0448003.MR Circuit Protection

Fuses

Fuses are specialized electrical devices used to protect electrical systems and equipment against excessive current. They are often installed in between the current source and the protected circuits, and senses current above a threshold during a fault condition. Upon detection of the fault current, the fuse opens to provide a low-resistance path, keeping the system protected from overload. A fuse is typically rated according to its maximum fault-current capacity, and is differentiated by its voltage rating.

MR Application Field and Working Principle

The MR type of fuse, also known as medium range, is designed specifically to be used for protecting telecommunications systems and other electronic applications. The MR fuse is available with 10A, 16A, 25A and 30A ratings according to its IEC 60060-1 standard classification. Its metal oxide resistor and ceramic body makes it suitable for direct current at voltages up to 48Vdc.

MR fuses have a unique operating principle thanks to two special segments they are composed of. One consists of a sensitive metal oxide resistor (MOR) in order to provide the Fuse Characteristic ‘C’ Curve, while the other is made from a dedicated ceramic body containing two active elements. One of these active elements is a protective fuse wire set at a certain overload point while the other is a constant, non-current carrying ceramic which provides the current-limiting characteristic.

Upon detection of a fault current by the MOR, the current across the body bumps up and overheats the protective fuse wire, causing it to open. This allows for an abrupt drop in the current because of the current limiting ceramic, which in turn reduces the temperature of the fuse wire, preventing it from damaging the protected circuit or indeed the fuse itself.

MR fuses are suitable for a variety of applications, such as protection from Current Harvests, Shorts, Current surges and overloads, and are used to protect telecom circuits, industrial and domestic power applications. Although this type of fuse can be used for higher voltage and current ratings with suitable modifications, it is primarily used for low-voltage DC applications.

Due to their smaller size and their relatively low cost compared to other electrical protective devices, MR fuses are gradually replacing bulky circuit breakers in a range of electronic applications.