OT - power outage anomaly

We had a power outage yesterday and I noticed something weird: the coffee maker's display was flickering (at several Hz). Unplugged it stopped, so there was no internal battery.

I put a DMM on a receptacle and saw about 9V! I tested its strength by shorting it and was able to get a (very small) arc! It went away with the main breaker off, so it was definitely coming from the street.

So, this was curious, but I'm wondering if it could affect anything more than the coffee maker. Especially if it could affect something more important than the coffee maker (TV, computer, DVD).

Would an appliance as cheap as a coffee maker have a power supply that would be susceptible?

Where do you suppose the 9v is coming from?

Thanks, Bob

Reply to
Bob Engelhardt
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Wild guess............. The power line was broken and both ends were contacting the earth.

Dan

Reply to
dcaster

I have a battery powered digital panel meter monitoring the AC line at my house 24x7. When the power dies, I sometimes see some residual voltage, usually on only one of the two phases. I've seen anywhere between about 20VAC and about 100VAC. The source is usually someone's emergency generator, back feeding the power line. Generally, such installations lack a proper transfer switch. I've also seen it once with a cheap grid connected solar installation, that was improperly installed.

Besides getting electrocuted by a back fed generator, some appliances such as refrigerators, do not like to operate at lower than normal voltages. Lots of articles on the topic: I don't think that 9VAC is going to do any damage. There are "brownout protection" devices sold everywhere, or you can build your own:

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

If there is a high voltage feeder on the same pole as you mid-voltage feed, it could pick up voltage by induction. On the main street near us, the 7200 V feeds are about 20 feet up, and then above that is a 45 KV feed that runs parallel for a mile or two. If there was a break, or the substation shut down that local feed but the higher voltage was still live, i could easily imagine the MV line picking up some voltage. You'd have that MV feed running for many blocks, with dozens of transformers mounted on nearly every pole, tapped onto that feed. Typical residential distribution systems (at least around here) are 7200 V, so the transformer has a 60:1 step-down to each side of the 240:120 secondary. So, your 9 V would be actually 540 V on the feeder.

Jon

Reply to
Jon Elson

** If another phase is still working in the street, any appliances connected across that phase and the dead one will transfer some power.

Draw it out.

.... Phil

Reply to
Phil Allison

Maybe from the AC battery that is supplying the DC transformer?

Reply to
Robert Baer

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