When Is 0.5ohms Not 0.5ohms?

[Darkwood]

You forget we have blinkered views on products that we buy will be to a certain standard just because we expect it... as well as this copper is a medium reactive substance in the presence of moisture so internal contacts will oxidise in an atmosphere that is regular subject to damp conditions and also mucky contaminated plug pins can leave a residue in the sockets all of which can easily equate to a very small 0.5 ohms resistance.

I found it quite common to have elevated R1+R2 reading on sockets in houses that have damp issues and mold up the walls but many negate this while testing and implement the reading through the socket as a wiring problem ... or even spuring out of the ring main

 

[spark 68]

Nick I would be more bothered about the slight differences r1 and rn on your end to ends, IME they usually measure within 0.01 of each other unless there is a slack terminal at one of the S/Os.

I have had where the conductors have been twisted together behind the S/O giving reasonable end to ends before but a high/er reading at the font of the outlet due to being slack at the terminals at the back.

Re: your parallel path, sometimes this has been a boiler spurred off the ring final somewhere and reading via the boilers supply CPC and the gas/water pipes' bonding.

Other than that, if a socket is giving quite a higher reading at the front I usually just replace it, sometimes you get quite a difference between the two outlets on the same double S/O, like DW said if it is more than just a little out then replace it.

 

[Badged01]

well...no doubt you walked off from this little post thinking `HA...thats told him then`.....

Now, now, dont get itchy under the collar. Im not into scoring cheap browny points!

All I'm doing is explaining my way of Zs'ing as opposed to yours of direct measurement. What you want to read in to that is upto you ......

 

[Darkwood]

I'm not sure I do concur, as I just said I trust Ohm's Law and my meter's R Low Ohms accuracy (got the calibration certificate to prove it and a resistance that persists even after 230V applied is not a mirage.

Well use Ohms law then and use your meters dc voltage as your value then use your operational voltage as the value and see the difference!



What kind of differences between front and rear measurement would you consider large and small in this context?

The kind that means a pass or fail!.... If is a pass either way then no issue if a fail like any situe' you do a few initial checks to ensure the basics are not the cause like a loose terminal etc on the socket.... if it passes testing behind but not from the front then we have either a test lead with plug adapter issue or a socket issue.

 

[NickD]

You forget we have blinkered views on products that we buy will be to a certain standard just because we expect it... as well as this copper is a medium reactive substance in the presence of moisture so internal contacts will oxidise in an atmosphere that is regular subject to damp conditions and also mucky contaminated plug pins can leave a residue in the sockets all of which can easily equate to a very small 0.5 ohms resistance.

Indeed. And my point is that if it is that mucked up, it should fail.

 

[NickD]

The kind that means a pass or fail!

I was kinda hoping for something off the top row of the keyboard? And I don't mean F1 F2 F3 etc.

 

[NickD]

Nick I would be more bothered about the slight differences r1 and rn on your end to ends, IME they usually measure within 0.01 of each other unless there is a slack terminal at one of the S/Os.

True, and up to a point a good reason for me to see if the customer cares to opt for having all the sockets swapped. I know that sounds like overkill but seriously, about two thirds of them I had to in-out-in-out-in-out switch-on-switch-off-switch-on-switch-off ad nauseam before they'd give reasonable readings.

 

[spark 68]

Just a thought, did you zero your meter correctly with the plug lead on ?, my megger MFT (separates) doesn't have a plug lead for the R1,R2 etc. I use a socket adaptor that takes the normal 4mm test prods, and I have been caught out before by this.

Usually I find most of the time where I have had slightly higher readings at the front it is just poor plug/socket/lead interfacing, operating the switch once or twice too sometimes helps.

 

[Darkwood]

Indeed. And my point is that if it is that mucked up, it should fail.

Its is a false fail as i said but i also said it may show signs of a actually contact switching issue (poor old/worn plates) and i would routinely replace such a situe'...

what im saying is in theory the test voltage your using is prone to false negatives when testing through sockets but because of the many different causes that create this its wise to flag the socket for replacement but not necessary fail the circuit par-ce' if proved behind it is compliant as its known by the ELI results a fault will clearly trip within specified time as the voltage is real to normal working conditions.

