TESTING ZS

[unphased]

I have never carried out testing on high bay luminaires. How is this carried out then? Would it be satisfactory to do just the R2 for example?

 

[Zeespark]

Kalzak, you cant subtract Ze from Zs and expect to find the exact value of R1+R2 due to not knowing the values of your parallel pathways.

AndyGuinness

 

[Andy™]

I would always strive to get a Zs by direct measurement as It is the preferred method.Then subtract the Ze for the R1+R2.If this is not possible do the R1+R2 during dead testing so long as Zs within calculated values

i hope that is not for new work... after all, you must carry out R1R2 before switching the power on... and you need to switch the power on before you can do Zs

 

[Kalzac811]

Kalzak, you cant subtract Ze from Zs and expect to find the exact value of R1+R2 due to not knowing the values of your parallel pathways.True Andy it won't be an exact value But if continuity proved In dead testing then measured Zs to compare calculated value is what we are looking for

---------- Post Auto-Merged at 17:00 ---------- Previous post was made at 16:52 ----------

[quote name='Andy

 

[Sidewinder]

You are not "allowed" to calculate Ze from Zs-(R1+R2)

 

[Kalzac811]

Hi Im saying get a value for R1R2 from subtracting measured Ze frm the measured Zs

 

[Sidewinder]

Same applies.

 

[Kalzac811]

If continuity has been established and not new instal why? Sidewinder

 

[Sidewinder]

Why do you think?

 

[green-hornet]

If you correctly follow the steps for testing Ze, you have to disconnect something, if you test Zs you do not. Which is why it is impossible to find R1+R2 on a circuit by the method you have described.

 

[Kalzac811]

I asked you first

 

[Sidewinder]

Because I know & I teach 2391, & I expect students to have a go at the answers first before I give them the correct versions.

If you don't like it sorry, but that is my way.

 

[Kalzac811]

No probs with me sidewinder that's why I joined this forum to learn from guys like you is it because it will give you a value of R1+R2 but with no value of parallel paths PS go easy on me it's my bday

 

[Sidewinder]

Happy Birthday Kalzac, have a few :Y & some Guinness then some :coffee in the morning & we'll chat tomorrow, it's late now & I've had a bad day, & too much :Y ; )

 

[Kalzac811]

Me too adios cheers

 

[unphased]

As I stated earlier in this thread it is too easy to get hung up on the value of test readings. Always go back to WHY we are testing and WHAT we are testing for. Take a 32A 60898 radial final circuit. It doesnt matter if the ZS reading is 0.45 ohms, 0.46 ohms, 0.47 ohms etc etc if the 80% max allowed is 1.15 ohms.

What are we testing for on dead tests? We need to know that the cpc is intact along its entire length.

How do we test it? Low ohms resistance tester by creating a loop with one of the other conductors, traditionally the line conductor, R1, and confirming there is a finite reading. This proves that BOTH conductors are continuous. If there is open circuit it could be that either the cpc is dicontinuous or the line is discontinuous. We don't yet know which. So we can create another loop with the cpc and neutral conductor, Rn, and try again. If that is continuous we know the break is in the line. So we find the break, correct it and re-test. Once we have proved the continuity of the cpc (and other conductors if the case is such) then we can record a value to prove it was tested. We can look at resistance-per-metre tables and compare whether the reading measured looks sensible. Experience will tell you after you have carried out hundreds of tests anyway. So having established that the circuit is complete and all conductors are continuous we can measure Ze. Ze can only be measured live. So we are measuring to see that there is an earth path. We are comparing the reading we get with expected maximums that are drummed in to our heads, TN systems, TNS or TN-C-S, and we record the value. The voltage alters all the time. When we measure the first time the voltage might be 244V and the reading might be 0.26 ohms. Test again and the voltage may have dropped to 242V and the reading will be different, we test a third time, we masy get same reading or a different reading, it really doesn't matter WHAT the reading is if it is within expected parameters.

Now we move on to Zs. Zs is a live test. We now have a complete circuit dead tested and we know Ze. We measure Zs and this varies amongst test meters whether it might be a two lead or three lead test. We know what value we are expecting witha simple check adding Ze to R1 + R2. The reading, whatever it is, is recorded on the test sheet. As long as it is around the expected value, or less if parallel paths come in to play, it doesn't matter. If we compare the measured reading with tables for our disconnection times or 80% value and it is witin, then job done. It is pointless getting hung up on values arrived at by chopping around formulas.

