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.


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

13 thoughts on “Three-way Lamp Socket: Fuse Test

  1. I bet you really feel safe, now! Seeing what largely unimpeded household current can do is always impressive to me. I was doing some electrical work in my attic, once, and was too damn lazy to climb up and down the attic ladder to the breaker so I was working on a live circuit. I have a Leatherman I carry in my pocket every day with two sizable chunks taken out of the pliers to remind me to be more careful when working on live circuits. :-) Anyway, in my case the breaker tripped and I got to work my way from the center of my attic back to the stairs in the dark. Lazy has a way of being thwarted, from time to time.

    1. doing some electrical work in my attic

      A long time ago, I’d turned off the breaker and tested the circuit in the room, before applying the big bolt cutter to the BX cable. Who knew they would wire a junction box with two separate circuits?

      The armor took the fault current and the glass fuse looked like a grenade went off inside; it was not a gentle poof.

      I think The NEC Code prohibits such things these days, but that was an old, old house…

    2. When I was rewiring my old house, about the first thing I did was wire in (to an as-of-yet unused breaker) a separate line with branches to the attic and crawlspace, into which went — only — worklights for the space in which I was working. All power tools and wiring changes were done on the remaining lines.
      Getting out of a nothing-but-insulation-and-studs attic in the dark is a claustrophobic, destructive project.

      1. Great idea! In the highly unlikely event I ever rewire another house, that’s on the to-do list.

        Getting out of a nothing-but-insulation-and-studs attic in the dark

        Throw in some bare-wire-and-knob action and you’re got a real challenge: somehow, you just know the bare wires will be live, no matter what else happens!

        FWIW, I got used to carrying a flashlight in my pocket as part of the bed bug experience and now I’m using the thing all the time. Just another few ounces…

  2. As far as I know, the only currently legal way to get “two” circuits in one box now uses 220 and a shared neutral. I use this approach a lot in my barn/shop, but the first outlet has to be a GFCI. The upstream neutral and the other hot wire go to another GFCI, and they act like normal110 circuits downstream. Mostly. I’ve had a couple gremlins where one GFCI tripping causes the other one to go. (Inductive spike, I suppose.)

    I had to do work at church where it used a shared neutral but both hot wires were in each box. It was pick-and-choose which phase wire to use. Didn’t help that the wiring method wasn’t proof against corrosion and we had a lot of moisture in some of the boxes from condensation. I found a few cover plates with insulation smoke and bits of oxidized bare wires including high current neutrals. Not fun, but the wiring just missed the aluminum era.

    Worst experience with dual circuits in in my mother’s 1950 vintage house. After I drew an arc replacing a light fixture, I pulled the main house fuse.

    1. 220 and a shared neutral

      I have some new-in-box dual outlets in the heap (one 220 and one 110) that I almost used, but then came to my senses: each heavy machine has a dedicated 220 outlet and all the 110 outlets, plus the lighting circuits, have GFIs.

      Some of the stuff that’s nominally legit scares me witless just looking at the drawings…

  3. Ed, having lived in Europe for 8 years, the first thing I learned was how cool having 220 or 230V standard to every outlet was. I’m not sure who the idiot was that decided 110/120 was safer. Point being, entire houses, building and even businesses can be wired with #18 safely for lights, and standard outlets. Stuff like dryers, stoves get 3 phase power probably with #12. Voltage is a wonderful thing because fault current is magnitudes greater than load current. It’s simple Ohms law, but wow, what a difference it makes in power distribution losses, wiring cost, and most of all fire safety. The incident you just had would never have happened. Let’s do the math, a 1500 watt heater or hair dryer at 110/120 takes over 10 amps. An air compressor or refirigerator can pull 20 or more. The breakers are designed aroudn those startup loads so they don’t trip right at 15 or 20 Amps (problem #1) and then because the true load has to be low enough resistance and enough current draw, the weekest link will take the brunt and turn it into heat. So that loose joint in a junxtionbox starts a fire in the wall, the insulation melts, but because the run is long enough and the now hot wire has enough resistance, it’s not a fault to the breaker at 110/120V.

    Start talking electric cars and people complain they cannot get a fast charge out of a normal outlet. What does that do to the grid? But change to 220 V, making 440V (old 220) really attractive and really efficent and cheap. Now, new contruction costs are halved or more, less incentive for copper thieves, and we solve a number of immediate social and economic problems that exist in front of us today.

    Again, you had the proof in front of you today, 110/120V is NOT safe, in fact it’s the opposite and then every home in America times the line losses, plus the copper cost, it just doesn’t add up.

    1. what a difference it makes in power distribution losses, wiring cost

      Had a long discussion with a guy who thought 12 VDC distribution from a photovoltaic solar system was just exactly the right thing for whole-house wiring. Couldn’t get across the notion that the IR losses in the wire would be an order of magnitude higher than for 120 VAC (or, heck, 120 VDC), unless you used an order of magnitude more copper, or that 1 V of IR loss is a much worse problem in a 12 V system than in a 120 V system.

      Somehow, Ohm’s Law just didn’t apply to solar-generated 12 VDC… [sigh]

    2. I once heard that the American frequency of 60Hz was supposed to be safer than our 50Hz in case you touch it. However, the lower voltage seems far more likely to be safer in that regard and intuitively it seems like that must’ve been the primary rationale. All I know is that there’ll be quite a bit less resistive loss at our 220/230V, which seems like a pretty good idea for all the reasons already noted. To top it off, my PSU runs perhaps as much as several percentage points more efficiently here than it would in the US and I doubt it’s the only piece of electrical equipment for which this is true. (NB That’s not including heat losses in the wiring leading up to the computer.)

      1. 60Hz was supposed to be safer than our 50Hz

        That certainly sounds like a great marketing concept, doesn’t it?

        Higher frequencies work better in brute-force iron transformers (hence military aircraft used 400 Hz), but that makes no difference with switching supplies. Nowadays only crackpots with antique equipment worry about the line frequency. [grin]

        1. That certainly sounds like a great marketing concept, doesn’t it?

          Iirc the rationale given had something to do with 60Hz making us spasm better than 50Hz, so that you had less chance of being electrocuted by remaining in contact. I can’t find anything to confirm or deny such a statement, but I find it sounds quite implausible.

  4. BTW, the GFI didn’t trip because the dead short wasn’t a ground fault. The current flowed between the line and neutral, so there was no current differential to trip the GFI.

    The house breaker probably didn’t trip because the impedance of the fault limited the current to (I’m guessing here) 10 times or so that of the breaker rating. If there was as much as 1 ohm of resistance, the fault current would have been 120 Amps which is only 8 times a 15 amp breaker rating. Most household breakers can take a full second to clear a fault of that magnitude. (See the Time/Current Characteristic curve for a Square-D “QO” breaker at .) So if it took a second for a hole to burn through the base of the lamp, the short probably opened right before the breaker had a chance to operate. (Or at least one would hope!) I don’t know the specifics – breaker size/type, wire size, etc., but you can play with the numbers to see that there could be a plausible explanation that the “failure” of the CB to trip was not, in fact, a failure.

    As for the power strip breaker, who knows- I’m not sure I would trust that device to provide true protection.

    1. the short probably opened right before the breaker had a chance to operate

      Wouldn’t that be an interesting thing to test? [grin]

      I was most impressed that the entire series of overcurrent / fault detection devices hunkered down until the lamp shell burned out; there’s still a vague expectation that something should pop as the lights dim, even though I know why it doesn’t.

      Which reminds me of the fuss over the original Thing-O-Matic DC extruder motor and why a tungsten lamp worked better than a fuse

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