A short circuit occurs when the hot wire in a circuit contacts either the neutral wire or the ground wire, and a hot-to-ground short is the more common of the two, according to the The Circuit Detective. If a light fixture is shorted out, the circuit breaker controlling the light fixture circuit will blow when you turn on the light because the short allows a current surge through the circuit, and it's the breaker's job to prevent that. You can usually make the repair yourself, but before you do, you should verify that the short is actually in the light fixture and not somewhere else.
Keep in mind that working with electricity is inherently dangerous. Always turn off the breaker controlling the circuit before touching any wires, and, just to make sure, test the wires with a voltage tester to make sure they are dead. Use insulated tools and, if practical, wear latex gloves to provide a layer of insulation between the wires and your hands.
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A Short and a Disconnection
If a light is not working but has power, that's not the same thing as a short circuit, but it's close. It means that there's an open circuit, which could be caused by a loose connection or a burnt-out light bulb. The break in the circuit stops power from flowing, but it won't trip the breaker. If one light in a chandelier is not working but the others are, that's an indication of disconnection rather than a short circuit. The light that's not working is probably just a burnt-out bulb.
The difference between an open circuit and a short circuit is that for a short to occur, the disconnected wires actually have to touch each other. One way this could happen is if a single strand from a braided copper fixture wire touches the opposite terminal or the ground terminal on the fixture. The wires don't have to come loose for this to occur, but it's more likely if the wires are loose.
Light Fixture Is Shorted Out
When a light fixture shorts, the circuit breaker will blow when you turn on the lights and everything on that circuit will lose power. Before assuming the light fixture is the culprit, you should conduct a simple test.
Turn off all the light switches on the circuit and unplug every appliance, then turn on the breaker and make sure it stays on. If it doesn't, either the breaker is faulty or the short is somewhere in the wiring. At this point, you're better off calling an electrician than trying to find the short yourself. If it stays on, however, you can proceed with the test.
Plug in the appliances one by one and turn on all the lights, leaving the suspect fixture for last. If the breaker blows at any time during this test, the culprit is the device you just turned on or plugged in, not the light fixture you originally suspected.
Locating the Short
If the breaker stays on until you reach the last fixture, you need to check the switch for loose wires. Finding none, turn your attention to the fixture. Turn off the breaker, unscrew the fixture from the ceiling mount and check the wire to see if they are loose or touching. If they are, you'll probably see some blackening and soot, which is characteristic of the heat generated by a current surge.
If there is no problem with the wiring, check the fixture for continuity using a continuity tester or ohmmeter. Disconnect the wires, remove the bulbs and place the tester leads on the fixture terminals. The fixture is shorted and needs repair if the continuity tester lights up or the meter shows zero resistance, indicating current is flowing, which shouldn't happen after you remove the bulbs.
Once you determine the fixture is shorted, you can find the actual short by looking for blackened wires or terminals. You may be able to make repairs, but the heat generated by a short generally causes enough damage to warrant replacing the fixture.