Ze Question again

[kiles1]

Hi All,

What are your views on this, on the Ze test the flat I'm working on has a shared MET with other flats, so do I do the test with the earth connected to my consumer unit and write it down on sheet or disconnect it from my unit and test like you would do normally. Cheers all for your help in this

 

[Evans Electric]

Just test it with your main gas and water bonds disconnected I would say, your Ze is that connection to the shared Main Earth Terminal .

 

[kiles1]

So do it like you would normally but write it on the sheet that shared MET

 

[Robojin]

Hi All,What are your views on this, on the Ze test the flat I'm working on has a shared MET with other flats, so do I do the test with the earth connected to my consumer unit and write it down on sheet or disconnect it from my unit and test like you would do normally. Cheers all for your help in this

Any test with the CU cpc connected back to a shared MET readings would be based parallel paths

However safety first with any live test, if in doubt don't disconnect, you could make an enquiry to the DNO or calculate by deducting R1+R2 from the Zs on a given circuit, 'Ze by calculation' is covered in the OSG ect

 

[green-hornet]

Your distribution board or consumer unit is part of a larger installation.

Ze is the test of earthing external to the installation and should be carried out as close to the supply as possible.

You can for the sake of common sense do the Ze at the cu your working on, ie the loop reading you get is the external earth path.

This could be written as Zdb as others have suggested, however it could be used as the actual Ze for that installation.

The PFC for that installation or cu could be recorded at this point also.

Your Zs readings will always be affected by the actual Ze and the relation with r1+r2.

It is easy to calculate from known figures to establish all the overall earthing arrangements and how efficient they are.

You will get parrallel paths recording Ze from a shared met however you could note on the certificate that the shared met was not disconnected during testing as a limitation.

 

[paulh]

The approved method is by enquiry. This is usually higher than a measured value so unless you are struggling to get low enough ZS readings, I would use this value.(getting ready to duck:coat)

 

[Evans Electric]

I can never see the value of the enquiry method.

Example :- Unknown to sparks the main earth cable has been cut by the builders, sparks decides to use the enquiry method instead of his tester, phones the network who say it should be 0.2 ohms there . Write down 0.2 everyone thinks the earthing is hunky-dory. Mmmmmmmm interesting.

 

[Andy™]

and i think everyone has came accross at least one TN-S supply with a high Ze due to faulty connection somewhere

 

[paulh]

That is true but it doesen't end at that point........... when you do your r1+r2 tests and add this to the DNO figure and you do your ZS tests shouldn't ZS be lower than the calculated ZS and if its not further investigate?(I really need to get my coat now:coat)

 

[jl-heating]

i know it always says you can calculate but i never do i always carry out the test req.

i would always pull out the main and test then as above add on comments shared met.

 

[Andy™]

generally, Zs will be lower than Ze+R1R2 due to parallel paths. more noticeable on TT

 

[Evans Electric]

That is true but it doesen't end at that point........... when you do your r1+r2 tests and add this to the DNO figure and you do your ZS tests shouldn't ZS be lower than the calculated ZS and if its not further investigate?(I really need to get my coat now:coat)

Don't you mean your cape

 

[paulh]

MR EVANSApplaud Smiley