Stupid question? Re. Insulation resistance

[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

 

[Leesparkykent]

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

No the lowest value measured individually on each circuit would be above 50Mohm.

 

[davesparks]

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.

Insulation resistance is the same as any other resistance and follows the scientific laws governing resistance.
Getting a result which displays on a meter as >200megOhm just indicates that the resistance is higher than the maximum reading that instrument is capable of, an instrument with a greater scale would return an actual value for it.
You may very well get a global test result the same as each individual circuit, but that could be that the global result is actually 201megOhm and the individual circuits are all 1005megOhm

 

[Dave OCD]

Yes I am aware of this !