Why this inconsistent resistance reading?

Today I've been trying to isolate a mains leakage tripping my circuit breaker. Using my DMM to measure resistance at certain connections, I'm puzzled why I get widely different readings on different ranges. On the 20 M Ohm range I see 1.6 M but on the 2 M range I get about 500 K.

If it's relevant, these are longish lengths of 3-core mains cable in my garden (yard).

Terry, East Grinstead, UK

 

[Phil Allison]

terry...@gmail.com wrote:

Today I've been trying to isolate a mains leakage tripping my circuit breaker. Using my DMM to measure resistance at certain connections, I'm puzzled why I get widely different readings on different ranges. On the 20 M Ohm range I see 1.6 M but on the 2 M range I get about 500 K.

** That is enough to indicate serious electrical leakage exists.

A good cable would read over 20mohms.

Leakage current increases in a non linear way with test voltage so must be tested at a realistic voltage level. Then it will probably increases with time and temperature.

If it's relevant, these are longish lengths of 3-core mains cable in my garden (yard).

** Hmm - I wonder if water is involved....



.... Phil

 

[Tom Biasi]

On 11/28/2014 5:38 PM, terrypingm@gmail.com wrote:

Today I've been trying to isolate a mains leakage tripping my circuit breaker. Using my DMM to measure resistance at certain connections, I'm puzzled why I get widely different readings on different ranges. On the 20 M Ohm range I see 1.6 M but on the 2 M range I get about 500 K.

If it's relevant, these are longish lengths of 3-core mains cable in my garden (yard).

Terry, East Grinstead, UK
Perhaps your resistance is not linear. This is consistent with corrosion.

 

[Jasen Betts]

On 2014-11-28, terrypingm@gmail.com <terrypingm@gmail.com> wrote:

Today I've been trying to isolate a mains leakage tripping my
circuit breaker. Using my DMM to measure resistance at certain
connections, I'm puzzled why I get widely different readings on
different ranges. On the 20 M Ohm range I see 1.6 M but on the 2 M
range I get about 500 K.

the short answer is that te thing you're measuring isn't a pure
resistance

> If it's relevant, these are longish lengths of 3-core mains cable in my garden (yard).

sounds like they're wet.

--
umop apisdn

 

[John Larkin]

On Fri, 28 Nov 2014 14:38:14 -0800 (PST), terrypingm@gmail.com wrote:

Today I've been trying to isolate a mains leakage tripping my circuit breaker. Using my DMM to measure resistance at certain connections, I'm puzzled why I get widely different readings on different ranges. On the 20 M Ohm range I see 1.6 M but on the 2 M range I get about 500 K.

If it's relevant, these are longish lengths of 3-core mains cable in my garden (yard).

Terry, East Grinstead, UK

Sounds like a bit of DC, fooling the meter. Use the voltage range to
confirm that idea.

A few wet connections could generate a little DC.


--

John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

Thanks both. Corrosion at some point is what I suspect, but I hadn't known that would present a voltage-dependent resistance. Looks like I have some messy work to do!

Terry, East Grinstead, UK

 

Thanks John. I'm pretty sure I did indeed see a low DC voltage. That was while switching between ranges, not deliberately looking for one, but I'll check again.

FWIW there's a 15V DC pair running at times a few inches away, alongside my 240V mains cable.

What's the physics behind this spurious DC?

I laid these cables about 15 years ago and didn't give much thought to their quality. If I replace (which might prove more expedient than isolating the failure), is there a specific type I should buy please?

Terry, East Grinstead, UK

 

[default]

On Sat, 29 Nov 2014 00:27:22 -0800 (PST), terrypingm@gmail.com wrote:

Thanks John. I'm pretty sure I did indeed see a low DC voltage. That was while switching between ranges, not deliberately looking for one, but I'll check again.

FWIW there's a 15V DC pair running at times a few inches away, alongside my 240V mains cable.

What's the physics behind this spurious DC?

I laid these cables about 15 years ago and didn't give much thought to their quality. If I replace (which might prove more expedient than isolating the failure), is there a specific type I should buy please?

