Buzzing light switch

[Tim H]

Home built in 1976, AL branch circuits. When testing the lights switch in a bathroom, there was a buzz that did not have the characteristc 'sputtering' associated with arcing, rather a low pithed humming buzz. This lasted for about 1-1.5 seconds, at which point the buzzing stopped and the intensity of the lights increased dramatically. Popped the cover and the first thing I notice is that the switch is rated (marked) 'CU ONLY'. Cycled the switch several more times, and did not see any arcing, nor was there visible evidence of previous arcing. Of course I called for the switch to be replaced and I recommended that the electrician check the other switches in the home. My question (just to satisfy my curiosity) is what may have been going on and was it potentially dangerous? What are the differences in switches manufactured for CU only v. AL/CU or AL only?

Thanks,

Tim

[Jim Katen]

Originally posted by Tim H

Home built in 1976, AL branch circuits. When testing the lights switch in a bathroom, there was a buzz that did not have the characteristc 'sputtering' associated with arcing, rather a low pithed humming buzz. This lasted for about 1-1.5 seconds, at which point the buzzing stopped and the intensity of the lights increased dramatically. Popped the cover and the first thing I notice is that the switch is rated (marked) 'CU ONLY'. Cycled the switch several more times, and did not see any arcing, nor was there visible evidence of previous arcing. Of course I called for the switch to be replaced and I recommended that the electrician check the other switches in the home. My question (just to satisfy my curiosity) is what may have been going on and was it potentially dangerous? What are the differences in switches manufactured for CU only v. AL/CU or AL only?

Thanks,

Tim

Hmm. I don't know what was happening.

As a general rule though, low-pitched humming/buzzing noises are not a good thing in light switches.

I doubt that the CU-only thing had much to do with it. The only difference between switches rated for aluminum and those that are not is the plating on the wiring connections. Since you say there wasn't any sign of arcing, I doubt it was a connection problem. Perhaps it was simply a bad contact in the switch itself and it had nothing to do with aluminum wiring. (Sometimes you don't see the lion because you're looking so hard for a bear.)

In this job, sometimes we're lucky to find the problem at all let alone fathom the cause. This might be one of those times when you just pass the ball to the electrician.

- Jim Katen, Oregon

[Terence McCann]

Originally posted by Tim H

This lasted for about 1-1.5 seconds, at which point the buzzing stopped and the intensity of the lights increased dramatically.

Thanks,

Tim

This would indicate an internal loose connection, contact points of the switch. The buzzing is caused by the loose connection and when the buzzing stops the light grows brighter, another sign of a voltage drop.

In any case replace the switch.