Bizarre readings

mhar · Dec 30, 2012

RFC step 1 tests :- l-l .31Ohms, n-n .31 Ohms, cpc-cpc 0 Ohms, repeated this numerous times and got same results. Remove crocodile clips from probes and touch cables directly and get sensible results. Repeated this exercise a few times as it didn't make sense to me but kept getting the same readings. Electrician co-worker did exactly the same (repeatedly) and got the same results (with my Ethos mft - he didn't have his mft with him).
Obviously I know the solution but any thoughts on the reason as I am now tending to give credence to the poltergeist theorists?

Steve_P · Dec 30, 2012

I'd be giving credence to the CEF theory, Ethos are a City Electrical brand aren't they?

There again, what are the sensible results and how are you touching direct?

mhar · Dec 30, 2012

This is on initial verification so, basically, just checking my own work where I would expect to have end to end continuity.
I was expecting the cpc end to end results to be higher than the live conductor results.
Touching direct was achieved by placing the bared conductors on a piece of wood and pressing the probes hard, my body not being in contact with the bare probes or bare conductors.
Just cannot understand why the meter will measure correctly without the crocodile clips but with them it shows a dead short on the cpc but not on the lives. cpc-cpc 0 Ohms, l-l .31, immediately back to cpc where it reads 0 again, immediately back to a live and .31.

Octopus · Dec 30, 2012

Have you checked that the CPC's aren't touching at the back of the CU??

Rockingit · Dec 30, 2012

I'm making an assumption here that you do actually have a continuous, non-shorted cpc on the ring, and it measures (if the other results are to be believed) somewhere around .45 ohms?

And also that your meter is doing an R1-R1 to R2-R2 each at a time on some kind of setting?

Three options - either knacked leads/clips, knacked software on the meter or a knacked operator.

And, if you're in the position of testing your own work, I'd really like to sleep better knowing that you don't need help to figure out which of those three it is.

Clue: do the test again on a known value of conductor / proving card, and just use the R1 settings for all conductors. You may need an old fashioned tool called a pen to write down the results instead of having the machine save them.

telectrix · Dec 30, 2012

You may need an old fashioned tool called a pen to write down the results instead of having the machine save them.

or a slate and stylus.

chrish_1977 · Dec 30, 2012

^^^^^^ hahaha

mhar · Dec 30, 2012

I test as I go. The tails are clearly visible for about 5m. There is no short circuit as proved by not using the clips.
I am able to test. I have the 2391.
The question is not about my competency although we can all make mistakes.
The problem occurs when using the clips. As already stated I know the solution - replace the clips. However I do not understand why I am getting the correct reading on the live conductors but not on the cpc when using the clips but I get what I expect when not using the clips.
Yes the meter is set to the correct scale, yes I null the leads when changing between with and without the clips.
I replaced the batteries in the meter when I first got these readings but it made no difference

Octopus · Dec 30, 2012

Did you try testing the RFC at a "mid" point??

mhar · Dec 30, 2012

No, but thanks for a constructive reply.
I know that everything is all fine. It tests out perfectly when not using the crocodile clips. Cpc-cpc is .51 Ohms from memory without the clips (and leads correctly nulled). With the clips (again correctly nulled) it will not read this figure but displays 0 yet transfer the leads to either the l's or the n's and I get a reading of .31 Ohms.
I just don't understand how this can be. It is not just me either as my friend was giving me a hand on this job (also 2391) got exactly the same results and was equally as mystified.

tony mc · Dec 30, 2012

I would invest in another Low Ohms meter as a back up if nothing else.

As you have pointed out the test results are what you would expect but now doubt has been put in your mind re the connection of the clips.

I had a issue with my leads and croc clips on a Megger I use but it was the spring connection on the probe that was worn which was not seating inside the clip correctly to make a sound connection!

ipf · Dec 30, 2012

It seems to be a mystery but I'm sure it's not. Not the sort of question to put forward here, I feel.

Octopus · Dec 30, 2012

mharries1 said:
No, but thanks for a constructive reply.
I know that everything is all fine. It tests out perfectly when not using the crocodile clips. Cpc-cpc is .51 Ohms from memory without the clips (and leads correctly nulled). With the clips (again correctly nulled) it will not read this figure but displays 0 yet transfer the leads to either the l's or the n's and I get a reading of .31 Ohms.
I just don't understand how this can be. It is not just me either as my friend was giving me a hand on this job (also 2391) got exactly the same results and was equally as mystified.

Hum.... did you do another check else where on the ring?? I certainly wouldn't be happy with what you are seeing.

sledgehammer · Dec 30, 2012

crap gear

My fluke did exactly the same a couple of times. I broke out my personal IR tester (megger) and it was fine. Dumped the fluke, M1552 sorted, never looked back. Fluke said you gotta do this n that, I said jog on.

Rockingit · Dec 30, 2012

mharries1 said:
I test as I go. The tails are clearly visible for about 5m. There is no short circuit as proved by not using the clips.
I am able to test. I have the 2391.
The question is not about my competency although we can all make mistakes.
The problem occurs when using the clips. As already stated I know the solution - replace the clips. However I do not understand why I am getting the correct reading on the live conductors but not on the cpc when using the clips but I get what I expect when not using the clips.
Yes the meter is set to the correct scale, yes I null the leads when changing between with and without the clips.
I replaced the batteries in the meter when I first got these readings but it made no difference

You're getting a result of half an ohm over a 5m run of cpc (presumeably 2.5T&E) in a ring??!! Come on man, THINK!

Geoffsd · Dec 30, 2012

Wrong end of stick ?

derek · Dec 31, 2012

I have never used Ethos gear, just wondering what symbol it uses to show an open circuit on continuity tests, it isn't an O by any chance? Or did the tester actually say 0.00 ohms for the reading?

mhar · Dec 31, 2012

Rockingit said:
You're getting a result of half an ohm over a 5m run of cpc (presumeably 2.5T&E) in a ring??!! Come on man, THINK!

Try READING. If I can see 5m of perfectly undamaged sheathed cable I can see that there is no dead short for at least 10m, therfore, even if there was a dead short just out of sight, I would expect a reading of 19.51 x 10 mOhms. Have you ever heard of a 5m ring main??!! Come on man, THINK!

Rockingit · Dec 31, 2012

mharries1 said:
Try READING. If I can see 5m of perfectly undamaged sheathed cable I can see that there is no dead short for at least 10m, therfore, even if there was a dead short just out of sight, I would expect a reading of 19.51 x 10 mOhms. Have you ever heard of a 5m ring main??!! Come on man, THINK!

Yep.....as I thought. What's the resistance of 10m's of 1.5mm2 copper conductor? (Try looking it up)