On Sun, 30 Nov 2008 12:34:42 -0500, Christopher A. Lee
wrote:

On Sun, 30 Nov 2008 15:08:16 +0000, Graham Murray
wrote:

David Hansen writes:

A dangerous impression. If there was no (electrical) danger from the
central conductor rail then it would not be mounted on insulators.

Insulators are a give-away that something is energised.

Does that necessarily follow? If the reason for using a 4-rail system
rather than a 3-rail with return via the running rails were to avoid the
problems such as electrolysis and interaction with signalling, would it
not be possible to do it by having the centre insulated rail at a
nominal ground potential but only bonding it to ground at the
substations?

There will always be leaks due to build up of dirt.

And it would assist the occurence of faults where the return path
breaks and the conductor on the load side achieves a good connection
via the wrong path (think RCD).