Today I've been trying to isolate a mains leakage tripping my circuit break er. 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 ra nge 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 gar den (yard).
Terry, East Grinstead, UK
Perhaps your resistance is not linear. This is consistent with corrosion.
aker. Using my DMM to measure resistance at certain connections, I'm puzzle d 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 t ime and temperature.
arden (yard).
** Hmm - I wonder if water is involved....... Phil
the short answer is that te thing you're measuring isn't a pure resistance
sounds like they're wet.
-- umop apisdn
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
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.
Maybe slightly dissimilar metals, in wet soil, or damp corrosive gunk on contacts? I'm not sure where you are measuring.
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.
-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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 m ajor cause. It was 3 ft of cable connecting a parallel mains socket half wa y down the garden (yard) for occasional use. Replacing that has taken the l eakage 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 sto mach under an immovable patio bench/arbour thing that wasn't there original ly.
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 c onnections, such as the 'spurs' (?) that branch off to the outlet sockets were made with plastic connectors generously wrapped in that self-sealing g affer 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 e ntrances, despite those appearing to be a snug fit. Digressing, I had a sim ilar problem a few years ago with a gadget I'd encased elsewhere in the gar den, for switching on the waterfall pump from a push-button or timer indoor s. It took several days before I discovered the cause of that circuit-break ing leakage: a slug inside a 3-pin mains plug, itself inside a largish comm ercial 'weatherproof' box, which in turn was wrapped inside a plastic bin bag.
Terry, East Grinstead, UK
Cable TV coax apparently doesn't:
-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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
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
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