An electrical short circuit is an easy thing to understand. The hot wire in the circuit is somehow able to contact the ground or the neutral wire, and this results in a large current surge. In a household circuit properly protected by circuit breakers, a short is usually nothing more than an inconvenience, but it has the potential to destroy electrical equipment and start a fire.
You know you have a light short when the breaker trips as soon as you turn on the light or plug it in. You might even hear the crackling of electrical arcing or see a spark. The experience can be shocking in more ways than one, but as long as there's no damage, you should be able to fix the light fairly expeditiously.
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How a Light Short Occurs
Things aren't very complicated inside a typical light fixture. A pair of wires, one white and one black, connect to the socket. There might also be a third wire, which is bare or green and is for grounding the fixture. In lieu of wires, some fixtures have terminal screws to which you attach the circuit wires.
A short can occur when any of the wire connections are loose or when the wire insulation has worn or melted, such as would happen if you use a larger-than-recommended bulb. Corrosion inside the socket or on the bulb threads can also cause a short by allowing the side of the socket, which is one of the socket terminals, to come in contact with the knob on the bottom, which is the other terminal.
You have to turn off the power and look inside the fixture to locate the problem, of course, but that usually isn't very difficult. The heat produced by arcing melts plastic and leaves soot-like deposits. Look for this damage, and you'll find the short.
How to Fix the Light
Once you determine where the short has occurred, you can decide how to fix it. If you see a melted wire, you can often cut it out and splice in a new wire of the same gauge, although this is more difficult if the wire is very close to the base of the socket. Sometimes, the best approach is to replace the socket or the entire fixture.
If the short was caused by a loose connection, all you have to do is make the connection tighter, but look carefully for frayed wires and twist the wires to consolidate the strands. Some shorts are caused by a single strand that sticks out and contacts another wire or terminal screw. It's also a good idea to replace charred terminal screws because the carbon deposits left by arcing can interfere with conductivity.
A Short in the Wiring
Long-term exposure to the heat generated by a light fixture can crack the insulation of wiring and expose the conducting metal. A short occurs when the exposed conductor comes in contact with the base of the fixture, a metal electrical box or another exposed wire. The short can occur anywhere along the length of the wire and may be due to something unrelated to the light, such as someone pounding a nail into the wall and puncturing the wire sheathing.
You can identify a shorted wire by disconnecting the fixture, turning off the power and doing a continuity test with a multimeter set to measure resistance. Touch the leads to the exposed ends of the hot and neutral wires. If the resistance is very high, or the meter display reads OL (open line), the wire is damaged, which means the fixture is okay, but you may need an electrician to help you find the short in the wire.