TESTING ZS

[LNRDF01]

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

 

[Doc Hudson]

Welcome to the forum. From the way you worded the question I presume you are referring to testing during periodic inspection? compared with during installation? There can be practical issues with what is feasible and reasonable to disconnect to carry out testing. During new installation work it is obviously easier to test some items before final connections. Is this a student question or are you already trading?

Doc H.

 

[LNRDF01]

Hi I am testing a property at the moment and have considered what i feel is a bit of an issue, also if a fault developed at the bathroom light fitting, with supplementary bonding in place would'nt the fault current be more liable to take the easier path to earth and go the pipework instead of the 1 mm cpc is was intended to go through.

 

[shed69]

And back down the main bond that hopefully exists..!

 

[Steve3948]

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

Have you tried with and without ? for R1+R2.

 

[Andy™]

Zs is a live test and is taken with all bonding etc connected. so its not a false reading.

 

[steptoe]

but your max reading is the greater of Zs or R1R2&Ze

 

[Doc Hudson]

Hi I am testing a property at the moment and have considered what i feel is a bit of an issue, also if a fault developed at the bathroom light fitting, with supplementary bonding in place would'nt the fault current be more liable to take the easier path to earth and go the pipework instead of the 1 mm cpc is was intended to go through.

I think you may be confusing the purposes of the CPC & the supplementary bonding. You are correct a CPC should be in place. Also that supplementary bonding could make a lower resistance path for an earth fault. Remember though that the pipework route may not be a lower resistance than the CPC. Any improved improved earth path, side effect of the connected bonding, will be a positive safety issue not a negative safety problem. It would be unsafe to do a Zs test with any bonding disconnected as it is a live test.

Doc H.

 

[Zeespark]

Think of earthing in general as a Faraday cage LNRD.

AndyGuinness

 

[LNRDF01]

Hi Andy,

All i was trying to say was that when completing a test report you are testing the cpc of a circuit for R1 R2 and Zs with supplementary bonding in the bathroom you may assume you are ,but in effect testing the supplementary bonding. It maybe that when testing you might not know that supplementary bonding has been done and as i said the CPC for the lighting circuit may have become dis-connected half way through the circuit. You would still get a reading if you tested to guidance notes 3 ( continuity test of lighting ccts)

 

[steptoe]

then you would know by your readings that something was afoot.

you do know how to interpet your readings and what readings you should expect to get on a 1mm cpc for the length of circuit I hope?

 

[SPECIAL LOCATION]

Hi Andy,All i was trying to say was that when completing a test report you are testing the cpc of a circuit for R1 R2 and Zs with supplementary bonding in the bathroom you may assume you are ,but in effect testing the supplementary bonding. It maybe that when testing you might not know that supplementary bonding has been done and as i said the CPC for the lighting circuit may have become dis-connected half way through the circuit. You would still get a reading if you tested to guidance notes 3 ( continuity test of lighting ccts)

I guess you are miss reading GN3... :_|

Is the illustration you refer Fig 2.1a, section 2.7.5... ?

Read the paragraph about mid way up page 34... It refers to potential problems of parallel paths for accurate readings..

BUT!

Remember this is ALL referring to INITIAL TESTING..

i.e. Chapter 2 Initial Verification..

When periodic testing existing installations it also mentions the limitations of being unable to remove parallel paths..

see.. Chapter 3 Periodic Inspection & testing

page 74..

3.10.3 Detailed periodic testing...

 

[unphased]

Zs is purely a live loop test to prove the continuity of cpc and verify that it is within max permissible values. Why would you be concerned whetehr parallel earth paths existed? It is a given that any circuit designed and installed correctly will always give test results within expected boundaries.

To prove the continuity of the cpc in a dead test use R1 + R2, or R2. In a live test ,Zs, will theoretically be Ze +(R1+R2). You can just as easily do Ze and Zs and sbtract the two to see whether the calculated result looks reasonable. The result will be satisfactory with a finite reading below the allowed maximum.

It would be a simple enough job to temporarily disconnect the supplementary bond from a bathroom ligh fitting and do a Zs, then repeat the test with the supp bond reconnected and compare results. I doubt there would be any significant difference, apart from the fact that, with supp bond connected, it should produce a lower reading. The Zs test with all earth paths connected will be representative of how the circuit would behave ina real earth fault situation anyway.

