What Are Circuit Protection Devices?

Electricity is paramount for powering the countless appliances and electronics that we rely on every day, but it is also a volatile force that needs to be managed for safety. When an excessive amount of current passes through wires due to a short circuit, a great amount of heat may be generated and sensitive components can be seriously damaged. Additionally, short-circuiting can even cause electrical fires and dangers to individuals which can be extremely dangerous for certain applications such as aviation. In order to prevent the damages and hazards associated with overload conditions, electrical circuits are guarded with circuit protection devices. The most common circuit protection devices are fuses, current limiters, and circuit breakers. As each device type has its own features and capabilities that dictate their use, it can be very beneficial to understand the differences between each Fuses are a basic component for circuit protection, and they are a thermal device with an element which exhibits a low melting point.

Fuses generally have low current carrying capacities, and they are designed to melt in the occurrence of a short circuit. With this method of operation, fuses can quickly interrupt a circuit for safety. The element of the fuse may be constructed from varying materials, and the most common are lead silver, tin, bismuth, and their related alloys. To provide ample protection for the circuit, fuses must be placed near the power source so that they may manage the entire cable. For aircraft applications in particular, fuses are often heavy-duty and may serve AC and DC sources. Furthermore, aircraft often employ fuses for both the circuit and devices for redundancy and further protection.

Current limiters are used when current needs to be capped at a defined level. Current limiters come in two types, those of which are thermal devices and limiting resistors. Limiting resistors in particular are mostly used for DC circuits where the initial current is high. As such, they can regularly be found within starter generators to limit the amount of current to a safe value. Current limiters are also generally useful for preventing the harmful effects of short-circuiting or overloads. Current limiters may be a single device or a group of devices depending on the application, and they may differ in design.

Circuit breakers are set apart from fuses and current limiters due to being an electromechanical device that may interrupt and isolate circuits. While circuit breakers may come in numerous forms, the most common working principle for actuation is the heating of a bi-metallic element which current passes through. In the instance that an overload condition occurs, the circuit breaker has the ability to automatically break contacts and quench arcs to interrupt a circuit. Unlike a fuse which must be replaced each use, a circuit breaker may be reset through automatic or manual means. Circuit breakers will also prevent the closing of contacts if a short circuit is still occurring, and the device will automatically trip again in such instances.

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