Neutral Fusing
Another practice that was common (and even required) in some older K&T designs was the installation of separate fuses in both the hot wire and the neutral (return) wire of an electrical circuit. The failure of a neutral fuse would cut off power flow through the affected circuit, but the hot conductor could still remain hot relative to ground, an unexpected and potentially hazardous situation. Because of the presence of a neutral fuse, the neutral conductor could not be relied on to remain near ground potential, and could in fact be at full line potential instead (via transmission of voltage through a switched-on light bulb, for example).
Modern electrical codes generally do not require a neutral fuse. Instead, they explicitly forbid configurations that might break continuity of the neutral conductor, unless all associated hot conductors are also simultaneously disconnected (for example, with ganged or "tied" circuit breakers).
Read more about this topic: Knob And Tube Wiring
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