[TowerTalk] Lightening

[jimlux]

On 6/26/17 6:09 PM, Stan K2STN wrote:
> May I ask if the re-bar in those blown tower foundations was as bonded
> together and to the tower?


>
> From what I've read it was the pulse jumping from rod end to rod end
> that concentrated the energy producing steam which did the damage.

properly installed rebar won't have any water containing gaps to turn 
into steam.

There are cases where the rebar sticks out from the side of the 
concrete, below grade, and gradually corrodes, leaving a small gap 
between rebar and concrete.  Once that gap forms, freeze/thaw or 
temperature cycling can pump water in and out of the gap (not to mention 
capillary forces).

Then you have a gap, with water, that might flash to steam, although... 
it's still pretty much a "everything has to be just so" to make it work

A water filled crack in the concrete, if lightning *hits the concrete* 
might cause spalling, the same as if you build a fire on a concrete slab.

But in the design situation - lightning impulse is in a conductor that 
passes into the concrete, and then flows out through the concrete to the 
surrounding soil, I don't think the current density can get high enough. 
  The whole point of a Ufer ground is that the concrete is in continuous 
and intimate contact with the electrode.

One of the *problem* that the Ufer ground (or the "grid ground" used in 
substations) was designed to address was the "smoking/steaming rod" 
where the ground has dried out a bit, and shrunk away from the rod, so 
the contact is not as good, and gets localized.  Then the current 
density goes up, and the temperature at that spot goes up, etc.

The idea of bentonite is that a) it's hygroscopic so it tends to hold 
the moisture and b) it's expansive, so it maintains good contact over 
the length of the rod (or buried cable, etc.)


I suppose you could have a 50 year old not so wonderful concrete chunk 
that is all cracked.  But is that a "proper concrete encased grounding 
electrode" any more than a 50 year old ground rod that is half corroded 
away and wiggles in the soil?

There's lots of really solid concrete that has lasted centuries (if not 
millenia) without cracking.  There's also lots of sort of sketchy 
concrete that crumbles when you walk on it.  The latter could easily 
spall with a lightning impulse.


(Are we getting to a True Scotsman argument?)




>
> Stan
>
>
> On 6/26/2017 4:29 PM, Gary Schafer wrote:
>> It can happen. Several years ago I had my boat docked at a friends
>> place and
>> a palm tree got hit by lightning. The concrete seawall about 10 feet away
>> had a large chunk blown out of it. This was a salt water canal. There was
>> rebar in the seawall.
>> It didn't do any damage to the boat except for the compass being off by
>> about 90 degrees for about a month and it slowly returned to normal.
>>
>> In another life I used to write subcontracts for two way radio tower
>> installations so I saw quite a few towers mounted on and in concrete. In
>> that time I did see a few foundations that cracked due to lightning
>> strikes
>> on the tower. However most if not all of those towers did not have
>> auxiliary
>> ground rods at the base of the tower.
>>
>> 73
>> Gary  K4FMX
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk