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
>In some installations, concrete very LOW conductivitity is desired.
>electric railways and sidewalk heaters, where the concrete may serve as an
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
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.
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
>any questions and ask for Sherman, W2FLA.
>TowerTalk mailing list
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
TowerTalk mailing list