electric fences

Submitted: Thursday, Feb 24, 2005 at 10:58
ThreadID: 20738 Views:2362 Replies:10 FollowUps:15
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Whilst driving recently along the side of an electric fence ( 1 or 2 inches from it) the thought occured to me as to what the consequences would be to my modern day computerised 4X4 if I were to inadvertantly come into contact with it. Would I fry the electronics or would it have no effect. Just wondering if anyone has experienced it or has any theory's.
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Reply By: flappa - Thursday, Feb 24, 2005 at 11:01

Thursday, Feb 24, 2005 at 11:01
It shouldn't have any effect should it ?

Wouldn't your rubber tyres insulate the vehicle ?
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Follow Up By: Member - Chrispy (NSW) - Thursday, Feb 24, 2005 at 11:06

Thursday, Feb 24, 2005 at 11:06
Hmmm... then why am I afraid of accidently touching our Cooma neighbour's electric fence around their house again? Last time I did I was zapped big-time... rubber-soled workboots and all...........
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Follow Up By: flappa - Thursday, Feb 24, 2005 at 11:15

Thursday, Feb 24, 2005 at 11:15
I'm not a lecky , but , if a Lecky line falls on your vehicle , you dont automatically get electrocuted. It needs to be grounded.
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Follow Up By: Nudenut - Thursday, Feb 24, 2005 at 21:22

Thursday, Feb 24, 2005 at 21:22
an electric fence discharge will escape the rubber insulation of normal boots...thats where a lot of fly fishermen get caught out....one has to remember its only high volts and its the volts that JUMP.....been there done that..hahaha
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Reply By: Member - Paul J (ACT) - Thursday, Feb 24, 2005 at 11:16

Thursday, Feb 24, 2005 at 11:16
Just dont WIZ on the electric fence, hehehe sorry couldnt help myself..........
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Follow Up By: Rick Blaine - Thursday, Feb 24, 2005 at 18:52

Thursday, Feb 24, 2005 at 18:52
Hmmmmmmmmmmmmm yes i do agree from personal experience....totally shocking...but the resuting erection put the Harbour bridge to shame.....
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Follow Up By: Member - Ed. C.- Thursday, Feb 24, 2005 at 20:19

Thursday, Feb 24, 2005 at 20:19
Geez, I hope my missus doesn't read this!!!
(she'll be rushin' out to buy an electric fence!!!)
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Then again..................... hmmmmm...
could be (ahem) "exciting"....
Confucius say.....
"He who lie underneath automobile with tool in hand,
....Not necessarily mechanic!!"

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Follow Up By: Michael ( Moss Vale NSW) - Friday, Feb 25, 2005 at 12:26

Friday, Feb 25, 2005 at 12:26
Hi ED C, My wife put an electrified fence down the middle of the bed.. I think it meant the opposite to your post. Anyway not so bad but i drew the the line when the Barbed wire went on the top of the fence.!!!!
.
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Reply By: Alan S (WA) - Thursday, Feb 24, 2005 at 11:31

Thursday, Feb 24, 2005 at 11:31
I have never considered it but your right i wonder what the consequences would be, a lot of electric fences dont just rely on the earth from the controller for the return but use the main fence as a return. This is particulay common in gravelly country.

Doesnt matter that the vehicle is on rubber tyres it just needs to touch the live wire and the rest of the fence to close the circuit.

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Follow Up By: flappa - Thursday, Feb 24, 2005 at 11:34

Thursday, Feb 24, 2005 at 11:34
why ?
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Follow Up By: Alan S (WA) - Thursday, Feb 24, 2005 at 12:05

Thursday, Feb 24, 2005 at 12:05
What the normal practice is to run the Positive from the controller to the live wire, the Neg is attached to a grounding post. This works well in moist ground as the current flows back to the controller when the live wire is touched.

Gravelly or dry ground is a bad conductor so as well as using a grounding post you attach the rest of the fence to the neg terminal.

All that is then need is to touch the live wire and the other fencing wire and zap you dont even need to touch the ground.

Is this what you mean with your question?

