On 11/26/14, 2:14 PM, Chris Albertson wrote:
You CAN (almost) lightening proof your system. The trick is to give
lightening a low impedence path to grind at very opportunity.
Start with the antenna mast and call. Use iron pipe for the mast and feed
the antenna cable down the center of the pipe. Place two large ground
clamps on this pipe and connect a large diameter wire that takes a straight
path to a group rod. This will go a long way to diverting energy to
ground because high voltage likes to flow on the outside of a conductor
which would be the pipe and not so much the antenna cable.
Not so much high voltage, as AC and skin effect. However, bear in mind
that lightning has a rise time of a microsecond or so: you can think of
it having a fundamental of 300-500 kHz (e.g. the first quarter cycle of
a sinewave), with most of the power below 1MHz.
Skin depth at 1 MHz in copper is 0.065mm.
In iron (using conductivty of 9.6 and relative mu of 1000) skin depth is
0.005 mm
So, steel/iron pipe is a terrible conductor for a lightning impulse,
compared to that nice copper coax next to it, or inside it.
The ground rod needs to be bonded to the rest of the building ground system.
Then the antenna cable passes through a metal bulkhead with a bulkhead
connector and all this is also grounded. After this is might be a high
voltage e on the center conductor. Use an "lightening arrester that is
bolted to the bulkhead.
From a electrical code standpoint, a grounded bulkhead connector isn't
compliant: you need one of those clamps that attaches to the shield in a
quasi permanent way. I'm not sure of the entire rationale, but I think
it's because connectors can become disconnected, but bolted connections
less so.
At this point you are reasonably safe. Remember that Ethernet is always
gavalically isolated by transformers
Which won't necessarily stand off a 10 kV lightning impulse. and, of
course, a common mode impulse carried on both wires of a pair might
couple via either capacitance, or more likely, through magnetic fields.
the ethernet galvanic isolation does a nice job dealing with the 10s or
maybe 100V common mode issues, and protects the network if there is an
internal short in a piece of equipment connected to the network.
Of course, with the increased prevalence of Power Over Ethernet, some
implementations of which are, shall we say, sketchy, that PoE system
might be a dandy conductor of transient energy. For instance, a point
to point microwave network terminal up on a mast running power for the
circuit up the network cable.
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