CBWI February 1997 BULKHEAD GROUNDING FOR TELECOMMUNICATIONS FACILITIES
Most telecommunications facilities in the Land Mobile Services, Broadcast, Amateur and Personal Services are woefully under-grounded. Each year insurance companies pay out thousands in claims for lightning damage to installations that are “fully lightning protected”. When inspecting these facilities it’s easy to see why lightning damage so easily caused, or why the station suffers from high, noise levels and lots of interference.
Proper grounding is almost always the culprit, and it is so often the result of the ground system being installed as a quick , cheap afterthought to radio equipment placement. A really good engineer knows that the first item, designed, constructed, and installed in a telecommunication facility is the ground system. And of the many ways that a grounding system can be employed, one type stands out as most often the best performer. Better yet, it costs less to build, involves less materials, and takes less time to implement.
Bulkhead ground systems are very easy to understand. They consist only of a place and a single metallic fixture. The bulkhead is most often a metal plate made from aluminum but almost any metal will do just fine. A bulkhead could also be a piece of heavy wire several feet long to which station cables and protective devices are attached. It’s not what you have, It’s what you do with what you have.
Most installations consist of an outside antenna with a coaxial line downlead connected to bonded equipment frames, and then a wire running from that point to ground. This has the unfortunate effect of placing the equipment Chassis (and the operator if he is there) in series with an incoming lightning surge, nearly always causing equipment damage and often injuring the operator. A bulkhead grounding system overcomes this deficiency by intercepting the incoming surges and shunting them to ground BEFORE they reach the equipment.
A bulkhead plate is defined as a high integrity, zero or low inductance earth terminal connection. For that reason a bulkhead plate should always be placed very close to or on the ground with its lead connections to earth entry point very short – preferably less than a foot long.
All incoming lines to the facility (coaxial cable, rotor lines, control lines, AC power lines, telephone lines, etc.) pass across the plate where they are connected to various lightning protective devices. The radio equipment should be located close by to take advantage of short distance grounding to the bulkhead plate, thus reducing or preventing harmful local interference in transmission and reception.
No other grounds should be employed or lightning current division may occur if the facility is struck. Division is what causes induced currents to flow across equipment chassis, resulting in circuitry damage.
What makes a good bulkhead? A commercial rely rack panel of 1/8″ thick aluminum is quite nice and easy to obtain through electronic distributors. Copper plate is ideal if available, but even a steel plate is acceptable. Mount antenna switches, transmitting and receiving filters, and lightning protective devices all on the same plate, and be sure to use an anti-oxidant between metal surfaces to ensure good long term metal-to-metal electrical bonding.
Examples of good anti-oxidants are Burndy Penetrox, Ideal Noalox, or I.C.E. Model 601 or 602. Mounting the bulkhead indoors against an exterior wall is suggested so that access may be obtained and protection from weather is assured.
Keep all leads short and connections tight. And try to obtain a plate that is bigger than what you expect to need so that some room is left for facility expansion. Once you find out how nice the bulkhead system works, you’ll probably want to build a bigger station. ©CBWI