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ANSWERS TO FREQUENTLY ASKED QUESTIONS, PAGE 6
Why Are Joules Important?
Joules are important because they indicate the power of an electric fence charger's shock. Specifically, they measure how much heat would be generated if an electric fence charger administered its maximum potential shock. That doesn't mean that a deer contacting a conductor powered by a 6-joule charger would actually receive a 6-joule shock, it just means that it is theoretically possible if a deer received the charger's full output.
This says something about effectiveness, because some of the electric energy that a charger puts on the fence line is dissipated by resistance on the line and also by electricity leaving the line through weeds and brush—all of which diminishes the remaining joules and also diminishes the voltage available to shock target animals. At some point the voltage dips below the 5 kilovolts needed to reliably shock a deer through its fur and skin; and at some further point it drops below the 2.5 kilovolts needed to reliably administer a shock on the nose or tongue.
In general, the more joules one has the better. For example, a very mild electric fence charger like the Fi-Shock SS-440, with its rating of 0.14 stored joules, can be rendered powerless to shock deer by a few weeds leaning on the fence. In contrast, the Parmak Super Energizer 3, with its rating of 9 stored joules, can charge many miles of electric fence, and can reliably shock deer despite heavy loads of weeds and brush.
But joules also relate to safety. That stands to reason. Other things being equal, a shock that generates a certain amount of heat while passing through a target animal is going to be safer than one that generates ten times that heat. Thus, the Fi-Shock SS-440, while generating high line voltages, administers shocks so small that they literally won't harm a sparrow; while shocks from the vastly more powerful Super Energizer 3 can harm small animals and demand respect.
Some experts claim that too much stress has been placed on joule ratings, because joules = volts x amperes x time; and so a weak electric fence charger generating a continuous output may have a higher theoretical joule rating than a powerful electric fence charger whose pulses only last a thousandth of a second. That's perfectly true. But the electric fence chargers best suited to controlling deer are so-called "low impedance" chargers (see below) that put out very brief pulses measured in thousandths or even ten-thousandths of a second. That being the case, their relative joule ratings do give a rough idea of their relative shocking power, potential hazard, ability to charge long fence lines, and ability to cope with weeds and brush.
What Is "Low Impedance"?
"Impedance," like "joule," is a technical term. In the world of electric fences it measures the resistance that the electrical state of a fence conductor presents to a particular electrical output moving down the conductor. If the resistance is high the impedance is high, and if the resistance is low the impedance is low.
One way to lower resistance on the line is to let electricity received from an AC outlet or battery build up for a second or so and then to release it all in a very brief interval, say one thousandth of a second, with a very high electrical potential (a high voltage). So long as there is relatively little potential remaining from the preceding brief pulse, the new pulse will move out relatively unimpeded—hence the name "low impedance" charger.
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