Under what condition may knob and tube wiring be considered acceptable?

Knob and tube wiring may be considered acceptable under the condition that it is not covered by insulation. This is crucial because knob and tube wiring was designed to operate without the added heat retention that insulation can provide. When this type of wiring is insulated, it can lead to overheating, which poses a significant fire hazard. Ensuring that the wiring is exposed allows for proper air circulation, reducing the risk of overheating and allowing the system to function safely within its intended parameters.

The other conditions mentioned do not specifically address the inherent risks associated with knob and tube wiring. For instance, while a proper circuit breaker system may ensure some level of protection, it does not mitigate the risks associated with inadequate heat dissipation due to insulation. Similarly, keeping the fuse system in place doesn't resolve the fundamental safety concerns linked to covered wiring. Lastly, the presence of fiberglass insulation does not allow for the same airflow characteristics that keep knob and tube wiring safe. Thus, the key factor for the safe use of knob and tube wiring is that it remains uninsulated, allowing it to operate within safe temperature limits.