RTcom Radio Modems - Manual (Page 8)

8
Lightning Surge Protection:
A direct lightning strike can never be totally protected against or predicted. Currents exceeding
10,000A can flow vaporising antenna, feeders, towers and other such structures. Lightning conductors
will give a degree of protection to the building but not always to the electronic apparatus within.
Generally the probability of a direct strike is very small, but a nearby strike with for example a 1km
radius can be quite a regular occurrence in many locations. Nearby strikes or "strokes" can lead to the
creation of large EM waves that can induce large voltages into antenna, feeders, signal wires and
power supplies.
The best form of protection is to use a surge arrestor. The arrestor is connected in series with the
antenna and the modem and is intended to safely limit the induced voltage. However a surge arrestor
can only be effective the impedance of the cable connecting it to ground is lower or equal to that of the
modem and the other connected apparatus and secondly they are all connected to a common Earth
point.
Note: Failure to connect the data terminal to the same Earth point compromises the protection of the
apparatus. If however is not practical to implement, then a second surge protection device should be
connected in series with the data and/or power connections, again bonded to the common Earth point.
RTcom
Modem
Surge Arrestor
Data
Terminal
Coaxial feeder
Antenna feeder
Data cable
Antenna
System Ground
PSU