Re: British electrification - voltage change over points

yyOn 24 Aug 2005 12:58:48 -0700,
billetelic_ferroequinologist@xxxxxxxxxxx wrote:

>
>3. Third rail DC systems (and overhead DC systems such as tramways) are
>NOT directly earthed. If they were, the traction return current would
>leave the running rails (causing electrolytic corrosion to the rail at
>the point where it did so) and then try to find some buried conductive
>material such as a pipe to flow along back to its substation, thence
>causing more corrosion when it leaves the conductor to flow back to the
>substation. If you can get near a Southern substation (without
>trespassing, of course), you will see the negative busbar is raised on
>insulators and has no direct connection to earth. On the Tyne and Wear
>Metro, the OHLE structures are mounted on insulated bases and then
>bonded to the running rails for the same reason.
>
The power supplies are (with rare exceptions) earthed on one pole. The
presence of insulation on the 0v line does not mean that a supply is
not earthed, the most common example being domestic mains supplies (a
"neutral" wire would not be "neutral" if it wasn't earthed at some
point). The absence of an earthed pole is one thing which will
increase the danger of persistent and/or dangerous stray currents due
to the uncertainty of the route that such currents will take. Reasons
for insulating the 0v/neutral conductor include:-
-under fault conditions (e.g. a break close to the power source) a
neutral conductor can itself be live - prevention of lethal contact
requires either insulation or local bonding to earth
-if it is not separated from surrounding conductive materials it can
itself become part of the return path for stray currents

Power supplies running with neither pole earthed are in practice
treated as if all wires are at the maximum available voltage which is
clearly not the case with railway and tramway supplies where for
electrical purposes at least one rail will have little or no effective
insulation from the ground.

LU power supplies (not NR supplies used by LU trains) are a special
case - neither conductor rail is directly earthed but the supply at
the substation is loosely tied to earth via a resistor network and
monitoring circuits are used to detect either pole becoming earthed on
the load side.
--
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+---------------------------------------------------+ |\\ //|
| Charles Ellson: charles@xxxxxxxxxxxxxxxxxx | | \\ // |
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Alba gu brath |//___\\|
.

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