[TowerTalk] Grounding

[Jim Lux]

On 11/17/12 4:40 AM, K1TTT wrote:
>> Dale - WD4IFR
>> In any lightning strike, even a 2" solid piece of copper takes on the
>> characteristics of a light bulb filament.
>
> It is simple statements like this that make the rest of what you say
> questionable.  The fact is that MOST lightning strokes hitting even small
> conductors will not significantly heat them.  To get any significant warmth
> in 2" copper is probably even beyond even the largest measured mega strokes.

I agree... take a look at the Preece or Onderdonk equations for fusing 
current.  Onderdonk takes into account the pulse length.

>> BTW -- You do not have to use exothermic welds exclusively for your
>> ground connections.  One of the companies that make crimp on grounding
>> connectors is "Burndy".  These clamps are designed to get away from
>> Cadwelding but still require the use of a 12 ton crimper to bond the
>
> I have never cadwelded a ground connection, most commercial electricians I
> know would probably look at you with complete confusion if you asked them to
> do that.  The basic brass clamp sold in every hardware store is what
> probably 99% of the ground connections in this country are connected with is
> just fine.  After all a discharge that has just jumped a mile to the ground
> isn't going to be stopped by a few microns of oxide in a connection between
> two conductors.  Even lightning rod installers use easily hand crimped
> splices and connectors.


the need for exothermic welding or good clamps isn't for lightning.  As 
'TTT says, a few thousandths of an inch gap or oxide isn't going to make 
any difference.

The good quality bond is for more mundane electrical safety reasons, 
when the 110V wire shorts to the metal case, you want the circuit 
breaker to trip, or, at least, the voltage on the case relative to your 
bare feet to be limited.








>
>
> Oh, one more while I'm at it:
>
>> copper braid that are cadwelded (exothermic) to the tower legs.  From
>
> Braid?? Cadwelded??  I always thought this was a no-no.  and who runs braid
> for a lightning or safety ground?  Don't all codes specify solid conductors?

You can use stranded in larger sizes. There's a lot of 2/0 stranded used 
for this kind of application, for instance.  (because handling solid 2/0 
is hard work?)

I'm too lazy to go get my code book and check, but there IS some 
threshold size for it.

I wouldn't use welding cable (zillions of tiny strands, so it's real 
flexible).  I had bad luck using welding cable for pulsed power 
applications, although I never spent much time figuring out why.

> I wouldn't trust braid for an outdoor application like that anywhere, unless
> maybe you are talking about braid that uses something  like 10ga strands.


AWG 2/0 stranded...19 strands of something like 16.
>