An Interesting Effect…. How can an HVAC system kill a PA system?

By Jim Sorensen

A severe electrical storm recently took out 3 churches PA system. The caused was their HVAC systems.  “How can an HVAC system kill a PA system?”

We recently had a very severe electrical storm here in Boca Raton with the result that a number of installations suffered from “lightning interruptus.” In the case of at least three churches in town the problem was ultimately caused by the HVAC systems.
Huh? How can an HVAC system kill a PA system?
The source of the issue goes back a few years. Back when these churches were built, they had utility transformers that were sized properly for their consumption and were located sensibly for their wiring plans. Then sometime in the 70’s or 80’s these churches either installed HVAC systems or added to their undersized HVAC systems. This results in the need for more power and that resulted in the need to add a transformer or “drop” for the new equipment’s demand.
Interestingly, in all the cases I saw there was one utility transformer on one end of the property and another on the other end. These were often interconnected in interesting ways, sometimes on purpose and sometimes by accident.
No, they weren’t “connected” to each other but there were phantom circuits created, especially in the ground circuits, that overlapped.
My home church had a set of three-phase “pole pigs” struck by lightning which energized the ground circuit on this set which then pulsed the ground circuit of the other transformer which is about 600 feet away. What’s more amusing is that when the utility company installed these transformers, they energized one from a high-tension line running from the west, with the other drawing power from a different line, from the east.
Now the new transformer powers two newer buildings which frees power from the old transformer set to run additional HVAC for the sanctuary and some of the older rooms. The power side is obviously separate…isolated from each other…but the grounds aren’t.
You can see, I hope, from this description that the lightning energy tried to find ground through an interesting maze of utility, AV, and computer wiring. Obviously the lightning will try for the lowest impedance path and in some cases this “path” might have mutiple legs.
Anyway, the result is thus: Lightning followed by…”zzzzsssstt” followed by…bad smells.
Worse is the fact that the computer network runs all over the place making a good deal of wiring common to both systems since the computer system simply plugs into whatever outlet is the closest without respect for which utility source it comes from.
The PA system, which is both dirt-simple and automatic (Thank you, Lord!) is only on one phase of the old three phase but it, too still references to that toxic ground. While it works very nicely it also suffered considerable damage from the lightning strike.
Most of the churches I look at were wired essentially the same way, with the multi-utility sources, common ground, and computer and ancillary wiring piled onto what are perhaps two wildly disparate ground potentials…when the lightning flies around.
Keep it out of the red!