That is of course accepting that two measurements ( Ze & R1+R2 ) added together is a calculation.
Old wife's tail, myths, and other stories we were told as an apprentice, before the days when BS 7671 started.
- Thread starter ericmark
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Right, I`ll put it another way (I know my tutors in class went to pains with some students to explain and again at Elex Shows Seminars could not grasp it and they were eloquent than I).
Firstly if we know Ze and R1 & R2 then we can surmise Zs approximately therefore the equation holds good and we can measure Zs to see if it confirms it is "in the parish" of what we expect - without bonds.
It is common on say small minor works to just measure the Zs with bonds etc attached and for that reason any Zs we actually measure might be quite optimistic so it is not relevant to deduct Ze to give R 1 + R2 or instead to deduct R1 and R2 to give Ze, it would be too unreliable
Firstly if we know Ze and R1 & R2 then we can surmise Zs approximately therefore the equation holds good and we can measure Zs to see if it confirms it is "in the parish" of what we expect - without bonds.
It is common on say small minor works to just measure the Zs with bonds etc attached and for that reason any Zs we actually measure might be quite optimistic so it is not relevant to deduct Ze to give R 1 + R2 or instead to deduct R1 and R2 to give Ze, it would be too unreliable
Yes agreed , at best it is part measurement and part calculation usually and an approximation at best. but often good enough within some constrasint
... and, as I've said, that is (in my opinion) how one should do it (if one doesn't actually measure Zs with bonding disconnected), since that then gives a 'worst case' answer (i.e. indicating what the Zs would be if parallel paths were to disappear.
... but what the, I wonder, did you think he meant when he wrote:
I took him to mean '... and any rearrangement of that equation', which would therefore include Zs = Ze + (R1+R2) - but you are suggesting that he actually advocated that (despite it being simply a rearrangement of an equation which he said is something that he said we would "never ever ever" use?
Kind Regards, John
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Well, in a literal sense I suppose that adding two things together is 'a calculation', albeit a very trivial one.
If one so wished, one could measure R1 and R2 separately and add them to get "R1 + R2", but I don't think it would be particularly helpful to call that "by calculation"
In terms of what the Zs would be if parallel paths to earth were to disappear, adding Ze and (R1+R2) gives an exact, not approximate, answer.
Unless one or more of one's measurement/determinations is incorrect, 'measured Zs' (with bonding present) cannot be higher than Ze + (R1+R2). In the presence of bonded extraneous-c-ps, it will be lower - but, as I keepo saying, it would be 'dangerous' to assume that thjose parallel oaths would always exist. As you go on to say ...
... but that (calculating Ze from Zs and R1+R2) is not what we're talking about. As you say, Zs measured with bonding in place will commonly be 'optimistic' (which I call potentially dangerous') because it relies on bonded parallel paths remaining present.
The danger obviously is that if one undertakes such a measurement on a circuit in a TN installation, and gets an answer that the Zs is low enough for OPD-mediated ADS, if the supply pipe creating the parallel path gets changed from metal to plastic the next day, one might then end up with a Zs that was too high to facilitate compliant disconnection times, which I would say would be 'potentially dangerous' (i.e. an EICR C2).
Kind Regards, John
I might have got confused by who said what and all the formulae. Just take what I wrote as the way I think it should be done.
As in your house, you cannot count the water pipe as part of the Ze or Zs in case it disappears one day.
As in your house, you cannot count the water pipe as part of the Ze or Zs in case it disappears one day.
Do you mean what you implied by what you wrote in post #60 - namely that Zs should be determined by adding Ze to R1+R2 ("to discount bonding"), and not by direct measurement (with bonding present)?
If so, that's essentially what GN3 seems to be saying firts mentioning tyhe 'calculation' (from Ze and R1+R2), before mentioning "by direct measurement" (with bonding present) - and then goes on to say that one should 'take into account' (as, I've asked, "how?") the fact that such a figure will, in some senses, be 'erroneous' (in a 'potentially dangerous' direction) if there are any bonded extraneous-c-ps.
