Final answer:
Thermal magnetic circuit breakers protect against overcurrent by using a bimetallic strip for thermal protection against sustained overloads and a magnetic element that trips the breaker instantly during short circuits.
Step-by-step explanation:
Thermal Magnetic Circuit Breakers and Overcurrent Protection
Thermal magnetic circuit breakers are essential safety devices in electrical circuits that protect against overcurrent conditions. These circuit breakers consist of two distinct protection mechanisms: a thermal component and a magnetic component. The thermal part typically uses a bimetallic strip that bends when heated by excessive current, causing the breaker to trip and interrupt the circuit. This thermal action provides protection against sustained overloads. The magnetic component, on the other hand, responds to short circuits, which are sudden and excessive spikes in current. When a short circuit occurs, the magnetic field created by the high current exerts a force on a moving contact within the breaker, causing it to open instantaneously.
For higher voltages, additional technologies are used, such as insulating gas or jets of gas, to prevent an arc from maintaining the current flow when the circuit breaker attempts to open. It is also noteworthy that AC current provides a natural advantage for extinguishing arcs since it crosses zero volts 120 times per second with a 60 Hz supply, hence breaking the circuit more easily than DC.