IS AC (ALTERNATING CURRENT) MORE DEADLY THAN DC (DIRECT CURRENT)?

Near the end of the nineteenth century when Westinghouse was challenging Edison’s dominance of commercial power generation and distribution, Edison set out to convince the public that AC was far more dangerous — deadly, even — than DC.

He quietly supplied financial support and resources to an electrical engineer and consultant named Harold Brown, who was publicly campaigning for legislation against AC generators and equipment in New York. As part of his campaign, Brown put on public displays designed to demonstrate just how much more dangerous AC was compared to DC.

In his first public display, he electrocuted a large black retriever in a lecture hall at Columbia College at 50th and Madison Avenue in New York City, much to the disgust of the members of the audience, which included members of the press, the New York City Board of Electrical Control, and various other interested parties.

Many of them walked out in the middle of the demonstration, but Brown persisted until an agent of the American Society for the Prevention of Cruelty to Animals forbade him to electrocute another dog. The demonstration ended, but Brown continued his deadly campaign, successfully electrocuting large dogs, calves, and horses in public.

After one of Brown’s demonstrations, a story ran in The New York Times describing the grim event. George Westinghouse wrote a letter to the newspaper in response to the article, defending AC. Brown, in turn, wrote a letter to the newspaper challenging Westinghouse to a bizarre contest.

“I challenge Mr. Westinghouse to meet me in the presence of competent electrical experts and take through his body the alternating current while I take through mine a continuous current….We will commence with 100 volts, and will gradually increase the pressure 50 volts at a time, I leading with each increase, until either one or the other has cried enough, and publicly admits his error.”*

Westinghouse didn’t honor him with a reply.

Is AC really more dangerous than DC? There are many factors that come into play when a person receives a shock. The severity of the shock depends on the size, weight, age, and body fat of the person, as well as voltage, frequency, duration of shock, contact area, contact pressure, temperature, and moisture of the skin.

Generally speaking, the impedance of the skin is the first line of defense against a fatal shock. It helps prevent current from flowing through the heart and causing fibrillation. The higher the impedance, the lower the current for a given voltage.

Like any other electrical circuit, the flow of current through a human body behaves according to Ohm’s law. It turns out that for AC current, the impedance of our skin decreases as the frequency increases, but the frequencies most likely to cause ventricular fibrillation are between 50 Hz and 60 Hz.

*Empires of Light, Jill Jonnes (Random House, 2004)

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