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Site Considerations for Equipment Installation, Grounding, and Wiring Manual
D301452X012
October 2019
For all systems it is best to have all communication equipment input/output grounds tied together.
However additional communication equipment lightning arresters and surge protectors should be
tied to the same system ground. System ground consists of the tower leg grounds, utility ground,
and bulkhead equipment ground stakes that are tied together via bare copper wire.
4.1.3
Ground Propagation
As in any medium, a dynamic pulse, like R.F., takes time to propagate. This propagation time
causes a differential step voltage to exist in time between any two ground rods that are of different
radial distances from the strike. With a ground rod tied to a struck tower, the impulse propagates
its step voltage outwardly from this rod in ever-expanding circles, like a pebble thrown into a pond.
If the equipment house has a separate ground rod and the power company and/or telephone
company grounds are also separate, the dynamic step voltage causes currents to flow to equalize
these separate ground voltages. Then if the coaxial cable (associated with a radio) is the only path
linking the equipment chassis with the tower ground, the surge can destroy circuitry.
4.1.4
Tying it all Together
To prevent this disaster from occurring, you must form a grounding system which interconnects all
grounds together. This equalizes and distributes the surge charge to all grounds and, at the same
time, makes for a lower surge impedance ground system. This interconnection can be done as a
grid (where each ground has a separate line to each other ground) or as a "rat race" ring (which
forms a closed loop – although not necessarily a perfect circle completely surrounding the
equipment house).
By making this interconnection, you must use proper I/O protectors for the equipment (which is a
requirement whether or not you use this grounding technique). Use I/O protectors for power lines
(even those these don't feed into a controller/flow computer), telephone lines, and to minimize
EMI pick-up from a strike. Ideally it is best to place all I/O protectors on a common panel that has a
low inductance path to the ground system. The controller/flow computer would then have a single
ground point from its chassis ground terminal/ground lug to this panel. In lieu of this, the
controller/flow computer in question should be tied to a ground rod that in turn is connected to
the earth/system ground created for the site.
Once connected to a common single ground system, your protected equipment is now like a bird
sitting on a high tension wire. When lightning strikes, even with a 50 ohm surge impedance ground
system, the entire system (equipment, ground system, building, etc.) rises together to the one
million volt peak level (for example) and decays back down together. As long as there is no voltage
differential (provided by protectors and ground interconnections), no current flows through the
equipment and no equipment damage occurs.
4.1.5
Impulse Protection Summary
Use more than one ground rod.
▪
Place multi-ground stakes more than their length apart.
▪
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Tie power, telephone company, tower, bulkhead, and equipment grounds together.
22
Lightning Arresters and Surge Protectors
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