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Short circuit duty

  1. Dec 27, 2011 #1
    Can someone explain what short circuit duty is? I see this a lot in my field but I don't know what it means. I see it in the context of new generation coming onto the grid and needing to interconnect to the current transmission system.
     
  2. jcsd
  3. Dec 27, 2011 #2

    jim hardy

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    when a piece of electrical gear, say a transformer or circuit breaker, is subjected to a short circuit overload, the current that flows is quite high.

    the high current heats the conductors.
    the high current exerts magnetic forces on the conductors.
    in a circuit breaker, when the contacts separate to interrupt the current, the current makes an arc that tries to melt the contacts or weld them together.

    short circuit duty is a description of the severity of overcurrent that a device can reasonably be expected to withstand.
    transformer windings must be stout enough to absorb the heat and withstand the magnetic forces during such an event.
    circuit breaker contacts must be robust enough to not melt, and in big ones there must be nonconductive vanes surrounding the contacts to break up the arc.
    cut open a big round industrial fuse and you'll probably find it stuffed with sand to quench the arc.

    next trip to a hardware store, examine a household circuit breaker like the ones in your electrical panel.
    on the back of say a 20 amp breaker you'll see something probably called 'interrupting rating' with a preposterously large number of amps like ten or twenty thousand.
    the breaker should successfully interrupt that much current, once.
    it's up to the electrical designer to assure that the breaker is never put into equipment that could ask more of it. to that end, it shouldn't physically fit into such equipment.

    there's a term (I squared t) , which is the product of current^2 X time(seconds) for an overload. It is proportional to the energy available during a short circuit. You'll find i^2t ratings in fuse and breaker datasheets, and power semiconductor datasheets always tell you what i^2t fuse to use.

    best introduction i ever saw to the subject was a little book from International Rectifier called "Semiconductor Fuses". There's also IEEE books on the subject, if you're a member (i'm not).

    i hope you're not offended by my oversimplification. it's best to get the basic concept then go out and fill in the details. hopefully this will help you clear up some of those mysterious numbers on datsheets.

    old jim
     
  4. Dec 28, 2011 #3
    That's very helpful. So the short circuit duty studies are essentially seeing whether the new generation being interconnected could result in the potential overcurrent being more severe than what the existing circuit breakers could withstand. If the current circuit breakers that are in place can't withstand it then upgrades need to be done.
     
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