Zs tests on radial circuits?

[nadger]

i have just watched their periodic inspection DVD today, and the same situation regarding Zs was reiterated in this one also.

Dont get me wrong i have no problems testing live for Zs but their information seems to contradict the scenarios their local inspectors will put us in apparently.

 

[Gtbadboy1]

can anyone answer the question below.
Calculate from the data below, the earth loop impedance of a radial circuit installed using 2.5mm conductors for line and CPC. show all calculations
Data
Ze 0.15 cable length 25m, conductor resistance 7041mΩ/m

 

[andyb]

Most people on this site could work it out Gtbadboy1, but you should be working it out yourself.
You've been given your conductor resistance per meter, multiply that by the length of the line conductor plus the length of the cpc, then add this to your Ze.

 

[telectrix]

either i am thick , short-sighted or what. the res.of the cable is given as 7041 mohms/m i.e 7.041ohms/m. is it copper or string? this gives R1+R2= 3705ohms

 

[andyb]

Don't forget to add 0.15 to that telectrix.

I'm assuming that he forgot to put a decimal point in there somewhere.

 

[Deleted member 26818]

I think you're short-sighted. I get 352.2Ω.
He stuck a zero where the decimal point should be.
Should be 7.41 mΩ/m.

 

[telectrix]

o.37 then. add o.15, Zs=0.52 ( multiply for the correction factor allowing for stretching due to the fact that the cable was 3" too short to terminate, )

 

[antimatter]

humm, bet Kewtech arnt happy then, after developing their extremely overpriced light fitting test thingys!!

They're not too badly priced I thought, I picked up the Lightmate (BNC) and socket (banana jack socket) both for £20 from Denmans

 

[gpz900jim]

on my assessment, the nic man said that the nic would rather you do a ze and add it to an r1/r2 to get zs, its less live testing

 

[Sparky Ninja]

on my assessment, the nic man said that the nic would rather you do a ze and add it to an r1/r2 to get zs, its less live testing

On this occasion NICEIC are having to stick with their DVD productions. It's up to your interpretation on what you consider best practice.
MCB's and RCD's can give off impedances that will provide a higher measured Zs than the simple calculation will prove.

I would go into the NICEIC's reasoning, however I dont have my EAW redgs in front of me so don't want to start misquoting them.


Oh and its (R1&R2) btw.

 

[Octopus]

I think you're short-sighted. I get 352.2Ω.
He stuck a zero where the decimal point should be.
Should be 7.41 mΩ/m.

How did you get 352.2?

 

[Deleted member 26818]

7041 x 25 = 176025 x 2 = 352050 / 1000 = 352.05 + 0.15 = 352.2.
Of course the correct answer using 7.41 is 0.5205Ω.

 

[Guest123]

I'm guessing he meant 7.41, as that is the value for a single 2.5mm conductor.

I'm going for 0.52ohms.

 

[Gtbadboy1]

i have worked it out to be 0.335
7.41*25= 185.25/1000=0.18525+0.15 =0.335

 

[telectrix]

i have worked it out to be 0.335
7.41*25= 185.25/1000=0.18525+0.15 =0.335

you'vge only got R1 there mate. its 7.41 x 50 for R1+R2