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Brobat

Installation resistance test querie

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Brobat

I hope this in the correct section.   New user.

 

I need to do an I R test on a circuit which I can disconnect the L & N from the rest of the installation ok,  but the CPC of this circuit is still

 

connected to the CPCs of the other circuits of the installation which can't be turned off. 

 

Is it best in this instance to seperate the CPC of the circuit I'm testing   (which will be a pain, but I'm used to that)   from the other CPCs or is it ok the leave it connected and test.????

 

Hopefully this hasn' t come across as a stupid question as it ain't meant to be.

 

Thanks in advance ,          Bro.

 

  

 

 

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Tony S

Do you know where the remote end CPC is connected? If you can disconnect one end there's nothing stopping you disconnecting both ends.

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Murdoch

Imho you should always conduct the IR test with the cpc connected to the cu ....

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Tony S

It would help if you explain why the CPC can't be disconnected for this circuit. As Murdoch pointed out, normally an IR test would normally have the CPC connected at both ends.

 

In the past I've had to isolate an entire cable from every connection from both earth and Ph-N conductors to test a parallel feeder but it would be exceptional circumstances.

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binky
1 hour ago, Murdoch said:

Imho you should always conduct the IR test with the cpc connected to the cu ....

 

why's that then?

 

I am assuming from the post that the CPCs are a jumbled mess in the CU and difficult to trace? Would it be that much bother @Brobat to shut down all the electrics, disconnect all the CPCs in the board and do the IR tests?

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Murdoch
1 minute ago, binky said:

 

why's that then?

 

I am assuming from the post that the CPCs are a jumbled mess in the CU and difficult to trace? Would it be that much bother @Brobat to shut down all the electrics, disconnect all the CPCs in the board and do the IR tests?

 

The way I was taught ....

 

So if you dis the CPC from the CU and just test the cable, any "issues" to anything else in the property won't show up..... so its not a valid test

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binky

Like what? The test is to verify that cable/ circuit is good. 

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Murdoch
10 minutes ago, binky said:

Like what? The test is to verify that cable/ circuit is good. 

 

Hum ......... an IR test as you suggest wouldn't pick up any connection between the cable and the earth "plain" of the installation.

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Sidewinder

If the circuit cable is not connected to earth then any testing to earth will not detect any fault to earth potentially.

 

Yes, I have actually had this exact scenario.

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binky

are we testing the insulation of Lines to CPC  for the cable or lines to the building?

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Sidewinder

Well both are required.

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binky

I spent many hours 1 day trying to trace a fault that turned out to be wet plaster. I would argue we are not required to test the resistance to the earth of the building, but resistance between conductors and CPC of that cable. Earths terminated at back boxes ( metal ones at least) would be connected to 'building earth' anyway. Given that CPCs are bare copper, a split in the insulation of a conductor and sheath should arc under test at 500V DC to the CPC anyway.

 

As it happens I usually can't be arsed to take CPCs out and test with them connected.

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Doc Hudson

If it is a ring circuit you will need to disconnect the two CPC ends to verify continuity of the CPC. However once that has been done, the on site guide and regulation 612.3.1 both suggest that the CPC must be connected to the earthing arrangement. Which I understand to mean IR between L's & N's (possibly L&N joined if items vulnerable are in circuit), with respect the circuit CPC and the whole installation earth.

 

Doc H.     

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Sharpend

Please obtain a copy of GN3, it is clearly explained that testing of Insulation Resistance must be done with CPC connected. 

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Sidewinder
19 minutes ago, binky said:

... I would argue we are not required to test the resistance to the earth of the building...

 

Well if when I was doing the 2391 examiner work you would have failed.

 

The requirement is.

Prior to connecting the cable, you verify that it is not damaged to itself.

Thus test between all cores.

Then you connect the CPC, then you test again to make sure that the live conductors are not connected to true earth, or that the cable is not compromised in some way that it is somehow connected to true earth when it should not be, or there is some other connection fault that should not be there.

 

I did make a mistake once, being lazy, and trying to claw back time on a job, I tested a cable from the load end.

The cable had been damaged such that the fault did not show at the load end, but, it had a dead short line to earth from the supply end.

That was a pop and a half!

It also cost me a new cable and 3 men on overtime on a Saturday to fix it!

Never again.

 

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binky

been  along time since I did 2391 - passed first time. 