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Follow Up By: Member - Jeff M (WA) - Thursday, Feb 24, 2005 at 12:17

Thursday, Feb 24, 2005 at 12:17
I agree with Alan S, being in the security industry myself I know first hand that it is common practice to eath to either the protection fence/poles or another section of the fence that is not live. In most security purposes you are trying to stop people jumping/clibming fences, where they are not always touching the ground. Plus the obvious earthing problems with shoes etc. Cattle might not wear shoes or jump but people certainly do! ;-)

And yes it is just a much more reliable way of doing things, you cannot gurantee that the person/animal is going to get a good earth just by standing on the ground out bush.
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Follow Up By: flappa - Thursday, Feb 24, 2005 at 12:39

Thursday, Feb 24, 2005 at 12:39
Yep , thanks for that.

Twas an honest question , didn't know the answer.
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Follow Up By: flappa - Thursday, Feb 24, 2005 at 12:40

Thursday, Feb 24, 2005 at 12:40
PS , the security fence cleared it up . . . hadn't thought of that.
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Reply By: Hero - Thursday, Feb 24, 2005 at 11:49

Thursday, Feb 24, 2005 at 11:49
If you happenend to be completely airbourne and touched the live wire only, you would not get zapped.
Rubber only dull`s the current, does not stop it compltely.
Not sure what would happen to a car, i imagine it may cause some damage.

You must be doin prettly well to drive within 1" of the fence and have not found out by now.
AnswerID: 99901

Reply By: *Peter* - Thursday, Feb 24, 2005 at 12:09

Thursday, Feb 24, 2005 at 12:09
"Would I fry the electronics.." Theoretically possible but probably wouldn't.

Electric fences operate with high voltages but low current. When you are dealing with high voltages or high frequencies the normal electrical principles taught don’t explain everything.

High voltages can track over rubber. I have seen rubber electrical insulation on a cable carrying 6600V with a 2mm wide track etched about 2mm deep from the electricity.

Also you would have to consider what happens to the voltage of your 4wd when it touches the fence. When you touch the fence the voltage of your 4wd will take a small amount of time for all of the 4wd to rise to the same voltage of the fence. During this time it is theoretically possible for the conditions to be right to fry the electronics.

Also electronics can be designed to withstand voltage spikes so it would also depend on how susceptible the electronics are.

Anyway if you do come to rest against an electric fence don’t step out of the car. It would almost be the same as peeing on the fence :-))
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Follow Up By: Member - DOZER- Friday, Feb 25, 2005 at 11:25

Friday, Feb 25, 2005 at 11:25
And that would be revolting....sorry couldnt help it.....i would say that 99 times out of 100, you would be alright if you touched the fence, and once, when you were in a hurry to get things done b4 church, you would be stranded....to look at it from another direction....the volts wouldnt do the computer any good.....remember that the sparkplugs are earthed to the motor block aswell, so maybe the fence black box wont like that either....
Andrew
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Reply By: Mike Harding - Thursday, Feb 24, 2005 at 19:52

Thursday, Feb 24, 2005 at 19:52
Because vehicle manufactures hate recalls more than anything else they go to a lot of trouble and expense to ensure the electronics in their vehicles are robust - and they know pretty much all there is to know about the reliability of the electronic components they use. Before they use a component it must be "automotive qualified" and each vehicle manufacturer will do that themselves for each component they use, last time I heard that process cost them around A$30k per component.

Especially during the development phase if (say) an integrated circuit fails it is normal for it to be returned to the IC manufacturer who is required to mill the top off the IC and perform an analysis of the die to find out why it failed.

Having said all that: the kind of situation you describe is (electrically and mathematically) _very_ complex and my answer is: I don't know - it might - I wouldn't do it.

Mike Harding
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Reply By: Splitpin - Thursday, Feb 24, 2005 at 22:44

Thursday, Feb 24, 2005 at 22:44
Well think about aircraft. They have heaps more electronic equiptment than a car and they struck by lightning all the time. Doesn't do them any harm. Mind you all the panels are electricaly bonded together so the lightning passes from one side to the other without you even knowing. I don't know much about cars but i have seen a strap that hangs down underneath them which i presume is some sort of static discharge sprap. Now if you were to make some sort of static discharge strap to help the electricity to pass through to ground quicker, then you would probably do less damage than just relying on the insulation in your tyres. true?
AnswerID: 100010

Reply By: Pauly - Thursday, Feb 24, 2005 at 23:27

Thursday, Feb 24, 2005 at 23:27
a company was putting up a pole light the crane touched the power line's above not sure if they were LV or HV but the tyres on the crane exploded one man died. im sure your car would be fine even thou its a 1000 volts or so very little current and it's supply is from a isolation transformer
AnswerID: 100027

Reply By: motherhen - Friday, Feb 25, 2005 at 01:26

Friday, Feb 25, 2005 at 01:26
Simple - don't crash into the fence!
AnswerID: 100046

Reply By: Member - Matt Mu (Perth-WA) - Friday, Feb 25, 2005 at 12:42

Friday, Feb 25, 2005 at 12:42
Apart from Induced EMF, voltage spikes and static electricity, isnt this all about the path of least resistance. ie wouldnt the electric pulse (most elec fences are pulsed) travel from the live wire, into the panel of the vehicle (or bull bar, side step etc) and straight back into the negative of the erected fence. The path would have to be less than a few inches!!

