are bonded together. It wouldn't matter to the occupant if the box (and everything in it) were connected to the live of the supply. Indeed, I believe this is sometimes done in some specialist situations (very high voltage test labs come to mind) where the test personnel may be inside a faraday cage at high voltage during a test - with only a measurement 'reference earth' brought in to the measuring equipment but not accessible to the test personnel.

Obviously, having a site office which is live would be hazardous to people and animals outside who are touching the 'earth' or other bits of conductive material which is earthed - not to mention the problem of people getting in and out between the two zones. So we earth the box for their benefit.

Earthing the box incidentally makes the bonded parts also earthed - but does not alter the purpose of the bonding. I think what you are seeing is people point out the difference and incorrectly conclude that they are arguing that the bonded items are not earthed.

 

[JohnW2]

Do we have an example?
To repeat: The proposition is that an electrician has made an installation compliant with BS 7671.
Yet this was deemed 'unsafe' and he should have known better.
By deduction, therefore, (that part of) BS 7671 is 'unsafe' - i.e. dangerous.

I'll try to think of a good example - those that have so far come to mind would probably be too easy to throw flak at!

However, I don't think that your deduction is necessarily sound. A set of regulations, be that BS7671 or whatever, cannot hope (and wouldn't pretend to be able to) cover every possible situation/eventuality - and I therefore don't think that the IET would ever claim that total compliance with BS7671 guaranteed that a particular installation would be safe.

I therefore think that it is probably quite possible for a particular installation to be totally compliant with BS7671 and yet, given the particular circumstances, also not considered to be very safe by those 'in the know'. Don't you think that could happen?

Kind Regards, John.

 

[JohnW2]

"Bonding" is the act of connecting a number of items together so that they cannot be at different potentials - principally to avoid a shock risk to someone in contact with the two items.
"Earthing" is the act of connecting something to 'earth' so that it cannot be at a significantly different potential to 'earth' and/or to ensure that sufficient current will flow during fault conditions to activate a protective device to disconnect the supply.

I think that I'd probably be more comfortable with those definitions if the bit I've emboldened wasn't there - since it sounds dangerously close to 'bonding'.

Having said that, I think you may have, probably unwittingly, illustrated where some of the confusions comes from. Consider a Class I metal electric kettle - what is the purpose of the G/Y insulated conductor in the cable to it? Certainly the second of the bits of your description of "earthing" above - but, in reality, also the other bit of your descriptrion (which I have emboldened). Given the only reason (I can think of) for wanting to prevent the kettle from being at a potential other than earth would be to prevent (reduce the risk of) it being at a significantly different potential from nearby things which are at earth potential (due to being connected to installation's CPCs and/or for more incidental reasons) - and that sounds rather like 'bonding' to me. So maybe the G/Y in that kettle lead is there to serve both functions?

Kind Regards, John.

 

[EFLImpudence]

A set of regulations, be that BS7671 or whatever, cannot hope (and wouldn't pretend to be able to) cover every possible situation/eventuality - and I therefore don't think that the IET would ever claim that total compliance with BS7671 guaranteed that a particular installation would be safe.

I therefore think that it is probably quite possible for a particular installation to be totally compliant with BS7671 and yet, given the particular circumstances, also not considered to be very safe by those 'in the know'. Don't you think that could happen?

Hmm. Probably not bearing in mind that the electrical regulations do not specify what to do but what to achieve.

I have not (quickly) thought of an example which would lead to the case in question.

 

[EFLImpudence]

"Bonding" is the act of connecting a number of items together so that they cannot be at different potentials - principally to avoid a shock risk to someone in contact with the two items.
"Earthing" is the act of connecting something to 'earth' so that it cannot be at a significantly different potential to 'earth' and/or to ensure that sufficient current will flow during fault conditions to activate a protective device to disconnect the supply.

I think that I'd probably be more comfortable with those definitions if the bit I've emboldened wasn't there - since it sounds dangerously close to 'bonding'.

Agreed.

 

[JohnW2]

I therefore think that it is probably quite possible for a particular installation to be totally compliant with BS7671 and yet, given the particular circumstances, also not considered to be very safe by those 'in the know'. Don't you think that could happen?

