Electrical Safety

The Fatal Current

Most fatal electric shocks happen to people who should know better. Here are some medical facts that should make you think before taking that last chance.

It’s the Current that KILLS

Offhand it would seem that a shock of 10,000 volts would be more deadly than 100 volts but this is not so. Individuals have been electrocuted by appliances using ordinary house currents of 110 volts and by electrical apparatus in industry using as little as 42 volts direct current. The real measure of a shock’s intensity lies in the amount of current (amperes) forced through the body, and not the voltage. Any electrical device used on a home wiring circuit can, under certain circumstances, transmit a fatal current.

While any amount of current over 10 mA (.01 amp) is capable of producing painful to severe shock, currents between 100 and 200 mA (.1 - .2 amp) can be lethal.

Currents above 200 mA (0.2 amp) , while producing severe burns and unconsciousness, do not always cause death if the victim is given immediate attention. Resuscitation consisting of artificial respiration will usually revive the victim. Artificial respiration must be applied immediately if breathing has stopped.

The Physiological Effects of Electric Shock

Figure 1 shows the physiological effects of various current densities. Note that voltage is not a consideration. Although it takes a voltage to make the current flow, the amount of shock-current will vary depending on the body’s resistance between the points of contact.

As shown in the chart, shock is relatively more severe as the current rises. At values as low as 20 milliamperes breathing becomes labored, finally ceasing completely at values near 75 milliamperes.

As the current approaches 100 milliamperes ventricular fibrillation of the heart occurs - an uncoordinated twitching of the heart’s ventricles.

Above 200 milliamperes, the muscular contractions are so severe that the heart is forcibly clamped during the shock. This clamping protects the heart from going into ventricular fibrillation and the victim’s chances for survival are good.

DANGER! - LOW VOLTAGE!

It is common knowledge that victims of high voltage shock usually respond to artificial respiration more readily than the victims of low-voltage shock. The reason may be the merciful clamping of the heart, owing to the high current densities associated with high voltages. However, lest these details be misinterpreted, the only reasonable conclusion that can be drawn is that 75 volts are just as lethal as 750 volts.

The actual resistance of the body varies depending upon the points of contact and the skin condition (moist or dry). Between the ears, for example, the internal resistance (less than skin resistance) is only 100 ohms while from hand to foot it is - closer to 500 ohms. Resistance may vary from 1000 ohms for wet skin to over 500,000 ohms for dry skin.

When working around electrical equipment, move slowly. Make sure your feet are firmly placed for good balance. Don’t lunge after falling tools. Kill all power and ground all high-voltage points before touching wiring. Make sure that power cannot be accidentally restored. Do not work on underground equipment.

Don’t examine electrical equipment when you are mentally or physically fatigued. Keep one hand in your pocket while investigating live electrical equipment. Above all do not touch live electrical equipment while standing on metal floors, damp concrete, or other well-grounded surfaces. Do not handle live electrical equipment while wearing damp clothing (particularly wet shoes) or while skin surfaces are damp.

Do not work alone! The more you know about electrical equipment, the more heedless you’re apt to become. Don’t take unnecessary risks.

What to Do For Victims

Cut voltage and/or remove victim from contact as quickly as possible–but without endangering your own safety. Use a length of dry wood, rope, blanket, etc. , to pry or pull the victim loose. Don’t waste valuable time looking for the power switch. The resistance of the victim’s contact decreases with time. Remember Ohm’s Law?  V/R=I, so current flow increases with voltage and inversely with resistance. The fatal 100 to 200-milliampere level may be reached if action is delayed.

If the victim is unconscious and has stopped breathing, start artificial respiration at once. Do not stop resuscitation until medical authority pronounces the victim beyond help. It may take as long as eight hours to revive the patient. There may be no pulse and a condition similar to rigor mortis may be present, however these are the manifestations of shock and are not an indication the victim has succumbed.

Precautions to be Used When Working with Electronic Circuits

  1. Practice a precaution used by experienced technicians. Try to keep one hand in your pocket, or behind you when you are making voltage or current measurements. If two hands are in contact with the circuit or if one hand is in contact with the circuit and the other hand is in contact with ground, (such as a metal panel or the case of a piece of test equipment), the current path is across the chest where the heart and lungs are located. This is extremely dangerous as you need to have your heart and lungs working at all times.
  2. Do not work on electrical circuits when the power is on.
  3. Electrolytic and other large capacitors can hold a charge for several hours after the power is turned off. Make it a habit to check that they have fully discharged by shorting them with a screwdriver or clip lead before working on a circuit.
  4. Do not work on electrical equipment while standing on a damp floor or when leaning on any metal object.
  5. Certain components such as resistors get quite hot. Give them time to cool before removing them.
  6. Make it point to know the location of the fire extinguisher and how to use it.
  7. Be sure equipment is in proper working order before you use it. Replace frayed, cracked cords.

Material contained in this advisory comes from The Department of Electrical Engineering, Massachusetts Institute of Technology and from a bulletin published by Graymark International entitled “Graymark Cares About Your Safety”

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