[TowerTalk] grounding (again)

Jeff DePolo jd0 at broadsci.com
Mon Apr 23 11:43:18 EDT 2018

> I'm using the formula from Rosa..That's quite a ways different.. what 
> formula are you using?

Grover.  I don't know how well this is going to come out in email without me making a typo, but...

Straight wire: Inductance = 2L * ln( (2L/D) * ( 1 + sqrt( 1 + (D/2L)^2 ) ) − sqrt( 1 + (D/2L)^2 ) + u/4 + (D/2L) )

Rectangle: Inductance = ( ln( 2L / (A+B) ) - ( 0.2235 ln( (A+B)/L ) + 1/2 ) ) * ur * L / (2 * pi) )

L = length
D = diameter
A,B = width/thickness
all in cm

For copper and other common non-ferrous conductors you can mostly ignore the terms involving permeability (i.e. set u to 0 and ur to 1).  Note that simplified inductance formulas are approximations only, and there will be differences in the results which, based on what I've read, tend to be most divergent at shorter lengths.

The inductance is also going to be affected by proximity to other objects.  In the instant case, with multiple conductors going through the same conduit, all bets are off as to the accuracy of any of these values which are based on the conductor being in free space.  

Numbers aside, my original point was that it wasn't until you got up to a very wide strap does it make a big difference in inductance.  When we build telecom and broadcast facilities, all of the underground bonding is done with #2 solid wire.  #2 stranded is the primary bonding conductor used internally between the master ground bar and equipment, as well as for the halo ground.  If the bonding conductor needs to be run across the floor to reach equipment (as is often the case in broadcast facilities with high ceilings) then we use strap.  I think we're in agreement that the use of small-width copper strap doesn't afford any improvement in terms of protection, and due to the higher cost, really isn't worth it unless it is needed for some particular physical/mechanical constrait such as passing through a slim opening.  In the instant case, with both the cables and bonding conductor in the same 2" I.D. conduit, it doesn't make sense to me to use strap.

				--- Jeff WN3A

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