August 2014

Low-voltage electricity

By Ellis Brasch

A series of unfortunate events that caused the 2007 death of a Florida construction worker show that even 120 volts can kill you in seconds. The medical examiner's report in the case also raises a question: What is low voltage?

The electrocution: 'Worker dies after electric shock from faulty cord'

The worker was standing on a plastic bucket using a refurbished, double-insulated saw to cut holes in the Sheetrock ceiling of a home under construction. The saw's power cord, which had an inadequate splice in it, was plugged into a 120-volt extension cord and the extension cord was plugged into a power strip. The power strip was connected to a series of three extension cords that went to an outlet in the house next door, which was equipped with a ground-fault circuit interrupter. However, the grounding pins in the three extension cords between the power strip and the outlet were missing.

The worker draped the 120-volt extension cord that powered the saw around his neck so that the plug was resting on the left side of his chest. It was a hot day, and he was sweating.

A witness heard the worker yell and saw him throw off the saw and cord. The worker stepped off the plastic bucket, took two steps, and collapsed. He was rushed to a local hospital where he was pronounced dead.

The electrical shock he received was not enough to leave burn marks on his body, but it disrupted the rhythm of his heart. The medical examiner's report listed the cause of death as acute cardiac dysrhythmia due to low-voltage electrocution.

How the electrocution happened

The saw's power cord had an inadequate splice in it.

Although the saw was double insulated, it would not protect the worker from the defective power cord. The "inadequate splice" in the power cord most likely was the energized conductor that allowed the current to flow through the worker's body. However, the fact that the current passed through the worker's body was a necessary – but not a sufficient – condition to cause the electrocution.

The three extension cords between the power strip and the outlet were missing their grounding pins.

The missing grounding pins in the cord sets meant that the cord sets were not grounded. Because electricity always seeks a path back to its source, the only remaining path for the current was through the worker's body.

The worker draped the 120-volt extension cord that powered the saw around his neck so that the plug was resting on the left side of his chest.

The path that electricity takes through the body affects the degree of injury. If the electricity passes through a person's chest cavity (from one hand to another, for example) the person is more likely to be electrocuted or receive a severe shock.

It was a hot day, and he was sweating.

The presence of moisture from standing water, wet clothing, high humidity, or perspiration increases the possibility of an electrocution. The level of current passing through the human body is related to its voltage and to the resistance of its path through the body, which can be expressed in units called ohms. Under dry conditions, the resistance offered by the human body may be as high as 100,000 to 300,000 ohms per square centimeter. Wet or broken skin may drop the body's resistance to 1 percent of that range, increasing the risk of a severe shock.

The electrical shock he received was not enough to leave burn marks on his body, but it was enough to disrupt the rhythm of his heart.

Thanks to Ohm's law, which describes the relationship between current, voltage, and resistance, it is possible to estimate the amount of current that passed through the worker's body: about 200 milliamps. Current between 100 and 200 milliamps causes ventricular fibrillation, but is generally not strong enough to cause burns. However, once fibrillation starts, death can happen quickly. (If the current that passed through the worker's body was 200 milliamps, he could have been electrocuted in less than one second.)

What is low voltage, anyway?

Low voltage is a term that everyone seems to understand – until you ask them to explain it. The National Electrical Code (NEC), the standard for safe electrical practices in the United States, does not define it and Oregon OSHA's rules for electrical work leave the term open for interpretation. For example, Oregon OSHA's general industry rules require "live parts of electric equipment operating at 50 volts or more" to be "guarded against accidental contact" while "rooms, or enclosures containing exposed live parts or exposed conductors operating at over 600 volts, nominal" must have signs that say "Danger – High voltage – Keep out." So, low voltage could be 50 to 600 volts, which has little educational value.

Remember: It is not necessarily the number of volts that will electrocute you (voltage is the force that causes electricity to flow through a conductor), but the amount of current, its path, and the time it takes to pass through your body.

Electric current and its effect on the human body

  • 1 milliamp (mA) or less – no sensation – not felt (1,000 milliamps equal 1 amp)
  • 3 mA or more painful shock
  • 5 mA or more – local muscle contractions – 50 percent cannot let go
  • 30 mA or more – breathing difficult – can cause unconsciousness
  • 50–100 mA – possible heart ventricular fibrillation
  • 100–200 mA – certain heart ventricular fibrillation
  • 200 mA or more – severe burns and muscular contractions – heart more apt to stop than fibrillate
  • Over a few amps – irreversible body damage

Source: "A guide to electrical safety," North Carolina Department of Labor, 2012  ▉

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