3SB 2

Fuses are electrical protective devices that interrupt electrical circuits if they become overloaded or short-circuited. Generally, fuses operate when a predetermined current level, known as the maximum current rating, is exceeded. Three styles of fuses have become industry standards: SB, Type 2, and Type 3.

SB fuses are used for large applications typically 99A and higher, such as switching contactors or similar mechanisms. SB fuses contain lead alloy fusible elements combined with support and insulating materials in an epoxy base. A ring, or tag, time delay fuse is an SB fuse that operates on a "time-delay" basis so that occasional large inrush currents or momentary overloads can be tolerated. These fuses are used for motor controls, transformers, and other applications where the possibility exists of some near-term overloads.

Type 2 fuse is a ferrule-style fuse of about 30 to 600A, and is designed for general use. An advantage of the ferrule-style fuse is the ease of removing and replacing the fuse element without tools. Type 2 fuses are Single-Use, meaning they are one-time use fuse that must be replaced after an over-current event.

Type 3 fuse, also known as a dual-element or "current-limiting" fuse, are medium-current devices (30A to 600A) are intended for installation near a branch circuit panel, where they provide very fast fault clearing time and extra protection for branch circuits. They are aided in this respect by a low melting point, so that they can interrupt all fault currents up to their melting point in a very short time. As with SB fuses, Type 3 fuses are available with either fusible wire or ribbon fusible elements.

The working principle of fuses is quite simple. When an abnormally high current flows through a circuit, the fusible element reaches the melting point and generates heat that exceeds the limit of the heating insulation material contained within the fuse enclosure. This heat then causes the fuse to "blow" or "trip" and interrupts the current flow. This protects the circuit and its components from being damaged by the abnormal current.

One of the most important aspects of fuses is their ability to quickly react to over-currents and tripping before any major damage can be caused to the electrical system. Although the design and application of fuses vary, each of the three fuse types discussed here have the same basic purpose: to create and maintain a safe environment and protect against damage resulting from electrical overloads.