When Lightning Strikes Alarm Systems
April showers bring May flowers, or so goes the old cliché. Warmer weather also brings with it thunderstorms and power surges galore. All too often the result is false alarms, lost data and the destruction of high-tech electronic equipment.
Although lightning is a major concern during good weather, it doesn’t have to be warm out for electric motors, smelting operations, heavy industry and other electrical concerns to spell big trouble for the equipment for which fire and burglar alarm technicians are responsible.
Lightning is a common concern for alarm technicians because of the suddenness with which it can strike. The damage that it can cause may involve a lone device connected to an internal low-voltage cable, like an addressable manual fire pull or an individual circuit board. Or it might involve an entire motherboard within a fire or burglar alarm panel.
Sometimes the damage is light while other times it can be catastrophic, tearing through the system from one end to the other. Having a fair understanding of how lightning works and by using good installation practices, along with quality equipment, technicians can limit the amount of damage that does occur.
Mechanics of a Lightning Strike
Any device that connects to a metallic wire can suffer damage at the hand of lightning. This is true whether the strike is direct to the cable itself or indirect by virtue of inductance. Sometimes no matter what you do, lightning will do irreparable harm.
Concerning a direct hit to equipment, lightning can enter through multiple paths from outside the structure. A good example is a campus environment where there are many buildings.
Lightning also can enter the structure through the public electric bus via outside power lines or one of the many paid subscription services now available. Examples include conventional telephone, cable television networks and satellite dish systems.
In a burglar or fire alarm system, once lightning enters the equipment through one of the methods cited above it can migrate to a signaling line circuit (SLC), initiating device circuit (IDC), notification appliance circuit (NAC) or some other low-voltage line. Another example is the coaxial cable or twisted pair unshielded wire that carries video images in a CCTV system.
“The focus is usually on SLC loops [burglar and fire alarms], low-voltage data [networks] and coax cable [CCTV],” says John Pecore, president of Stormin Protection Products Inc. of St. Petersburg, Fla.
Manufacturers are well aware of the problem lightning poses and for this reason they commonly build a limited amount of surge protection into their electronic systems at the point of manufacture. In most cases this is done right on the motherboard itself.
This ploy most often includes add-on printed circuit (PC) boards and any number of subsystems, such as smoke detectors, glass breakage sensors and others. And yet damage or total destruction can and often does occur when lightning appears on the scene.
According to Pecore, one fire alarm manufacturer uses optical isolation on the data loops to minimize lightning damage. Damage in this case often occurs when lightning induces a high voltage current in the internal wiring of a building. This can be especially problematic in campus environments where data lines travel under ground between buildings.
“This design incorporates the idea of isolating the grounding differential from the other buildings. Nice little spark gaps and it does help, but the problem is, when incorporating second-rate surge suppression, you get a feedback loop up the case ground into the smart chip,” says Pecore.
When you consider the multiple pathways over which lightning can travel, you must also think about distance with regard to proximity as it pertains to the surge protection device that acts to clamp the surge voltage and route the strike current to earth ground.
This is true whether it involves external surge protection or those internal to the device itself. Many years ago an engineer told this writer that lightning will often travel well beyond the point where the surge voltage is clamped. All too often this ends up damaging critical circuits, not to mention the possibility of arcing within the panel itself.
Pecore says that most panels can handle some of this current, but not all of it. This is especially true where there’s a significant voltage differential between equipment grounds of all the interconnected equipment.
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