From EC&M's Electrical Calculations Handbook

Overcurrent Devices: Fuses and Circuit Breakers

A tremendous amount of electrical energy is available in almost every electrical power system, so every part of an electrical installation must be protected from excessive current flow. Excessive current flow can be considered in two distinct categories:

  1. Instantaneous current from inrush on start-up or from a short circuit

  2. Long-time overload current

Overcurrent devices are available in several forms. At low voltage, the most common forms are

  1. Non-time-delay fuse

  2. Time-delay dual-element fuse

  3. Magnetic-only, instantaneous-trip circuit breaker

  4. Thermal-magnetic-trip circuit breaker

Figure 12-1 shows the time-current characteristics of the most common of these types of overcurrent devices for a standard 20-ampere (A) device. Note that for instantaneousonly protection, a magnetic-only circuit breaker unlatches and trips (opens the power circuit) immediately on reaching the preset ampere value, as does the thermal-magnetic-trip circuit breaker. However, the instantaneous-trip setting on a thermal-magnetic-trip circuit breaker is normally set at a higher ampere rating than would be a magnetic-only breaker because the thermal element of the thermal-magnetic-trip circuit breaker adequately provides protection within the ampere range of maximum safe operating current. The thermal-magnetic-trip breaker curve and the curve of the time-delay fuse are very similar to each other because the thermal-magnetic-trip breaker curve is designed to mimic the curve of the time-delay fuse.

Figure 12-1: Time-current characteristic curves of typical 20-A overcurrent devices.


A fuse heats internally due to I 2 R heating, and after enough heat builds up, the thermal element in the fuse simply melts, opening the...

Products & Services
Circuit Breakers
Circuit breakers are mechanical switching devices capable of breaking currents under specified abnormal circuit conditions.
Fuses protect electrical devices and components from overcurrents and short circuits that occur in improperly operating circuits.
Thermal Cutoffs and Thermal Fuses
Thermal cutoffs and thermal protectors are nonresetting, thermally sensitive devices designed to protect domestic electrical appliances and industrial equipment from fire. They are sometimes described as thermal one-shot fuses.
Power Entry Modules
Power entry modules are composed of a connector and a mounting case with features to produce the highly conditioned output necessary for medical or sensitive instrumentation.

Topics of Interest

Designing Circuits for Various Electrical Loads The normal procedure used to determine circuit sizes and characteristics for typical loads is to determine the wiring method and conductor and...

3.8 MOTOR FEEDERS AND STARTERS Introduction Motors comprise a significant portion of a building's electrical system loads. They are needed to power fans and pumps for basic mechanical building...

Follow these guidelines to avoid the 12 most common mistakes of specifying circuit protection for electrical equipment. Many engineers design electrical equipment with too little or too much circuit...

Interest in complete overcurrent device selectivity has increased due to the addition of selectivity requirements to articles 700, 701, and 708 of the National Electrical Code (NFPA70). Many users,...

National Electrical Contractors Association, Inc. Washington, D.C. GLOSSARY Frequently used terms are defined here. More complete definitions are available in the latest edition of the National...