[time-nuts] Lightning arrestors for GPSDO antenna

Jim Lux jimlux at earthlink.net
Fri Oct 17 12:11:31 EDT 2014

On 10/17/14, 8:17 AM, Chris Albertson wrote:
>> You can use metal conduit as the bonding conductor between grounding
>> systems, for one thing.
> That works fine, but I think it is disallowed  by the electrical code.   If
> you used metallic conduit it MUST be grounded but you can't use it for
> grounding.  That said, it does work.   I think the danger the electric code
> addresses is that connections between conduit sections become loose over
> time and might corrode.

The metallic raceway (code speak for conduit) is allowed to be the 
bonding conductor  (bonding conductor = "greenwire" or "electrical 
safety" ground in code speak).  Properly installed conduit will have a 
good connection, etc..

When interconnecting multiple grounding electrodes or electrode systems 
is where the requirements for particular gauges of wire come in, and 
mostly it has to do with mechanical strength and reliability.  You can 
use a smaller conductor if it is protected inside something, for 
instance. The other rule is that the bonding conductor has to be 
continuous (the concern you mentioned about connections becoming loose, 


is a very nice summary

Mike Holt (http://www.mikeholt.com/) has a great website on all code 
related issues, and he's written a bunch of articles that explain the 
code and the rationale behind the requirements.

And when it comes to antennas and the like, you're in a different 
section of the code 810, 820, and the requirements for the grounding 
conductor (and whether coax shield can be that grounding conductor) are 
all laid out there.

In many case, the coax shield can serve as the grounding conductor, but 
only if there are no connectors in the path (i.e. you have to have a 
clamp that directly contacts the shield where it interconnects with the 
building grounding system).  A barrel feedthrough in a grounded metal 
panel doesn't meet the strict requirements of the code (although 
personally, I think it's a fine solution)

One thing to remember about the NEC requirements is that the "threat" 
they are protecting against with the grounding and bonding requirements 
is NOT a lightning strike.  It's contact with an energized conductor 
(e.g. a power line touches your antenna or supporting structure). 
That's a whole lot more common (wind storms, etc.)  NFPA 780 is the 
lightning protection code, and has a lot more "lightning protection" 

The NEC cares almost nothing about transient protection, the concern is 
more about electrical shocks and burning the building down. 
Furthermore, the NEC really only regulates the wiring in your building, 
and nothing that is connected to it, nor does it regulate the wiring of 
the power company.

There are two tomes of reference I use for transient protection: one is 
IEEE 1100 (the Emerald Book) which has gone under many names over the 
years (politics.. computer manufacturers did not want their equipment 
described as "sensitive electronic equipment")

The other is "Protection of Electronic Circuits from Overvoltages" by 
R.B. Standler.

And, if you're at the Dover Pubs store.. take a look at the books about 
lightning from Martin Uman.  Very readable, lots of technical info.

> I think the threaded conduit would work fine.  That stuff is like water
> pipe but smoother inside.

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