For questions generally relating to safety devices used to automatically stop the flow of electric current in the event of a fault condition.

Circuit breakers are devices designed to automatically stop the flow of electrical current, in the event of a fault condition. Similar to a fuse, though circuit breakers are resettable, and typically offer more than just thermal overcurrent protection.


Circuit breakers use various techniques to offer different types of protection, often providing at least thermal and magnetic protection.

Thermal Protection

A bimetallic device, is the most common way to provide thermal protection. As current flow it generates heat, the higher the current, the more heat that's generated. Since at a specific ambient temperature, a specific current will generate a specific amount of heat. Knowing the specific values, a bimetallic device can easily be designed to react to temperatures above a predetermined range. As current flows through the bimetallic device, the device heats up. If the temperature gets too high, the device opens contacts and stops the flow of electricity.

The current at which the breaker will trip, is often listed on the handle of the circuit breaker. However, since this is a thermal device, there will be a time delay between the overcurrent and the opening of the circuit. This time delay will be described by what's know as a trip curve, and will be documented by the manufacturer.

Magnetic Protection

This type of protection is typically handled by a solenoid, and may be referred to as instantaneous trip. This type of protection is designed to detect very high current short-circuits, and open the circuit quickly when they are detected. When the high current flows through the circuit breaker, it generates a large magnetic field. The magnetic protection device uses this magnetic field to physically open the contacts, stopping the flow of electricity.

The current at which the breaker trips is often listed on the circuit breaker, and will be in the tens of thousands of amperes range.

Other protections

There also exist circuit breakers that provide ground-fault circuit interruption (GFCI), arc-fault circuit interruption (AFCI), combination arc-fault circuit interruption (CAFCI), as well as dual function circuit interruption (DFCI) which offer both AFCI and GFCI protection. These devices typically offer the specialized protections, in addition to thermal and magnetic protection.

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