Circumventing an electric fence

[HorseBox]

If you leaped onto an electric fence would I be right in assuming you wouldn't get electrocuted as long as you don't provide a path for the charge to neutralize itself?

 

[vk6kro]

The voltage on an electric fence is not really in the form of a charge.

It is a brief pulse of about 3000 volts and then there is nothing until the next pulse.

Also, this pulse is at high voltage but it is unable to deliver much current. High current would deliver a possibly fatal shock when the intention is that there will be a deterrent effect only.

So, you could avoid the unpleasant shock if you were to jump onto the high voltage wire and not make contact with any other wire, or the ground. Birds do this all the time when they perch on high voltage wires.

There will be a return path for the high voltage back to the generator. It may be the ground or it might be another wire strung near the one with high voltage on it.

 

[cjameshuff]

Note that humans have considerably more body capacitance than birds...roughly 100 pF. This can be enough to allow a jolt to be felt even if there's no DC current path through your body, as your body charges up from the fence and discharges back into it with each pulse. It'll probably not be nearly as bad as touching the fence while well grounded, though.

Also note that usually not all wires on the fence are electrified, and some may be grounded to some degree. Even an unpowered wire that's well-insulated from the ground will have substantial capacitance, and again you will feel a jolt if you touch it and the hot wire at the same time.

 

[vk6kro]

humans have considerably more body capacitance than birds...roughly 100 pF

A human body is effectively a conductor, so this might act as one plate of a capacitor. The dielectric may be air or a pair of insulated boots and the other plate may be the ground or the return wire of the electric fence.

It is not reasonable to claim a person would have 100 pF to ground unless you knew the exact situation

 

[cjameshuff]

humans have considerably more body capacitance than birds...roughly 100 pF

A human body is effectively a conductor, so this might act as one plate of a capacitor. The dielectric may be air or a pair of insulated boots and the other plate may be the ground or the return wire of the electric fence.

It is not reasonable to claim a person would have 100 pF to ground unless you knew the exact situation

I did not say mutual capacitance with ground or the return wire, I said body capacitance. The usual human body model is capacitance of about 100 pF (a good match to the the self capacitance of a 90 cm sphere, though likely derived from actual measurements, or at least a more detailed model) in series with a 1.5 kohm resistance. The exact value will indeed depend on the situation, but birds have far more capacitance simply due to being smaller...that they perch on the wire and feel nothing is no guarantee that you would feel nothing if you did the same.

Remember that the fence is not "charged" with DC or 60 Hz AC. It's generally pulsed with high voltage spikes from an induction coil, and can easily have enough high frequency content to be felt via capacitative coupling through a low capacitance. You don't need another plate connected to ground, free space can be enough.

 

[sophiecentaur]

As the fence will not be monitored (I assume) you could just Earth it to a stake in the ground, temporarily, then pull out the stake when you're over / through it. That's the Engineer's solution - not the Physicist's.
Or you could wear a Faraday Suit or learn to pole vault.

btw, cjameshuff, don't you mean that birds would have lower capacitance?

 

[cjameshuff]

As the fence will not be monitored (I assume) you could just Earth it to a stake in the ground, temporarily, then pull out the stake when you're over / through it. That's the Engineer's solution - not the Physicist's.
Or you could wear a Faraday Suit or learn to pole vault.

Depending on the fence, you could also weigh wires down or otherwise push them apart with something, either a good insulator or taking care not to touch the object while it's in contact with the fence. Or push one of the neutral wires aside and squeeze through. Or just brave the shocks and go through/over...depending on the charger, you could have a decent chance of getting through between pulses, and if you get hit, we're talking about a mildly unpleasant experience, especially if you're poorly grounded. I wouldn't try this with a fence meant to deter humans, though.

btw, cjameshuff, don't you mean that birds would have lower capacitance?

Oops...yes. Or as I put it the first time, that humans have more.