[TowerTalk] Sparky Pays a Visit

[Jim Lux]

At 11:37 AM 8/3/2005, Jim Brown wrote:
>On Wed, 03 Aug 2005 11:28:18 -0700, Jim Lux wrote:
>
> >They can add things to the concrete mix to
> >enhance the conductivity as well (used in, for instance, electric railroad
> >installations).
>
>In some installations, concrete very LOW conductivitity is desired. 
>Examples are
>electric railways and sidewalk heaters, where the concrete may serve as an
>insulator.
>
>Jim
Indeed. Interesting that railroads need both high and low conductivity. As 
I recall the high conductivity was to reduce step potentials in fault 
conditions.  Obviously, if you have concrete ties, etc., you want low 
conductivity for those.

Anyway, you can change the conductivity of concrete substantially with 
admixtures of one sort or another.  The run of the mill structural concrete 
used in foundations, etc., apparently has a high enough conductivity for 
most applications.

BTW, there's several discussion on the web about the impact of the vapor 
barrier (usually a thin sheet of plastic) on the grounding effectiveness 
for Ufer grounds.

I think it probably makes some difference for DC or low frequency AC (line 
frequency).  For transients, the dielectric is so thin that the capacitive 
reactance will be so low that it's not even there.

Example: slab that's 10x10 meters (about 1000 sq ft) on a 4 mil (0.1 mm) 
polyethylene sheet would have a C of about 8.85 uF.  At 1 MHz, that's about 
0.02 ohms.  Lightning rise time is on the order of 2 microseconds, so 
you're still in the sub 1 ohm reactance, even with the plastic sheet.




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