Insulation Resistance Test

Joined
11 Feb 2009
Messages
680
Reaction score
9
Location
London
Country
United Kingdom
I am about to screw and glue down T&G flooring in a new loft, I have laid wiring underneath, ready for an electrician to come do the final wiring, test and certify at a later date.

To make sure my cables have been routed correctly (ie away from insulation) and will carry the correct current without any issues, I have bought one of these to test the laid cables.:

http://www.ebay.co.uk/itm/Digital-I...0B-/261127402305?ssPageName=ADME:L:OC:GB:3160

I understand a voltage, twice the operating voltage (LV 240v), should be used to test (500V), with a resistance reading of 2 Mega Ohm or greater deemed satisfactory. Could someone advise if that is correct, thanks.
 
Links in this post may contain affiliate links for which DIYnot may be compensated.
Sponsored Links
STOP: An electrician who is signing off your work needs to see all of the installation cabling. He will not be able to do that if you have covered them up.

How did you decide the size and type of cabling to use ?
 
Some common sense needs to be used. A length of cable 20 meters long should give us far more than 2MΩ and under 20MΩ I would be looking for a fault, but measuring a whole insulation I would expect under 2MΩ.

612.3.2 states 1MΩ often items like neons in FCU etc are the cause for low readings and once one knows why the reading is low then one can allow for it.

Do remember that any wiring needs inspecting as well as testing so if your not completing the installation certificate then the person doing so needs to inspect before any boards are laid.

In the main loft wiring is for lights and 1mm² cable can take the 6A of a lighting circuit even with Reference Method 103# (in a stud wall with thermal insulation with cable not touching the inner wall surface) where the limit is 8A.

However with for example a shower cable then ref 103 would mean 16mm² cable.
 
The instrument you mentioned will not test the current carrying capacity of your cabling.

Having cable with perfect insulation does not mean it will carry any amount of current. Safe current carrying depends on

[1] correct design
[2] correct installation
[3] assured quality of cable manufacture

That instrument cannot measure or verify any of those 3 points.

PLEASE get an electrician involved now to at least verify your design is using the correct size of cables.
 
Sponsored Links
Thank you for your replies.

I have used:

1.0mm for lighting
2.5mm for ring (sockets)
16mm for power shower

I have observed the 'Safe Zone' guidance, cables 50mm away from floor or ceiling, clipped to joists, and away from any insulation.
 
I have confirmation an electrician is happy to work with my laid cables provided I have observed the 'Safe Zone' guidance, cables 50mm away from floor or ceiling clipped to joists, and away from any insulation.
 
To make sure my cables have been routed correctly (ie away from insulation) and will carry the correct current without any issues, I have bought one of these to test the laid cables.:
As has been pointed out, the instrument you have bought will, unfortunately, not help you at all in relation to either of those issues about which you 'wish to make sure'.

Kind Regards, John
 
Some common sense needs to be used. A length of cable 20 meters long should give us far more than 2MΩ and under 20MΩ I would be looking for a fault, but measuring a whole insulation I would expect under 2MΩ.
Indeed. However, to be frank, if (as I think is probably the case) we are talking about relatively short lengths of cable that are not currently connected to anything, I would be concerned if I could measure an insulation resistance at all (i.e. with my meter, I would expect to see ">500MΩ").

Kind Regards, John
 
Ok, thanks, it is just a precaution before I stick all the boards down, the electrician has said he is happy for me to go do that, I just want to work as competently as possible.
 
Sorry but I have to ask-

Are you under the impression that the 'Insulation Resistance test' is related to the thermal insulation in the floor?
 
As mentioned in quite a few replies an IR test instrument is only going to measure the insulation resistance of the cables insulator, it is not going determine whether the design and construction of the circuit is safe with regards to the core size, current rating, reference method, de-rating factors and compliance to the holing of joists and chasing of walls.
You state that you have maintained a 50mm gap top and bottom of cables to prevent damage to cables, did you also follow the compliance of holing joist within part A of the building regulations, with regards to the span, hole location and size?
Also the first test that should be carried out are the continuity of conductors and dead polarity, insulation resistance tests mean very little without confirming continuity and polarity!
And as already mentioned in previous posts, I would be expecting a reading on new cables of a circuit of infinity of the test equipment, also just to add I cannot see where it mentions that the meter you have linked to has been calibrated.
 
You state that you have maintained a 50mm gap top and bottom of cables to prevent damage to cables, did you also follow the compliance of holing joist within part A of the building regulations, with regards to the span, hole location and size?
Whilst a theoretically valid question, I'm not sure that it is, in reality, all that relevant 'after the event'. If the answer to your question were 'no' (i.e. joists had been drilled 'in the wrong places') then, short of replacing the joist, there's not really anything which could sensibly be done about it. To attempt to 'rectify' the situation by drilling yet more holes ('in the right places') in a joist which already had holes drilled 'in the wrong places' would, IMO, be totally crazy and counter-productive!

Kind Regards, John
 
Whilst a theoretically valid question, I'm not sure that it is, in reality, all that relevant 'after the event'. If the answer to your question were 'no' (i.e. joists had been drilled 'in the wrong places') then, short of replacing the joist, there's not really anything which could sensibly be done about it. To attempt to 'rectify' the situation by drilling yet more holes ('in the right places') in a joist which already had holes drilled 'in the wrong places' would, IMO, be totally crazy and counter-productive!

So we may as well ignore Part A, and go whoops!
 
So we may as well ignore Part A, and go whoops!
Of course not - but I think it only makes sense to raise the issue before, not after, the holes have been drilled :) As I said, to drill extra holes to 'rectify' the fact that holes had already been drilled in non-compliant places would surely go even more against the spirit of Part A?

I'm just trying to be realistic. You could,of course, suggest that someone should replace the joists if they had already drilled holes 'in the wrong places' to be compliant, but .....!!

Kind Regards, John
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top