Also I think of the mass of the body of the vehicle. You dont get electrocuted if ur in the ocean when lightning strikes, unless you were unlucky enough to be inside the upstrike radius(the leader bolt before the main?).

What about welding on a veh, we disconnect the battery as a precation but all the earths of the electrics are still connected to the body of the vehicle!! Why dont we fry the computer then, again I think this comes back tot he path of least resistance. If that current flow had to flow throught he electronics then yes, you would 'let the smoke out' (fizz them) but it doesnt does it. It goes from the stick into the body and back to the earth clamp.

Im pretty sure there wouldnt be a problem with brushing up against an electric fence, but hey anyone want to rush out and crash into an electric fence?? Im suyre the property owner would be suitably impressed!!!

Matt.

AnswerID: 100112

Follow Up By: Mike Harding - Friday, Feb 25, 2005 at 15:46

Friday, Feb 25, 2005 at 15:46
>Apart from Induced EMF, voltage spikes and static electricity,
>isnt this all about the path of least resistance. ie wouldnt the
>electric pulse (most elec fences are pulsed) travel from the
>live wire, into the panel of the vehicle (or bull bar, side step
>etc) and straight back into the negative of the erected fence.

Nope :)

That's why it's complex. Of course Ohms law applies to the DC components of the current as does Kirchoffs Law
http://physics.bu.edu/~duffy/PY106/Kirchoff.html
so some DC currents will flow through paths of more resistance.

However brushing an electric fence with a vehicle would also
produce a lot of AC currents and how they would behave would take a Cray Computer and a better engineer than me to analyse. Actually the DC component would be bloody complex too!

Like I said before - it might be OK, but I wouldn't do it :)

Mike Harding
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Follow Up By: Richard Kovac - Sunday, Feb 27, 2005 at 02:11

Sunday, Feb 27, 2005 at 02:11
Sorry Mike
i don't see were your coming from
Kirchoffs law applies to parallel circuits of resistance (ie resisters) not the entry and exit point of the electric current?
As Matt says if the vehicle touches the wire the shortest path will be taken to ground, if there is no path to ground then there can be no flow off electrons, as I can see it.
Both AC & DC need the flow of electron to create current the only difference is one crosses the line - to + and some impedance.
my only knowledge is that I left school
Regards

Richard Kovac
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Follow Up By: Mike Harding - Sunday, Feb 27, 2005 at 07:56

Sunday, Feb 27, 2005 at 07:56
Hi Richard

>i don't see were your coming from

I sometimes wonder, myself! :)

>As Matt says if the vehicle touches the wire the shortest path
>will be taken to ground, if there is no path to ground then
>there can be no flow off electrons, as I can see it.

This is one of those things which would be easy to explain with a pencil and paper, anyway we'll give it a go....

Essentially (if one pole of the fence is connected to ground) there are four return paths for the current ie. 4 tyres. I'm assuming the tyres would not be perfect insulators at high voltage. From the point of contact with the fence _some_ current will flow to ground via _all_ four tyres - how much will return via _each_ tyre will be dependant upon the resistance (for the DC component) and the reactance (for the AC component). Measuring or calculating the impedance (resistance and reactance combined) through each of those paths would be next to impossible. Things get even more complicated because each of those four paths will have many sub paths through which current will flow and one of those sub paths _may_ be through the vehicle electronics? And that may cause damage.

Think of the vehicle as being thousands of resistors in parallel (Kirchoff) between the fence and earth - each resistor will carry some current but the lowest value resistor will carry the most and the highest value resistor the least.

Your are correct about current flow being the flow of electrons from atom to atom - well, actually, I think it's the "holes" which move - but I never understood all that stuff - personally I think it's all black magic :)

>Still Getting there

Aren't we all! :)

Mike Harding

mike_harding@fastmail.fm
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