Strange Zs Results

[Joe S]

Hi, I'm new on here and have my NIC inspection on friday. I have had a lot of help from a mate of mine and dont really want to keep phoning him all the time.

I have done the testing on a shed that I have wired and all my Zs readings are up around 35 - 36 ohms! This is higher than the max Zs allowed per breaker. Not quite sure what to do??

The breakers are 1x 16a 1x 10a 1x 6a

 

[acat]

Hi Joe what system are you testing on is it a TT TNCS or TNS.


Chris

 

[Joe S]

TT mate. Sorry, should've said that at the beginning.

 

[acat]

Could it be that you were mis reading the meter or had it on the wrong setting max Zs for TT is given in the book as 21ohms but somewhere else it states that if Zs goes above 200 ohms it becomes unstable just looking for that at the moment.

Chris

Have a look at 411.5.3 of BS7671 page 50 does that help


Chris

 

[Joe S]

I did double check the meter as I thought it was wrong. I'm having to test Zs with the Zs RCD function switched or else it trips will testing. Could this throw my readings out?? I'll just go get my book to look at that page.

 

[scotsparky]

cat the 21ohm figure is what the supply authority says the earth at there transformer will be.

when doing an eqrth test on a tt system deduct 21ohms from your figure ant that will give you the true reading for your earth.

The 200ohm number is a recomendation. Above that the earth may be "unstable" meaning climate conditions may affect it. Normally i would look for under 100ohms not including the supply authoritys 21ohm figure.

Hope that makes sence.

 

[Joe S]

When I do the sum of 0.03xZs I get 1.188 which is obviously less than the 50v recommended in table 41.5. With the Max Zs values per breaker in table 41.3, my reading of 39ohms would fail?? Is that right?

 

[Spudnik]

Agree with the others.

However, you could just stake the shed and then disconnect the earth from the house in the shed CU.

 

[Joe S]

Sorry for bumbling but I'm still very new to this testing lark and am bricking my inspection on Friday as I have a hard cookie from what I've heard!!

 

[Spudnik]

Its V=I/R

so PFC divided by your Ze will give you your voltage.

 

[Joe S]

Jason S - Thats how its set up at the moment.

 

[Spudnik]

Jason S - Thats how its set up at the moment.

If thats the case then just measure your true Ze and PFC.

Then divide the PFC into your Ze and that will give you your voltage.

Whatever it is, it will be fine as you are well under 100Ohms anyways.

 

[Joe S]

My PFC=6.34 kA and my Ze= 39.3ohms. That gives me 161.3V???

 

[trebor]

Sorry for bumbling but I'm still very new to this testing lark and am bricking my inspection on Friday as I have a hard cookie from what I've heard!!

Cant see what the problem is Joe, if the garage is TT and you are measuring Zs via the earth electrode then the results you have are very good, you are never going to achieve the max Zs figures quoted in the device tables through an electrode, as someone has already said the rec max for TT is 200ohms as this allows for varying soil conditions freezing drying out etc, remember a 30ma RCD will operate in required times with a Zs of 1667 ohms

50/30 x 1000 = 1666.66

 

[Joe S]

Thanks Trebor. I think I'm panicking a bit too much but I have never liked exams!! I dont completely understand everything and feel as though I am going to get asked some awkward questions.