Terry, East Grinstead, UK

Is this a ground fault interrupter breaker?

If so I'd go with Phill Allison's answer. Hi pot testers (used to
measure insulation resistance, typically use ~500 volts to test the
insulation resistance)

A similar thing happens at the other end of the spectrum, very low
(micro ohms) resistance's require substantial current to get accurate
results.

 

[John Larkin]

On Sat, 29 Nov 2014 00:27:22 -0800 (PST), terrypingm@gmail.com wrote:

Thanks John. I'm pretty sure I did indeed see a low DC voltage. That was while switching between ranges, not deliberately looking for one, but I'll check again.

FWIW there's a 15V DC pair running at times a few inches away, alongside my 240V mains cable.

What's the physics behind this spurious DC?

Maybe slightly dissimilar metals, in wet soil, or damp corrosive gunk
on contacts? I'm not sure where you are measuring.

I laid these cables about 15 years ago and didn't give much thought to their quality. If I replace (which might prove more expedient than isolating the failure), is there a specific type I should buy please?

You Brits have strange (to us) wiring. And we don't have GFDs in our
breaker boxes. There are "direct burial" cables you might research.
And you can pull wires through PVC conduit, provided it won't fill
with water somehow.

A heavy-duty extension cord type cable would be OK (but probably not
legal) for direct burial.

I have a similar problem today: see Bloggsy's whiney (whingey?) post
about materialism in sci.electronics.design.

https://dl.dropboxusercontent.com/u/53724080/Food/Lemons_Post-Rat.JPG

https://dl.dropboxusercontent.com/u/53724080/Food/LemonsAndLight.JPG



--

John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Maynard A. Philbrook Jr.]

In article <3a96e6b0-3de4-464e-8ba6-10f99d869add@googlegroups.com>,
terrypingm@gmail.com says...

Thanks John. I'm pretty sure I did indeed see a low DC voltage. That was while switching between ranges, not deliberately looking for one, but I'll check again.

FWIW there's a 15V DC pair running at times a few inches away, alongside my 240V mains cable.

What's the physics behind this spurious DC?

I laid these cables about 15 years ago and didn't give much thought to their quality. If I replace (which might prove more expedient than isolating the failure), is there a specific type I should buy please?

Terry, East Grinstead, UK

Your lucky it lasted that long. I am sure you didn't use direct burial
cable or ran it through a water tight pipe of any kind.

Direct burial wire has components in the PVC to help prevent the bugs
and rodents from eating it.

Putting that aside, people tend to use garden tools when grooming their
turf, ignoring what is under it. So a chip here and a slice there, you
have all the ingredients for oxidation, galvanic and polarized voltage
conduction. Even the soil can generate small amounts of DC if
you have the correct minerals in it. This of course is assuming your
insulation is exposed.

If you can't get your hands on some direct burial then get some
electrical PVC pipe and glue so you can lay it in the ground and use
plumbers putty or something like it on the pipe outlets. They do make
putty fill for electrical, some of which can spray and it expands like
foam all around the wires.

Jamie

 

Thanks for those helpful follow ups.

Spent around 8 hours pursuing the problem yesterday and finally found the major cause. It was 3 ft of cable connecting a parallel mains socket half way down the garden (yard) for occasional use. Replacing that has taken the leakage resistance up to 6 M. I don't think I'm going to be able to improve on that without an unrealistic effort and expense. Involved lying on my stomach under an immovable patio bench/arbour thing that wasn't there originally.

I'd guess about 80% of the 100 ft run from my house/garage to the furthest point (sockets and garden lamp) is inside PVC tubing, in several sections. But I wish I'd sealed it more efficiently. I like the putty idea. Various connections, such as the 'spurs' (?) that branch off to the outlet sockets were made with plastic connectors generously wrapped in that self-sealing gaffer tape. Which proved very difficult to remove for testing.

