Surging power can cause serious injury, death

Measuring or monitoring for power quality problems usually requires making some sort of electrical connection between the instrument and the electrical system. Despite the promotion of electrical safety by industry organizations and NFPA 70E, there are some who like to tempt fate by involving human contact with live circuits. In these instances, even the 120V AC used in residential circuits can kill. Electrocutions may have decreased in recent years, but some still lick their fingers and touch an AC line to see if it is live; so a brief review of the dangers of electricity seems warranted.

Both AC and DC voltages can be dangerous. Whereas DC voltage is more likely to cause muscle contractions and deep tissue burns, AC voltage can cause disruptions to the body’s electrical system, including forcing the heart to fibrillate and muscles to contract. One of the biggest problems with electric shocks is the heart lies more or less on the path between two hands or a hand and a foot—the most common paths for an electric shock to travel. Current passing through the head can quickly cause loss of consciousness. Nerve cells use electrical pulses to tell muscles to contract. Subjecting muscles to an electrical current can therefore cause a contraction that cannot be released until the current has been removed, increasing the damage to the body.

The following current levels flowing through one’s body are approximate and vary from person to person and should not be taken as hard limits to test on oneself (or others):

  • Most people can feel 1–10 mA of AC power and 5–10 mA of DC power.
  • For AC power, less than 5 mA typically is considered harmless.
  • At around 16 mA, involuntary muscle contractions would begin, and it would be difficult to release one’s grasp. This is especially dangerous if gripping the conductor providing the shock.
  • At 20 mA, paralysis of respiratory muscles begins. This causes ventricular interference, pain and respiratory difficulty.
  • 100 mA is approximately the threshold for ventricular fibrillation, where the heart’s contractions are no longer coordinated. Blood will cease to circulate and death follows if the situation is not corrected.
  • At 2–6A, the cardiac system is at a standstill, and internal organ damage can follow if the current level is sustained. However, a defibrillator uses this level to cause a sustained ventricular contraction and then release to allow the heart to reset its rhythm. Note these are well below the 15–20A required to operate a typical residential circuit breaker or fuse.

These current levels are mostly meaningless out of context, however. Ohm’s Law tells us how much current flows through the human body with a 120V potential across it if the resistance is known. However, humans’ resistance varies depending on the thickness of the skin, the amount of fat and liquid in the body, the dryness and cleanliness of the skin and several other factors. The amount of skin in contact with the conductor also determines the resistance.

While the skin can have up to 100K§Ù of resistance if it is dry and clean, blood is a fairly good conductor. Broken or punctured skin lowers your electrical protection by penetrating the protective skin barrier. Sweaty or broken skin can lower the resistance to less than 1K§Ù. Wet skin allows the current to flow into the body at more points than just the physical contact with the conductor. Metal rings, bracelets, etc., have the same effect and should not be worn while doing electrical work if possible. Returning to Ohm’s Law, 120V with 100K§Ù is 12 mA and 120V with 1K§Ù is 120 mA, where the latter clearly is in the range that can cause ventricular fibrillation.

As noted above, fuses and circuit breakers are not too useful in preventing electrocution due to their trigger range being so much higher than a lethal amount of current. Even still, their reaction times may not be quick enough to prevent injury or death. Ground-fault circuit interrupters (GFCIs), however, can be designed to trigger on a 5 mA or less difference in current between the hot and neutral lines and trigger in approximately 25 msec. This protection is very useful if the current is flowing from the hot line to ground but usually cannot protect against someone touching both the hot and neutral lines simultaneously.

The best protection is, therefore, to make sure the circuit is de-energized and that it remains that way for the duration of the service following the proper lock-out/tag-out procedures. When that is not possible, proper personal protective equipment is even more necessary, based on the arc flash and other potential hazards. For example, properly maintained and rated rubber gloves can increase one’s resistance to 20M§Ù, where negligible current would flow. Many other types of gloves can absorb moisture and lose their effectiveness.

Compromising safety is like Russian roulette—maybe one can get away with it this time, but it is just a matter of time until the worst happens.       EC

BINGHAM is pursuing a master’s degree in engineering at Cornell University and can be reached at stb25@cornell.edu.