Consumer unit change testing

[GL4Spark]

I understand some people have stated you cannot use Zs - Ze to calculate R1 + R2 due to parallel paths being present... but aren't they also present during continuity tests? I have always been told you can't backwards calculate but now I'm curious as to why

 

[Toneyz]

It is the resistance of the line and cpc conductors of that circuit,I'm not saying that in most cases the result is about the same.eg you do a loop test of a circuit take a immersion heater circuit and that circuit cpc is loose/broken etc loop test shows earthing ok due to a paralell path of the copper pipe via water bond pipework gets altered plastic intersection T added to pipe for other water supply that paralell path is no more then there is no earth to that circuit.

 

[unconditional]

Low current testing is also more revealing of faults like high resistance connections than plugging in a loop tester. Is voltage sensitive equipment still an issue with modern testers at 500v? I know the current limiting on mine stops at quite low voltages on lower resistances, and only applies 500 on big resistances.

 

[GL4Spark]

But if you were to do an (R1 + R2) test on said immersion circuit with the broken cpc you would also get a reading would you not? I understand what you're saying but the regs don't require us to remove bonding conductors for the test, so we would have parallel paths in both cases.

 

[Risteard]

There is no merit in this Zs-Ze approach. The whole point of a dead continuity test is to ensure the cpc is there BEFORE energising. If a live Zs test is just done then there is absolutely no point in pretending R1+R2 was measured just to fill in a box.

So do an R2 or R1+R2 before energising, or if you just do the Zs then at least state that that is all you have done rather than making up useless figures.

 

[westward10]

This is not an Initial Verification and tests can be carried out as if it was a Periodic Inspection in which case it is perfectly acceptable to use the EFLI test as proof for earth continuity.

 

[Risteard]

This is not an Initial Verification and tests can be carried out as if it was a Periodic Inspection in which case it is perfectly acceptable to use the EFLI test as proof for earth continuity.

Well, it is an initial verification (hence the Electrical Installation Certificate). BUT the certified work relates ONLY to the replacement of the distribution board (which will be clear in the extent of the installation work undertaken box), and essentially the tests carried out following the inspection are simply to ascertain that the existing circuits are safe to put back into service on the new distribution board. You are not taking ownership of the existing installation. After all you cannot verify the route the wiring takes or any number of other things about the prior installation methods.

It still doesn't explain why you feel the need to fill in a box for something which you are stating that you have not tested and have also stated that you do not need to test. If that is your position then do not fill the box for it in! To do so is both misleading and pointless.

 

[westward10]

Has been tested by way of EFLI.

 

[Risteard]

Has been tested by way of EFLI.

Earth continuity has been verified with a Zs test. So why are you filling in a box stating R1+R2? You haven't done this test (which is not a criticism) - but why are you filling the box out? That is misleading and serves no purpose.

 

[westward10]

Earth continuity has been verified with a Zs test. So why are you filling in a box stating R1+R2? You haven't done this test (which is not a criticism) - but why are you filling the box out? That is misleading and serves no purpose.

I don't recall stating which boxes I fill in, I just agreed that Zs-Ze=R1+R2.

 

[Risteard]

I don't recall stating which boxes I fill in, I just agreed that Zs-Ze=R1+R2.

It doesn't really. It can verify cpc continuity in an already energised installation which is fair enough. But some people are advocating noting down a reverse engineered value under the dead continuity test boxes which is frankly a nonsense.

 

[HandySparks]

It still doesn't explain why you feel the need to fill in a box for something which you are stating that you have not tested and have also stated that you do not need to test. If that is your position then do not fill the box for it in! To do so is both misleading and pointless.

Totally agree. My certs usually have loads of unfilled boxes. If I didn't check or measure something, it doesn't go in the box. And if I measure something once it only gets written down once. So, for RCD times, the values go on a line for the RCD and not on the lines for every circuit it protects.

 

[Risteard]

So, for RCD times, the values go on a line for the RCD and not on the lines for every circuit it protects.

Personally for an RCCB I list the times next to each protected circuit rather than list the RCCB separately on a line of its own. But so long as what you have is clear then I don't have a problem with it being done that way.

 

[westward10]

It doesn't really. It can verify cpc continuity in an already energised installation which is fair enough. But some people are advocating noting down a reverse engineered value under the dead continuity test boxes which is frankly a nonsense.

It isn't necessarily nonsense, where there are no parallel paths the reading is probably quite accurate. This isn't much different to people calculating Zs which I have seen recommended on this forum where a live reading should be obtained. For example dead testing R1+R2 then adding on Ze.

 

[Risteard]

It isn't necessarily nonsense, where there are no parallel paths the reading is probably quite accurate. This isn't much different to people calculating Zs which I have seen recommended on this forum where a live reading should be obtained. For example dead testing R1+R2 then adding on Ze.

A calculated Zs will err on the side of safety. To reverse engineer the R1+R2 will err on the side of danger. It also serves no purpose when you are verifying cpc continuity on an already energised installation with live (Zs) testing. Filling in a box simply because it exists on the form has no merit when you are excluding dead continuity tests. If you are excluding it then exclude it!

 

[Andy78]

It isn't necessarily nonsense, where there are no parallel paths the reading is probably quite accurate. This isn't much different to people calculating Zs which I have seen recommended on this forum where a live reading should be obtained. For example dead testing R1+R2 then adding on Ze.

It is entirely different to calculating Zs.
Calculating Zs from (R1+R2)+Ze gives a value based on two measured readings, so it is not really calculated, merely an addition of measured values. It also gives a worst case scenario, a maximum figure for Zs for that circuit without any connected parallel paths to earth that, if compliant, can only mean that a live (connected) Zs will be less and therefore also comply with disconnection times. That is the reason this value is permitted to be entered as a value on a schedule of test results to BS7671

Calculating R1+R2 from Zs by subtracting Ze accounts for no connected paths and therefore will not be accurate. As Risteard says, this is an valid enough method of verifying continuity of protective conductors on an already energised circuit, but not a valid way of obtaining a reliable value of R1+R2.

 

[davesparks]

I may be wrong, (I am new to this). but i thought you could use 250v as long as the results comply with 500v

No, 250V is below the peak voltage it will normally be exposed to so is a useless test as far as proving the integrity of the insulation.

 

[davesparks]

Joking im not. Zs-Ze does equal R1+R2, its a very popular method of testing and not to mention quicker.

That is not a method of testing, it is a method of calculating an estimated value.

It is not a viable method of attaining a value for the continuity of the CPC, for initial verification it is a requirement that the R1+R2 or R2 be measured prior to making a circuit live,

 

[E.E.I]

What about when live testing through RCD's, this gives a higher value on your zs reading. This would effect the calculated R1+R2

 

[davesparks]

What about when live testing through RCD's, this gives a higher value on your zs reading. This would effect the calculated R1+R2

Just don't live test through the RCD, use a more suitable test method