BTW, the two sockets are theoretically weatherproof, but I found all sorts of grunge inside, presumably from bugs, flies, etc, entering at the cable entrances, despite those appearing to be a snug fit. Digressing, I had a similar problem a few years ago with a gadget I'd encased elsewhere in the garden, for switching on the waterfall pump from a push-button or timer indoors. It took several days before I discovered the cause of that circuit-breaking leakage: a slug inside a 3-pin mains plug, itself inside a largish commercial 'weatherproof' box, which in turn was wrapped inside a plastic bin bag.

Terry, East Grinstead, UK

 

[John Larkin]

On Sat, 29 Nov 2014 18:11:36 -0500, "Maynard A. Philbrook Jr."
<jamie_ka1lpa@charter.net> wrote:

In article <3a96e6b0-3de4-464e-8ba6-10f99d869add@googlegroups.com>,
terrypingm@gmail.com says...

Thanks John. I'm pretty sure I did indeed see a low DC voltage. That was while switching between ranges, not deliberately looking for one, but I'll check again.

FWIW there's a 15V DC pair running at times a few inches away, alongside my 240V mains cable.

What's the physics behind this spurious DC?

I laid these cables about 15 years ago and didn't give much thought to their quality. If I replace (which might prove more expedient than isolating the failure), is there a specific type I should buy please?

Terry, East Grinstead, UK

Your lucky it lasted that long. I am sure you didn't use direct burial
cable or ran it through a water tight pipe of any kind.

Direct burial wire has components in the PVC to help prevent the bugs
and rodents from eating it.

Cable TV coax apparently doesn't:

https://dl.dropboxusercontent.com/u/53724080/Truckee/Cable_Chewed.jpg


--

John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Jeff Layman]

On 29/11/2014 08:27, terrypingm@gmail.com wrote:

Thanks John. I'm pretty sure I did indeed see a low DC voltage. That was while switching between ranges, not deliberately looking for one, but I'll check again.

FWIW there's a 15V DC pair running at times a few inches away, alongside my 240V mains cable.

What's the physics behind this spurious DC?

I laid these cables about 15 years ago and didn't give much thought to their quality. If I replace (which might prove more expedient than isolating the failure), is there a specific type I should buy please?

The short answer is "Yes". There are numerous requirements for outdoor
cabling in Part P of the Buildings Regulations. If you are going to
replace the cable, at the very least it will have to be armoured. Even
if you do all the work yourself, it will probably have to be certified
by a registered electrician before the new wiring is connected to the
consumer unit.

If I were you I'd repost this question in uk.d-i-y newsgroup. You will
find there are a number of electricians who lurk there and will be able
to give you good advice (they aren't touting for business, either!).

--

Jeff

 

[Maynard A. Philbrook Jr.]

In article <3m7l7atpch9mmpqoprsbemdkrugpo5h102@4ax.com>,
jlarkin@highlandtechnology.com says...

On Sat, 29 Nov 2014 18:11:36 -0500, "Maynard A. Philbrook Jr."
jamie_ka1lpa@charter.net> wrote:

In article <3a96e6b0-3de4-464e-8ba6-10f99d869add@googlegroups.com>,
terrypingm@gmail.com says...

Thanks John. I'm pretty sure I did indeed see a low DC voltage. That was while switching between ranges, not deliberately looking for one, but I'll check again.

FWIW there's a 15V DC pair running at times a few inches away, alongside my 240V mains cable.

What's the physics behind this spurious DC?

I laid these cables about 15 years ago and didn't give much thought to their quality. If I replace (which might prove more expedient than isolating the failure), is there a specific type I should buy please?

Terry, East Grinstead, UK

Your lucky it lasted that long. I am sure you didn't use direct burial
cable or ran it through a water tight pipe of any kind.

Direct burial wire has components in the PVC to help prevent the bugs
and rodents from eating it.

Cable TV coax apparently doesn't:

https://dl.dropboxusercontent.com/u/53724080/Truckee/Cable_Chewed.jpg

Yes, rodents love some forms of foam

A lot of the PVC's used for wire have organic fillers in it, it's no
wonder the rodents and bugs love it.

Jamie