Sub blowing Christmas lights?

The Sub power supply is sloppy and the fluctuating power needs of the sub are messing up the voltage in the A/C supply. The mains are being drawn down and bouncing way over the standard voltage by the sub.

I think that you may very well be pointing in exactly the right direction, Elizabeth. I'm thinking that what is known as an inductive kickback effect, from the power transformer of the sub's amp, may be putting a large voltage spike into the lights.

Inductive kickback, which results in what can often be an extremely large voltage spike, is what happens when the current through an inductance is abruptly changed. Power transformers have lots of inductance. The abrupt change might be related either to the sub's "fluctuating power needs," as you put it, or conceivably to the sudden change in line voltage that may occur when the lights are plugged in and start drawing current.

"Back emf," though, is probably not the best term to use for this. The back emf voltage produced by a woofer or other speaker driver results from a different effect -- the motion of the voice-coil continuing for some amount of time after the input signal has stopped or changed, which causes the driver to act as a generator until its motion gets back in sync with the signal.

Sprink, it would be helpful to know some further details: Does the fuse blow precisely at the moment the lights are plugged in, or at random other times? When it blows, is the sub turned on, and is it playing music?

Regards,
-- Al

That's a logical question, Paulsax, but I can't envision how either a missing ground or a miswired outlet could cause something like this. No matter how the outlet were miswired or ungrounded it would seem that no more than the normal 120V or so line voltage would be put across the light string (unless, as Elizabeth and I suggested, the sub is adding a transient voltage spike on top of the 120V).

Regards,
-- Al

Jim, the famous fictional detective Sherlock Holmes once remarked something to the effect that when you've ruled out everything else, whatever remains, no matter how improbable, must be the explanation :)

In this case the only explanations that seem to present themselves are the ones you and others have offered, the voltage spike possibility (which of course would result in a proportional current spike), and the possibility that it is all coincidence (meaning that the fuses will eventually blow on other outlets as well).

Sprink's words that "the lights have blown twice WHILE plugging in to this outlet" would seem to give credence to the spike theory, because the spike would be adding to the already large current surge that occurs during the first second or two after the lights are connected (due to their low resistance when cold). You appear to be saying that the spike would be either too brief or too small to blow the fuse, but given all the foregoing my feeling is that it is a close call as to whether that possibility or coincidence is the least unlikely possibility.

Sprink -- When you respond, it would also be helpful to know the ratings of the fuse (amperage, fast blow/slow blow, etc.), and how many lights are in series with it, and what their voltage and wattage ratings are.

Regards,
-- Al