Points are being made on here that we are "not allowed" to arrive at certain values by doing this that and the other. All we are trying to do is satisfy ourselves as installer, inspector, designer, whatever, that the circuit is safe. Schedules of test results are proof that the test has been carried out and the values recorded are proving that maximum tabulated values in the regs are not exceeded. How the values are arrived at is immaterial if the TEST has been conducted correctly. If the circuit we are testing has been energised already and been used for decades then it is folly to worry whether we are allowed to record R1 + R2 by subtracting ze from Zs. It is just a value for all intents and purposes, a need to fill in a box on a form. A correctly measured Zs with satisfactory test result will tell you whether anything is looking dodgy. A Zs test will tell you that there is an earth path, it will tell you whether polarity is correct and it will highlight a potentailly high Ze. On an energised circuit if you try a Zs test and the test meter tells you something is wrong you won't get a test and you need to delve. If you want to RECORD R1 + R2 then subtracting Zs from Ze is just an exercise in maths. Else you can LEAVE THE BOX BLANK. Why would you want to swith the circuit off and do it? It is already energised and you haven't been able to verify it dead. This is the point I am trying to make.

So when faced with testing existing circuits use some common. When installing a new circuit or several you do the entire test procedure as per the book and fill in all the boxes. Does that make the circuit any safer? I'll leave that one to you.

 

[Manator]

Very good post unphased, some points you raise I personally do not agree with but a good post all the same.

 

[Sidewinder]

Similar to my thoughts Manator, I'm a bit busy tonight, but I would like to discuss and explain where and why my views differ with unphased, whilst the post is not incorrect, I feel that there could be some additional info to it that could be added to explain to others the why's & wherefores.

 

[SPECIAL LOCATION]

I think what Unphased is basically saying... {which I agree with..}

When adding brand new circuits or major alterations to an existing circuit you will fully test everything to make sure everything is connected as it should be....

and that all readings and tolerances are within accepted limits.

i.e. Dead tests THEN Live tests..

Whereas:-

With an existing installation {e.g.during a periodic} no mater how much you may want to do...

very few customers would be willing to pay for comprehensive tests of everything!

So the extent and limitations has to be agreed,, this would include the amount of testing that is feasable...

Plus a level of sampling has to be taken anyway.. :C

All you are aiming to establish is if the installation is satisfactory or unsatisfactory and if there are any immediate dangers..

It is not too difficult for a competent person to estimate what the expected values of Zs may typically be...

allowing for type of circuit size of conductors probable parallel bonding paths,..

and if they are within max permited values...

IF readings are getting close to max permitted and/or appear ridiculously high for the type/length of circuit..

then you'd investigate further...

Otherwise you can assume it is reasonably safe without knowing the exact R1+R2!

Back to the Ops original post....

when testing ZS at a bathroom light fitting ( last light in circuit) and a supplementary earth has been connected from the pipe work in 4mm, surely this would give a false Zs reading. Also when carrying out continuity test for the same circuit, guidance note 3 shows placing a link between the earth bar of the consumer unit to the phase conductor ( cct breaker in off position)with supplementary bonding in place, if the cpc of the lighting circuit had become dis-connected say at a light fitting, one would still get a continuity reading and believe that all was fine. i think the cpc for the lighting circuit should be dis-connected from the earth bar and a continuity test may then be carried out without taking the supplementary bonding into account. Also i think a warning notice should be on consumer unit advising that supplementary bonding exists in the bathroom and to dis-connect it for testing purposes. Comments please

If you really want to know the R1+R2 ...

DON'T use the method shown in the GN3 or OSG...

Just take BOTH Line & CPC out of the board and connect them together with a lever Wago connector,...

Then go to your light and read the R1+R2 at the fitting!

WIRING REGULATIONS are NOT black and white instructions...

We all should use our training and skills to establish the best way to gather sufficient information on any particular installtion...

Anyone blankly following the books WILL come unstuck at some point!

I have often used different methods on different installations .....

{I think Mr N Tessla has a few "unorthodox" methods for getting an R1+R2 reading!!! :innocent

especially on a TNCS when N & E are joined at the suppliers cut-out!}

but I couldn't possibly comment :Blushing

:C

 

[LNRDF01]

Hi All , That was my point from the begining, i always dis-connect the CPC from the earth bar at the consumer unit and connect to the phase conductor,its the only way to be sure of continuity and ignoring parallel earth paths, Thats why i disagree with the procedure in guidance note 3 for continuity of a lighting circuit. (linking earth bar to phase conductor). Also please confirm question i asked before. By connecting a 4mm earth cable to the earth terminal of a bathroom light for supplementary bonding, wouldnt it be possilble for a fault current that may develope at the light fitting to elect to travel down the supplementary earth , thereby passing possibly 100-150 amps of fault current down the pipework and back to the main earth terminal