[TowerTalk] braid and high current

[Bill Aycock]

Pardon me, but this is the biggest collection of garbage on this subject 
that I have seen. The braided copper conductors I have seen in explosives 
storage were about 1/2 inch in diameter, and were braided to allow limited 
flexing on installation.  The individual wires were at least #12, and more 
likely (from appearance) #10. Also, Army Ordnance, at least, thought they 
were there for lightning grounding and dissipation, not just to calm the 
fears of the peasants. Next time, get some facts before expounding .
Bill-W4BSG

At 11:17 AM 7/19/2006 -0700, you wrote:

>Hello all,
>
>   The first problem with braid is that the stuff forms a coating of 
> copper oxide around each strand that is nonconductive. This coating makes 
> each wire like a individual conductor with a resistance according to its 
> gage. Now think of the flexing due to vibration, heat cycling, or any 
> other thing which will cause the strands to rub against each other thus 
> breaking the copper oxide surface. The crossover points of the braid will 
> be alternatively be conducting and then reoxidizing to nonconducting 
> state. This makes all the strands at different resistances which when the 
> 20,000 to 200,000 amp surge hits them will cause some of them to take 
> more of the current than others and vaporize. An avalanche effect occurs 
> and you have meltdown and arcs.
>   Second we have the idea that all the current will travel on surface 
> with the skin effect. This is true for normal current amounts but at the 
> current levels of lightning there are not enough free electrons at 
> accommodate the current flow and the conductor depth comes into account. 
> If not enough depth is available the current has to flow outside of the 
> conductor in a arc as there is no where for all those electrons to go.
>
>   The use of braided strapping to protect ammunition was to prevent a 
> static discharge from detonating the charges not the high current of 
> lightning. In the workers minds they thought it was for lightning but all 
> you can do with lightning is shunt the current, not stop flow. Even if 
> you shunt 99% of current there is enough voltage to arc at even micro 
> amps which is enough to make fireworks out of your munitions. The only 
> safe place was inside of conductive box with lid closed. (i.e. ammo 
> boxes, metal lined bunkers, etc.)
>
>   That leaves us with either large stranded (each strand #12 or 
> larger)  or solid conductors. Even solid has enough flex to allow a 
> fold-over to work if you disconnect(or never use ) the ground opposite 
> the hinge.
>
>   Save your braid for your indoor station ground applications.
>
>   73
>   Nick
>   WB7PEK
>
>
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Bill Aycock - W4BSG
Woodville, Alabama