Electric Fences

Mike Mason said:

Went for a walk over Hard Knott yesterday. At most styles over the electric
fence was a notice saying fences can cause compasses to be inaccurate within
30 metres.

Is this correct.

Yes. An electric fence is an electro-magnetic device. Next time get
your compass out and see how it is affected. Even an ordinary wire
fence will have some effect on a compass as will ice axe heads if they
are steel. Phones and GPS recievers will also have some effect on a
compass.

Quoted message said:

Wasn't a problem yesterday. Lovely weather, great views and I was using a
gps anyway.

But was the fence on the map or was it a new one?

--
Phil Cook looking north over the park to the "Westminster Gasworks"

Mike Mason said:

Went for a walk over Hard Knott yesterday. At most styles over the electric
fence was a notice saying fences can cause compasses to be inaccurate within
30 metres.

Is this correct.

Sounds a bit dubious to me. A current in a coil of wire will generate
a magnetic field - that's how a relay works - but not, I think, in the
straight wires of a fence.

Colin

Quoted message said:

Sounds a bit dubious to me. A current in a coil of wire will generate
a magnetic field - that's how a relay works - but not, I think, in the
straight wires of a fence.

Actually a current in a straight piece of wire does generate a magnetic field.
In a coil the field is bigger because all the fields surrounding each part of
the wire add up. That's why you should "flake" rather than coil any spare
extension wire when you're mowing the lawn, that way the fields subtract rather
than add.

Cheers,

--
Boo

Boo said:

Quoted message said:

Sounds a bit dubious to me. A current in a coil of wire will generate
a magnetic field - that's how a relay works - but not, I think, in the
straight wires of a fence.

Actually a current in a straight piece of wire does generate a magnetic field.
In a coil the field is bigger because all the fields surrounding each part of
the wire add up. That's why you should "flake" rather than coil any spare
extension wire when you're mowing the lawn, that way the fields subtract rather
than add.

True enough - it's all coming back to me now, right hand rules and all
that. I'd be surprised, though, if the field was strong enough to
influence a compass 30m away, but it's worth experimenting if someone
has a nearby electric fence they can play with over the weekend.

Colin

Might do. after all isn't the magnet N really weak anyway?

Boo was right - there is inductance in every piece of wire... also would
depend on the voltage going through the wire - what is it on fences? 50v ac
?

' Be interseting to see how much defelection if any..

Dave said:

Might do. after all isn't the magnet N really weak anyway?

Boo was right - there is inductance in every piece of wire... also would
depend on the voltage going through the wire - what is it on fences? 50v ac
?

Flipping 'ech, you lot. A magnetic field is only produced by a *current*
not a voltage. There will be some current in an electric fence, depending
on varius things like leakage to ground and the like, but not much. The
voltage in the fence, if nothing is touching it, will be about 10,000 volts,
but this is only present for about 1/2000th of a second (500us) once a
second - the rest of the time there is no voltage. If something is touching
the fence the voltage will be less than that and there may be a substantial
current flowing during the short pulses.

I'd have said that any current flow in a fence would be hard pressed to
influence a compass but I don't know without trying it so I wouldn't
want to bet on it. The fact that any current's only there for one 2000'th
of the time, and is pretty small in any case seems to rule out any
discernable influence, though.

--
Nobby Anderson

Dave said:

Might do. after all isn't the magnet N really weak anyway?

I thought that at first, but if it's that weak then wouldn't the
compass be affected by a mobile phone or a GPS or overhead power lines?
(Maybe it is - I'm just speculating.)

Quoted message said:

Boo was right - there is inductance in every piece of wire... also would
depend on the voltage going through the wire - what is it on fences? 50v ac
?

Don't know what the standard voltage would be, if there is one, but the
magnetic field strength is goverened by current rather than voltage and
the low current in an electric fence suggests a low magnetic field. No
idea how it translates into actual distances though.

Some field research is required. (Get it? "Field" research??)
Colin

"> But was the fence on the map or was it a new one?

Quoted message said:

The fence was put up after the last Foot and mouth outbreak.

Good old Tony needlessly killed all the sheep, but being Herdwicks they are
hefted to where they are born. So to prevent the re-stocked flocks simply
walking back to where they came from the electric fence was put up.

It was intended as a temporary measure, 5 years being its envisaged
lifespan, but like most things with this taxation for the sake of it
government, funds are not being released for its upkeep. It has been knocked
down in several places already, although the flocks do seem to have
remained, so perhaps its usefulness has passed. Not sure if there is any
current flowing through it either!

I love this group - everyone is so fiesty and experts on evrything.

Anyway - i'll just throw back at you - For a fixed resistance (the wire) if
you increase voltage then current would also go up.

I = V/R

You can't have current without voltage.

So surely changes in voltage = change in current = change in magnetic field.

"Nobody Here" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Dave said:

Might do. after all isn't the magnet N really weak anyway?

