GN3: IR test, whole installation vs final circuit

[happysteve]

Eh up

Someone tell me where I'm going wrong.

I'm reading up on testing theory, using GN3 (6th edition, incorporating Amd 1, 2012). Section 2.7.7, pp40-41.

"Simple installations that contain no dist. ccts should pref be tested as a whole, see fig 2.4a."



"Test should be carried out with main switch off, all fuses in place, switches and cct breakers closed, lamps removed..." blah blah.

Got it. All good

Next:

"Example (ii) - IR test of a final cct. Fig 2.4b shows an example of testing a single final circuit at a CU (only the line to neutral test is shown)."



Unlike the first example (fig 2.4a, for the whole installation) there is no accompanying text to say, "main switch like this, fuses like that or CBs like the other," all you've got to go on is the picture.

But all the breakers are set to "on". Which means, all your circuits will be in parallel, wont they (as far as an IR test is concerned)? And you're not doing an IR of just one circuit at all, you're doing the whole installation again. Aren't you? Should you, in fact, turn the breakers off / remove the fuses (to IR test individual circuits)?

Or am I going wrong somewhere?

 

[telectrix]

deletde.

 

[happysteve]

deletde.

Aw, before I even saw it an' all! Was it rude?

 

[telectrix]

nope. t'waS A BRAIN FART.

 

[telectrix]

looking at the 2 pics. the only difference is that the 2nd example is through the MCB so will also give a global reading. unless i'm missing something due to the beer.

 

[happysteve]

looking at the 2 pics. the only difference is that the 2nd example is through the MCB so will also give a global reading. unless i'm missing something due to the beer.

That's what I thought! Ta

 

[Baker1988]

Eh up

Someone tell me where I'm going wrong.

I'm reading up on testing theory, using GN3 (6th edition, incorporating Amd 1, 2012). Section 2.7.7, pp40-41.

"Simple installations that contain no dist. ccts should pref be tested as a whole, see fig 2.4a."

View attachment 26208

"Test should be carried out with main switch off, all fuses in place, switches and cct breakers closed, lamps removed..." blah blah.

Got it. All good

Next:

"Example (ii) - IR test of a final cct. Fig 2.4b shows an example of testing a single final circuit at a CU (only the line to neutral test is shown)."

View attachment 26209

Unlike the first example (fig 2.4a, for the whole installation) there is no accompanying text to say, "main switch like this, fuses like that or CBs like the other," all you've got to go on is the picture.

But all the breakers are set to "on". Which means, all your circuits will be in parallel, wont they (as far as an IR test is concerned)? And you're not doing an IR of just one circuit at all, you're doing the whole installation again. Aren't you? Should you, in fact, turn the breakers off / remove the fuses (to IR test individual circuits)?

Or am I going wrong somewhere?

yeah you are right the breakers are still on and the cable is terminated into the breaker so it would just do a global line to neutral again with the illustrations its basically the same test, it should show the breaker off or to take the cables out of the terminals and test it by its self (that's what i do when test one circuit). its definatly interesting haha

 

[Baker1988]

Just looking at this in gn3 and if you look down to fig 2.4c page 42 it says when ir testing the armorings of swa the armorings need to have a connection to earth :/ i did not know that but usually when i have tested swa it is normally teminated to a metal board which is earthed so i suppose it is earthed any way.

 

[happysteve]

Yeah, there's a bit of commentary about that (need to connect the protective conductor to the earthing arrangement when IR testing between the live conductors and the CPC) on P39. Apparently it's a change that came into effect with the 17th Edition in 2008.

 

[Silly Sausage]

I can think of a good reason for that.

First correct answer gets a £5 off Paul!

 

[Baker1988]

I can think of a good reason for that.

First correct answer gets a £5 off Paul!

I have amhad a good few jd and cokes but I cant think why you woukd have to connect to the earthing arrangements when you ir as you are only testing to make sure no live cables are touch the earthing conductor

 

[Silly Sausage]

I have amhad a good few jd and cokes but I cant think why you woukd have to connect to the earthing arrangements when you ir as you are only testing to make sure no live cables are touch the earthing conductor

Try tomorrow!

 

[Simonslimline]

Could this be to make sure cables have not been damaged during the installation process and still have sufficient insulation resistance values from the means of earthing and what is connected to it. Surely if a cable was tested disconnected from the MET then you are only testing to check the cable cores are all properly insulated from each-other and not that the live cores are separate from everything else connected to the means of earthing?

 

[Baker1988]

Could this be to make sure cables have not been damaged during the installation process and still have sufficient insulation resistance values from the means of earthing and what is connected to it. Surely if a cable was tested disconnected from the MET then you are only testing to check the cable cores are all properly insulated from each-other and not that the live cores are separate from everything else connected to the means of earthing?

I have ony just come in from pub and that is what i am thinking to make sure every core is not connected to earth by some means or another

 

[Simonslimline]

I can think of a good reason for that.

First correct answer gets a £5 off Paul!

Is your good reason anywhere near what i put Archy?

I am curious now :computer:

 

[Silly Sausage]

Forgot about this!

Re: the CPCs being connected to Earth, No, but what you said is good.

I'm thinking of, if they weren't and there was a fault of the old negligible impedance ...

 

[Simonslimline]

Forgot about this!

Re: the CPCs being connected to Earth, No, but what you said is good.

I'm thinking of, if they weren't and there was a fault of the old negligible impedance ...

Not quite sure i follow you here!

If we are carrying out Insulation Resistance testing then the circuit or circuits (depending) will be safely isolated from the supply.

I may be missing something very obvious here though. Thanks:smile5:

 

[Silly Sausage]

Last clue.

0 Ohm L-E fault, cpc not Earthed, put 500V on it, what's the V on the cpc/exposed parts?
Other trades wandering around...