A couple of months ago we dealt with lightning and the fast transients that can be coupled onto the electrical distribution system of the utility or within a facility, and the use of TVSS (transient voltage surge suppressors) to minimize the potential damage from such. What I didn’t cover is the need to protect not just the electrical circuit path, but also the communication circuit paths. Lightning can couple into the phone wires and/or the cable television circuits. Besides the destruction that can occur to modems, telephones, cable modems and other data communication equipment, it can also be the path to get into the electrical system and propagate the destruction to additional equipment.
Unfortunately, I didn’t think of writing it earlier in the year so that it would have been published before the thunderstorm season, and a friend of mine could have read it before his house was hit by lightning. The lightning initially followed the path along the aluminum fascia to the aluminum siding and then coupled to his DSL line, before eventually coupling to the electrical system in his house. Coming from the power quality instrumentation industry, my first reaction was that it would have been great to have had a few monitors at the house to capture the waveforms. He, of course, didn’t share the exact same point of view, as there was damage to his HVAC system, sump pump control, computer and other equipment in his house.
Fortunately, there was no resulting fire, and TVSS strips on another computer did their job, though they also were “sacrificed” in the process. The large current flow tripped two breakers, which was the initial indicator that something was wrong when they returned home in the evening. In discussing the path of destruction and what he could have done that might have minimized the damage, the idea came up to put lightning rods on his roof.
While the lightning rod does provide a path of less resistance to earth ground, lightning rods also act as a preventive measure. The tip of the lightning rod comes to a point, to help the charge that has built up on the earth and the structure to bleed off into the air, reducing the potential difference between the charged clouds and the structure. For lightning to complete the path, there must be a significant voltage potential, with either the cloud charged positively and the earthbound structure being negatively charged, or vice versa, in order for the voltage to jump through air and complete the path. A step leader comes down from the clouds while another comes up from the earth. When these two meet, the ionized path is complete, and the big bright flash that most people relate to as lightning strike occurs.
Another potential problem in his house was the way that he ran the electrical wiring when he built the house. To save wire and not run each circuit out to the end of an extended garage where the service connection was, he ran them all to a subpanel within the house, and then ran a cable over to the main breakers near the meter. Once the lightning pulse coupled into his electrical system, it came to the point where the subpanel was through one circuit, but then had to go through another length of wire to get to the ground rods.
We must remember that lightning is a high-frequency transient with a tremendous amount of current associated with it. The typical lightning pulse is usually measured in microseconds and kiloamps. The relatively large gauge wire used in a properly designed and installed lightning protection system, as well as the grounded conductor path, may measure in the fractions of an ohm with a multimeter. But that is at low frequencies, not the high frequency of the lightning transient.
Impedance due to the inductance of the wire goes up proportionally to the frequency. Let’s assume that the combination of the resistive and inductive impedance of the ground wire is 0.1 ohm at 0.2 MHz. And then let’s assume that the lightning was a 10kA current. That would push the common grounding point in his subpanel to 1,000V, and send that voltage back through the rest of his circuits. This may well exceed the breakdown voltage of equipment, as it did in his house. In particular, low-voltage transformers with one wire on the secondary side grounded didn’t fare well at all.
It goes back to the “weakest link” adage, or in electrical terms, the path of least resistance. Lightning will seek that out and follow it throughout the facility on its way to Mother Earth. Protection should not just be limited to the power system, but to communication devices and any other equipment that would be in the path. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.