Comments on: Three-way Lamp Socket: Fuse Test

By: Ed

Ed — Tue, 02 Oct 2012 23:47:49 +0000

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...

By: EngineerZ

EngineerZ — Tue, 02 Oct 2012 21:26:17 +0000

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 http://tinyurl.com/94drk9j .) 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.

By: Frans

Frans — Sun, 23 Sep 2012 14:19:21 +0000

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.

By: Ed

Ed — Sun, 23 Sep 2012 12:35:32 +0000

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]

By: Frans

Frans — Sun, 23 Sep 2012 10:52:15 +0000

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.)

By: Ed

Ed — Fri, 21 Sep 2012 19:41:13 +0000

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...

By: smellsofbikes

smellsofbikes — Fri, 21 Sep 2012 18:37:41 +0000

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.