Pojistky

Pojistky are protective devices used in electrical circuits to interrupt current flow when it exceeds a predefined value. Their primary purpose is to prevent damage to wiring, components and to reduce the risk of fire by isolating a faulted part of the circuit. In Czech usage, the term can also refer to insurance policies, but this article focuses on electrical fuses.

The basic principle is simple: a fusible element, such as a thin wire or metallic strip, melts when it overheats due to overcurrent. Once the element disappears from the circuit, the circuit is opened and power is cut off. Some designs also include thermal or magnetic mechanisms to respond to short surges more rapidly.

Pojistky come in several forms. Cartridge fuses are common in domestic electrical systems; they contain a fusible element inside a cylindrical body. Blade fuses are widely used in automobiles. Fuses may be glass or ceramic-bodied, and they are categorized as fast-acting or slow-blow (time-delay) depending on how quickly they melt under overcurrent. Older or specialized systems may use rewirable fuses, but these are less common in modern installations.

Ratings are given by current (amperes) and voltage, and the physical bodies are often color-coded or marked with their ratings for easy identification. Safe use requires replacing a blown fuse with an identical type and rating and installing it in an appropriate fuse holder or panel. Bypassing or replacing a fuse with an improper rating is dangerous and should be avoided. In many newer installations, circuit breakers have largely replaced fuses due to their resettable nature, but fuses remain common in simpler or legacy systems.

Note: in Czech, pojistky can also mean insurance policies; this article covers the electrical devices.