Archive for September 18th, 2012

Three-way Lamp Socket: Fuse Test

After un-bending the top of a pole lamp that suffered an untimely collision with the floor, I discovered that the entire stock of three-way bulbs in the heap had at least one burned-out filament each; I’d acquired them when Mom moved out of the Ancestral House, so they dated back a long time. So I figured I’d insert a decently sized single-filament bulb and be done with it.

Three-way lamp sockets have an additional tab contact between the usual central contact and the outer shell:

Interior of 3 way lamp socket

Interior of 3 way lamp socket

The shell forms the common contact for the filaments and the switch counts in binary: off / off, off / on, on / off, on / on. In principle, the tab sits low enough to not contact the shell of an ordinary bulb.

I was doing this in the Basement Laboratory Workshop Wing, with the lamp plugged into the outlet strip along the front edge of the bench; that way, I simply poked the power strip button to remove line voltage from the lamp while swapping bulbs. So I:

  • turned the power strip off
  • unscrewed the last dead three-way bulb
  • threw it away
  • screwed in an ordinary bulb
  • turned the strip on

At which point all the fluorescent overhead lights in the Laboratory went dim, the shop resounded with a deep resonant groan, and the acrid smell of electrical death filled the air. Elapsed time less than a second, tops.

Come to find out that the socket’s contact tab stuck up a little bit further than it should, producing a dead short across the line:

Melted bulb base

Melted bulb base

Of interest: the branch circuit breaker didn’t trip, the GFI on the circuit didn’t trip, and the pop-out breaker in the power strip didn’t trip.

Huh.

I harvested the pole sections, the base counterweight, and the line cord. The rest of the corpse joined the bulbs in the trash…

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