A fuse is designed to break current flow in the circuit when there’s excessive current flowing through a circuit.
A fuse contains a strip of metal, which when exposed to higher current than designed for will melt/break before any damage can be done to the wires in the circuit.
Example: A circuit with #14 wire is rated for 15A with a suggested 15A fuse. If you have 18A flowing through this circuit, the wire insulation could become damaged and possibly start a fire. The fuse is designed so the metal link inside the fuse will heat up and break the flow of current.
Plug and cartridge are the two main types of fuses.
Plug fuses come in sizes 15A, 20A, and 30A. They screw into your electrical panel and are most common in older residential houses.
Type D Plug Fuses: Type D fuses allow for a time delay. Your circuit can withstand a surge of electricity without blowing a fuse right away. Type D fuses would be used in circuits running motors or electrical heat
Type P Plug Fuses: Type P fuses are non-time delay standard fuses. These would be used in standard circuits which contain lights and ordinary household appliances
Type S Plug Fuses: Type S fuses are time-delayed fuses and also tamper-resistant. You cannot use an incorrect fuse as you have to screw them into a base designed for that specific amperage fuse