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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
 
A

acat

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


Chris
 
  • Thread Starter Thread Starter
  • #3
TT mate. Sorry, should've said that at the beginning.
 
A

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
 
Last edited by a moderator:
  • Thread Starter Thread Starter
  • #5
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

-
Arms
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.:D
 
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  • #7
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?
 
S

Spudnik

Agree with the others.

However, you could just stake the shed and then disconnect the earth from the house in the shed CU.
 
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  • #9
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!!
 
S

Spudnik

Its V=I/R

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

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.
 
T

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
 
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  • #15
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.
 
G

Guest123

Hey.

For a TT system the max Zs values in 7671 are not applicable, on the next page you will se for TT systems the the max is 1667ohms to enable operation of a 30mA RCD in the desired times at a safe touch voltage of 50V.

Really the RA should be well below 200ohms as this is the accepted value (about 100ohms is spot on) allowing for soil conditions weather etc. As you have 39ohms you have an excellent reading there so no need to worry at all.;)
 
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  • #17
Thanks Lenny ;). Its just nice to hear someone else saying its ok. I always seem to question too much!!
 
A

acat

Joe never say sorry for BUMBLING thats what this forum is all about helping each other with the things that bug us in the job. If you learn we succed and how many people have learned from this thread as well the time will come when I will BUMBLE and someone like you is gonna help out:)

Chris
 
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  • #19
What a good way of putting it!! Thanks for that. Its put a smile on a very tired face!
 
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  • #20
Thanks for everyones help on this. I passed my inspection yesterday!! Will definately recomend this site to my sparky mates! ;)
 
G

Guest123

Well done Joe.;)
 
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  • #22
Cheers mate. What a nerve-racking 3.5 hours! But he was a nice guy and turned out to be as much of a car-nut as I am so we had a good chat!
 
I

iceman

blimey m8 3.5 hours i did mine last week he was only with me 1.5 hours i must of been jammed up lol
 
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  • #24
Hope you passed mate ;). The guy I had was supposed to be a hard cookie from what I have heard from a few mates in the trade. I found him a very easy to talk to guy who is very knowledgeable. I got a lot of questions answered by him about he does things and to be honest, we're all in the same game but they've just got to another level!!
 
B

baldsparkies

Just been reading your thread Joe and wanted to add my congrats. The NIC inspectors are a good bunch even, if they are bookworms.:D Most can quote the regs without even looking it up.
Just wanted to be sure you are clear on a couple of points with regards to your tt readings.
Mainly that you need to relate two methods of fault current protection in a TT system.
a) is your fault current from phase to earth that is dealt with by your 30ma rcd, mainly because with the higher Ze values its the only way to achieve disconnection times. Remember ohms law and the magic triangle V=IxR. The value of 1667 ohms makes sense if you apply it to the 50v rule in other words I= 0.03 x R=1667=V which is our 50 volts.
b) is your fault current from phase to neutral and this is taken care of by your mcb, ok an mcb is quite capable of giving earth fault protection in lower earth resistance path systems like tn-s 0.8 ohms or
tn-c-s (pme) at 0.35 ohms (max recommended) But as said our rcd is coping with that one on TT.
Anyway for phase to neutral we can rely on table 41.3 because our phase to neutral loop will (or should) be much lower and hence our generated pfc much higher,again ohms law will bear this out.

I expect a few of the lads on the forum will be having a chuckle for my waffling on. But I have seen some good sparkies make simple mistakes for years and not realise, just because they haven't grasped the basics early in there careers. For example taking an earth loop and there pfc from the same reading
(Highest pfc will almost always come from phase to neutral) or high ring main continuity (Forgot to null the leads)
Anyway I will shut up now;) Well done to you for getting through.
 
T

tim1

install rcd protection which will cover the high efli measurement
 
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