[TowerTalk] Lightning Protection Methods

[Jim Brown]

On Thu, 29 Apr 2010 12:11:10 -0400, Dick Green WC1M wrote:

>While I'm a big believer in suppressors, the units on the market are
>designed for coax connections to radios and rotor cable connections to
>simple rotor controllers. But I don't believe that MOV-based devices are
>adequate for some of today's new solid-state devices like SteppIR
>controllers, advanced rotor controllers and advanced stack controllers.

Dick,

Thanks for your detailed account. As to MOVs and other similar protection 
devices. These are SHUNT MODE devices -- that is, they operate by 
attempting to short out the lightning event, usually to "ground." In 
addition to any issues with the protectors themselves, that "ground" itself 
is a major part of the lightning damage problem. 

The reasons are simple. First, lighting is not DC, it is a monster IMPULSE, 
and any impulse has infinite harmonics. IEEE working groups have shown that 
the ENERGY in lightning is broadly centered around 1 MHz, with strong 
components several octaves (2:1 frequency ratio) above and below. In other 
words, it's 100 kHz to 10 MHz.  

Second, "ground" is not a single point, nor is the earth an ideal 
conductor, and our connections to it have both resistance and inductance, 
at at all frequencies above about 10kHz, inductance dominates the 
impedance. 

Third, equipment is interconnected. So we have "grounded" towers, 
"grounded" shacks, "grounded" telephone and CATV wires, and "grounded" 
power entry, usually "grounded" at different points. These points SHOULD be 
bonded together (short fat wires), but often they are not. So when a 
lightning event hits, current tries to go to the earth in all of them, and 
thanks to their impedance (mostly inductance), the voltage at every piece 
of equipment rises to a DIFFERENT HIGH voltage. Another reason for the 
DIFFERENCE is that any given strike couples more into one circuit than 
another based on proximity. 

Thus -- the single most important element of lightning protection is the 
bonding together of all of their equipment grounds. The second most 
important element is to avoid shunt mode protection and use SERIES mode  
when possible. Series mode suppressors are only made for AC power. The only 
mfrs I know of are SurgeX and Brick Wall. They aren't cheap, but those of 
us working in pro audio learned about them at least 15 years ago and use 
them extensively. 

When we MUST use shunt mode protectors (rotor controllers, SteppIR, coax), 
it is CRUCIAL that they be bonded to a very low impedance ground, AND that 
ground is bonded to all other grounds in your home, AND to all of the 
interconnected gear. 

A major exception to the advice against shunt mode protection is at the 
power service entrance -- whole house protectors use big shunt mode 
devices, and they ARE a good thing IF properly installed with good bonding 
of all grounds.  

73,

Jim Brown K9YC