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DC cable puzzle

Started by Terry Pinnell August 23, 2018
Tidying up my shed workshop I got an electric shock from a cable pair
that had earlier been disconnected from its 14V DC power supply. So I'd
expect it to have virtually zero voltage across it, apart from ac noise.
Its destination is a garden lamp relay box about 80 ft away from the
supply. For some of that distance it runs close to a mains cable
(unloaded at the time).

My DMM consistently reports 21V AC. And 0.56 mA AC shorted across the
pair.

Before I dig around in the undergrowth to get access to the
(weather-proofed) relay box...
Q1: Does that quite high and steady level of voltage result from a
capacitive effect?
Q2: Is that really enough to give me a shock?

Terry, East Grinstead, UK
In article <opitndl9nmspps61dhd6u7n33b5jqe56bm@4ax.com>, 
me@somewhere.invalid says...
> > Tidying up my shed workshop I got an electric shock from a cable pair > that had earlier been disconnected from its 14V DC power supply. So I'd > expect it to have virtually zero voltage across it, apart from ac noise. > Its destination is a garden lamp relay box about 80 ft away from the > supply. For some of that distance it runs close to a mains cable > (unloaded at the time). > > My DMM consistently reports 21V AC. And 0.56 mA AC shorted across the > pair. > > Before I dig around in the undergrowth to get access to the > (weather-proofed) relay box... > Q1: Does that quite high and steady level of voltage result from a > capacitive effect? > Q2: Is that really enough to give me a shock? > > Terry, East Grinstead, UK > >
If a wire is close to another wire with voltagae on it, there is a coupling effect, either capacitance or inductive like a transformer. I doubt that 21 volts would shock you unless your skin is wet.Usually anything under 1 or 2 miliamps will not be very noticable as far as the shock.
On Thu, 23 Aug 2018 16:13:32 +0100, Terry Pinnell
<me@somewhere.invalid> wrote:

>Tidying up my shed workshop I got an electric shock from a cable pair >that had earlier been disconnected from its 14V DC power supply. So I'd >expect it to have virtually zero voltage across it, apart from ac noise. >Its destination is a garden lamp relay box about 80 ft away from the >supply. For some of that distance it runs close to a mains cable >(unloaded at the time). > >My DMM consistently reports 21V AC. And 0.56 mA AC shorted across the >pair. > >Before I dig around in the undergrowth to get access to the >(weather-proofed) relay box... >Q1: Does that quite high and steady level of voltage result from a >capacitive effect? >Q2: Is that really enough to give me a shock? > >Terry, East Grinstead, UK
You aren't seeing any capacitive effect there. That's got to be leakage through the soil. You might try measuring it then sprinkling it with water (assuming you know where the actual wire run is) - one would expect the leakage current to increase. Use test instruments, you never know when the current may increase dramatically.
On Thu, 23 Aug 2018 12:21:07 -0400, Ralph Mowery
<rmowery28146@earthlink.net> wrote:

>In article <opitndl9nmspps61dhd6u7n33b5jqe56bm@4ax.com>, >me@somewhere.invalid says... >> >> Tidying up my shed workshop I got an electric shock from a cable pair >> that had earlier been disconnected from its 14V DC power supply. So I'd >> expect it to have virtually zero voltage across it, apart from ac noise. >> Its destination is a garden lamp relay box about 80 ft away from the >> supply. For some of that distance it runs close to a mains cable >> (unloaded at the time). >> >> My DMM consistently reports 21V AC. And 0.56 mA AC shorted across the >> pair. >> >> Before I dig around in the undergrowth to get access to the >> (weather-proofed) relay box... >> Q1: Does that quite high and steady level of voltage result from a >> capacitive effect? >> Q2: Is that really enough to give me a shock? >> >> Terry, East Grinstead, UK >> >> > >If a wire is close to another wire with voltagae on it, there is a >coupling effect, either capacitance or inductive like a transformer. > >I doubt that 21 volts would shock you unless your skin is wet.Usually >anything under 1 or 2 miliamps will not be very noticable as far as the >shock.
You can feel 12V DC with salt water and a few minor cuts.. 21 VAC is ~30 volts peak and it is probably enough to feel. I'd categorically rule out coupling, it may be enough to cause audio hum in signal wire but 21VAC sounds unbelievable to me. I've wound my own induction coil and transformers, built more than a few capacitors for Tesla coils. It takes some serious magnetic fields (which fall off rapidly with separation in air - I assume soil would be similar) and a fair amount of surface area and very tiny space between plates, to make a small cap. Damp soil and exposed conductors is a different story and more believable.
On Thu, 23 Aug 2018 13:11:09 -0400, default <default@defaulter.net>
wrote:

