basicbill said:
I'm not aware of the need for a third fuse.
Putting fuses in the neutral was done away with in the early 1900s. I believe it is closely related to the expanding uniform practice of bonding the neutral to other conductive systems (water pipe, steel, etc.) and to earth, as the local voltage stabilization problems of AC transmitted over long distance was studied and understood.
The shared, or common, neutral of the standard 240 / 120 Volt single phase AC service, has its origin in DC. This multiwire circuit was, and still is, called an Edison Circuit. Thomas Edison's power distribution scheme required lots of small 240 /120 V generators located no more than a quarter mile from the most distant load. DC transformers were not economic or practical.
Until 1900, it was common for the generators to be supported on beds that were monitored to maintain isolation from earth or other conductive systems. Whether to ground the generator, or not, was a design choice.
The early National Electric Codes were written including rules for ungrounded generator frames.
For an ungrounded generator customer, if a neutral conductor on a branch circuit shorted to ground, and there were other shorts of hots (from the same generator) to ground elsewhere in the system, the greatest current would occur in the neutral, the fuse would clear, and trouble would be indicated, and, presumably, the electrician would be brought in.
From the perspective we have today, it's hard to imagine life in a world where electricity was available only in small areas (think of an urban area 5 or 6 blocks in diameter) and that was on for only part of every 24 hours. And the last big energy shake up that the "old timers" would talk about was when lighting with whale oil gave way to the new fangled petroleum. And now they're pushing this electric "smokeless" light that I can't light whenever I want? ? ?
