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In Reply to: RE: Crowbar ? posted by Jon Risch on November 13, 2010 at 18:20:20
At least I know all of our stuff is now required to have a fuse in series with any MOV.
I am not sure if it is a UL or CSA standard but I can ask the agency guys next time I am at the agency lab.
They also make MOV's now with integrated fuses.
Follow Ups:
My neighbor had an old Monster power strip for his video gear. The ganged MOVs blew up and took out a new dishwasher on another circuit. He asked me to take the strip apart, as he is not comfortable with electricity. The charred remains of the MOVs were an impressive sight.
"The ganged MOVs blew up and took out a new dishwasher on another circuit."
OK, I'll bite. How?
Seems more likely to me that if they are correlated at all that the same event that blew the MOV's also took out the dishwasher.
Rick
My house is on the same utility transformer and we had no indications of a problem.
I believe the MOVs shorted and the current surge on the neutral created a voltage that added to the normal line voltage on the dishwasher circuit.
Al, I've been thinking about it (always dangerous) and can't visualize how it could damage a device on another circuit.
As far as I know the neutral and safety ground are always hooked together in the breaker box (at least around here) and that node is grounded to the local ground and the common from the transformer. So what's the path through the dishwasher?
We just had to put in a new dishwasher this year as the old one could no longer hold it's water, as it were, and although a cheap one it's got a microcontroller and a sea of buttons and LED's on the front instead of a rotating timer that I could make sense out of. But it actually washes a lot better much to my surprise. Now we shall see how long it lasts...
Regards, Rick
The USA system, with a grounded center tap on the utility transformer, provides a neutral wire that is grounded at both the transformer and the panel, as you observed. The reason for the two grounds is that the earth in between is not generally a low impedance conductor.
Thus, when there is a line-to-neutral fault on one hot leg, there may be considerable voltage developed across the neutral wire (Ohm's law) until the breaker opens. This voltage is subtracted from the faulted hot leg, but adds to the opposite hot leg voltage.
Appliances should withstand voltage surges within reason. My neighbor's dishwasher should not have failed. However, it was new and the failure may have been something about to happen in any case.
OK,
Thanks for the explaination, I see what you mean. It's not something I would expect to cause problems considering the low resistances involved but who knows what the reactance looks like or the susceptibility of the dishwasher.
We are in the process of looking for a new stove and at Sears they are pushing their long-term warranty and have a controller assembly laying in front of the sign as an example of something expensive (at least it looks expensive) that is at risk. I should have looked more closely but I think it actually used mechanical relays to control the burners so that should provide pretty good isolation back to the drivers. Seems like poor marketing to me "be sure to get an extended warranty because this new model is far less reliable than the one you are replacing despite costing five times as much."
I can still get parts for he old one but they aren't cheap and I'm not sure I want to yard out the asbestos wiring which is probably getting pretty friable after forty years. Too many unknowns...
Rick
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