Stupid question? Re. Insulation resistance

[Golden_Boy]

If when carrying out a "whole instalation" test from L+N to Earth the result is say 50 Megohms, and there are ,say,5 circuits would that mean that ALL the circuits pass the test or do each one still need testing?

 

[davesparks]

What it means is that the combined insulation resistance of the five circuits in parallel is 50megohms

 

[Golden_Boy]

Yes , thats what i said. as there are 5 circuits the each one can be no lower than 50 megs? would each individual circuit still need testing?
Im talking about the real world here ...

 

[TJ Anderson]

If you have 50 meg on a global then you don't need to test further. All 5 of those circuits will individually be greater than that figure due to them being parallel resistances when tested together. BS7671 states 1 meg as minimum, not 10.

 

[Golden_Boy]

If you have 50 meg on a global then you don't need to test further. All 5 of those circuits will individually be greater than that figure due to them being parallel resistances when tested together. BS7671 states 1 meg as minimum, not 10.

Thats what i thought ...... cheers

 

[Paul.M]

A Global IR test shows up the lowest result for the weakest insulation on a certain circuit. Do an IR test on each circuit and find out which one is low and then find out why?

Global IR testing is fine but when something pops up, it i time to test each circuit.


ps, drop the attitude.

 

[davesparks]

A Global IR test shows up the lowest result for the weakest insulation on a certain circuit

Actually it will show the combined resistance of the IR of every circuit as per the normal rule for resistances in parallel.

 

[davesparks]

Yes , thats what i said. as there are 5 circuits the each one can be no lower than 50 megs? would each individual circuit still need testing?
Im talking about the real world here ...

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

 

[TJ Anderson]

A Global IR test shows up the lowest result for the weakest insulation on a certain circuit. Do an IR test on each circuit and find out which one is low and then find out why?

Global IR testing is fine but when something pops up, it i time to test each circuit.

Parellel resistances will show lower than and individual cct reading. Also I would be happy with 50 meg on a global

 

[davesparks]

Personally I'm not happy with a global test

 

[Golden_Boy]

A Global IR test shows up the lowest result for the weakest insulation on a certain circuit. Do an IR test on each circuit and find out which one is low and then find out why?

Global IR testing is fine but when something pops up, it i time to test each circuit.


ps, drop the attitude.

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

 

[TJ Anderson]

Personally I'm not happy with a global test

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

 

[davesparks]

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

Which is about as helpful as a chocolate fireguard!

 

[TJ Anderson]

Which is about as helpful as a chocolate fireguard!

Don't understand that really. It can confirm if an installation has satisfactory insulation resistance overall and if it complies with minimum acceptable values in BS7671. Pretty useful I think.

 

[davesparks]

Don't understand that really. It can confirm if an installation has satisfactory insulation resistance overall and if it complies with minimum acceptable values in BS7671. Pretty useful I think.

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