>On Thu, 23 Aug 2018 16:13:32 +0100, Terry Pinnell ><me@somewhere.invalid> wrote: > >>Tidying up my shed workshop I got an electric shock from a cable pair >>that had earlier been disconnected from its 14V DC power supply. So I'd >>expect it to have virtually zero voltage across it, apart from ac noise. >>Its destination is a garden lamp relay box about 80 ft away from the >>supply. For some of that distance it runs close to a mains cable >>(unloaded at the time). >> >>My DMM consistently reports 21V AC. And 0.56 mA AC shorted across the >>pair. >> >>Before I dig around in the undergrowth to get access to the >>(weather-proofed) relay box... >>Q1: Does that quite high and steady level of voltage result from a >>capacitive effect? >>Q2: Is that really enough to give me a shock? >> >>Terry, East Grinstead, UK > >You aren't seeing any capacitive effect there. That's got to be >leakage through the soil. You might try measuring it then sprinkling >it with water (assuming you know where the actual wire run is) - one >would expect the leakage current to increase. > >Use test instruments, you never know when the current may increase >dramatically.
Right. This should be investigated. Sounds like bad insulation on the mains cable. Measure its current. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Thanks all, mains leakage sounds the culprit. I&rsquo;ll get on it today. It&rsquo;s a long run, with an intermediate outdooor mains socket, then a (supposedly) waterproof case with the relay.

Terry, East Grinstead, UK
On Thu, 23 Aug 2018 22:52:26 -0700 (PDT), terrypingm@gmail.com wrote:

>Thanks all, mains leakage sounds the culprit. I&rsquo;ll get on it today. It&rsquo;s a long run, with an intermediate outdooor mains socket, then a (supposedly) waterproof case with the relay. > >Terry, East Grinstead, UK
Maybe something underground is full of water or ants or something. Possibly the wire insulation got nicked. There are some fun instrumentation possibilities, like probing the soil at the surface and measuring/listening for AC potentials. A map of the surface potentials of some chunk of land, color coded by frequency, would be fun. We were on the roof yesterday and speculated how cool it would be if we could see RF, all the microwaves and cell phones and wifis and transmitters. That's not possible. But a surface potential map is. -- John Larkin Highland Technology, Inc lunatic fringe electronics
John Larkin wrote:
> > We were on the roof yesterday and speculated how cool it would be if > we could see RF, all the microwaves and cell phones and wifis and > transmitters. That's not possible. But a surface potential map is.
I've often thought of that. You'd need eyes bigger than Zoey Daschanel and Irene Bordoni combined. You know that famous chaotic function that can be modelled with a circuit of 2 multipliers and fed into a scope in XY mode to produce a pair of spirals? There's a complete circuit on Paul Horowitz's website. I might have made one for fun if the multipliers weren't $20 each. Well, it would also be neat to put one in the cabinet under that old scope in your lobby.
John Larkin <jjlarkin@highlandtechnology.com> wrote:

>On Thu, 23 Aug 2018 22:52:26 -0700 (PDT), terrypingm@gmail.com wrote: > >>Thanks all, mains leakage sounds the culprit. I&rsquo;ll get on it today. It&rsquo;s a long run, with an intermediate outdooor mains socket, then a (supposedly) waterproof case with the relay. >> >>Terry, East Grinstead, UK > >Maybe something underground is full of water or ants or something. >Possibly the wire insulation got nicked. > >There are some fun instrumentation possibilities, like probing the >soil at the surface and measuring/listening for AC potentials. > >A map of the surface potentials of some chunk of land, color coded by >frequency, would be fun. > >We were on the roof yesterday and speculated how cool it would be if >we could see RF, all the microwaves and cell phones and wifis and >transmitters. That's not possible. But a surface potential map is.
To my surprise the mains cabling seemed OK, giving readings outside my DMM's 200Mohm at various points. A side issue arose has me puzzled. I found about 1V DC across a twisted pair cable, disconnected at both ends, buried in several places. How does that come about? Even the short section I've cut out, with gaffer tape around connector blocks, has about 0.5V DC. Terry, East Grinstead, UK
On 25/08/18 08:32, Terry Pinnell wrote:
> John Larkin <jjlarkin@highlandtechnology.com> wrote: > >> On Thu, 23 Aug 2018 22:52:26 -0700 (PDT), terrypingm@gmail.com wrote: >> >>> Thanks all, mains leakage sounds the culprit. I&acirc;&euro;&trade;ll get on it today. It&acirc;&euro;&trade;s a long run, with an intermediate outdooor mains socket, then a (supposedly) waterproof case with the relay. >>> >>> Terry, East Grinstead, UK >> >> Maybe something underground is full of water or ants or something. >> Possibly the wire insulation got nicked. >> >> There are some fun instrumentation possibilities, like probing the >> soil at the surface and measuring/listening for AC potentials. >> >> A map of the surface potentials of some chunk of land, color coded by >> frequency, would be fun. >> >> We were on the roof yesterday and speculated how cool it would be if >> we could see RF, all the microwaves and cell phones and wifis and >> transmitters. That's not possible. But a surface potential map is. > > To my surprise the mains cabling seemed OK, giving readings outside my > DMM's 200Mohm at various points. > > A side issue arose has me puzzled. I found about 1V DC across a twisted > pair cable, disconnected at both ends, buried in several places. How > does that come about? Even the short section I've cut out, with gaffer > tape around connector blocks, has about 0.5V DC.
What are you using to measure the potential difference? A high input impedance meter? What's the reading if you have a, say, 10kohm resistor between the conductors?