 

[spark 68]

Damn you DW, lol

You are getting too many likes from me lately. you must stop talking so much sense. lol

 

[NickD]

Just a thought, did you zero your meter correctly with the plug lead on?

Indeed I did.

 

[NickD]

the test voltage your using is prone to false negatives when testing through sockets

Flipping the coin, what you're saying is that the resistance is heavily non-ohmic, and so we should trust that a resistance that is quite clearly there when accurately tested with low test voltage will become negligible when 230V is applied. I don't see any reason to go with that hypothesis, but if you have one I'd like to hear it. Plus we know bog all about the nature of the resistance (slack contacts? tarnish? oxidation? general muck?) so how can we make confident predictions that it'll start conduction happily at 230V?

I suppose the bigger philosophical questions is whether the fixed wiring system ends at the terminals at the back of the socket or the socket receptacles at the front....

 

[Darkwood]

I totally understand NickD's concerns but there is a sense of drummed in modern teaching that gives you a parrot fashion response yes pass or no fail... we were taught to investigate to check for false negatives or positives and evaluate if you need to adjust your readings ... if the circuit is fine from behind the socket how can it be justified to say your going to fail it when it passes ELI at socket front but fails R1+R2 at socket front ... this shouts out your test voltage is a large factoring issue so from there you then weigh up your options and yes if a large difference id be recommending a new socket as expressed before as the cause cannot be always pinpointed.

 

[spark 68]

Actually the 30/32 A OCPD (and Zs) is to protect the RFC, so if it measures okay at the back the circuit itself is fine, as DW correctly points out the S/O should be flagged for replacement, but to whether that is an actual fail is open to debate.

 

[NickD]

how can it be justified to say your going to fail it when it passes ELI at socket front but fails R1+R2 at socket front

Because the other thing that's different between those two scenarios, as well as the test voltage, is the presence of parallel paths. R1+R2 done properly is immune to them, ELI is at their mercy. I would bet my car that I'd have failed my 2394/2395 if I'd live tested Zs at a socket then used it as a Schedule Of Tests sheet result.

 

[Darkwood]

Flipping the coin, what you're saying is that the resistance is heavily non-ohmic, and so we should trust that a resistance that is quite clearly there when accurately tested with low test voltage will become negligible when 230V is applied. I don't see any reason to go with that hypothesis, but if you have one I'd like to hear it. Plus we know bog all about the nature of the resistance (slack contacts? tarnish? oxidation? general muck?) so how can we make confident predictions that it'll start conduction happily at 230V?

I suppose the bigger philosophical questions is whether the fixed wiring system ends at the terminals at the back of the socket or the socket receptacles at the front....

Thats what the Plug in ELI check will confirm ..forget its a socket and tell me how would you do the test (cooker point, spur etc ) your drop it forward and your point is really lost as what if the spur contacts are the same etc you would still miss them through routine testing. I admire anyone who goes this deep to understand their career as many don't give it a thought but try not to overthink this one..we are here to ensure safety and as of yet y advise would not compromise safety merely give you more flexibility in deciding if to pass a circuit or fail it.

 

[NickD]

Actually the 30/32 A OCPD (and Zs) is to protect the RFC

Rugby Football Club? I guess you mean the fixed wiring but the abbreviation escapes me.

 

[Silly Sausage]

What does 0.5 Ohm actually represent?
These measurements are taken getting towards the arse end of the instrument's range.
Then there are all the errors to take into account, probe contact, etc. etc. etc.......

Plus, I usually find a smearing of copper ease on plugs prongs always improves things!

 

[Darkwood]

Because the other thing that's different between those two scenarios, as well as the test voltage, is the presence of parallel paths. R1+R2 done properly is immune to them, ELI is at their mercy. I would bet my car that I'd have failed my 2394/2395 if I'd live tested Zs at a socket then used it as a Schedule Of Tests sheet result.

