Under what condition are fuses and circuit breakers permitted to be connected in parallel?

Fuses and circuit breakers are permitted to be connected in parallel when they are factory assembled in parallel and listed as a unit. This is crucial because it ensures that the devices are designed to work together safely and effectively. When electrical protection devices are created as a unit by the manufacturer, they undergo testing to confirm that their performance is reliable when operating simultaneously in a parallel configuration.

Using factory-assembled units guarantees compliance with safety standards and ensures consistent trip characteristics, which is essential for preventing overloads and short circuits. This parallel configuration allows them to share the load while ensuring that each component can independently provide protection if necessary. Other arrangements, such as connecting different models or types of devices in parallel, can lead to imbalances in load sharing and reduce the overall effectiveness of the protection system. Therefore, adhering to the requirement of factory assembly and unit listing is key to maintaining electrical safety and system integrity.