What is your concern, OP?

 

[Manator]

Zs is purely a live loop test to prove the continuity of cpc and verify that it is within max permissible values. Why would you be concerned whetehr parallel earth paths existed? It is a given that any circuit designed and installed correctly will always give test results within expected boundaries. To prove the continuity of the cpc in a dead test R1 + R2, or R2 or in a live test ,Zs, the result will be satisfactory witha finite reading below the allowed maximum. It would be a simple enough job to temporarily disconnect the supplementary bond from a bathroom ligh fotting and do a Zs then try it again with the supp bond reconnected and compare. I doubt there would be any significant difference, apart from the fact with supp bond connected it should produce alower reading.

What is your concern, OP?

I fully agree with your post unphased but I would urge everyone never to disconnect any bonding during live Zs testing. Even for experimental purposes unless the environment is under strict controlled use.

 

[unphased]

Hi manator. Yes if safe to do so. I thought that would go without saying, but I accept your well made point. I edited my post during your reply so unfortunately it varies slightly.

 

[LNRDF01]

Surely dis-connecting the supplementary bonding for checking the correct measurement of the lighting CPC (R1 R2) or Zs is no different to dis-connecting the Main Earth and removing any parallel earth paths ,when carrying out a Ze test ( with the consumer unit switched off).this is done under testing conditions with no one in the premises. To my mind you are confirming that the CPC of the lighting circuit is actually what you are testing and that result is what you would confirm on the schedule of test results.

 

[Manator]

The Zs reading is the highest reading taken under live conditions with all bonding in place. R1+R2 or r1+r2 are the readings taken under dead testing and no harm would be done should any bonding be removed. There is also a safe working practice that sometimes does not allow you to take a Zs reading when doing any periodic inspection never mind remove bonding during the test, and Zs must then be calculated. Both calculated and measurement are allowed.

Zs does not verify the continuity of the cpc. That is done when dead testing the circuit BEFORE live testing.

 

[unphased]

Hi LNR

That is not quite the same. If you disconnect the main earth conductor you remove the final connection to earth, and you cannot do a Zs in this situation The test instrument will "see this" as a fault and won't allow the test to be carried out. However, all earths/cpcs within the installation itself can still present parallel paths. The point I was trying to make is simply that the purpose of the dead test R1+R2 or R2 is to prove the continuity of the CPC. There is no point getting hung up about perfect readings, ie readings calculated from tabulated values of resistance per metre and concerning yourself with perfect values. That is not what testing is about. Focus on the information that the test instrument is telling you and, provided the test reading is below the maximum allowed values you can rest easy that the circuit you are testing is safe. If you wish to compare test results then you can do as many tests as you, as the tester, see fit. For example, do the Ze, do the R1+R2, even do the R2 if you want, and see how they all compare when you do the Zs. You are looking for unusual results not finite perfect values.

The value recorded on the test sheet is the value your tester tells you. If you record R1+R2 and you find that the Zs is much lower than the theoretical Ze plus R1+R2 then so what? I hope you understand what I am trying to explain. Don't get hung up on the actual values, just focus on the PURPOSE of the test and satisfy yourself that the circuit is testing out within expected maximums. Thats all you need to do.

Regards

---------- Post Auto-Merged at 09:34 ---------- Previous post was made at 09:29 ----------

The Zs reading is the highest reading taken under live conditions with all bonding in place. R1+R2 or r1+r2 are the readings taken under dead testing and no harm would be done should any bonding be removed. There is also a safe working practice that sometimes does not allow you to take a Zs reading when doing any periodic inspection never mind remove bonding during the test, and Zs must then be calculated. Both calculated and measurement are allowed.Zs does not verify the continuity of the cpc. That is done when dead testing the circuit BEFORE live testing.

Hi Manator. When testing an existing installtion a Zs test would be used to verify cpc and max values would it not? After all, the system has been energised so the dead test need not be necessary. Just a thought.

 

[Manator]

Not really Unphased, testing Zs has its own risks and sometimes due to health and safety regulations is not permitted, an example would be the high bay lighting in a factory where access is required at heights and on a metallic platform. Yes the test can be carried out in a safe manner,but would it really be required given the fact that it can be calculated without any due risk. You can prove earth paths by dead testing.

 

[Kalzac811]

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