It's not such a problem in my house, because it is 'declared' and treated (by me) as TT, so people would presumably not be expecting the Zs of any circuit to be low enough for OPD-mediated ADS (and would, instead, look for the RCD protection).
As I've said, the 'real potential danger' of Zs measurements with bonding connected is in TN installations - since those measurement may 'confirm' that Zs figures are low enough to achieve the required disconnection times with the bonded metal water pipe present, but Zs might become too high to achieve those required disconnection times if (as can certainly happy=en) the metal pipe were to "disappear one day". As I've said,that would sound like a "C2" to me - to be acceptable, I think I would want to see a Zs measured with bonding disconnected, to conform that disconnection times would still be achieved even if the metal pipe were to disappear.
Kind Regards, John
Yes, I mean that.
Sorry, I must have misread The thread.
Ok.
A measurement, from the farthest point of the circuit, lower than [Ze + R1+R2] will show that the circuit is connected correctly and actually has a Zs measurement.
Yes, I know - as it should be.
Hence why it should be done as I am saying.
OK - so you, myself and (partially) GN3 say that's how it should be done, but GN3 also says that Zs may be determined "by direct measurent" (with bonding connected) and I strongly suspect that such is what many people do (plug in their MFT and press the button") - in which case, as I keep saying, the result can be misleading
Yes, it shows that - but, as I said, if it's done with bonding connected, it can give a misleading answer, because and apparently 'low enough' Zs (to achieve required disconnection times) might cease to be 'low enough' if the bonded extraneous-c-pwere to disa[ppear.
As above, we seem to be agreed about that, BUT it remains my suspicion that ('supported by GN3") many people probably just 'measure Zs' (with bonding connected).
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Absolutely John, that was what I was trying to imply. Although we might accept a Zs knowing that in some cases that Zs is potentially optimistic we must not use it in the equation in order to calculate back. Too risky.
As a pure maths exercise yes you can do that but as equation for electricians to use the NO. Dont do it, tends to get hammered into us at college etc and quite rightly.
If you want to measure or calculate Ze then fair enough, similarly with R1, R2, Zs, but never use the results as a "back calculation" far too much change of additional error in that method, there are already more than enough chances of error using our best efforts (meter accuracy, events on the day, ambient temp etc etc)
Yes, exactly, so is only allowed on minor works like additions etc, any proper work, (rewire, CU change) then no, the bonds etc must be removed (even with all the potential danger it brings in, which must be mitigated)
I don't really understand your position since you, EFLI and myself seem agreed that it is not 'safe' to rely on a direct measurement of Zs with bonding connected - so that (if Zs is to be measured with bonding connected) the only 'safe' way to determine Zs is by using 'the equation' [based on (R1+R2) and a measurement of Ze without bonding ] - but you seem to be suggesting the opposite in the above.
What am I missing ?
See above. If Zs is going to be measured with bonding connected, it seems that the only 'safe' way of determining Zs (the Zs that there would be if parallel paths were to disappear) is by using the equation?
Again, what am I missing?
I on't really understand what you mean by 'back calculation' - if one arrangement of an equation is acceptable, then so is any other arrangement - as with Ohm's law (i.e. given any two of current, résistance and voltage, one can calculate the third).
Can you clarify? As you will understand, I don't really follow your reasoning.
As I keep saying/implying all of these 'problems' would go away if the guidance was that Zs should be measured with bonding disconnected and I somewhat struggle to think of any significant downsides that would result from that approach.
Kind Regards, John
Needless to say, I agree - but the only 'guidances' I've ever seen say that one should not disconnect bonding when measuring Zs, and I don;t think I've ever seen an electrician do that.
As for "...only allowed on minor works", I'm not sure where that 'not allowed' comes from (I've seen it in no guidance) and, in any event, the 'potential danger' of measuring with bonding connected is no less if the work has been 'minor' than if it is 'major'
Kind Regards, John
Yup, basically , in an ideal world, Ze should be measured without bonds etc. If you are wiring a complete installation from scratch or a complete rewire of an existing installation then, hopefully, you should enough skill and knowledge to minimise the risk for a fairly limited time.
The equation applies (as near as it ever can) and could be transposed and should be just as correct as pure maths could show.
