Re: pole pig question (fwd)

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Original poster: List moderator <mod1@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Mon, 21 May 2007 11:34:35 -0400
From: Jonathon Reinhart <jonathon.reinhart@xxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: pole pig question (fwd)

How is that possible? There is a neutral/grounded wire that runs along with
the three phase wires from pole to pole.  It is the neutral conductor that
carries the return current back to the substation transformer.  That wire
running down the pole is one of many hundreds of similar wires that run down
(just about) every other pole that the wires travel along.  It simply keeps
the grounded neutral conductor at ground potential (~0V).  If it were to
break, there shouldn't be much problem, as the current is still traveling
down that neutral conductor back to the substation.

Grounded circuits, such as the 120/240V in your house, and the 7.2/14.4kV
systems in your neighborhood do not use the earth as a return path.  In a
fault condition in the wiring in your home (say a hot conductor comes lose
and touches a grounded metal box in the wall), that current is NOT seeking
to return to "ground", meaning the earth.  This is a common misconception
due to the fact that the third wire ground system is connect to an earth
grounding rod. However, the third wire ground is bonded to the neutral
conductor in your panel box (in many installations, they simply use the same
terminal bar).  Thus, the fault current returns to the transformer then via
the grounded neutral conductor.  The reason this point on the system is
grounded is again, to keep the whole system's "ground" voltage level at the
same voltage level we are in constant contact with ( ~0V, the earth).  If
your house's ground/neutral were allowed to float at any 0V (relative) it
wishes, the fault system would still work. However, a "grounded" metal box,
could be considerably higher potential than the earth, and a human could
receive a shock.

Imagine if there were no neutral conductor running along the poles. How
would you ever get electricity?  The current would have to travel
tens/hundreds of miles along the utility lines to your transformer. Then how
would the current return to the substation? Through the earth? Imagine the
voltage drop over miles and miles of earth!  Does anyone have any rough
guesses to the ohms/foot of earth at 8' depth? It would not work.

It is a terrible thing that happened to that poor kid, but something else
had to have gone wrong. Perhaps the connection to the neutral line up on the
pole was weak at some point, and there was fault current traveling to earth.

Jonathon Reinhart




> That innocuous looking wire that runs down the pole into the ground (it
> often ends in a spiral coil stapled to the bottom end of the utility
> pole) plays a critical function.  A teenager around here was killed a
> few years ago when he scraped a utility pole with a lawn mower, breaking
> the down wire, and essentially running 14.4 KV into the mower deck.
>
>
>