Ze Problem for Extended Supply

[Piratepete]

Hi Guys

The PME 100Amp 3 phase supply at the meter in my local church has a Ze of 0.3, 0.29 and 0.33 Ohms at the meter for each of the 3 phases.

I'm proposing a 3ph DB 20 metres away (in a new annnexe) via a 40 Amp switchfuse by the meter.

As Ze has to be measured at the DB I have a problem because the 4 core SWA 6sqmm supply cable I was going to use will add 0.27 Ohms - the total then easily exceeding the max allowance 0.35 Ohms. Even if I use a 10sqmm cable I will be adding 0.16 Ohms.

Am I correct in my musings. Any solution? The church supply cannot be improved! Would it be allowed to give the new DB a TT earth even though the annex is only separated from the main building by a door?

Thanks
Pete

 

[Engineer54]

Ze is taken at the origin of the installation (which you have already tested)!! Any board/CU downstream is classed as Zs or commonly referred to as Zdb!! So as long as your Zs values meet the minimum allowable for your protective devices your fine to go!! lol!! Don't they teach this at collage anymore??

Don't forget, with a PME supply, if this extension to an annex, is outside of the current equipotential zone and has extraneous services such as water/gas pipes or the like you will need at least a 10mm bonding conductor to run with your 6mm SWA cable, or increase the SWA cable size and core number to suite!! lol!!

 

[Silly Sausage]

You're creating a problem where there isn't one!

Z[SUB]e[/SUB] only exists at the intake.
The EFLI at your new board is a Z[SUB]DB




[/SUB]
edit...got to be quick around here! Eng already answered!

 

[Piratepete]

Thanks
Not thinking straight!. Years of measuring Ze at the mainswitch and calling that the 'origin' has addled my brain!

Yes - I do need a seperate 10mm bonding conductor, pain in the a that it is.

Note that the new Generic Schedule of Test Results now asks for 'Zs at the DB' and not 'Ze at the Origin' like it used to do.

 

[sparkymalarky]

Just to be clear and I know you are probably aware of this Piratepete, Z[SUB]e[/SUB] is measured with the main earthing conductor disconnected from the main earthing terminal.

P.S Archy, check me out with my subscripts, two can play your game

John

 

[Engineer54]

Thanks
Not thinking straight!. Years of measuring Ze at the mainswitch and calling that the 'origin' has addled my brain!

Yes - I do need a seperate 10mm bonding conductor, pain in the a that it is.

Note that the new Generic Schedule of Test Results now asks for 'Zs at the DB' and not 'Ze at the Origin' like it used to do.

Ze and Zs are two completely different animals!! lol!!

Zs at a DB/CU is commonly called Zdb, but is still in reality, a normal Zs test with all bonding and other parallel earths in place. The Zdb notification, is just indicating that the value indicated was taken at the DB/CU...

 

[Silly Sausage]

Just to be clear and I know you are probably aware of this Piratepete, Z[SUB]e[/SUB] is measured with the main earthing conductor disconnected from the main earthing terminal.

P.S Archy, check me out with my subscripts, two can play your game

John

¡sʇdıɹɔsqns ʎɔuɐɟ ɹnoʎ ɥʇıʍ noʎ ʇǝפ

 

[Piratepete]

Just to be clear and I know you are probably aware of this Piratepete, Z[SUB]e[/SUB] is measured with the main earthing conductor disconnected from the main earthing terminal.John

Yup. No probs. I do know the difference.

 

[wade88]

I always remember it like this *in strong German accent*

Zs = Zee system
Ze = Zee external

Works for me.... lol