Boo was right - there is inductance in every piece of wire... also would
depend on the voltage going through the wire - what is it on fences? 50v
ac
?

Flipping 'ech, you lot. A magnetic field is only produced by a *current*
not a voltage. There will be some current in an electric fence, depending
on varius things like leakage to ground and the like, but not much. The
voltage in the fence, if nothing is touching it, will be about 10,000
volts,
but this is only present for about 1/2000th of a second (500us) once a
second - the rest of the time there is no voltage. If something is
touching
the fence the voltage will be less than that and there may be a
substantial
current flowing during the short pulses.

I'd have said that any current flow in a fence would be hard pressed to
influence a compass but I don't know without trying it so I wouldn't
want to bet on it. The fact that any current's only there for one 2000'th
of the time, and is pretty small in any case seems to rule out any
discernable influence, though.

--
Nobby Anderson

"Gordon Burns" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

"> But was the fence on the map or was it a new one?

Quoted message said:

The fence was put up after the last Foot and mouth outbreak.

Good old Tony needlessly killed all the sheep, but being Herdwicks they
are hefted to where they are born. So to prevent the re-stocked flocks
simply walking back to where they came from the electric fence was put up.

It was intended as a temporary measure, 5 years being its envisaged
lifespan, but like most things with this taxation for the sake of it
government, funds are not being released for its upkeep. It has been
knocked down in several places already, although the flocks do seem to
have remained, so perhaps its usefulness has passed. Not sure if there is
any current flowing through it either!

Yes there was a notice on one style to that affect. Didn't see any knocked
down parts on the section I walked but the thing wandered off into the
distance towards the Crinkles. Picked it up at NY 22977 01830 as I came in
from Border End and left it at approx NY 23412 03982. Didn't try touching it
either so don't know if there is current flowing through it. Fence not on
the map, at least on my edition of OS 1:25,000 OL6 edition a revised 1997.
That's the one with the yellow cover rather than the orange one.

Mike

Mike Mason said:

Went for a walk over Hard Knott yesterday. At most styles over the electric
fence was a notice saying fences can cause compasses to be inaccurate within
30 metres.

I sent a complaint to LDNP about that fence because there was no "dog
gate" on the stiles... but it seems that the fence was put up by NT..
who said they would sort it...
Did you notice that the dog-gates had been installed... like the NT
promised?
Cheers,
MalJ

Boo <reply_to_group_not_me@spam_me_no_spam.net> wrote

Quoted message said:

Quoted message said:

Sounds a bit dubious to me. A current in a coil of wire will generate
a magnetic field - that's how a relay works - but not, I think, in the
straight wires of a fence.

Actually a current in a straight piece of wire does generate a magnetic
field. In a coil the field is bigger because all the fields surrounding
each part of the wire add up. That's why you should "flake" rather
than coil any spare extension wire when you're mowing the lawn, that
way the fields subtract rather than add.

More importantly, it prevents heating of the coil!
I always run out the full length of extension leads from their housings,
remembering when my teenage kids used one for a 2kw heater and melted
the whole cable and casing. 8-(
--
Gordon Harris

The message <[email hidden]>
from Boo <reply_to_group_not_me@spam_me_no_spam.net> contains these words:

Quoted message said:

Quoted message said:

Sounds a bit dubious to me. A current in a coil of wire will generate
a magnetic field - that's how a relay works - but not, I think, in the
straight wires of a fence.

Quoted message said:

Actually a current in a straight piece of wire does generate a
magnetic field.
In a coil the field is bigger because all the fields surrounding each
part of
the wire add up. That's why you should "flake" rather than coil any spare
extension wire when you're mowing the lawn, that way the fields
subtract rather
than add.

No idea what particular configuration you mean by 'flake' but as your
basic assumption is wrong it may not help. In a mains extension the
magnetic fields generated by the live and neutral wires cancel anyway.
The reason why keeping a live lead coiled is a bad idea is heat
retention, not magnetic forces.

--
Roger Chapman

Dave <[email hidden]> wrote

Quoted message said:

Might do. after all isn't the magnet N really weak anyway?

Boo was right - there is inductance in every piece of wire...

And capacitance.

Quoted message said:

also would
depend on the voltage going through the wire - what is it on fences? 50v ac
?

Voltage doesn't "go through" wires, voltage is applied between one end
of the wire fence and earth, and current flows through it if anything
touches it and forms a circuit to earth.
The voltage drives the current, and in the case of wire fences, the
voltage is high (thousands of volts), but the regulation of the circuit
would not cause an injurious current to flow through a hooman bean or
animal.

The saying goes; (It's volts that jolts, but mills (milliamps) that
kills).
Mains voltage is dangerous because it has good regulation, ie - plenty
of current can be drawn before the voltage drops.

Quoted message said:

' Be interseting to see how much defelection if any..

Very little, I would guess.
--
Gordon Harris

In message <[email hidden]>

Dave said:

I love this group - everyone is so fiesty and experts on evrything.

