Zs tests on radial circuits?

[nadger]

Just taken delivery of my very own NICEIC DVD: "inspection and testing - initial verification for domestic properties" and after watching it i have a question.

In the chapter about testing for Zs on radial circuits, the NIC bod says that testing on exposed live parts is a direct contavention of the Electricity at work act section 14, there for the NIC require only a calculated Zs entering on their EIC's using Zs = Ze + (R1+R2)

is this correct? because i'd have thought testing live installations would fall within the boundaries of a "competent person"

 

[Deleted member 9648]

NIC policy is not to Zs test at luminaires for safety reasons.....suits me....balancing on a pair of steps trying to get the flaming leads onto a tiny connector block and just as the test starts the poxy earth croc clip drops off.....then on the 15th attempt you manage to keep them connected for long enough for the test to start only for the effing type B mcb to drop out.....i'ts an overated pastime.

 

[Des 56]

Ah the good old Nic again

In all fairness they are probably following Ewr regulations closely
There is no need to work live if it can be avoided

In the case of initial verification, it is considered unnecessary and its safer to calculate from dead tests than work live

 

[Spudnik]

I mentioned this on my last assessment and he told me there is no reason not to do a live Zs on a radial lighting circuit.

 

[nadger]

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

 

[Des 56]

He is not speaking with fluent Nic speak then

On a Pir it would more than likely be un avoidable,but their policy I believe is different to the guy doing the assessment

 

[Des 56]

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

You mean the adaptors for testing R1 + R2

 

[nadger]

i havnt really looked at them to be honest, a mate bought some and moaned like mad at the cost and said dont bother if we were thinking of buying them

 

[nadger]

He is not speaking with fluent Nic speak then

On a Pir it would more than likely be un avoidable,but their policy I believe is different to the guy doing the assessment

so when my local assesor asks me to perform a Zs on a radial and i state Tony Cable says i shouldn't do it, will i fail

 

[Spudnik]

Yep......

 

[Des 56]

Re visit fee as well

 

[Guest123]

I'm afraid regardless of all the red tape and bureacracy and he said, she said, I do and will continue to test Zs with my trusty meter.

 

[Des 56]

Yeah I have the same attitude
The litigation culture may have reared its ugly head
They may need to give this advise,but personally, they probably dont agree with a lot of the nonesense they have to preach

 

[Guest123]

The way I see it I am my own health and safety officer and everything I do, I do taking into account all risks associated with the procedure. Plus I think I'm big enough to make up my own mind now. My mother hasn't cleaned my face with a licky tissue for years.

 

[RexT]

Hi guys,

It is always interesting how this works out.

I thought you were allowed to use calculation, so if you added R1+ R2 to Ze it would be Zs. So I guess if you used the R1 + R2 measurements (dead test) and added them to the Ze (live test) then it would give the Zs. I know there is a live test but it would only be one (or at least a couple of verified goes) compared to lts of Zs tests at the final circuits.
I have watched the videos by Tony Cable and like them a lot. He is very competent, straight forward and has a down to earth view which is great.

I believe for the 2391 exam they expect demonstration of testing, measurement and knowledge of the circuits. Of course it goes without saying that safety must be shown at all times so I would agree with compliance with the Electricity at Works Regs 1989 comments, however they primarily use Guidance Note 3 as the reference document, so I believe that this should be your first port of call and follow that.

Good luck

Rex

 

[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

 

[Gtbadboy1]

you are correct mate double the length will include the R1 and R2. I must be blond

 

[Deleted member 26818]

Of course, that's without applying any correction factors.
My understanding is that you would multiply that figure by 1.2 to obtain the Zs for a 70° conductor temperature.

 

[telectrix]

either that or use the corrected value for the max.Zs (as in OSG).