Im not telling you to totally rely on ELI results against obscureR1+R2 Im saying evaluate any difference and do a few small checks to ensure the socket isn't the cause then make an educated judgement to whether fail or pass the circuit on the merits of the front and rear results.

This subject goes very much deeper into areas you haven't even strayed across before like taking an PSCC reading when the 11kv sub TX is very close then see what crazy inconsistent readings meters give you due to resolution issues.

 

[spark 68]

Because the other thing that's different between those two scenarios, as well as the test voltage, is the presence of parallel paths. R1+R2 done properly is immune to them, ELI is at their mercy. I would bet my car that I'd have failed my 2394/2395 if I'd live tested Zs at a socket then used it as a Schedule Of Tests sheet result.

I am not so sure on that statement, when I did my 2391 it was generally accepted that the measured Zs could be different from the measured/calculated Zs= Ze + (R1+R2), usually lower due to parallel paths.

There are two distinct schools of thought on this, some people prefer the "worst case" ie. measured /calculated (R1+R2)+ Ze, and others who prefer the in service measurement.
Both are acceptable.

Edit: RFC = Ring Final Circuit

 

[NickD]

forget its a socket and tell me how would you do the test (cooker point, spur etc ) your drop it forward and your point is really lost as what if the spur contacts are the same etc you would still miss them through routine testing.

An excellent point. Yes testing the socket front is picking up a fault you would never pick up with a fused connection unit to a fixed appliance or whatever. So is failing a socket for excessive Ze+R1+R2 measured at the front a True By The Book Failure or not? Probably not. Do you fail it in practice? Depends on the scenario I think. What is the risk if you don't fail it? A house fire, maybe. A dead family. What is the risk if you do fail it? Maybe an argument, maybe a client who doesn't use you again, maybe (depending on the contract) getting taken to court. This thinking inclines me towards failing it.

I admire anyone who goes this deep to understand their career

Only way I know how

 

[NickD]

These measurements are taken getting towards the arse end of the instrument's range.

Disagree. Measuring very low resistance is the bread and butter of the R Low Ohms range of the MFT and it has a calibrated accuracy capable of making the measurement, to an order of magnitude better than the 0.5ohm being measured.

Then there are all the errors to take into account, probe contact, etc. etc. etc.......

Yeah, I know, bimetallic voltages etc. But these should be catered for *within* the meter's tolerances in typical usage.

 

[Darkwood]

I have a socket that passes both R1 + R2 and ELI then i plug in a 30m extension that means the extension outlet falls well below pass value if i tested from it! ...where do i stand then???? We could go on forever here thats why i said you overthink this situe and subject to difference at front and back you kind of get an educated guess whether to recommend changing the socket.

 

[NickD]

I have a socket that passes both R1 + R2 and ELI then i plug in a 30m extension that means the extension outlet falls well below pass value if i tested from it! ...where do i stand then????

Thirty metres away.

 

[Darkwood]

Should have seen that coming good response ...my bed time cheers! :stooge_curly:

 

[NickD]

I am not so sure on that statement, when I did my 2391 it was generally accepted that the measured Zs could be different from the measured/calculated Zs= Ze + (R1+R2), usually lower due to parallel paths.

Indeed, and that's the point, parallel paths can offer an unfair assist to help pull a Zs assessment down below the maximum value. Go back to basics - why when taking a Ze measurement at a DB do we not just clip onto the earth bar or MET, but instead have to whip out the earthing conductor and clip onto that? This is a standard assessment test/check in exams, practicals, Part P registration assessments and not doing it or not knowing why you do it is a good way to crash and burn. Not placing reliance on parallel paths in other situations is just an extension of that logic.

 

[NickD]

Should have seen that coming good response ...my bed time cheers! :stooge_curly:

Before you judge a man, walk a mile in his shoes. That way you're a mile away from him and you've got his shoes.

