Leading Australian distributor of professional sound, light and staging systems Jands explains the significance of fuses in electrical or electronic equipment, and the importance of consulting with a qualified technician to resolve fuse-related problems.
A fuse acts as a sacrificial device to provide overcurrent protection of either the load or source circuit in any electrical or electronic equipment. Consisting of a metal wire or strip that melts when excess current flows through it, a fuse protects the equipment from damage by interrupting the circuit. Short circuits, overloading, mismatched loads, or device failure are the prime reasons for excessive current.
A blown fuse is the result of the fuse interrupting excessive current so that further damage by overheating or fire is prevented. Overcurrent protection devices are essential in electrical systems to limit threats to human life and property damage. The time and current operating characteristics of fuses are chosen to provide adequate protection without needless interruption.
Slow-blow fuses for example, are designed to allow harmless short term currents over their rating while still interrupting a sustained overload. A self-resetting fuse automatically restores the circuit after the overload has cleared. Such fuses are typically seen in aerospace or nuclear applications where it may be physically impossible to replace a blown fuse.
A slow-blow fuse finds application in electric motor protection, where inrush currents of up to ten times the normal operating current are experienced every time the motor is started from a dead stop. A fast-blowing fuse in such an application would never allow the motor to start as normal inrush current levels would immediately blow the fuse. A slow-blow fuse features a fuse element with more mass, but not more amp capacity than an equivalent fast-blow fuse, meaning it has more resistance, which allows it to slowly heat up for a given amount of current.
Glass cartridge type fuses are commonly used in the professional entertainment world. Fuses are rated in amps, which is the primary measurement used to determine current. Although fuse operation depends on the self-generation of heat under conditions of excessive current by means of the fuse’s own electrical resistance, they are engineered to contribute a negligible amount of extra resistance to the circuits they protect. This is largely accomplished by making the fuse wire as short as practically possible; a fuse wire of a given material and gauge will blow at a predetermined current, regardless of its length. The melted ends of the once-continuous wire will be separated by a gap, with full supply voltage between the ends. If the fuse isn’t made long enough on a high-voltage circuit, a spark may be able to jump from one of the melted wire ends to the other, completing the circuit again, and causing additional unwanted damage.
The Jands Service department often gets this question from customers, “Why did the fuse blow in my gear?” The answers are many: there may have been a nuisance trip; the fuse could have been damaged or weakened during transportation; or the inrush current was slightly greater than usual and blew the fuse. However, most likely there is a real fault with the equipment that needs to be addressed.
Before you attempt to replace a user accessible fuse, it is important to note what happened before the failure occurred: Did the device smoke? Were there sparks? Perhaps an undesirable noise was emitted at the time of the failure? Does the equipment have a burnt smell? If you answered “yes” to any of these questions, get the equipment checked by a qualified technician before trying to replace the fuse or restart the unit.
Fuses that are not user accessible are usually located inside a piece of equipment requiring a screwdriver or other tools to access, and should only be replaced by a qualified technician. These internal fuses almost certainly will blow when there is a serious problem. Replacing the fuse without checking the circuitry in this situation will cause more damage and additional expense at repair time.
Blown fuses should be replaced with the same type and rating of fuse. Failure to do so can make the equipment dangerous to operate; in the event of a ground fault, it could make the chassis ‘live’, which could prove fatal to the user.
So, the next time a fuse blows, get the equipment checked by a professional to avoid damage from an improperly replaced fuse.