Lightning is the biggest cause of damage and destruction of electrical and electronic equipment in industrial facilities, commercial buildings and homes, as stated in the 2008 report by the U.S. National Lightning Safety Institute. Fires started by lightning occur mostly in the late afternoon and early evening in the summer months, and these fires cause an average of nine civilian deaths, 53 civilian injuries, and $451 million in direct property damage per year (other sources list the annual losses in the billions of dollars).


Contrary to the saying that lightning never strikes the same place twice, it does, and it can do so hundreds of times in a single season, as those in the Empire State Building can attest to. With a properly designed, installed and maintained lightning-protection system, business can go on as usual despite such events. Many residential owners, though, don’t install a lightning-protection system; they use surge protection devices (SPDs). However, an SPD doesn’t fully protect a home from even indirect lightning strikes.


A lightning bolt can produce tens of thousands of amperes of current, peaking in less than 10 microseconds and restriking numerous times in a second. That tremendous amount of current needs to go somewhere, and like all electrical problems, it also follows Ohm’s and Kirchhoff’s Laws. Looking for a good (low-impedance) path to an earthy ground can cause it to enter a structure in numerous ways, especially using any metallic object or wire that penetrates the structure walls. Whereas the direct strike path is obvious, hitting an object adjacent to a structure can also start the process rolling. 


Some of you may remember the ­October 2006 ELECTRICAL CONTRACTOR article “Things That Go Boom in the Night,” about lightning hitting a tree, coupling in the pond pump motor in the rear yard of a house, following the wiring into the house, and significantly damaging the electrical infrastructure and equipment plugged into the electrical outlets and the telecommunications system throughout the house.


A somewhat similar thing occurred recently in central New Jersey, when lightning struck a house and coupled into the wiring for the under-the-earth dog fence. The energy followed the wiring into the garage, where it encountered the dog fence’s power source device. Even though the alternating current (AC) line cord was not plugged into the adjacent receptacle, a ground wire was connected between the device and the metal part of the receptacle where the cover is screwed in, which is electrically connected to the grounding conductor. This receptacle was fed from a ground-fault circuit interrupter (GFCI) receptacle on the back wall of the garage. This GFCI was fed by another GFCI in the pantry before the wires headed to the breaker panel in the basement. Other receptacles in the garage were also daisy-chained off of the garage GFCI load-side connections.


As the top picture shows, diodes in the dog fence’s power source exploded, and there were burn marks on the wall. The plastic cover had blown off and was in pieces. The green wire at the center bottom went to receptacle ground using the cover plate ground screw to the left (not shown). The twisted-pair on the right went outside along the perimeter of the property and had insulation failures near the device. The wires on the bottom left went to the AC transformer that was not plugged into the receptacle at the time of the strike. 


The lower picture shows the receptacle where the ground wire from the dog fence device was connected. The damage to the screw threads is apparent at the bottom, with clear evidence of arcing and melted metal deposited from the eyelet to the receptacle.


Both of the GFCIs—in the garage and upstream in the pantry—failed with the load side open-circuited, so no power fed any of the garage outlets anymore. Also damaged, but not shown, was the garage door opener located electrically and physically closest to the receptacle where the dog-fence device was. The 15A breaker in the basement that protected this circuit did not trip. The homeowner found no other damaged equipment in the house or the garage, though damage from lightning and transients in general is not always apparent to the eye.


Protection against lightning, causing damaging transients to equipment and infrastructure, requires diligence in finding every possible conductive path and providing it with adequate SPDs and a lower impedance path to ground than through the building’s electrical system. Given that lightning strikes somewhere on earth every second, it’s just a matter of time before it finds your weak link.