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Kitchenaid microwave KBMC140HSS0 blowing circuit breaker

I have a Kitchenaid over-the-range microwave KBMC140HSS0 that blows the circuit breaker every so often. Repair guys have replaced the fuse a few times and it works better for awhile and then does it again. Another repair guy came yesterday and said it can't be the fuse because when he used touchpad and pushed one button another would come on and one time he just walked by it and it came on. He thinks we need a new touchpad/keypad but he seems perplexed too. Any experience or advice with this?

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SOURCE: JMC7000ADW Blows circuit breaker

I've had this problem twice with our microwave. The hint to where the problem lies is that the breaker blows when you open the door at times. Inside the microwave are three microswitches; primary and secondary interlock micro switches and an Interlock Monitor switch. The switches are supposed to shut down the microwave if it is running and the door is opened. But, I've found the upper primary switch can stick internal at times and when the door is opened the stuck switch will, because of the way it is wired as a safety switch, cause either the fuse or possibly the main circuit breaker for the oven to pop. The reason the stuck switch blows the breaker or fuse is when the door is opened the interlock switch will cause the neutral wire from the AC power to be applied to the Upper Primary Interlock switch which normally should be open when the door is opened, but if the switch is intermittent or the contacts weld themselves shut the neutral line is connected direct to the "hot" side of the power line through the fuse and thermostat. The first time my microwave failed the switch had melted the contacts together inside the switch and the second time the switch became intermittent and would blow the breaker to the over ever so often. I suspected the switch the second time this happened and utilizing an ohm meter I checked the switch several times by opening and closing the door and once in a while it would remain closed instead of opening when the door was opened. Of course while trouble shooting the power cord to the microwave must be disconnected and use all safety precautions when working around the high voltage areas inside the microwave. I wrote Jenn-Air about the bad switch and the way it is wired into the circuit, but never received any acknowledgement. I suspect these microswitches are under rated for the amount of current that passes through the switch, thus they overheat and eventually arc the contacts together. I hope this helps explain the intermittent problem and could explain many of the intermittent blown fuse problems I see in these internet help sessions

Posted on Oct 11, 2008

SOURCE: Microwave fuse blowing???

I laid out the stuff I wrote above in nice, neat lists and paragraphs, but this neanderthal editor/mail system jams it all together.

I think if I posted it as a solution instead of a clarification request, the format would be better preserved.

So here it is again hopefully in a better format.



Correct polarity is:

  • Wide blade = neutral = white wire = silver screw on outlet
  • Narrow blade = hot = black wire = brass screw on outlet
  • Grounding pin = ground = bare copper or green = green screw on outlet


With a multimeter set to Volts AC, the readings would be:

  • Between Wide & Narrow Blades = 120 Volts (nominal)
  • Between Narrow & Ground = 120 Volts (nominal)
  • Between Wide & Ground = 0 Volts or, a very small induced voltage reading

With multimeter set to ohms and breaker off:

  • Between Wide and Narrow = Infinite ohms (open circuit)
  • Between Wide and Ground = near 0 ohms (closed circtuit)
  • Between Narrow and Ground = Infinite ohms (open circuit)
My experience is that most homes will have at least a couple of circuits with reversed polarity. The black/brass white/silver schema is not a conclusive indication. For example, white wires are often used as black/hot wires -- by code these are supposed to be marked -- but 99% of the time they are not. Plus, it is very easy, when in production mode, to get these wires reversed somewhere in the circuit, whcih reverses everything downstream.

Reversed polarity is not a major issue unless the device plugged into the outlet uses its chasis/frame/cabinet as the neutral side of the circuit. In this case, the chasis becomes hot and can be very dangerous if there are grounding sources nearby. If the chasis is grounded as well as being used as neutral, reversed polarity can create a short (because hot is grounded).

Charlie


P.S. There is also a plug-in circuit tester available at hardware stores which will test for all the above.

Posted on Aug 30, 2009

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I've had this problem twice with our microwave. The hint to where the problem lies is that the breaker blows when you open the door at times. Inside the microwave are three microswitches; primary and secondary interlock micro switches and an Interlock Monitor switch. The switches are supposed to shut down the microwave if it is running and the door is opened. But, I've found the upper primary switch can stick internal at times and when the door is opened the stuck switch will, because of the way it is wired as a safety switch, cause either the fuse or possibly the main circuit breaker for the oven to pop. The reason the stuck switch blows the breaker or fuse is when the door is opened the interlock switch will cause the neutral wire from the AC power to be applied to the Upper Primary Interlock switch which normally should be open when the door is opened, but if the switch is intermittent or the contacts weld themselves shut the neutral line is connected direct to the "hot" side of the power line through the fuse and thermostat. The first time my microwave failed the switch had melted the contacts together inside the switch and the second time the switch became intermittent and would blow the breaker to the over ever so often. I suspected the switch the second time this happened and utilizing an ohm meter I checked the switch several times by opening and closing the door and once in a while it would remain closed instead of opening when the door was opened. Of course while trouble shooting the power cord to the microwave must be disconnected and use all safety precautions when working around the high voltage areas inside the microwave. I wrote Jenn-Air about the bad switch and the way it is wired into the circuit, but never received any acknowledgement. I suspect these microswitches are under rated for the amount of current that passes through the switch, thus they overheat and eventually arc the contacts together. I hope this helps explain the intermittent problem and could explain many of the intermittent blown fuse problems I see in these internet help sessions

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