> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Nick Pair
> Sent: Wednesday, July 19, 2006 1:18 PM
> To: gm3sek@ifwtech.co.uk; towertalk@contesting.com
> Subject: [TowerTalk] braid and high current
>
> 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.
If the strands touch each other or not that does not change the total
resistance of the cable.
> 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.
This does not change the resistance of the cable. It will not cause some
strands to vaporize.
It can change slightly the inductance of the cable but if there is enough
resistance between strands to cause an arc, the impedance of an arc is quite
low.
> 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.
So we should not use thin copper strap for that reason?
>
> 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.
Large stranded wire has the same susceptibility to corrosion as does braid.
The real reason for avoiding stranded or braid for outside lightning
conductors are the termination points. It is all but impossible to properly
clean and renew an outdoor connection on stranded or braided cable.
The performance difference between all will be very small otherwise.
73
Gary K4FMX
>
> Save your braid for your indoor station ground applications.
>
> 73
> Nick
> WB7PEK
>
>
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