 

fair enough Sharpend, as I said I normally do this anyway, and certainly on industrial ccts with singles in metal trunking I would always test this way, but I was intrigued as to what other faults Murdoch could find, but seeing as he can't be arsed to explain I don't see us being enlightened with his pearls of wisdom.

 

Little puzzler for you, single cable run up through concrete car park. At each floor, Steel conduit run from steel trunking splits into 4 runs of lights. Radial ccts of course. End of line test, Zs readings fine. Zs tested at point of split to lighting runs, no result, can't get a Zs. So how did we get a Zs at end of line(s)? Steel conduit and trunking not be used as CPC.

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Barx

I got pulled on assessment for disconnecting cpc's. think he said leaving them connected came in in 17th (he maybe even said 3rd ed) I never checked up to previous versions though.

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Sharpend
40 minutes ago, binky said:

 

 

Little puzzler for you, single cable run up through concrete car park. At each floor, Steel conduit run from steel trunking splits into 4 runs of lights. Radial ccts of course. End of line test, Zs readings fine. Zs tested at point of split to lighting runs, no result, can't get a Zs. So how did we get a Zs at end of line(s)? Steel conduit and trunking not be used as CPC.

 

For clarity, you say single cable, is that a single cable as in 6242 type or 6491's, are we saying 4 independent radials? 

 

Is each floor separate electrically and mechanically? 

Edited by Sharpend

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Doc Hudson
16 minutes ago, Barx said:

I got pulled on assessment for disconnecting cpc's. think he said leaving them connected came in in 17th (he maybe even said 3rd ed) I never checked up to previous versions though.

The IR tests described in 612.3.1 first appeared in 17th 2008, Red book. But that was also when parts 6 & 7 were swapped over. So it would have been somewhere in the 713-xx's if it had been in 16th.

 

Doc H.

  

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Murdoch
44 minutes ago, binky said:

but I was intrigued as to what other faults Murdoch could find, but seeing as he can't be arsed to explain I don't see us being enlightened with his pearls of wisdom.

 

 

 

I am am not sure what you are on about .... I, and others on here have stated that ir tests need to be completed when the cpc is connected to the earth bar.....end of

 

 

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binky
18 minutes ago, Sharpend said:

 

For clarity, you say single cable, is that a single cable as in 6242 type or 6491's, are we saying 4 independent radials? 

 

Is each floor separate electrically and mechanically? 

single cable run (singles) to each floor, then splits to 4 cable runs to cover the parking area on each floor. Each floor is separate electrically and mechanically, 14 floors in all.

2 hours ago, Murdoch said:

 

 any "issues" to anything else in the property won't show up.....

 

This is what I'm on about, what other issues do yuo expect the test to show?

Edited by binky

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Murdoch

Can't you work that out for yourself ?

 

obviously not as you don't test properly 

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Brobat

I can't turn the whole installation off as its industrial. They wouldn't be too keen in me shutting the whole place down to test a faulty outside lighting circuit. 

 

I am aware that the regs state that the CPC should remain connected,   The fault is that its tripping the dedicated RCD as soon as the timer sends power to the circuit. I've disconnected the L & N  coming out of the RCD and applied power to it and it doesn't trip which suggests that the RCD itself is not at fault.  however the CPC in question is the armour of some 2 core armoured flex which is connected on some trunking at a fairly awkward position to get at due to the amount of various bits of other lettuced that have been added over the last 20 years or so. 

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binky
13 minutes ago, Murdoch said:

Can't you work that out for yourself ?

 

obviously not as you don't test properly 

yes thanks, and you know nothing of how I work. But you raised the point so enlighten us with your wisdom if you are able? Failure to do so which point ot you being a person who posts glib statements and fails to back then up with actual facts. Worked out my little puzzler yet? Bet you can't, but it is very relevant to this discussion.

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binky
12 minutes ago, Brobat said:

I can't turn the whole installation off as its industrial. They wouldn't be too keen in me shutting the whole place down to test a faulty outside lighting circuit. 

 

I am aware that the regs state that the CPC should remain connected,   The fault is that its tripping the dedicated RCD as soon as the timer sends power to the circuit. I've disconnected the L & N  coming out of the RCD and applied power to it and it doesn't trip which suggests that the RCD itself is not at fault.  however the CPC in question is the armour of some 2 core armoured flex which is connected on some trunking at a fairly awkward position to get at due to the amount of various bits of other lettuced that have been added over the last 20 years or so. 

fair enough,  chances are it's water in the light(s). Think I would test at RCD with CPCs connected LN-E

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