Minimum IR values

It is a somewhat muddled situation. In practice these days often each final circuit is tested individually - partly because RCDs (both RCCBs and more especially RCBOs with an functional earth connection) tend to mess up global tests, and partly because the model forms seem to require per-circuit results to be recorded. But as you say the regulation itself suggests a test should be done globally on a DB (along with it supply circuit).

As Mike says, it's all a bit arbitrary - if you look back over previous regulations there have been all sort of different approaches - with acceptable values varying from one edition to another and the basis of the test varying between and entire installation, through per-DB, to dividing the result by the number of points served. The general gist has been though that large values (tens or hundrends of MegOhms) are good, very small values (kilo Ohms) usually bad.

The other gotcha is that the tables are specified only for initial verification of new work ... for existing installations there's room for more engineering judgement - which can be handy for some situations (like an old damp Church full of old MICC cableing).

   - Andy.

It is a somewhat muddled situation. In practice these days often each final circuit is tested individually - partly because RCDs (both RCCBs and more especially RCBOs with an functional earth connection) tend to mess up global tests, and partly because the model forms seem to require per-circuit results to be recorded. But as you say the regulation itself suggests a test should be done globally on a DB (along with it supply circuit).

As Mike says, it's all a bit arbitrary - if you look back over previous regulations there have been all sort of different approaches - with acceptable values varying from one edition to another and the basis of the test varying between and entire installation, through per-DB, to dividing the result by the number of points served. The general gist has been though that large values (tens or hundrends of MegOhms) are good, very small values (kilo Ohms) usually bad.

The other gotcha is that the tables are specified only for initial verification of new work ... for existing installations there's room for more engineering judgement - which can be handy for some situations (like an old damp Church full of old MICC cableing).

   - Andy.

Thanks for the reply Andy,

looking through GN3 which states that the installation should preferably be tested as a whole and for new a new installation results should yield higher than 20Mohm, which should easily be achieved if the wiring was installed correctly, so usually this problem won’t exist when carrying out I&T.

GN3 also states that simple installations that contain no distribution circuits should preferably be tested as a whole but doesn’t say it’s a requirement, so on a simple domestic installation directly connected to the origin if the global test comes to less that 20Mohm, each individual circuit can be tested individually and if all circuits are satisfactory then no worries.

However with more complex installations where there are distributions circuits supplied from an LV switchboard for example, according to 643.3.2 distribution circuits should be tested separately with all final circuits connected and  with current using equipment disconnected. 

I interpret this to state the whole consumer unit, including the cable supplying the consumer unit should be tested together and the results of the distribution circuit itself with all the parallel resistances for all final circuits recorded on the schedule of test results for the distribution circuit, which as I stated in another post could result in all final circuits being satisfactory but the distribution circuit itself being under the recommended value, which seems a bit odd!

I hope that make sense I’m not the best with words, just trying to make sense of it all