Help please! Cheers guy!!

[cagedanimal2010]

Am doing a inspect and test on some work iv done board, re-wired light,
So tncs system ze .16 , PFC 1.5 , all circuit have >500 I.r, R1+R2 readings are what I would expect for run length etc. Problem is my zs on all circuits is coming out .70 more, not the .16 I was expecting!(based on zs=(R1+R2)+ze), Any ideas guys??

 

[trebor]

Why were you expecting 0.16 as Zs ? if youre Ze is 0.16 then the Zs has to be higher without considering any parallel earth paths

 

[cagedanimal2010]

Sorry I didnt explain that clearly,e.g what I ment was ze.16 + R1+R2 . 20 was expecting zs of .36 there or there abouts what I'm getting is zs of .90????

 

[trebor]

That does seem strange, assuming youare taking Zs at same place as R1 R2 you would expect it as you rightly say to be around the sum of ZE plus R1 +R2 or lower is the norm. Have you got another loop tester to double check

At the end if the Zs is compliant we can be happy but I share your confusion over the difference as it is substantial, lets see if the clever ones can help

 

[cagedanimal2010]

Yep readings took at same place! And the zs are compliente, strange tho!

 

[cagedanimal2010]

any of the big dogs got any idea?

 

[nickblake]

Woof .. no im a puppy , did you disconnect the main bonds to water gas etc ,and if you did did you do it befor or after you did the Ze ,although when you carry out the R1 R2 test you get a reading you'd expect ,when you do your Zs test the fault current of the circuit now if you have any slightly loose or poor connections the readings could increase to a higher than expected reading for instance doing a Zs on a socket if the terminals are worn when the fault curent is applied it may not make the contact as it should thus increasing the Zs level , i have come across this loads of time on older properties new socket readings improve

 

[cagedanimal2010]

the thing is, it is all the zs readings on the board 8 circuits in total?

 

[IQ Electrical]

Are you getting a reading through the rcd main switch (es) or rcbo's when you are measuring the Zs?

This often increases expected results.

 

[cagedanimal2010]

its a split load board so yes, 2 separate rcd main switches, iv installed loads of these before tho and never seen readings like this before???

 

[gnselectricals]

Hi are the circuits RCD/RCBO protected as you may be measuring the resistance of those when carrying out your Zs test,
Ive encountered this esp. with certain brand RCBO's such as Dorman Smith
Regards, Graeme

 

[Had8Lives]

Is it your test leads? Maybe your test lead fuses or meter connections need cleaning.

 

[Sparky Ninja]

I'd suspect one of two things..

i) Are you using a loop tester on the 'no trip' setting? If so then this will most likely give in inaccurate reading?
ii)Have a crack at measuring your Ze again, but instead of the supply side, carry the test out on the load side of the MCB/RCD/RCBO's etc. This will enlighten you on wether you are experiencing increased resistance through the devices. Obviously for this, you will need to close the protective devices, so removing the circuit cables first would be a good idea.

 

[nickblake]

As IQ says it could be a multitude of reasons , at the end of the day if the R1 R2 readings are satisfactory and the Zs readings are below the max permitted then you have nothing to worry about ,and even less if the installation is fully RCD protected

 

[cagedanimal2010]

leads are shiney new! tested ze 3 times and zs on all circuits 3 times, no change, tester is however on "no trip" setting, R1+R2 readings are fine and zs reading are fine, just one of those "wonder whats causing that" things!

cheers guys really appreciate it
danny