Surge protector strikes out lightning

Toronto--A local transit agency had to insure traffic light operations during thunderstorms. Typical of many modern, remote-operating electronic systems, such as oil rig controls, railroad crossings, toll booths, and ski-lift equipment, these devices are solid-state based--chock-a-block with sophisticated processors and semiconductor chips for fast, efficient control and communications. But because they lack the ruggedness of previous heavy-duty electromagnetic devices, surge protection against the effects of lightning strikes, direct or on power lines, is critical.

Traffic-signal electronics are near to the signals, many themselves on poles. These devices not only regulate the lights but often monitor traffic and may provide signal timing to facilitate traffic flow. The agency used iron-core transformers to filter high frequencies from strikes to protect the older relay-based devices. Newer equipment brought concern that a long-duration strike (20 microsec or longer) could not be sufficiently handled by the transformer alone, damaging the controller and even the transformer insulation.

After comparison tests, including withstanding a 3,500V "wave" that peaked in 1.2 microsec, the transit authority selected the two-stage Wieland (Burgaw, NC) WB.700 Series surge suppressor to harden the signals. This is placed in front of the primary winding of the existing transformer, which still handles the first few microseconds of a pulse. According to Rob Ray, Wieland protection devices group manager, the suppressor choke coil then reflects the voltage pulse back upon itself. Two metal oxide varistor (MOV) stages are next. They drop their resistance in presence of a proscribed voltage pulse--the first lower-voltage threshold stage bleeds off power to dissipate the leading edge of the remaining transient (it also clamps regularly occurring low-energy transients). The second, higher-threshold stage evens the voltage wave out to a normally shaped waveform.

As "hits" on the suppressor accumulate over time, the first MOV bank will wear out, causing its LED indicator to go out, signaling replacement. Ray notes, "When the first stage dies, only the secondary wave shaping is removed. Nothing harmful is seen at the equipment." The lower-threshold portion is disconnected from the circuit. Models in use with the traffic signals can handle a 10 kV surge. He adds. "The latest, more robust versions, can, for example, tackle over 900 Joules of energy and 3,200A." And new applications for the DIN-rail-mounted, screw-connector suppressors include protecting RS485 interfaces as well as control voltage and current loops.