2 Questions

I

iamgary25

Hi all

I'm using a fluke multimeter and when i test insulation resistance i get a reading of about 300Mohms on a cooker circuit but if i disconnect the cooker switch and link the cable straight to the cable going to the cooker connection unit (no cooker connect) i get >500Mohms. so is it ok that the switch lowers my reading?
BTW this is a new circuit. Also on the ring finals the tester goes from 350ish to >500... whats that all about?

Next question...

I have a metal CU and SWA going to an outbuilding with a metal CU but i'm going to TT the building. Which means there will be a parallel earth i guess this is perfectly acceptable?

Thanks in advance
 
How long did you apply the IR test for?

As for the second part, guessing isn't really a good way to do an electrical design, try referencing some regulations instead. But if you are going to guess I'd say you should guess again!
 
1. I wouldn't be reading in to it too much 350M ohms is still a very high reading and would be no cause for concern even on a new build. Think about resistors in parallel, that's all it may be.

2. If your TT'ing the sub main then I would consider earthing the armour solely from the TN end where it will be supplied from and then either installing an insulated consumer unit in the outbuilding and not using a fly lead at that end OR glanding in to an adaptable box first and then feeding in to your metal CU from there. How come you are favouring a TT Earth for the outbuilding anyway?
 
iamgary25 said:
Hi all

I'm using a fluke multimeter and when i test insulation resistance i get a reading of about 300Mohms on a cooker circuit but if i disconnect the cooker switch and link the cable straight to the cable going to the cooker connection unit (no cooker connect) i get >500Mohms. so is it ok that the switch lowers my reading?
BTW this is a new circuit. Also on the ring finals the tester goes from 350ish to >500... whats that all about?

Next question...

I have a metal CU and SWA going to an outbuilding with a metal CU but i'm going to TT the building. Which means there will be a parallel earth i guess this is perfectly acceptable?

Thanks in advance
Click to expand...


Plenty of older threads on this subject. Have a search.
 
The steel on the SWA i mean... i know you shouldn't connect the CPC conductor at both ends but i've been told by some people to bond at both ends and just one end by others which means it shouldn't matter if the CU is metal or not? i'm just asking for more advice. And the outbuilding is about 70 something meters away. Hope this clears up my question
 
iamgary25 said:
The steel on the SWA i mean... i know you shouldn't connect the CPC conductor at both ends but i've been told by some people to bond at both ends and just one end by others which means it shouldn't matter if the CU is metal or not? i'm just asking for more advice. And the outbuilding is about 70 something meters away. Hope this clears up my question
Click to expand...

Maybe get an electrician in if your not sure.
 
When you TT the outbuilding, neither the CPC or the armour should be connected at the outbuilding end.
 
spinlondon said:
When you TT the outbuilding, neither the CPC or the armour should be connected at the outbuilding end.
Click to expand...

ok, i agree with this... but it will be glanded into a metal CU will this matter? or as tomspark93 said, i should connect it into an adatable box?

thanks
 
You can connect into a plastic adaptable box, use an insulating (plastic) gland into the CU, or I have seen the gland made off into a kind of bracket with just the Line and Neutral then taken to the CU.
 
iamgary25 said:
i would gland into the metal CU as its extraneous conductive part, i can't find why you can't, you ask for help and just get mocked, i see why people say don't go on here
Click to expand...

Your not being mocked...I just asked a question. I doubt the metal CU is an extraneous conductive part! If you are making it a TT system at the far end then you need to isolate the suppliers earth from this part of the installation.
 
Last edited by a moderator:
I'm getting the impression you shouldn't connect straight into the metal CU, which doesn't tie up with other advice i've been told, i'm trying to do the job correctly and have had a look in the book but couldn't find anything thats why i ask... thanks simon, didn't know those were available
 
iamgary25 said:
I'm getting the impression you shouldn't connect straight into the metal CU, which doesn't tie up with other advice i've been told, i'm trying to do the job correctly and have had a look in the book but couldn't find anything thats why i ask... thanks simon, didn't know those were available
Click to expand...

How come you want to make the remote end TT?
 
Leesparkykent said:
Your not being mocked...I just asked a question. I doubt the metal CU is an extraneous conductive part! If you are making the it a TT system at the far end then you need to isolate the suppliers earth from this part of the installation.
Click to expand...

i meant the steel of the SWA not the metal CU, sorry

and where can i find this in the book please?
 
iamgary25 said:
As its about 70 something metres away and will be cheaper to install 2 core as its 25mm
Click to expand...

What is at the far end? Shed, garage, house, etc.

The price difference between 2core and 3core will be about £1 a meter so about £70. You will more than likely recoup this in labour/material if you don't make the far end a TT arrangement.

Is there anything extraneous at the far end?
 
Last edited by a moderator:
542.1.3.3
Where a number of installations have separate earthing arrangements, any protective conductors common to any of these installations shall either be capable of carrying the maximum fault current likely to flow through them or be earthed within one installation only and be insulated from the earthing arrangements of any other installation.
 
Leesparkykent said:
What is at the far end? Shed, garage, house, etc.

The price difference between 2core and 3core will be about £1 a meter so about £70. You will more than likely recoup this in labour/material if you don't make the far end a TT arrangement.

Is there anything extraneous at the far end?
Click to expand...

Its a big storage shed with steel structure
 
If you go down the route of making the far end TT, the glands Simon linked to are good or gland in to a plastic dappy box.
 
Leesparkykent said:
If you go down the route of making the far end TT, the glands Simon linked to are good or gland in to a plastic dappy box.
Click to expand...

ok, thanks for your help, i am trying to do the job properly, i don't know everything and when i don't i try and find out, and asking someone is sometimes easier, as i couldn't find anything in the book, and i've been told to do it incorrectly by other electricians
 
iamgary25 said:
ok, thanks for your help, i am trying to do the job properly, i don't know everything and when i don't i try and find out, and asking someone is sometimes easier, as i couldn't find anything in the book, and i've been told to do it incorrectly by other electricians
Click to expand...

The armour or any protective conductor in the cable must not be connected to and must not be simultaneously-accessible with any exposed-conductive-parts in the outbuilding if the outbuilding is being made TT.
 
by far the better method is to use 3 core SWA. it's 25mm, so the 3rd core is more than adequate for bonding, even if it's PME. the extra cost in using 3 core is outweighed by the fact that you won,t need to install earth rods to end up with a pitiful 10 ohm or greater Ra.

and without getting my brain in gear after ale, i'm sure 16mm would give a within limits VD. ( going by your 20A and 70m. about 6V off top of head)
 
Last edited:
telectrix said:
by far the better method is to use 3 core SWA. it's 25mm, so the 3rd core is more than adequate for bonding, even if it's PME. the extra cost in using 3 core is outweighed by the fact that you won,t need to install earth rods to end up with a pitiful 10 ohm or greater Ra.

and without getting my brain in gear after ale, i'm sure 16mm would give a within limits VD. ( going by your 20A and 70m. about 6V off top of head)
Click to expand...

thankyou, advice taken, and its for solar PV so only have 1%
 

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