I understand basically that the current, or amps, are what cause harm, but based on Ohm’s law, V=IR, for the amps to be so low with such high voltage, the resistance must be very high. So why do you need such a high voltage in the first place? Wouldn’t that require a transformer to step up the voltage? And doesn’t such a high voltage create a safety risk, for instance if a conductor with lower resistance came into contact with the voltage source? If you want low amps as not to electrocute animals/people, would it not be easier to use lower voltage?
Just curious and trying to understand. Electricity is like magic to me.
In: Physics
It turns out that *time* or the duration of a current is also a factor in the risk of electrocution.
Electric fences store a Static *Charge* using a fairly small Capacitor.
*Current* is the rate that electric charges travel through an object. Capacitors are similar to batteries in that they store electric charge, and can produce current by pushing those charges through a closed circuit, due to the voltage difference between the positive and negative terminals or ends. Likewise, as the charge in a battery or a capacitor runs out, the voltage and current drops. However unlike batteries, the latter can release their stored charge extremely quickly, producing a brief burst of high current.
The high voltage on the fence capacitor causes a brief current to travel through the skin, enough to cause pain at the contact site. However, because the capacitor is small (technically speaking it has low *capacitance,*) the charge stored therein runs out extremely quickly, in a matter of a few tens of milliseconds.
As the charge runs out, the current from the capacitor also drops extremely rapidly and so does the voltage. In practical terms this means the capacitor is not capable of sustaining a current for any significant period of time that would be life threatening. It’s merely enough current to confuse pain sensing neurons in your skin into believing you’ve just been physically wounded.
The circuit driving the fence is capable of supplying charge to the primary capacitor at high voltage but at a very low current. The informal term is Trickle Charging. Usually takes several seconds to recharge. This compared to the short duration of the discharge current. The charge circuit supplies less than a milliamp which would barely be noticable under normal circumstances.
You can think of the driving circuit like stretching a rubber band. The force needed to stretch out a band is trivial and not likely to cause pain. However once stretched, if the band is released suddenly against your skin, could be pretty painful.
It also turns out that the energy stored in a stretched rubber band and the energy stored in the capacitor of an electric fence is roughly comparable. Strickly speaking a stretched band may contain around 10-50 times as much energy, however an electric current is more efficient at causing pain.
Lastly something of note- electric fences usually have an isolation circuit that cuts off the capactor from the fence wires after it’s discharged, allowing it to be recharged in isolation. This prevents an animal or human leaning against the fence from producing a permanent path to ground, which would prevent the capacitor from being able to store any charge. Once a certain voltage is reached on the cap, it’s switched back into the fence wires, which might in the mentioned situation, cause another jolt.
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