Control Overload Current Hazards
When a current exceeds the current rating of equipment or wiring, the wiring cannot handle the current without heating up or even melting. Not only will the wiring be damaged, but the high temperature of the conductor can also cause a fire.
Overcurrent Protection Devices
To prevent this from happening, an overcurrent protection device (circuit breaker or fuse) is used in a circuit.
These devices open a circuit automatically if they detect current in excess of the current rating of equipment or wiring. This excess current can be caused by an overload, short circuit, or high-level ground fault.
Circuit Breakers. A circuit breaker is one kind of overcurrent protection device. It is a type of automatic switch located in a circuit. A circuit breaker trips when too much current passes through it. A circuit breaker should not be used regularly to turn power on or off in a circuit, unless the breaker is designed for this purpose and marked "SWD" (stands for "switching device").
Fuses. A fuse is another type of overcurrent protection device. A fuse contains a metal conductor that has a relatively low melting point. When too much current passes through the metal in the fuse, it heats up within a fraction of a second and melts, opening the circuit. After an overload is found and corrected, a blown fuse must be replaced with a new one of appropriate amperage.
Not Allowed in Hazardous Environment
Overcurrent protection devices are not allowed in areas where they could be exposed to physical damage or in hazardous environments. Overcurrent protection devices can heat up and occasionally arc or spark, which could cause a fire or an explosion in certain areas. Hazardous environments are places that contain:
- flammable or explosive materials such as flammable gasses or vapors (Class I Hazardous Environments),
- finely pulverized flammable dusts (Class II Hazardous Environments), or
- fibers or metal filings that can catch fire easily (Class III Hazardous Environments).
Knowledge Check Choose the best answer for the question.
7-11. What type of overcurrent protection devices contains a metal conductor that melts when too much current passes through the metal?
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