 

[spark 68]

Indeed, and that's the point, parallel paths can offer an unfair assist to help pull a Zs assessment down below the maximum value. Go back to basics - why when taking a Ze measurement at a DB do we not just clip onto the earth bar or MET, but instead have to whip out the earthing conductor and clip onto that? This is a standard assessment test/check in exams, practicals, Part P registration assessments and not doing it or not knowing why you do it is a good way to crash and burn. Not placing reliance on parallel paths in other situations is just an extension of that logic.

I did say there were two schools of thought on this, my own preferred is the "worst case method" measured Ze + measured (R1+ R2), thus not relying on parallel paths that may or may not be reliable, if this is compliant then I am happy, however I always record the actual measured Zs, unless it is impractical to do so.

BS7671 deems either method to be acceptable.

On an EICR it is quicker to do the measured, and just the R2 wandering lead method is usually sufficient, obviously proving polarity as well, horses for courses and whatever method is quicker or easier in a given situation.

 

[NickD]

I did say there were two schools of thought on this

But as I say, go back to basics. There is no two schools of thought on how you test for Ze at the origin, correct? There is "remove the earth conductor and measure directly onto that", and anything (other than something that achieves electrically exactly the same effect) else is simply wrong, correct? And if one understands why it is that way, then I find it difficult to see how one can fail to extend that principle more generally into one's testing wherever it is critical to be correct (EIC, EICR). I can understand why it's fine on a MWC where you just want a basic assurance. That's how I feel about it anyway.

BS7671 deems either method to be acceptable

But I challenge anyone to find where it says that it is perfectly OK to take and use a Zs measurement even if it incorporates parallel paths which are not any part of the by-design earthing arrangement for the item under test.

 

[spark 68]

Then why give you a choice of (R1+R2) or R2 on the schedule of test results ?

It depends who you talk to in the industry, both sides gives pro's and cons.

The Ze measurement is slightly different, and even that may be obtained by enquiry, sometimes where it is not possible to power down for example a Zdb measurement may be used.

I take it you are fairly new to testing then ?, and/or limited to domestic ?

Obviously you would not design a circuit that relied on parallel paths, but sometimes on an EICR there are limitations as to how far you can T&I.

 

[NickD]

Then why give you a choice of (R1+R2) or R2 on the schedule of test results ?

Don't understand the relevance, sorry?

The Ze measurement is slightly different

In what way? The core point is exactly the same - avoidance of the influence of not-to-be-relied-upon parallel paths in a critical measurement.

I take it you are fairly new to testing then ?, and/or limited to domestic ?

Domestic and (on paper) small commercial. New to testing but have a certain amount of relevant experience from 20+ years of electrical and electronic engineering plus stuff in my own time. Was an MIEE before there was even an IET to be an M of Would still be only I resent paying their bloody annual fee. Got an Electrical & Electronic Engineering degree of 1991 vintage. Someone mentioning Norton and Thevenin has scared me by reminding me how much I've forgotten

 

[spark 68]

You obviously don't understand what I am saying to you, if ever you test an industrial install that may have literally thousands of parallel paths you would understand.

Sometimes we are not permitted to power down, then we have to rely on either enquiry for the Ze, or a Zdb reading, in this case we would have to use a Zs measurement too, because calculated is not good enough to prove effective earthing exists, this would also include all parallel paths.

BS7671 allows us to record either (R1+R2) or R2 so long as we have a Zs measurement in the case of an R2 only measurement and the polarity is checked.

By using the (R1+R2) method we are checking the polarity at the same time, and we may also use a calculated Zs by adding the measured (R1+R2) to the measured Ze, this would not factor in any parallel paths, and this is my preferred method where possible.
Plus I like to include a measured Zs which may be lower than the calculated value, just to prove what value is actually present.

In the R2 method the measured Zs includes all of the parallel paths by default, and that is what is recorded.

Either method is acceptable, have a look at the schedule of test results in BS7671.

Both methods have their uses.

