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I plugged a G-4 bulb into a GU-4 slot and it burned up the unit yesterday. What does that U mean?

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The pin size is a little bigger in the GU-4 than the G-4. There's probably a risk that this could damage the socket, but they are pretty close. More importantly, I don't see anything that specifies the wattage for these bulbs, so it's possible that you replaced a lower wattage bulb in a higher wattage fixture.

  • Yeah, embarassingly, that's what I did... Will replacing the sockets fix my problem (in your opinion)? – Rikon Oct 17 '11 at 15:54
  • @Rikon, hard to say, it really depends on what burned up. If all it did was smoke a little before you turned it off, then you may be able to just put a new bulb in. But if it overheated and melted any components in your fixture, then it may not be safe to use without first replacing any damaged parts. – BMitch Oct 17 '11 at 16:14
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    Wattage will not cause this. If the bulb is lower wattage than the fixture, it's fine (a lower wattage bulb of the same technology is just not as bright -- keep in mind a CFL will be lower than the equivalent incandescent but just as bright). If the bulb is higher wattage, it's dangerous because it's hotter than the fixture is designed for, so may blow a fuse in the fixture. – gregmac Oct 17 '11 at 17:49
  • @gregmac, doh, you're right, I was thinking voltage. – BMitch Oct 17 '11 at 19:28
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Most likely what happened is a 12V bulb was used in a fixture that runs at 24V or a higher voltage.

My second guess being the bulb was cracked/damaged during install, since as @BMitch says, the pin sizes are a bit different (GU4 diameter is 0.95-1.05 mm, while G4 is 0.65-0.75 mm. Both are 4.0 mm pin-center to center).

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