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I have a UL-approved lamp that I've had on my desk since about 2004; it's the kind with two metal arms rising from a heavy base that connect to a halogen bulb fixture. The other day I was working at my desk with a piece of sheet metal (don't ask), and it slipped and briefly contacted both arms of the lamp. Sparks shot everywhere and the lights dimmed. I took the thing apart looking for the broken wire, and instead discovered to my surprise that there are no wires; instead the arms themselves function as the conductors. There's a transformer in the base, presumably so the current won't be enough to kill you, but that obviously isn't enough to prevent problems if there's metal nearby. Can this seriously be how the thing is meant to work?

halogen desk lamp

  • What's the output voltage label on the transformer say? – ThreePhaseEel Feb 18 at 0:13
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    @ThreePhaseEel 12V 20VA – dlf Feb 18 at 0:20
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    I totally want that lamp now. – Harper - Reinstate Monica Feb 18 at 16:29
  • "Sparks shot everywhere and the lights dimmed..." I'd get rid of it. Sell it to Harper. – Greg Nickoloff Feb 18 at 22:11
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Lamps below the low contact voltage are not uncommon. I have installed many “fancy” systems that the conductors are not insulated and the lamp holders clip on to the wires . Some fixtures have fancy looking loops with the same exposed wiring. I am sure it scared the #%%^ out of you but it is not dangerous. I think it is 15vac and 30vdc

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    Not dangerous to me maybe, but couldn't the sparks themselves have done some damage? And what if a delicate piece of electronics fell against the arms? – dlf Feb 18 at 0:21
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    Would have shorted out tx and electronics case would probably be grounded. – JACK Feb 18 at 0:50
  • Initial packaging might have warned against shorting out arms. – JACK Feb 18 at 0:51
  • I installed two of those fancy systems. the customers still call me to change out the bulbs.. They don't want to get near the wires.+ – JACK Feb 18 at 1:09
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Most such lamps use 12 volts AC, which is considered safe in terms of electric shock.

The reason for this is that a minimum current is required before muscles contract, preventing one from letting go of the wires. For most people, 10 mA at 60 Hz is considered safe, other than if the current were to go directly through a vital organ (e.g. during open-heart surgery).

Human skin resistance, when damp, can be as low as 1,000 ohms for each contact, and the body resistance is about 300 ohms, so the maximum current that can flow at 12 V is about 5 mA. Therefore, it's unlikely you could get a dangerous electric shock from 12 VAC.

That said, as you found out, bare conductors can easily be bridged by metal, e.g., a dropped screwdriver or pen. There probably is a fuse by the transformer that would have blown had the sheet metal stayed in contact, but you removed the short circuit quickly enough to save the lamp. Underwriters Laboratories testing changes with time, and represents best safety practice at the time the approval is issued. Most likely, such lamps are no longer approved, though I wouldn't consider one dangerous, knowing its foibles.

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  • I had the thing sitting on top of my aluminum PC case for years. It makes me wonder what would have happened if the arm had sagged and contacted the edge... – dlf Feb 18 at 0:43
  • It probably would have shorted out the tx. Your PC case is grounded so no damage to it. – JACK Feb 18 at 0:48
  • @JACK Well yes, shorted out the transformer and then what? Just blown a fuse? Or started a fire? Or both? OP said sparks shot "everywhere" and the lights dimmed, which does not inspire confidence in the lamp having a built-in fuse. – user253751 Feb 18 at 17:35
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    It does actually have a fuse. I... um... tested it again, just to see what would happen. There were more sparks, and then the fuse did blow, but not for a second or so. – dlf Feb 18 at 19:50

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