DC cable puzzle

[Terry Pinnell]

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

 

[Ralph Mowery]

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.

 

[default]

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.

 

[default]

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.

 

[John Larkin]

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’ll get on it today. It’s a long run, with an intermediate outdooor mains socket, then a (supposedly) waterproof case with the relay.

Terry, East Grinstead, UK

 

[default]

On Fri, 24 Aug 2018 08:41:16 -0700, 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’ll get on it today. It’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.

When I worked at a Navy DF/listening site we had "Time domain
reflectometers" that could locate a buried cable malfunction to within
inches. I wonder if something similar couldn't work on buried mains
cables?

We had an antenna array with 150 elements whose cable length had to be
accurate to an inch or less. The reflectometer was a scope where a
sig-gen sends a step waveform down the line then measures and displays
the amplitude of the reflected signal. Glitches in the waveform were
displayed against a horizontal time base. One of those "A intensified
by B" type textronics gizmo let you dial in the exact distance.

 

[default]

On Sat, 25 Aug 2018 08:32:50 +0100, Terry Pinnell
<me@somewhere.invalid> 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’ll get on it today. It’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

Don't trust high impedance meters on long wire runs. Some devices
respond fast enough to "see" radio waves and background noise as
voltages. A high value load resistor should nip it if that's what it
is.

I built this differential thermometer and rigged a DC motor to display
micro differences in temperature between two thermistors in a bridge
circuit. Worked like a champ (would sense radiant heat from my hand a
few inches from one sensor) The geared DC motor was just a meter
movement to drive a long pointer.

The heatsink on the motor was always way hotter than it should have
been. (especially with the bridge in balance) I tracked it down to
radio waves that were picked up by the (short) wires to the 10K
sensors. They were flying between the power op amps I was using for a
motor driver and saw the .01uf cap across the motor brushes as a dead
short. The motor wouldn't even jitter a little bit so I had no clue
to the heat problem until I got a scope on it.

Measurement technique and equipment means a lot when you want to
really know what is going on.