[time-nuts] lightening protection of a GPSDO system / optical isolated distribution amp

Jim Lux jimlux at earthlink.net
Wed Nov 26 19:03:54 EST 2014

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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