Engineering Expert Witness Blog

     Ever been jolted with electric current?  Like the time you’d just gotten out of the shower and went to plug in a lamp with damp hands?  So what do you think the voltage was that caused that nasty biting feeling that resulted from your momentary lapse in good judgment? 

     Once, while operating a subway car at a railroad museum at which I was a member, I was inadvertently “electrocuted.”  I went to turn on the lights inside the car, and unbeknownst to me the light switch was faulty.  When I touched it I instantly became connected to the car’s 600 volt lighting circuit.  With just a split second of contact the current passed through the tip of my right index finger, along my right arm, down the right side of my body, and out the tip of my big toe, finally exiting into the metal railcar’s body.  The current actually burned a hole where it had exited through my boot.  The experience was both frightening and painful, but fortunately did not result in any real injury.  I was lucky that the current had bypassed my heart, because if it hadn’t, I might not be alive today.

     That was 600 volts.  Now imagine being jolted by the 4000 volts present in a microwave oven’s internal high voltage circuitry.

     Last week we discovered how the high voltage circuit in a microwave oven converts the ordinary, everyday 120 volts alternating current (AC) present in our homes into a much higher voltage approximating direct current (DC).  This is done by an internal component known as the capacitor.  The capacitor is capable of storing large amounts of electrical energy, and this can result in microwave ovens presenting a danger even when unplugged.

     A microwave oven capacitor is shown in Figure 1.  If you happened to touch its wire terminals while it’s still charged, its power can rapidly discharge high voltage electrical current throughout your body.  The electrical current from the capacitor can even stop your heart from beating, and this is exactly what caused the demise of a person featured on a soon to be released Discovery Channel program, Curious and Unusual Deaths.  While being interviewed as an expert for the program, I was asked to explain this rather unique phenomenon of latent stored energy, and how it may present a threat.

Figure 1 – A Microwave Oven Capacitor

     Remember, a microwave oven capacitor can remain charged with dangerous electrical energy for hours, even days, after the microwave oven plug is pulled from the wall outlet.   The bottom line here is that you should not attempt to fix your microwave oven, unless you are trained and certified to do so. 

     Next week we’ll switch to a different topic, namely an electrical device known as a “wall wart.”  That’s the black plastic adapter you plug into electrical outlets to power your cell phones, laptops, and other small electronics. 

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