 

[Badged01]

Spare 10 mins so just had a skim of GN3/OSG on the whole Zs saga to try and get some clarity around where I've got my understanding from. When you've been doing it a particular way you tend to forget why you do it that way!! Anyway salient points condensed as I interpret them:

Filling in of the 'Schedule of test results' (P100 GN3 -- for EICR or EIC) refers to using either of the methods on P52 for Zs.

P52 gives ......
1) 'Live' Zs is made with neither bonding or earth removed.
2) Zs can be a measured value or a calculated value.

OSG P162 also mentions test or measure for Zs .... why didnt I think of looking there earlier!

So everyones a winner and is right in the way they test, you've all passed the 2395(?) ........ if thats how you interpret the requirements of BS7671

 

[Guest111]

looking at the original post and some of the replies regarding the difference between the voltage used for the test ie very low and the operating voltage,230 and how this affects readings ie 230 wont "see" the odd 0.05 ohm but the tester will,well it's a bit like hunting down a fault isn't it? sometimes you don't find it with 230 but IR at 1000v and all is revealed.

 

[NickD]

(GN3) P52 gives ......
1) 'Live' Zs is made with neither bonding or earth removed.
2) Zs can be a measured value or a calculated value.

OSG P162 also mentions test or measure for Zs ....

Yep, agree with that and GN3 actually states it is down to personal preference. And not a mention of parallel paths. In terms of what the books say, you're right. BS7671 seems to add nothing useful on the point. Well well. I have to admit to being pretty gobsmacked about them being apparently so relaxed about their implied suggestion of "just measure Zs if that suits your mood, don't worry about how much of the R2 is actually a parallel path through a boiler that's being ripped out next week".

 

[NickD]

it's a bit like hunting down a fault isn't it? sometimes you don't find it with 230 but IR at 1000v and all is revealed.

Yebbut that's for an obviously physical reason of stuff breaking down at higher voltages (plus on my meter for example, on 250V IR test the reading hits end of range at 200Mohm; on higher ranges it's 1000Mohm so anything between 200 and 1000 will be picked up as a reading).

 

[spark 68]

Well well. I have to admit to being pretty gobsmacked about them being apparently so relaxed about their implied suggestion of "just measure Zs if that suits your mood, don't worry about how much of the R2 is actually a parallel path through a boiler that's being ripped out next week".

That is because you are thinking in terms of domestic with only a few circuits.

The second method is allowed because that may be the only way to test a larger install.

Think of even a small factory unit with all metal clad accessories installed in metallic conduit and metal trunking which may be fixed to the structural steel, there will be many parallel paths and it would be virtually impossible to try and remove them all just to carry out an EICR it is simply not practical.
On even larger complexes with metal pipework, metal ducting and machines etc. it would be nigh on impossible unless you were ripping the whole place apart.

 

[Badged01]

.... "just measure Zs if that suits your mood, don't worry about how much of the R2 is actually a parallel path through a boiler that's being ripped out next week".

I think it boils down to you can't predict whats going to happen in the future, you can only play with the cards dealt to you now! Plus of course we all know people only use "competent/registered tradesmen" to do their work for them. And through their training and experience these tradesmen would clearly be appreciative of the problems they could cause .....

 

[NickD]

That is because you are thinking in terms of domestic with only a few circuits. The second method is allowed because that may be the only way to test a larger install.

No, I do understand that, absolutely; but my point is, why not clearly point out the parallel paths risk that comes with the direct Zs measurement rather than declare it to be a matter of personal taste? Something like "Measure Zs direct by all means if you must, but beware the parallel path Jabberwock, my son."

 

[spark 68]

No, I do understand that, absolutely; but my point is, why not clearly point out the parallel paths risk that comes with the direct Zs measurement rather than declare it to be a matter of personal taste? Something like "Measure Zs direct by all means if you must, but beware the parallel path Jabberwock, my son."

Because as a "competent person" you are supposed to know this, lol, GN3 and associated literature is only to guide you as far as the regs are concerned, it not meant to teach you how to T&I, only how to comply with the requirements of BS7671.