This piece was originally published in the October 2018 issue of electroindustry.

Nick Offerman, Field Application Engineering Manager, Eaton’s Bussmann Division

Fuseology is defined as the study of a fuse’s operating principles and its unique role in circuit protection. These principles include ratings, operating characteristics, and constructions that enable the fuse to act as a highly effective overcurrent protective device (OCPD).

Reliable OCPDs help prevent and minimize costly equipment damage while enhancing electrical safety and avoiding downtime. Many industry professionals rarely encounter the basics of reliable overcurrent protection during their courses of study, but it’s important to be aware of the three key principles for applying OCPDs.

These principles, referred to as Friemel’s Laws of Overcurrent Protection, provide a solid foundation for industry professionals to leverage regardless of National Electrical Code^® (NEC) changes. While these principles are not all-inclusive of the considerations in applying OCPDs, an understanding of these three key concepts will lead to a safer, more reliable, and code-compliant electrical system:

• Friemel’s first law—interrupting rating: OCPDs shall be applied with an interrupting rating equal to or greater than the maximum available fault current.
• Friemel’s second law—component protection: OCPDs shall be selected and installed to clear a fault without extensive damage to electrical equipment and components.
• Friemel’s third law—selective coordination: A properly engineered and installed electrical system will restrict outages to only the nearest upstream OCPD for the full range of overcurrents and associated opening times, leaving the remainder of the system undisturbed and preserving service continuity.

These laws encompass the unique operating principles of modern-day current-limiting fuses, including high interrupting ratings to safely open very high fault currents; current limitation to “limit” fault currents to low values for optimum component and equipment protection; and simple selective coordination for the full range of overcurrents to prevent blackouts caused by upstream OCPDs cascading open when applied with the correct ampere rating ratios.