I am fascinated by the history of the electrical industry. More than a few of the authors of the hundreds of electrical books I collect were the Einsteins of the electrical industry, and I have known several of the more recent ones personally. Some early legends of the National Electrical Code were still in the industry when I started in 1971. (Of course, some people assume I’m old enough to remember the 1897 panel.) Sometimes I feel that way, but I didn’t join the NEC Panels until 1988. I have had the pleasure of learning from these icons about their knowledge of the reasons for certain parts of the NEC and NFPA 72, the National Fire Alarm and Signaling Code.
I am going to delve a little into the history of fire alarms, smoke detectors, smoke alarms, heat detectors and other similar fire alarm equipment to better understand the functions for these life safety systems in the NEC and in NFPA 72.
Discovery by cigarette
Francis Robbins Upton, an associate of Thomas Edison, patented the first automatic electric fire alarm in 1890. The first heat detector was patented in 1902 in Europe by George Andrew Darby.
In the late 1930s, Walter Jaeger, a Swiss physicist, attempted to invent a sensor for poison gas. He expected the poison gas, entering the sensor, to bind to ionized air molecules and thereby alter an electric current in an internal circuit of the detector.
Unfortunately, his device did not achieve its purpose, since small concentrations of poisonous gas molecules were not high enough to affect the sensor’s conductivity. Frustrated, Jaeger lit a cigarette and was surprised to notice that a meter on the device had registered a drop in current flow. Unlike poison gas, the smoke particles from his cigarette were able to alter the circuit’s current. Jaeger’s experiment was one development that paved the way for the modern smoke detector.
In 1939, Swiss physicist Ernst Meili devised an ionization chamber device capable of detecting combustible gases in mines. He also invented a cold cathode tube that could amplify the small signal generated by the detection mechanism so it was strong enough to activate an alarm. The first sale of an ionization smoke detector in the United States occurred in 1951. These detectors were primarily used in major commercial and industrial facilities due to their large size and high cost.
In 1955, a simple heat detector was developed for use in homes, but it responded only to a fire’s heat. Smoke was and still is the cause of most deaths, so a smoke detector was more desirable.
The first single-station smoke detector was invented in 1970 and started selling in 1971. A single-station alarm is defined as “a detector comprising an assembly that incorporates a sensor, control components, and an alarm notification appliance in one unit operated from a power source either located in the unit or obtained at the point of installation.”
Between 1971 and 1976, there were several major developments, including the replacement of cold-cathode tubes with solid-state electronics. This greatly reduced the detectors’ cost and size and made it possible to monitor battery life. These detectors could also function with smaller amounts of radioactive source material in the ionization chamber, and the sensing chamber and smoke detector enclosure were redesigned to make operation more effective. Rechargeable batteries were often replaced by the manufacturers with a pair of AA batteries and a plastic shell encasing the batteries for easy replacement.
The photoelectric (optical) smoke detector with light sensing was invented by Donald Steele and Robert Emmark from Electro Signal Lab and patented in 1972. In 1995, the 10-year, lithium-battery-powered smoke alarm was introduced, and the requirement to replace electronic smoke detectors every 10 years was implemented.
The risk of dying in a residential fire is cut in half in houses with working smoke alarms and detectors. The NFPA reports 0.53 deaths per 100 fires in homes with working smoke detectors, compared to 1.18 deaths without them. Commercial and industrial installations usually have lower statistics since people are not sleeping there. Next month’s article will cover the actual installation based on Article 760 of the NEC and operation of smoke and heat detectors, as well as notification devices.