Shocking, The inventor of the GFCI…. Charles Dalziel

Charles Dalziel

Link to his research paper- Deleterious effects of electrical shock

In his study he conducted a series of test to determine the minimum current required to produce  ventricular fibrillation of the heart. Some of this was based on an earlier study,”AC shocks of varying parameters affecting the heart” using sheep and other animal subjects. Turns out that the sheep's heart was most vulnerable to shocks that fell during the T phase of the electrocardiogram of the heart.

Out of all of this research it lead him to invent the Ground Fault Current Interrupter (GFCI). Which he received a patent on in 1961.

Just three years after Dalziel received his patent manufacturers produced a commercially viable product that the National Electrical Code then required swimming pool underwater lighting systems to have Ground Fault Circuit Interrupter protection. The first introduction of GFCI protection.

Some basic numbers discovered out of his on other research is that the resistance of hand to hand or hand to foot is about 1000 ohms(there are variables on this and is used as a basic guideline). It was also found that the heart works on 40 hertz cycle, which is not too far from the 60 hertz cycle used in North America (50hz in Europe and other countries), so we are very close to tuning the AC circuit to the hertz cycle of the heart. It was found that around even around 30mA of 120v/240v at 60hz can cause fibrillation and it can be as low as 1mA (and lower) if it is a direct enough path, GFCI’s work on a 5mA threshold.

Typically around 10 to 20 mA you reach the “let go” threshold when the current will create a muscle contraction that you will not be able to override (it’s about 1mA per 10 lbs). As a musician in my early days I learned to always test potential “Hot” objects with the back of my hand. That way if you are shocked and have an involuntary grasp reaction that is was easier to pull away. I test panel covers and other electrical components this way today.

OSHA- How Electrical Current Affects the Human Body

At the time of Dalziel patent there was about 1,100 accidental electrocutions per year and now that has reduced to over half that number even though electrical usage and components have grown exponentially.

It is very hard to know exactly how many lives have been saved because of these devices but if one may have been your child I think most would think it is worth it.

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Red Adair

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