Hmm. Probably not bearing in mind that the electrical regulations do not specify what to do but what to achieve.

In the sense that there are a number of very vague 'catch all' clauses in the regs, I suppose that, in a literal sense you are probably right - i.e. that anything which was regarded by experts as unsafe would probably thereby be a violation of one of those 'catch alls' [much the same as with Part P - which consists only of a single 'catch all' statement!]. However, I was thinking more about compliance with the letter of all the specific/explicit requirements of the regs. I'm sure one could be compliant with all them and yet not be safe enough for me!

Kind Regards, John.

 

[JohnW2]

I think that I'd probably be more comfortable with those definitions if the bit I've emboldened wasn't there - since it sounds dangerously close to 'bonding'.

Agreed.

Glad you agree - but how do you feel about the second part of what I wrote? Do you also agree that the G/Y in a kettle lead (to a Class I metal kettle) is probably there for bonding as well as earthing purposes? [before you answer 'no', please consider a L-E fault of sufficient impedance not to operate the OPD with the prevailing Zs].

Kind Regards, John.

 

[EFLImpudence]

Do you also agree that the G/Y in a kettle lead (to a Class I metal kettle) is probably there for bonding as well as earthing purposes? [before you answer 'no', please consider

I don't think there is anything to consider before answering NO.
Are you suggesting your kettle is an extraneous part and should be 'main' bonded?

considering a L-E fault of sufficient impedance not to operate the OPD with the prevailing Zs].

How is that to happen?
There is an inbuilt impedance of sufficient impedance not to operate the opd (13A fuse) of 29Ω (2kW kettle) .
Are you considering a fault L to E of, what, 10Ω ? - in the kettle making it 'a bit live'? Wouldn't the kettle and cpc just become a neutral and the fault burn away?

What would be the point of 'supplementary' bonding it to the toaster?

 

[JohnW2]

Do you also agree that the G/Y in a kettle lead (to a Class I metal kettle) is probably there for bonding as well as earthing purposes? [before you answer 'no', please consider

I don't think there is anything to consider before answering NO. Are you suggesting your kettle is an extraneous part and should be 'main' bonded?

No, of course not!

considering a L-E fault of sufficient impedance not to operate the OPD with the prevailing Zs].

How is that to happen? There is an inbuilt impedance of sufficient impedance not to operate the opd (13A fuse) of 29Ω (2kW kettle) . Are you considering a fault L to E of, what, 10Ω ? - in the kettle making it 'a bit live'? Wouldn't the kettle and cpc just become a neutral and the fault burn away?

To be honest, I was overlooking the 13A fuse, and thinking of a B32 protecting the circuit I also should have written "...not to operate the OPD quickly enough to necessarily save life/limb....".

A better example would be a cooker on a circuit with a B40 MCB and an L-E fault within it of about 1.5Ω. That would represent a fault current of about 150A, which would take a B40 about 20 seconds to disconnect. With a Zs of, say, 0.8Ω, for that 20 seconds the cooker would be at about 80V above (true) earth - and 80V for 20 secs can certainly be damaging or fatal if one is unlucky.

However, this whole discussion is totally academic and semantic, since there would usually be no hazard in the situation I've just postulated. IF there were a way of 'earthing' the cooker (so as to provide the required disconnection functionality) without also 'bonding' it to the installation's CPCs, then the above would represent a hazard - but that is obviously just not possible. In reality there obviously is no way of earthing the cooker without also 'bonding' it to (rendering it roughly equipotential with) anything else connected to the CPC network. Hence, in reality, that 80V above true earth for 20 seconds (or for 20 years!) would not usually represent a hazard, since any other exposed-conductive-part (and also any bonded pipework etc.) should also be at roughly the same potential.

However, using the definitions with which you have agreed, the reason why that 80V is not dangerous in the situation I postulated is not because the cooker is 'earthed' (the earthing is not going to provide rapid enough disconnection to necessarily save life/limb) but, rather, because it is, via CPCs, 'bonded' to (rendered roughly equipotential with) any nearby exposed-conductive-parts (or bonded pipework etc.).

Kind Regards, John.