Protect Your Site From Lightning

Part 2

Feed Line

by
W.C. Alexander

Fall is an ideal time to inspect and shore up our lightning defenses. Spring may be six months away, but the first wave of thunderstorms may come as early as February — and let’s not forget about those damaging winter static discharges. In the first part of this series, we explored some of the characteristics of a "typical" lightning strike and discussed some of the measures that broadcast engineers can take out at the tower to protect their equipment from damage. This time, we’ll wrap up with a discussion of protecting the equipment inside the transmitter building.

Central Ground

The heart of any effective lightning protection scheme is a central ground system. Some call this a "star" grounding scheme because of the way all the ground conductors return to a central point or reference ground. If the transmitter building is located very near the tower, this ground can be the same as that for the tower itself. In most cases, however, there will be some distance between the tower and transmitter building, and in those instances, another array of ground rods should be provided.

The best place for the array of rods is on the tower side of the building. As at the tower, use four or more rods long enough to reach below the deepest frost line into the water table. The rods should be placed a distance of at least two to three times their length away from one another and should be joined together with bare copper wire at least 1/0 in size. Cad-welding is the preferred method of connection, as mechanical clamps do not provide a joint with low-enough resistance. A cad-welded joint will not oxidize or corrode as a mechanical clamp junction is prone to do.

All conductors operating at ground potential that enter or leave the transmitter building should be bonded to this ground array. That includes the outer conductors of all transmission lines. The method for connecting a ground to a transmission line outer conductor was discussed in the last part of this series.

A conductor from the ground rod array should be brought into the transmitter building via the shortest and straightest route possible. The point where it enters the building becomes the center of the "star", or the point to which everything in the building is grounded. We will call this the "station reference ground". All grounds in the building, including the safety ground of the electrical system (service entrance ground) and the ground conductors from all the equipment and outlets, then connect to this point. Figure 1 is a diagram of a properly-designed station grounding scheme.

I prefer to disconnect the service entrance ground from the rod installed by the electricians and connect it to the station reference ground. Check your local electrical code before you do this. You may have to plan the layout of your ground rod array so that one of the rods is driven immediately adjacent to the service entrance to accommodate such a connection. It is worth doing, however, whatever lengths you have to go to. Having a separate rod connected can spell t-r-o-u-b-l-e, as a huge potential can develop between the station reference ground and a separate rod outside the system.

If you have a ground strap or terminus of the ground system coming to the transmitter building from the tower(s), be sure to connect it to the station reference ground. If there is no such strap coming from the tower(s) you do not specifically need one, but an advantageous location for a transmitter building is often at the end of the ground system at the end of the transverse ground strap. If this is the case, that strap may have been extended to connect inside the building.

When connecting transmitters, racks and other equipment to the reference ground, it is important to do so in such a way that lightning currents will not flow through the equipment cabinets enroute to ground. On a transmitter, for example, make the ground connection as close to the RF output connection as possible. In that way, lightning currents coming in on the transmission line outer conductor can flow through the short copper path to the ground conductor and not through the metal of the cabinet. Remember that such currents create a strong magnetic field that will induce currents into nearby unshielded conductors. By keeping surge currents out of the cabinet steel, you can keep them out of your transmitter’s wiring harness as well.

Wye Connection

If your site has three-phase power, when it comes to lightning protection, it is hard to beat a "wye" secondary on your utility power feed. This type of connection has several advantages, the most important of which are that every leg is referenced to ground (balanced with respect to ground) and the lower voltage (208) is easier to clamp in surge conditions.

Unless you specify a 208-volt wye, the utility company will probably provide you with a delta. Worse, they will probably save themselves a transformer and give you an open delta, which is terrible from a lightning protection standpoint.

Most all broadcast transmitters will operate just fine on anything over 200 volts, so switching to 208 volts will pose no problems. A change of taps and you’re all set. The thing to watch for is the increase in current. Service conductors and disconnects sized for 240 volt operation may be too small for use at 208 volts, so be careful, lest you create a fire hazard.

Surge Suppressor

A good surge suppressor is the only way to minimize lightning transients on the incoming utility power. These devices range from inexpensive "kamikaze" devices that work one time and have to be replaced, to very expensive series/shunt devices. Somewhere in between is an economical device that will adequately protect your equipment without breaking the bank.

The metal-oxide varistor (MOV) is at the heart of most shunt-type surge suppressors. These devices conduct when the potential across them exceed a threshold voltage. The devices must be rated to carry most of the anticipated lightning current. This may seem like an impossible specification, but the device only has to carry the current for a very short period of time.

Modern surge suppressors are available with fused MOVs in many voltage ratings that will hold up well under typical lightning surge conditions, clamping the AC line to ground during the surge and thus protecting equipment downstream. The fuses are designed to act slowly, holding their state for the short duration of the surge but blowing if the MOV becomes shorted as a result of excess current. The affected MOVs and fuses can then be replaced and the effectiveness of the surge suppressor restored.

Be sure to install the surge suppressor downstream of the main fused disconnect at the site. The ground connection from the surge suppressor must connect to the station reference ground. All the conductors to the surge suppressor must be relatively large, as the instantaneous currents that they will be called upon to carry can be substantial.

When it comes to surge suppression, buy all you can afford. If your budget can sustain a $10,000 series-shunt type, go for it. If you can only afford the little "kamikaze" cans, buy and install them. Any working surge suppression is better than none. In practical terms, the insurance deductibles and premium increases you will save may well pay for one of the more expensive units in just a few years.

Toroids

The final step in creating an effective lightning protection scheme is to build a low-pass filter into all your power, control and monitor cables. This is easily done by placing a toroid core over the conductors. This effectively forms an RF choke that is a very high impedance to fast rise-time lightning energy. Such cores are available from most mail-order electronic parts houses, and they come in a variety of sizes.

One such core should be placed over each of the cables entering a transmitter cabinet or rack. Run all the AC power wires through a single toroid. Pass the remote control cable through a core, and do the same with any small coaxial feeds (RF drive, mod monitor sample, etc.). Finally, for transmission lines up to and including 1_", install one or more cores on the cable just above the connector.

Larger, rigid transmission lines should be installed so that they form a "trombone" section, making at least three 90-degree turns before connecting to the transmitter. The 90-degree bends also present a high impedance to lightning energy.

Think Like Lightning

When giving your site a lightning protection checkup, you’ve got to think like a lightning surge. If you were a surge coming in on the transmission line, where would you go? What is more attractive, the short path to ground outside the building, or the path through the transmitter cabinet to the utility ground? This may sound ridiculous, but if you will analyze each and every possible lightning current path at your site in this way, you will begin to uncover the weak spots. Then you can deal with them and harden your site against lightning damage.

Although the focus of this series of articles has been on transmitter site lightning protection, the same principles can be applied at studio and other locations as well. There is no substitute for good surge suppression on the incoming studio AC power feed, and watch those transmission lines coming in from the STL tower. The studio is a place where multiple grounds can easily exist, especially if the building has been expanded over the years, so pay close attention to this.

There is no way to completely lightning-proof a site, but you can take some effective steps to prevent damage from all but the most severe strikes. Apply these principles and you will have a much happier, more restful storm season.