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Archive for December 27th, 2009

Failed Switch

Switch Innards

Switch Innards

When I flipped this switch on, it started fizzing and emitting ozone-scented smoke while the lights it controlled flickered. This is not a nominal outcome. I toggled the switch a few times, but it continued to misbehave, so I installed a replacement switch and laid the old one out on the desk for an autopsy.

It’s an old-school mechanism, as suits the 1930-vintage structure it came from. The lyre-shaped arch with the spring swings back and forth on its tabs, which rest in the small recesses near the middle of the switch body. The peg on the toggle handle engages the spring, thus providing the over-center snap action.

The switch action takes place at the bottom of the arch, where those two very small tabs stick out. They wipe on the grubby-looking bottom tabs of the oddly shaped flat-brass doodads, the U-shaped ends of which surround the screws that clamp the copper wire to the switch.

I expected to find a scorched contact or perhaps an insect in the mechanism, but nothing seemed out of the ordinary. Apart, that is, from the layer of congealed grease covering everything inside. I suspect the grease was applied in the factory to help prevent contact corrosion, but the volatiles are long gone.

Switch Contacts

Switch Contacts

A closeup of the switch contacts shows (what I think is) the problem.

All the contact points are covered in grease, but the lyre-shaped gizmo looks like it’s been painted: its contact points were black and resisted cleaning by fingernail scraping.

As nearly as I can tell, all the current passed through a very few high spots that were wiped somewhat clean as the contacts closed. As those spots heated up, the grease melted and flowed over them, increasing the resistance and the heat.

The switch had been working for many decades, as the BX armored cable in the box had fabric-covered rubber (stiff rubber) insulation. I managed to install the replacement switch without breaking the insulation, but it was ugly in there.

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