Anyway - i'll just throw back at you - For a fixed resistance (the wire) if
you increase voltage then current would also go up.

I = V/R

You can't have current without voltage.

So surely changes in voltage = change in current = change in magnetic field.

I don't know how electric fences work, but that's only partially true.
The wires in an electric fence could be at any voltage you like
relative to ground, but if there's no complete circuit from the fence
to ground then you'll get no current flowing, and therefore no
magnetic field. I guess that is how they do work, otherwise the
battery powering it will be wasted for no use.

You get current when there is a difference in electrical potential
(difference in voltage) between two points in the same circuit, and
it's that difference (and the resistance) that determines how much
current flows.

--
Simon Challands

Nobody Here said:

Flipping 'ech, you lot. A magnetic field is only produced by a *current*
not a voltage. There will be some current in an electric fence, depending
on varius things like leakage to ground and the like, but not much. The
voltage in the fence, if nothing is touching it, will be about 10,000 volts,
but this is only present for about 1/2000th of a second (500us) once a
second - the rest of the time there is no voltage. If something is touching
the fence the voltage will be less than that and there may be a substantial
current flowing during the short pulses.

Glad someone said that at last - now the question is could you use the compass
as a shocked sheep detector when the mA flows?

Geo

MalJ said:

Mike Mason said:

Went for a walk over Hard Knott yesterday. At most styles over the electric
fence was a notice saying fences can cause compasses to be inaccurate within
30 metres.

I sent a complaint to LDNP about that fence because there was no "dog
gate" on the stiles... but it seems that the fence was put up by NT..
who said they would sort it...
Did you notice that the dog-gates had been installed... like the NT
promised?
Cheers,
MalJ

Nobody's brought this point out yet. A magnetic field around a
conductor occurs when a current changes direction - ie on a 50Hz Ac
supply twice per second. On an electric fence a capacitor charged to
many thousands of volts discharges a momentary current flow producing a
momentary magnetic field. I have no idea at what range this brief
magnetic influence might affect a compass - not at all I would think.

bob watkinson said:

MalJ said:

Mike Mason said:

Went for a walk over Hard Knott yesterday. At most styles over the electric
fence was a notice saying fences can cause compasses to be inaccurate within
30 metres.

I sent a complaint to LDNP about that fence because there was no "dog
gate" on the stiles... but it seems that the fence was put up by NT..
who said they would sort it...
Did you notice that the dog-gates had been installed... like the NT
promised?
Cheers,
MalJ

Nobody's brought this point out yet. A magnetic field around a
conductor occurs when a current changes direction - ie on a 50Hz Ac
supply twice per second. On an electric fence a capacitor charged to
many thousands of volts discharges a momentary current flow producing a
momentary magnetic field. I have no idea at what range this brief
magnetic influence might affect a compass - not at all I would think.

Good God did i say twice per second? I did of course mean twice per
cycle, or 100 times per second.

Dave said:

I love this group - everyone is so fiesty and experts on evrything.

I wouldn't claim to be an expert on evrything, but I am en electronic
ingineur with a PhD in the subject. Not that that counts for much
in the general scheme of things, but I can probably claim a little
bit of knowledge when it comes to electrickery, specially Ohm's law.
You know how it goes - six mumfs ago I couldn't even spell ingineur
now I are one.
:-)

Quoted message said:

Anyway - i'll just throw back at you - For a fixed resistance (the wire) if
you increase voltage then current would also go up.

I = V/R

You can't have current without voltage.

So surely changes in voltage = change in current = change in magnetic field.

Except that unless something is touching the fence the resistance is
for all intents and porpoises infinite. There is a small number of
milliamps in the fence (because the resistance is not actually infinite)
but that amount is insignificant as far as generating a magnetic field
goes.

Actually, you can have a current without a voltage by substituting
R=0 in your equation above. Similarly you can have a voltage without
current by substituting R=infinity.

Norbert

Quoted message said:

"Nobody Here" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Dave said:

Might do. after all isn't the magnet N really weak anyway?

Boo was right - there is inductance in every piece of wire... also would
depend on the voltage going through the wire - what is it on fences? 50v
ac
?

Flipping 'ech, you lot. A magnetic field is only produced by a *current*
not a voltage. There will be some current in an electric fence, depending
on varius things like leakage to ground and the like, but not much. The
voltage in the fence, if nothing is touching it, will be about 10,000
volts,
but this is only present for about 1/2000th of a second (500us) once a
second - the rest of the time there is no voltage. If something is
touching
the fence the voltage will be less than that and there may be a
substantial
current flowing during the short pulses.

I'd have said that any current flow in a fence would be hard pressed to
influence a compass but I don't know without trying it so I wouldn't
want to bet on it. The fact that any current's only there for one 2000'th
of the time, and is pretty small in any case seems to rule out any
discernable influence, though.

--
Nobby Anderson

--
Nobby Anderson