Stupid question? Re. Insulation resistance

[TJ Anderson]

It tells you if the installation meets an arbitrary value in the book without any regard for the situation.
An installation with 5 circuits could have an IR of 2 megohms and bee deemed satisfactory, and an installation of 50 circuits could have the same IR and be deemed satisfactory. It doesn't take much thinking to realise that the smaller installation is in a poorer condition, but both pass?
And what does it tell you when you look back at previous test results? Absolutely nothing as you cannot see which circuits are deteriorating and which are not.
Once upon a time the minimum value of IR was calculated based on the specifics of an installation, either from the full load current or the number of circuits/points, which makes sense as then it is related to the size of the installation.
But following a blanket 1megohm rule is just daft and symptomatic of the constant simplification of the trade to suit the lowest common denominator instead of striving to achieve a good standard

I know the value of individual circuit testing and don't disagree with your points above. BS7671 is the regulations though, so compliance with that is all that is required. Anymore than that is our personal preferences.

 

[Paul.M]

Personally I'm not happy with a global test

Same here. Deterioration of an individual circuit is much better than just doing and logging a global result. Pin point the offending circuit, rewire/repair or replace the insulation at it's weak point and the job is done. Saves time for the next man if/when certs are availible.

Sorry, didnt mean to be offensive... Thanks for the reply...

Everything is cool Golden_Boy :39::rockon2:

 

[7029 dave]

It definately has a place in quickly confirming if an installation has an acceptable insulation value overall.

Yeah especially with cheap EICR's. lol

 

[TJ Anderson]

Yeah especially with cheap EICR's. lol

Based on your global reading you can determine if individual circuit testing is required. That is what differentiates an electrician who understands the results and one that doesn't.

 

[7029 dave]

Based on your global reading you can determine if individual circuit testing is required. That is what differentiates an electrician who understands the results and one that doesn't.

Yes agreed, only playing TJ.

 

[Dave OCD]

I'm talking about the real world too, it's just a simple network of five resistances in parallel. It could be five circuits with 250megohm each, or any other combination of resistances which totals 50

Are you absolutely sure about this Dave ? If each circuit had a I.R. of 250 Megohms I think that's what you'd see on the 'Global' test....

 

[Andy78]

are you absolutely sure about this dave ? If each circuit had a i.r. Of 250 megohms i think that's what you'd see on the 'global' test....

1/rt = 1/r1 + 1/r2 ?

 

[Leesparkykent]

Are you absolutely sure about this Dave ? If each circuit had a I.R. of 250 Megohms I think that's what you'd see on the 'Global' test....

Edit: Andy beat me to it

 

[davesparks]

Are you absolutely sure about this Dave ? If each circuit had a I.R. of 250 Megohms I think that's what you'd see on the 'Global' test....

No you wouldn't, five equal resistances in parallel will have a combined resistance of 1/5 the individual resistance.

 

[NDG Elecs]

1/rt = 1/r1 + 1/r2 ?

Taking me back to my college days there Andy!

 

[Richard Burns]

Remember that for any set of resistances in parallel the total resistance will be lower than the lowest value of any of the individual resistances.
The worst case scenario is when they are all equal as Davesparks has said.

e.g. if you had five circuits with very high insulation resistance and one circuit with an IR of 2MΩ, then the total resistance when measured as a "whole installation" would be less than 2MΩ.
Say the high values were all 200MΩ then the 1/r for each circuit would be 0.005 (so a total of 0.025 for five circuits) but the 2MΩ 1/r value would be 0.5 therefore the contribution of the "high" values is very small and the total resistance would work out to (1/0.525) or 1.9MΩ.

 

[telectrix]

gawd, RB, it's sunday after ale.too heavy atm.

 

[Dave OCD]

I don't doubt you're all correct but we're talking about insulation resistance or integrity so maintain that if every circuit [4 circuits for example] IR tested say >200 Megohms you'd also see similar as a 'global' reading.

 

[Andy78]

I don't doubt you're all correct but we're talking about insulation resistance or integrity so maintain that if every circuit [4 circuits for example] IR tested say >200 Megohms you'd also see similar as a 'global' reading.

Taking your example, if 4 circuits measured individually at >200MOhms it is possible that a global reading would also read the same if the actual readings were very high.
But also possibly not. If all 4 circuits measured 299MOhms then the global reading would be around 75MOhms for example.

 

[newtothis]

e.g. if you had five circuits with very high insulation resistance and one circuit with an IR of 2MΩ, then the total resistance when measured as a "whole installation" would be less than 2MΩ.

Does that mean the lowest IR reading the OP would get if he measured the 5 circuits separately would be 50MΩ ?
Sorry confused.com