Chapter 6

6.1.1 Electrocution

• Typically, it is the current passing through the chest area that causes electrocution. Though current passing through the head alone can be fatal, contact is usually made with one arm, the current passing through to the other arm or to a leg. The amount of current passed, not the voltage involved, is critical.

• The most dangerous current range is roughly from 100 mA to 3 A. In this range the heart is sent into unsynchronized contractions called ventricular fibrillation. Without medical defibrillation intervention death will follow.

• At currents higher than about 6 A the heart and breathing muscles are paralyzed, but often may be restarted by someone with CPR (Cardio-pulmonary Resuscitation) training.

• It is primarily skin resistance that (combined with the applied voltage) determines the current passed through the body. There have been electrocutions with a 22 volt source when the skin resistance was lowered by abrasion. There are two reasons why many accidental electrocutions involve people in water: (1) the water provides a relatively low resistance current path outside the body and (2) the water substantially reduces the resistance of your skin. Solutions of electrolytes (salts, gatorade, etc.) are even more effective.

• The most dangerous voltage source is the 110 Vac that powers the labs, shops and offices. Not only is it sufficient in many cases to pass the deadly 100 mA of current, but there are so many opportunities to come in contact with it.
  6.1.2 Damage Due to Reflex Action

• Currents over 10 mA can cause violent involuntary muscle contraction. Such contractions can result in bodily damage and/or equipment damage. In addition, such contractions may make it impossible to release one's grasp on the voltage source.
  6.1.3 Burns

• Currents over about 2 A will result in burns at the point of contact. This is a major factor only for those working with high voltage sources.

  
  

  6.1.4 Burned Out Electronics

• Instead of damaging or killing yourself, your electrical accident may destroy your electronics. The destroying currents may either pass through your body or through some tool. It takes less than 50 volts to cause major damage in some CMOS circuits, so it is clear that 110 Vac is again a major hazard.

  
  

  6.1.5 Radio Frequency (RF) Field Injury

• Microwave fields are efficiently absorbed by water containing materials, such as the human body. This fact is put to use in microwave ovens. The threshold is only about one milliwatt/cm2 for damage to the eyes. This damage is not associated with pain, so one must take care not to work with microwaves that are not enclosed in waveguides if the power densities can exceed this level.

  
  6.2 Safety Procedures

  Be extremely careful about touching any metallic equipment if there is water on the floor. Such water enables the hand-through-body-to-leg current path.

  Whenever possible unplug a circuit before working on it. Additionally, ensure that someone else does not try to use a piece of electrical equipment that you are working on: if you leave it, at the very least put a note on it. Do not just turn off the switch; you may make contact with the "hot" side of the power line on the source side of the switch. If the circuit involves voltages over about 100 volts, you should use a grounded insulated wire to touch all possible high voltage points before working on the circuit. Capacitors can store dangerous charges for fairly long periods of time (up to hours or more).

  Do not try to help someone ensnared in an electrical shock situation without first turning off the power source involved, preferably at the breaker box. If you do not take this precaution you are likely to also become ensnared.
  If you must work on an energized circuit there are several precautions you can take to minimize the danger. Do not work alone. In case of an accident you need someone to turn off the power, or do CPR, or call for help. Keep one hand in your pocket. This simple stratagem avoids the arm-to-arm shock. Be sure your legs or seat are not grounded to avoid other shock paths. Wear rubber-soled shoes; sit on a wooden or plastic stool...the objective is to maximize the resistance through all paths that include your heart. THINK!

  Be sure power cords are correctly wired using the conventional color code. An amazing number of physics students do not know proper wiring; one of them could have built the equipment you are using! Always use three wire plugs for your circuits. The blade with the brass screw (this is the "hot" line, the line with potential 110 Vac different from ground) should be connected to the black lead in the power cord. This black lead is also the one in which the mandatory on/off switch and fuse are inserted. The blade with the silver screw is the neutral and should be connected to the white lead in the cord. This is the lead that carries the current back to the source. The third (middle) prong on the plug is the ground connection, and should be connected to the green wire in the cord. This green wire should be connected to the metal part of your instrument (such as the cabinet), which would be most dangerous if accidentally electrified from the black lead. The green wire is for your protection. Never cut off the ground prong on a plug. Without the case-to-ground connection, an internal short of the hot line to the case results in a death trap. With the case-to-ground connection this same short will announce itself by blowing a fuse or tripping a circuit breaker. Some special circuit breakers, called Ground Fault Interrupters (GFls), sense the current in the ground lead and will open the power lead if even a few mA are flowing in that lead. The same color rules hold when wiring an outlet as when wiring a plug.

  
  

  Quick Reference Table

  Wire Color:		Black		White		Green
  Screw Color:		Brass		Silver		Sometimes Green
  Voltage to Ground		110 ac		~0		0

  Avoid exposed dangerous voltages; replace worn cords before they cause a shock, or possibly a fire. Enclose circuits. If the voltages involved are high, place interlocks on the doors to the enclosure in which the high voltage exists; these will de-energize the voltage should the doors be accidentally opened.

  If you work with very high voltage, you should keep in mind that the breakdown electric field of air is about 10 kV/cm. Thus, you do not need to actually touch a conductor to be shocked. This is particularly important if you are using an exposed coil from an induction heater.

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