Why Earth before IR testing?

[Woofie]

A question for testers to settle a debate at work - GN3 & regulation 612.3.1 state that when doing insulation resistance, the CPC/armour of swa MUST be connected to Earth.
Does anyone know why this is please?

 

[Octopus]

in case the cable is damaged .........

when IR testing, all CPC's should be connected to the earth bar!

 

[westward10]

If they are debating this then maybe testing should not be part of their job description.

 

[Adam W]

Because the IR test is between line and CPC/earth - if the CPC isn't connected to anything then you could have a short between line and CPC/earth which the test wouldn't identify.

 

[Ian1981]

By completing the R1+R2 testing at each point on the circuit during initial verification you are confirming the cpc is continuous and present then you can move on to IR testing , assured that the cpc’s will be included in the test and is not discontinued to give the tester false or incorrect readings.
There is a reason that the testing sequence in initial verification is there and it’s worth remembering that any issues found during IR may mean you have to repeat the previous tests.

 

[Woofie]

Much appreciate your answers so far but I don't feel anyone has actually given a compelling reason (other than the regs which I already quoted in the OP).

 

[westward10]

When you say the cpc / armour connected to earth are you just referring to swa cables and how would it not be connected to earth if glanded to a metal enclosure.

 

[PEG]

The compelling reason,over-rides the regs,as it is a fundamental requisite,of complete and thorough testing.
Out of interest,why do you think,it is necessary?

 

[Adam W]

It has been explained but you don't seem to have understood.

You have to have a CPC for the IR test to work.
You need to test the CPC before you do any other test that needs the CPC.

It's like making sure your tester works and your leads aren't broken before you start testing.

 

[westward10]

This seems to be specific to the armour of a cable.

 

[Adam W]

It's the same idea - if the live is making contact with the armour then an IR test will show that up unless the armour isn't earthed, in which case it'll just liven up the armour to 500v. The test might show you have a high resistance, but then when you energise the circuit the armour will become live.

 

[westward10]

Yes I know but have they installed these cables with no earth to the armour.

 

[Strima]

Because there maybe a fault via a parallel path which may not be identified if you test without the armour and CPC connected to earth.

 

[Wilko]

Apologies if this already said - isn't it so a L/E fault that isn't via the cable armour shows up during cable tests.
Edit - @Strima beat me to it

 

[diditrain]

Perhaps the op is getting a little confused and is referring to the requirement to discharge the conductors of an swa to earth following an insulation resistance test. I could be barking up the wrong tree though.

 

[Woofie]

Too right I'm confused. The question isn't specifically about SWA but taking that as an example, if there were a phase to armour fault that would be picked up even if the armour were not connected to earth.

 

[diditrain]

Not if the armour was isolated from earth

 

[Pete999]

When you say the cpc / armour connected to earth are you just referring to swa cables and how would it not be connected to earth if glanded to a metal enclosure.

Might not be a very good termination, judging by some I have seen,

 

[Richard Burns]

However if there was a fault from a live conductor to another piece or earthed metal nearby the test from armour to live conductor woudl show as OK.
If the armour was connected to the earthing system then the short would be shown on the test result.

 

[telectrix]

However if there was a fault from a live conductor to another piece or earthed metal nearby the test from armour to live conductor woudl show as OK.
If the armour was connected to the earthing system then the short would be shown on the test result.

View attachment 41916

excellent explanation.