RFC testing shenanigans

[NDG Elecs]

Hello Chap/ettes,

Testing a RFC today. End-End values spot on: 0.19/0.21/0.34. All IR tests good bar N1-CPC2 which was 0.0 mega ohms.

N2-CPC2 was >299 mega ohms.

Why, if I am getting 0.21 and 0.34 on the end-end am I not getting 0.0 m ohms on both neutrals?

Is it to do with the resolution of my meter (MFT1552) for the different tests....I am guessing that IR tests are nowhere near continuity tests in terms of the resolution for each type of test.......me confused, though doesn't take much!

Please take pity on my frazzled brain!

Ta.

*I used the word 'mega' just for you Trev....I know you love it really!

 

[telectrix]

user error.

 

[Octopus]

user error.

Deffo. Redo the tests.

 

[Edd]

IR tests are Done at 500V and end to end is done at 12V, A fault at 500V will show as 000 but at 12V may not show up at all.

 

[NDG Elecs]

Thanks for replies. I did redo the tests three times, as was puzzled by it. Double checked nowt was plugged in etc. I also checked the MFT was functioning correctly twice. I have my tester out of its case at least three times a week so am confident I use it correctly!

I see what you are saying Edd but the fault was there and then gone depending on which neutral I tested, both at 500V.

 

[Sparky Ninja]

Why are you testing IR from one part of the leg to the other?

You only need to test from conductor to conductor

I think you have confused yourself with the testing. You haven't mentioned your R1&R2 and R1&RN, are you doing them?

 

[Octopus]

Or are you testing IR with the main switch on and the "other" leg of the N connected??

 

[Edd]

I really should read the whole post properly before replying

 

[trev]

*I used the word 'mega' just for you Trev....I know you love it really!

Yes, I spotted that. Bloody wind up merchant
Looks like you're confusing your continuity readings with your IRs mate, it's easily done. It's Friday.
Just test between your conductors as opposed to along them like you're doing now and all will become clear.
We all have brain farts from time to time.

 

[NDG Elecs]

Hi,

I am not being rude by not replying; I did start to reply earlier from my phone and promptly fell asleep due to a hard wks graft!

 

[NDG Elecs]

Why are you testing IR from one part of the leg to the other?

You only need to test from conductor to conductor

I think you have confused yourself with the testing. You haven't mentioned your R1&R2 and R1&RN, are you doing them?

Hi Widdler,

I am 99% certain of what I am saying mate. r1r2/4 figures were spot on, only seven sockets on circuit and all appeared to be
part of RFC (no spurs) according to test results. r1rN equally satisfactory.

I do realise that you can IR test from one leg only (providing no open ccts on end to end). Testing across both legs is just something, maybe peculiar, I do, don't ask me why! When I say across both legs I will test L1-CPC1, L1-N1, L1-CPC2, L1-N2......and so on.

Cheers

 

[NDG Elecs]

Or are you testing IR with the main switch on and the "other" leg of the N connected??

Are you thinking PME here? TT install.

I confess main switch was on Murdoch! Both legs were fully disconnected though, and I could clearly see the outer sheath of the T&Es leg so was not mixing up cables. The board, approx 30 years old, was actually quite neat (for a pleasant change!), and easy to work on.

 

[NDG Elecs]

Yes, I spotted that. Bloody wind up merchant
Looks like you're confusing your continuity readings with your IRs mate, it's easily done. It's Friday.
Just test between your conductors as opposed to along them like you're doing now and all will become clear.
We all have brain farts from time to time.

Couldn't resist Trev!! Haha!

Was not IR testing as in N-N. I Cont. tested N-N etc, but all IR tests were L-N....L-CPC...N-CPC...

Cheers.

My original post was not that clear, I just re-read it and I sound like I have no idea!

I know what I say does not make sense, that's why I am really puzzled by it.....

 

[tony mc]

Think about what you are asking. If your rn end to end is continues how can you get the results you have posted ? And why would you test the IR this way ? So if you are getting 0.00 Meg what is the reading in Ohms or K ohms when ?

 

[NDG Elecs]

Think about what you are asking. If your rn end to end is continues how can you get the results you have posted ? And why would you test the IR this way ? So if you are getting 0.00 Meg what is the reading in Ohms or K ohms when ?

I have no idea, hence the question!! As stated, it's just something I do. Regarding your last sentence, I forgot (CRUCIALLY!) that I did get a continuity figure of approx 1.5k ohms on the N-CPC on which I was also getting the 0.0 meg ohms IR.

The unaffected (>299 megohms) N-CPC was open cct as it should be on continuity testing....

The 0.0 meg ohms was N from one leg and CPC from another.

Please, nobody else, go on about why I IR test across the legs! I just do!! Unless of course it is bad to do in some sense, then please enlighten me..I am not the most experienced spark by any means and welcome thoughts etc.

Cheers.

 

[NDG Elecs]

So after Tony has made me recall the 1.5k ohms bit, which I am beyond stupid to forget! As said brain is frazzled! I am left pondering.

1500 ohms resistance on cont reading. So why am I getting open cct on other N-CPC reading, if the meter was accurate enough surely I would get 1500.21 ohms, if you follow my train of thought!!

 

[telectrix]

seems to me to be case of over thinking.

 

[Richard Burns]

If you have continuity on r1 r2 and rn then, whilst you may get a low insulation resistance between cores, it would not be possible to get different IR values from either ends of the cable.

The scale differences would obliterate any variance in IR readings based on tenths of an ohm.
1.5kohms on continuity could well show up as 0.0Mohm on IR.

The only thing, apart from operator error, that I can think of would be that the method of moving the cables to test them is causing a fault in one case and not in the other.