A switching mystery, resolved...

A switching mystery, resolved...  Yesterday I replaced a few more electrical switches.  I've been chipping away at this project, working my way through the house replacing all the switches and outlets, and cleaning up the (all too numerous) wiring problems I've found in the process.

One of the switches I replaced is located near our front door.  It controls four outdoor floodlights, which we use every evening and morning to light up the yard when we walk the dogs.  We only know of that one switch for controlling those lights, so I was surprised to discover – when I removed the existing switch – that it was wired as a “three-way” switch.  The fact that it was wired as a three-way switch implies (rather strongly) that there is another switch somewhere that also controls these same lights.  That was a big surprise to us – we have no idea where that switch might be, and we don't know of a logical place for it to be located.  In this house, though, it could be located in some completely nonsensical place.  We joked that maybe it was in the basement, where there are several switches whose purpose we haven't yet deciphered.

That's not the mystery that the title of this post refers to, though, as we haven't yet resolved it.  The resolved mystery was a different one: I replaced the switch and then tested it.  I test these rewiring jobs every time, even when (as in this case) the rewiring is trivial.  It's pretty difficult to miswire a three-way switch, and there is no way to dangerously miswire one.  I was confident this one was wired correctly, as the “hot” wire was easy to identify visually.

So I was very surprised when it didn't work.

I took the new switch out, and connected the wires directly.  I saw a spark, but no lights came on.  I checked the circuit breakers; no breakers were tripped.  WTF?  There's so little that can go wrong with a circuit like this!  With the power verified (the spark and no breakers tripped), what's left to go wrong?  All four bulbs coincidentally burn out?  Just in case that was somehow the problem, I replaced one.  No joy.  I went round-and-round on this for about an hour, and finally just gave up.  I decided to reinstall the switch, button things back up, and troubleshoot it later.  I had other things to do.

A couple hours later, while moving some boxes around, I was thinking about the switch problem.   It was bugging me, because it was such a simple electrical wiring problem and yet it had me buffaloed.  Then I remembered something I'd observed that surprised me: the spark made when I connected the wires manually was quite small – smaller than I'd expect for a circuit with 600 watts of lights being switched.  Why would that be?  Then a few seconds later, I had it – I knew why the lights weren't working.

Did you figure it out?

I hypothesized that somewhere in that circuit there must be a photoelectric control that turned those outdoor floodlights on at night, and off in the day.  That control would draw a small amount of current even in the day, when the lights would be off – hence the small spark, and no lights.  At night the photoelectric control would allow the lights to come on.

So at dusk I walked Mo'i, and turned on the light switch.  The lights all came on.  All that time I spent troubleshooting the switch earlier in the day was for naught – it was wired correctly in the first place, and was working all along.  The lights simply aren't allowed to come on during daylight hours!

I still have no idea where that photoelectric control is located.  Nor do I know where the other switch is.  Yesterday I also replaced another “mystery switch” located near our front door.  This one is a simple on/off switch (not a three-way), located in our entranceway.   The circuit is live, and it draws current when turned on (about an amp, so around 100 watts).  We haven't detected anything that it's turning on and off.  My working theory is that the former owner installed a light bulb inside a wall somewhere, and that's what's being controlled :)