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Broken devices due to increased voltage

  1. Dec 9, 2015 #1

    A few days ago I read a story in the newspaper.
    In a cityblock the voltage of the grid was accidently increased to 400V (instead of the standard 240V).

    Clearly quite a few appliances died because of this.

    My question is how far can I get explaining these breakdowns using Ohm's law and the Joule effect.
    Basically an explanation at the high school level.

    If we simply replace the device by a resistor it is clear that through ##P=\frac{U^2}{R}## we get a lot more power that has to be dissipated.
    From this we can expect the resistor to melt if we simplify the system in an extremely naive way.


  2. jcsd
  3. Dec 9, 2015 #2


    Staff: Mentor

    You are correct, but devices can also fail when electrical arcs jump from point to point through air or through insulators because the voltage is too high.

    In an incident that happened near me, a 3000 volt line touched a 240 volt line, causing all the TVs in the neighborhood to catch fire at the same time. The fire department had a busy night.
  4. Dec 10, 2015 #3


    User Avatar
    Gold Member

    There are two basic effects (other than transients that might also accompany the event)

    1. The input voltage causes devices subjected to it to breakdown because they are rated for a lower voltage
    2. Higher currents cause devices to overheat or fuses to blow.

    I expect #1 is most likely, but that will also generally cause #2 to happen.
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