by Dwight Bues, Systems EngineerA few years back, I consulted on the construction of a trash-to-power plant in New Jersey. My job was to provide filtered, dry, oil-less air for control valve actuation as well as status instrumentation. The plant took in the refuse stream, separated recyclables and then burned the rest. The plant was also instrumented to monitor the efficiency of the burner and flue gas scrubbers as well as to directly tax its operation by levying fines for release of certain toxins from its effluent gases. Once the site of a major landfill, this plant was slated to be “The Future” of refuse disposal.
The filter and dessicant-dryer system for supplying control air was “my baby.” I supervised construction and the initial checkout using the local controls and all went well. Later, when we tested the remote controls from the power plant control room, I was quite confused as to why they wouldn’t work. We checked the switchboard wiring, “buzzed out” the circuits to the equipment and everything checked “good.”
One day, after many trips from Baltimore to New Jersey, I was amazed to discover that the indicator lights on the local controls were indicating that the system was performing normally, while a short walk away in the power plant control room, the lights were flickering randomly. This was definitely odd. This was not a “failure mode” that I was familiar with. The reason that this was an unusual problem is that the control system in question was designed for an industrial environment - no low voltage signals here, these signals were set up for standard 110 VAC!!!
The indicator assemblies employed standard NE-2 neon bulbs with a dropping resistor, so that they would fire at about 85V and then back off to about 70V to provide extended life. I recall that these bulbs had about a 30,000 hour life (about 10 years of intermittent operation). The other advantage to the neon bulb is that it is much lower power than an incandescent bulb, thus making the driver circuitry in the control unit less expensive and more reliable to boot.
More weeks of testing verified my initial suspicions: SOMETHING was getting onto our signal wires, but how? Every once in a while, solutions come from an unlikely source. The successful engineer has to learn to rely on them. Either “Serendipity” strikes, or you use assets that you don’t usually call on (reference the garbage man in the DILBERT comic). I called my recently-graduated nephew (EE from Georgia Tech) and he provided the suggestion that “solved the puzzle.”
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