Posts Tagged ‘electric shock’

Electrocution by Microwave Oven

Sunday, August 21st, 2011

     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. 


Ground Fault Circuit Interrupters

Sunday, June 26th, 2011
     I’ve been talking about how I was asked to be a subject matter expert for an upcoming series on The Discovery Channel titled Curious and Unusual Deaths.  Most of the accidents discussed involved electrocutions, and in each case the electrocution occurred because the victim’s body, usually their hand, inadvertently contacted a source of current.  When that happened their bodies essentially became like a wire, providing an unintended path for current to travel on its way to the ground.  Why does it travel to the ground, you ask?  Because electric current, by its very nature, always wants to flow along a conductor of electricity from a higher voltage to a lower voltage.  The ground is the lowest voltage area on our planet.  When electricity flows to ground along an unintended path it’s referred to as a “ground fault,” because that’s where the electricity is headed, to the ground, or Earth.  By “fault” I mean that something in an electrical circuit is broken or not right, allowing the electrical current to leak out of the circuit along an unintended path, like through a person’s body.

     For example, in one of the Curious and Unusual Deaths segments I was asked to explain how a fault in wiring caused electrical current to flow through a woman’s body to the ground that she was standing on.  This happened when she unintentionally came in contact with a metal door that was, unbeknownst to her, electrically charged from an unanticipated source.  The current was strong enough to cause her death.  Where did the electric current originate from?  Watch the program to find out, but I’m sure you’d never guess.  To say that it was an unlikely source is an understatement.

     When ground faults pass through a person’s body, bad things often happen, ranging from a stinging shock to stopping your heart muscle to burning you from the inside out.  The severity depends on a number of factors, including the strength of the current to the amount of time your body is exposed to it.  It might surprise you to know that if your skin is wet at the time of contacting a current, you risk a greater chance of injury.  Water, from most sources, contains dissolved minerals, making it a great conductor of electricity.

     But what exactly is electrical current?   Scientifically speaking it’s the rate of flow of electrons through a conductor of electricity.  Let’s take a closer look at a subject close to home, a power cord leading from a wall’s outlet to the electric motor in your kitchen hand mixer.  That power cord contains two wires.  In the electrical world one wire is said to be “hot” while the other is “neutral.”  The mixer whirrs away while you whip up a batch of chocolate frosting because electrons flow into its motor from the outlet through the hot wire, causing the beaters to spin.  The electrons then safely flow back out of the motor to the wall outlet through the neutral wire.  Now normally the number of electrons flowing into the motor through the hot wire will basically equal the number flowing out through the neutral wire, and this is a good thing.  When current flow going in equals current flow going out, we end up enjoying a delicious chocolate cake.

     Since the human body can conduct electricity, serious consequences may result if there is an electrical defect in our hand mixer that creates a ground fault through the operator’s body while they are using it.  In that situation the flow of electrons coming into the mixer from the hot wire will begin to flow through the operator’s body rather than flowing through the neutral wire.  The result is that the number of electrons flowing through the hot wire does not equal the flow of electrons flowing through the neutral wire.  Electrons are leaking out of what should be a closed system, entering the operator’s body instead while on its way to find the ground.

     Next time we’ll look at a handy device called a Ground Fault Circuit Interrupter (GFCI) and how it keeps an eye on the flow of electrons, which in turn keeps us safe from being electrocuted. 


Tech Quiz Number 1

Thursday, August 13th, 2009
Test your knowledge of science and engineering…
1. It can take as little as ___________  of electrical current to kill a human being.

2. You keep bending and unbending a wire coat hanger.  It eventually breaks at the bend because of ____________.

3. When you boil one pound of water off to steam in an open pot, its volume expands by a factor of over ____________.

4. The proper combination of ______________ is needed for combustion to occur.

5. The National Electric Code (NEC) is also known as _________________.

A Few Words About Electric Shocks

Sunday, July 26th, 2009
     For an electric shock to occur, a person must become a part of an electrical circuit in such a way that electric current passes over their skin or through their body.  Under certain conditions, even momentary contact with an energized metal object can result in serious injury and even death.  According to an article in the American Journal of Industrial Medicine:

“Contact with live electrical wiring, equipment, and light fixtures was the main cause of electrical deaths and injuries among electrical workers, followed by contact with overhead power lines. Among non-electrical workers, contact with overhead power lines was the major cause of death. Other causes included contact with energized metal objects, machinery, power tools, and portable lights.”

     From an engineering viewpoint, the body’s electrical resistance is an important variable.   Electrical resistance of an object is a measure of how freely electrical current can flow across the object when a voltage is applied across it.  Resistance is measured in units called “Ohms.”    Resistance of a person’s body can depend on skin dryness, perspiration level, thickness of the skin, the distance that the electrical current travels through the skin, and other factors. The typical human body has a hand-to-hand electrical resistance somewhere between 1,000 and 2,000 Ohms, but resistance across other parts of the body can be much higher.  There is a wide variation in body resistance between individuals, so the same voltage level may result in different effects.

     Most electrical injuries occur from alternating current (AC) at levels above 50 Volts at the low frequencies typically maintained by electric utilities.  The North American utilities normally generate power at a frequency of 60 cycles per second.  The “cycles per second” unit of frequency is also referred to as “Hertz,” usually abbreviated as “Hz.”  At 60Hz frequency, the threshold for perception occurs with electrical currents as low as 0.0001 Amps.  The “can’t let go” electrical current for adults is approximately 0.010 to 0.015 Amps. This is the current that causes involuntary muscle contractions severe enough to prevent the person from letting go of the source of the electrical shock. 

     Electric currents as low as 0.050 Amps at 120V, 60Hz, have been known to cause death.  Just to give you an idea of how small that current is… a table lamp with one 40 Watt incandescent bulb draws 0.333 Amps from a 120 Volt, 60 Hz household electrical outlet!  In the interest of electrical safety, the National Electrical Code (NEC) considers 0.005 Amps at 120V, 60Hz to be the safe upper limit for children and adults.