When Is 0.5ohms Not 0.5ohms?

[NickD]

why can't you use a 'live' test Zs value? Its a legitimate value in that it represents in-service conditions much more than a low voltage (R1+R2) test?

Because some of those in-service conditions might not be part of the design earthing arrangements, and therefore you would be in error to rely on them in a test. It's like how you have to pull the earthing conductor before measuring Ze. Take my case - I know there is an extra parallel path of 0.6ohms somewhere between my CPC and the MET. No idea what it is, how it is formed, what its nature is, whether I have any right to rely on it. Indeed the whole point of it not being part of my by-design CPC arrangement means I would be totally out of order to rely on it.

Let's say my existing CPC is too large to ever meet Ze+R1+R2 > Zsmax, and must and should fail. A Zs test takes advantage of that extra 0.6ohm in parallel that I have no right to take advantage of, and indicates a pass. Then the final circuit that the parallel path sits within is taken out of service. My circuit no longer meets 0.4s disconnection time, I have a dangerous situation, and my test failed to pick this up because it relied on a parallel path that I had no right to.

You may also loose your earth at a later date when the plumber is busy disconnecting/cutting all this green'n'yellow stuff getting in his way!

But then he's cut a by-design means of earthing / CPC that I had every right to rely upon. I can't mitigate against that. But I can mitigate against relying upon things that I shouldn't be relying upon.

 

[Badged01]

Then I think its best you keep to your way of testing and I keep to mine

 

[Silly Sausage]

Too many ifs and buts Nick.
How much time and money are you prepared to spend on a Periodic, on what is essentially a safe installation at the moment in time you test it? Remember, what happens in the future is not your responsibility.

 

[GLENNSPARK]

Then I think its best you keep to your way of testing and I keep to mine

different testers for different jesters

 

[Badged01]

There would be no need for one's test gear to meet GS38 really then!

Tut, tut! Most definately is if your are on 'live' equipment, circuits etc ..... EAWR permits one to go 'live' when its impracticable to do it 'dead'

 

[guest119]

Continuity testing is at something like 11volts and if tested at the rear and found to be good then fair enough.
You are basically trying to fail a socket because it wont respond correctly (to you) to a voltage way below what it was designed for.
ELI testing will inject a simulated 26amps (at the designed voltage) down the line and give you your satisfactory reading, what more do you want?

Boydy

 

[GLENNSPARK]

Tut, tut! Most definately is if your are on 'live' equipment, circuits etc ..... EAWR permits one to go 'live' when its impracticable to do it 'dead'

no Ze today then?...

 

[Badged01]

no Ze today then?...

Unless you have a method to do it "dead"?

 

[Silly Sausage]

Testing and Working Live are totally separate things.
I've explained my stance on this in the past and I'm not typing it all out again.
You'll never change my mind on this subject!

 

[Badged01]

I'm not into changing peoples minds, Im just into exchanging views and having some health debate and banter!

I personally don't know your view on working/testing but I gather you see them as two seperate points whereas I don't. Hell the forum would be boring if everyone agreed on everything and we all became little clones ........

 

[GLENNSPARK]

Unless you have a method to do it "dead"?

well you crack on then getting your Zs from (R1 R2)...i`ll just whip round with the loop tester shall i....

 

[Badged01]

well you crack on then getting your Zs from (R1 R2)...i`ll just whip round with the loop tester shall i....

Both methods work ...... take your pick!

 

[GLENNSPARK]

Both methods work ...... take your pick!

i think i`ll take a measured value of Zs thanks...

 

[Darkwood]

As we can all concur outlets like sockets can give a false negative to a R1 + R2 reading and using the BS7671 there is no call to actually make the test through the outlet. Whats happened here really is modification of test instruments to allow quick and easy plug in testing where as in the by-gone days dropping of the fronts was normal so we now have a situation where a circuit complying can fail due to a DC extra low voltage test where in reality a LV will not see this resistance.

I personally see no issue if you find an elevated reading off a socket from dropping it forward and confirming the reading behind and using that reading, if you have a large difference then a socket change just to omit any bad contact issues but no need for any replacements if we are talking a small difference.

If anyone finds a fail at a socket outlet they should by their own professional nature just drop the socket forward and ensure tight terminals and whilst they are there retest from the rear.

 

[Badged01]

i think i`ll take a measured value of Zs thanks...

No problem, I use both!

 

[GLENNSPARK]

thing is...theres no set value for r1 r2 & rn...and thus (R1 R2) in the case of a ring..

can this be said for loop?

 

[NickD]

Continuity testing is at something like 11volts and if tested at the rear and found to be good then fair enough.
You are basically trying to fail a socket because it wont respond correctly (to you) to a voltage way below what it was designed for.
ELI testing will inject a simulated 26amps (at the designed voltage) down the line and give you your satisfactory reading, what more do you want?

Fair question. I think the following is my take on it. There's sod all to a socket that if working correctly should add any appreciable impedance and certainly not half an ohm. (Bearing in mind 13A through 0.5ohm dissipates 84W). Nor is there anything clever about the technology of a socket that means it is only capable of delivering negligible resistance when supplied with 230V. A contact resistance of 0.5ohm which appears to disappear under 230V but reappears after is not a mirage, it is there. Might be non-ohmic a bit and vary somewhat with voltage, but it is there. I trust Ohm's Law and I trust the spec of my meter which says a 1ohm reading is accurate to 0.06ohms.

I don't know the details of how EFLI testing works but I can't believe it puts a full fault or load current on. My guess is it very accurately measures the supply voltage, then applies a smallish accurately known load resistance and measures the small drop in the voltage (then takes off the load and probably measures the voltage again). And then calculates Ze as load resistance x (voltage drop / offload supply voltage). Bit like how a car alarm works. Would explain why supply voltage noise can significantly affect the results. But the point is it's a simulation of the fault and will have its limits. Instinctively I trust this less than a simple, basic high-accuracy low resistance measurement, which is not that challenging to engineer.

And as already done to death there's the parallel path issue to contend with, some of which may be things not safe to rely on.

 

[NickD]

As we can all concur outlets like sockets can give a false negative to a R1 + R2 reading

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.

if you have a large difference then a socket change just to omit any bad contact issues but no need for any replacements if we are talking a small difference.

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

 

[Badged01]

thing is...theres no set value for r1 r2 & rn...and thus (R1 R2) in the case of a ring..

can this be said for loop?

As I interpret it you can derive your value of Zs either by calculation ( R1 + R2 + Ze) or direct measurement then compare either to the BS7671 values.

I do R1+R2 measurement as my way of proving polarity and CPC on circuits. You could equally take an R2 reading for your CPC -- again both acceptable.

As a final check I take a measured Zs reading to confirm there is no high resistance through the C/B etc.. that my R1+R2 doesn't pick up.

Thats my preference, I like to work 'dead' as much as I can, so I derive by measurement and confirm by test!

But there are many ways to skin the cat ......

 

[GLENNSPARK]

As I interpret it you can derive your value of Zs either by calculation ( R1 + R2 + Ze) or direct measurement then compare either to the BS7671 values.

I do R1+R2 measurement as my way of proving polarity and CPC on circuits. You could equally take an R2 reading for your CPC -- again both acceptable.

As a final check I take a measured Zs reading to confirm there is no high resistance through the C/B etc.. that my R1+R2 doesn't pick up.

Thats my preference, I like to work 'dead' as much as I can, so I derive by measurement and confirm by test!

But there are many ways to skin the cat ......

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

only prob is that this....is standard practice for confirming polarity...isn`t it...

but i guess i didn`t know that till now...