No problem.
So new/rewired or major works ( and EICRs/PIRs) justifies this risk and the equation holds good.
When adding a socket, lighting point etc etc then the risk of disconnect all para paths including bonding is difficult to justify, both for safety and the additional costs/upheaval that might bring.
A quick test of Zs at that socket to confirm it "looks like it might be OK" is often all that is done, an R1, R2 test might be done, but it would be unusual to disconnect bonds etc to test Ze. Any use of the formula by using Zs then subtracting R1 and R2 to give Ze would be unreliable therefore no use (not much) .
That`s why we don`t use it (we say it doesn`t work - well actually as an equation does work, after all it is an equation just like any other but we say "doesn`t work" to bring to attention it is unreliable so not much use).
As any electrician if they actually remove bonds to confirm Ze then tests Zs before reconnecting bonds on a small addition to an existing circuit (like an extra socket or an extra lighting point) an there will be extremely few.
where I said "far too much change of additional error in that method, there are already more than enough chances of error using our best efforts (meter accuracy, events on the day, ambient temp etc etc)" the word "change" was a typo, sorry it should have read "chance".
Metered tests are always dependant upon the actual accuracy of the meter at best, plus additional field errors on the day, huma errors plus other additional sources of error (such as how steady your source at origin is on that particular minute/hour/day/week/month and say ambient temperature. Any tests you make at the time of Ze, Zs, R1,R2 etc etc are only so good as plus/minus 1000 percent or 1000000 percent or something else, we minimise this to a reasonable extent but do not take extra step of using derived equations.
At best, any results we obtain, using our best efforts, might only be about just about good enough for our purpose if we cross our fingers hard enough and say a few hail Mary`s and our Fathers as well.
It is considered allowable to measure Ze without bonds with an instalation isolated if required and undertaken using robust procedures but is never considered reasonable to increase risks by energising a circuit(s) or parts thereof with bonding not connected because of risks.
Proper testing of Ze without and real world testing of Zs with. Renders the equation not useable - like comparing apples with oranges.
The equation applies (as near as it ever can) and could be transposed and should be just as correct as pure maths could show.
No problem.
So new/rewired or major works ( and EICRs/PIRs) justifies this risk and the equation holds good.
When adding a socket, lighting point etc etc then the risk of disconnect all para paths including bonding is difficult to justify, both for safety and the additional costs/upheaval that might bring.
A quick test of Zs at that socket to confirm it "looks like it might be OK" is often all that is done, an R1, R2 test might be done, but it would be unusual to disconnect bonds etc to test Ze. Any use of the formula by using Zs then subtracting R1 and R2 to give Ze would be unreliable therefore no use (not much) .
That`s why we don`t use it (we say it doesn`t work - well actually as an equation does work, after all it is an equation just like any other but we say "doesn`t work" to bring to attention it is unreliable so not much use).
As any electrician if they actually remove bonds to confirm Ze then tests Zs before reconnecting bonds on a small addition to an existing circuit (like an extra socket or an extra lighting point) an there will be extremely few.
where I said "far too much change of additional error in that method, there are already more than enough chances of error using our best efforts (meter accuracy, events on the day, ambient temp etc etc)" the word "change" was a typo, sorry it should have read "chance".
Metered tests are always dependant upon the actual accuracy of the meter at best, plus additional field errors on the day, huma errors plus other additional sources of error (such as how steady your source at origin is on that particular minute/hour/day/week/month and say ambient temperature. Any tests you make at the time of Ze, Zs, R1,R2 etc etc are only so good as plus/minus 1000 percent or 1000000 percent or something else, we minimise this to a reasonable extent but do not take extra step of using derived equations.
At best, any results we obtain, using our best efforts, might only be about just about good enough for our purpose if we cross our fingers hard enough and say a few hail Mary`s and our Fathers as well.
It is considered allowable to measure Ze without bonds with an instalation isolated if required and undertaken using robust procedures but is never considered reasonable to increase risks by energising a circuit(s) or parts thereof with bonding not connected because of risks.
Proper testing of Ze without and real world testing of Zs with. Renders the equation not useable - like comparing apples with oranges.
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