Ring continuity testing on a MICC circuit

[RF Lighting]

Hello all.

I've come upon a slightly unusual query which has even left me a little unsure of the best way to proceed.

I'm testing a ring final circuit wired in MICC. I'm fine until the low ohms end to end testing.

Obviously line and neutral are the same as normal, but I'm not sure what to do with testing the CPC.

As it is wired in MICC each connection has two ends of the ring glanded into the same metal enclosure, so which ever point I test at, I would just be reading across the enclosure, rather than the whole ring circuit.

Would you just prove there is a CPC connection to each point, and as long as Zs values are acceptable, leave it at that, or would you try and break continuity somewhere to do the test?

Also what about R1 + R2? Surely this will be a misleading reading too?

Thanks chaps

 

[NotHimAgain]

Well first I should declare that I am not a fan of some of the testing methods in the OSG and GN3 - so you should bear that in mind as you may wish to disregard my comments.

You don't say if this is PIR or EIC testing but if it is the latter you should note that 610.1 requires that you carry out some testing during erection if this is a necessity.

A similar example - as part of a contract I had to supply test values for R2 on SWA cable. The problem was that this was on an industrial site and there were many SWAs running to each point of utilisation (lots of parallel paths). This meant that the test had to be carried out before each cable was inserted in its gland plate - difficult to arrange on a large site with many electricians.

Next bear in mind that the ring main R1 + R2 test only works if there are no parallel paths so you may have a problem there. The R1 + R2 test is not specifically required by BS 7671 - you only have to verify continuity and you don't even have to give a reading - a tick in the box will do . The test was actually invented to try and ensure that you carried out a polarity test before switch on .

If this is a PIR I would certainly consider not breaking the cpc for testing. A PIR on anything a bit more complex than a domestic installation will often be based largely on information gather by inspection rather than testing.

That is not to say that testing is not useful but it should be seen as a support to inspection. How many times do you see someone working away testing a distribution board and filling in forms - when if they would only take a proper look they would discover far more about the installation and its condition.

In deciding whether or not to break the cpc by removing the MIs you must decide if the benefit outweighs the risk of damage to the installation. You don't say what type of glands and pots have been used - if its the wedge type run a mile .

If you can see every part of the circuit I would not - I would then base my report on inspection only for this circuit.

 

[RF Lighting]

That make sense

It's a new installation. I never really thought about testing before the installation is complete. That makes sense though.

Thanks for your indepth reply

 

[333rocky333]

Is it worth also doing a test from the board to the midpoint, which should read about 1/4 of the L-L or N-N readings.
To give you an idea of the continuity of the earth.

 

[RF Lighting]

Gosh you are all very helpful on this site. I might have to visit more often...

 

[plugwash]

Obviously line and neutral are the same as normal, but I'm not sure what to do with testing the CPC.

If there are no paralell paths you could measure the resistance to the approximate midpoint and see if it agrees with your calulated values.

 

[securespark]

Personally, RF, I have to say I am a little taken aback.

Knowing you as a stickler for gettings things done just so, I am surprised at your question.

I would expect nothing less of you than to go round separating all legs of the ring and doing it properly.