Odd Ze test result.

[jackdor]

Hi all, just wondered if one of the members could shed a light on my result of a Ze test. I have just changed my ccu in my ex council house in order to add 2 new circuits(mains powered smoke and heat detectors and also a mains feed for my alarm). The house was rewired about six years ago so the cables and fittings are in good condition and the correct sizes, but in the old colours. The ccu that i changed feeds all circuits in the house except for the electric storage heaters that are on their own ccu and operated by an economy 7 radio time switch. I have tested every circuit to BS7671 2008 (both dead tests and live tests). Everything seams ok and has been recorded on the apropriate cert, except for the Ze test. The 16mm earth to new ccu, the earth to the heaters ccu and the 10mm earth bonding to water(no gas feed in house) are all connected to the earth terminal enclosed in the TNCS enclosure which houses the 100amp cut out fuse. fortunatly ther is no seal on the cut out so i was able to remove fuse and gain access to the earth terminal to remove the other earth to do the test. I isolated the heater ccu before dissconnecting its earth. I then dissconected the earth from the new ccu and connected my megger1552 to the correct point and tested the Ze. The result i got was very low and caused some disbelief on my part( 0.07 ohms) as i was expecting around 0.34ohms. i did this test serveral time with the same results. The megger did indicate that ther was some Interference some where, but all circuits were isolated apart from the radio time switch so not sure if this result is valid. This has to be acuratly measured as this job is to be used for 1 of my assessments for self cirtification. Also the meter cables are the old colours is it ok as on is off meter and one off cut out (seald Meter). cheers

 

[Guest123]

Hi there.

Are you saying you disconnected all other earthing conductors from the MET and left just the earth under test there by eliminating all parallel paths??

It could be that you are very close to a sub-station transformer and with it being a TN-C-S system the earth is very low.

What reading did you have for PFC?? was this high?? If it is the case I would expect a reading of around 3285A.

 

[Mr Mark Sparks]

.07 is fine its quite common in my area to be low because you are never far from the transformer
the job in the photos in the show us your installs section i posted has ze of .10 and they were recorded by the dno and were happy
as lenny said just watch your ka reading

 

[jackdor]

Thanks, yes i dissconnected all other earthing conductors from the MET. My PFC reading was 3.3Ka. As for the meter tails which are 25mm black and red do i need to change these as 1 is from a henley block which i could change and 1 is from the meter which has a seal on ang therefore not permissable touch. Thanks again, knowledge is power.

 

[scotsparky]

it the tails are 25mm dont worry about the colour. If you want to be correct wrap some tape round them to mark them. you dont have to change them. besides what means you got to isolate the main incommer to D.B ?

 

[jackdor]

Cheers. The cut out has not been resealed since the house was rewired 6 years ago by some contractor working for the coucil. see attached pic of ccu,s.

 

[jackdor]

See pic off install

 

[Jamal]

HI everyone Just Ihad exam 2391, the question was many special location in domestic installtion?
My answer, was nine special location,is that right or wrong

 

[Spudnik]

HI everyone Just Ihad exam 2391, the question was many special location in domestic installtion?
My answer, was nine special location,is that right or wrong

Please start a new thread in the relevant forum.

Thanks.