Fuse Basics: Time-Delay & Common Types Explained
Choose the right protection with an overview of time-delay fuses and popular types of fuses like RK5, RK1, and Class J.
A time-delay fuse is designed to withstand temporary current surges without opening, making it ideal for motor branch circuits and applications where inrush current is common. Unlike standard fuses, a time-delay fuse allows harmless start-up surges while still providing:
Overload protection (sized closer to motor full-load current).
Better short-circuit protection than standard fuses.
The ability to use a smaller disconnect switch in some applications.
Time-delay fuses are the most popular choice for motor protection because they balance both short-circuit safety and tolerance for inrush currents.
Types of Fuses
There are many types of fuses used in electrical and industrial applications, each designed for specific protection requirements. In this guide, we focus on three of the most common and widely used in motor branch circuit protection: UL Class RK5, RK1, and J (LPJ/AJT).
UL Class RK5 (Fusetron™ / Tri-onic®)
- Most economical option for general applications.
- Provides adequate protection for most circuits.
- Dimensionally interchangeable with RK1.
UL Class RK1 (Low-Peak™ / Amp-Trap®)
- Designed for applications requiring a higher degree of current limitation.
- Offers better component protection and system coordination.
- Interchangeable with RK5.
UL Class J (LPJ / AJT)
- Introduced in 1983 and has since become an increasingly popular choice.
- A time-delay fuse with higher current limitation than RK1.
- Roughly half the physical size of RK5 and RK1 fuses.
- Particularly well-suited for IEC-style motor starters and contactors, which are smaller and have lower withstand capabilities than NEMA starters.
- Best choice for IEC protection when sized at 1.25–1.50 times motor full load amps.