As we all know by now, or will find out, microprocessor-based systems\devices and other sensitive [CMOS, etc.] electronics have a great aversion to voltages, currents, and frequencies they are neither designed nor intended to handle. Historically, a number of different schemes have been applied to the problems of lightning and surge protection, with varying degrees of success. In the case of Life Safety Systems [Fire Alarm, Mass Notification, Security, CCTV, Surveillance, Access Control, etc.], our main concern is preservation of specific systems, sub-systems, and devices in fully operational condition, and that is what we will focus on in this series.
The popular comedian Ron White is probably most famous for his assertion that “You can’t fix stupid”. Our first rule for the protection of sensitive electronics should be, “You can’t fix bad wiring with new equipment.” “Bad wiring”, for purposes of this discussion, includes wiring and electrical distribution systems that are somehow deficient, particularly with respect to proper wiring methods, selection of conductors, separation of Class 1 conductors from other Class conductors, and the granddaddy of many disasters electrical, inadequate and\or improper grounding and\or bonding within an electrical distribution system or sub-system. Having said that, we also have to realize that certain unalterable geological and\or geographic factors may be in play that can seriously affect the measures we may have to employ to protect critically important systems; buildings or facilities situated on wet ground, on the tops of hills or mountains, or near high power\high frequency transmission towers are some examples.
Background & History
Protective devices can be Series-wired or Parallel-wired. They can be PCB mounted, or employed separately, usually at wiring termination points. Most rely on a low impedance ground to function. As it turns out, many of the power related problems encountered within buildings’ and facilities’ sub-systems can often be traced back to improper\inadequate\deteriorated grounding systems, or the local absence of a suitable [low impedance] ground.
Lightning arrestor\TVSS use has been common for decades throughout the Telephone, Central Station, and Municipal Fire Alarm industries, to name a few. Early devices were parallel wired, utility pole mounted, which would take a leg of a protected circuit to ground if subjected to a lightning strike, or power cross with a fallen utility power line. Unfortunately, once these arrestors “fired”, that leg was shorted to ground, and any subsequent transients could find a path through the “protected” equipment to ground.
Another generation of parallel wired devices employs a normally non-conductive gas tube design. A suitably high voltage applied to either leg of the protected circuit causes the gas in the tube to ionize and become conductive. When “fired”, the devices simultaneously short the pairs to each other and to ground, reducing “back door” hits to the protected loads. These devices have the added benefit of being much smaller, and also self-restoring, if not physically destroyed by the triggering event. However, the satisfactory operation of these devices presumes an effective, low impedance ground, and as suggested above, such a ground may not always be present. The other down side to such devices is that they generally have a very wide window of operation, and as we know, most alarm and computer electronics are not designed or intended to operate outside 85% to 110% of their nominal rated input voltages. Some modern systems, such as medical imaging, as well as other delicate systems, may have an even narrower tolerance for input voltage abnormalities.