Fuses and Breakers

Single Element Fuses

-consists of a link or several links contained in a tube filled with arc quenching filler -if an overload current of more than rated current is continued for a sufficiently long period of time, the fuse link melts. -a short circuit instantly melt several sections of the link and the surrounding filler aids in extinguishing the arc

Dual Element Fuses

-the thermal cutout gives dual element fuses a time delay on overloads, such as motor starting current. -On a low overload current the short circuit fuse link remains entirely inactive but if it flows long enough to raise the temperature of the center mass to the melting point of the solder, the connector is pulled out of place by the spring, thereby opening the circuit. -a short circuit instantly melt several sections of the link and the surrounding filler aids in extinguishing the arc

Thermal-Magnetic Circuit Breaker

-the thermal strip provides a time delay for momentary overloads. -the magnetic coil provides instantaneous trip on high or short circuit currents and protects the bimetallic strip from excessive overheating that can destroy it.

Ambient Compensating Circuit Breaker

-used in areas where high temperatures are encountered. -has an overload bimetallic strip, and a compensating bimetallic strip -the breaker trips only when the excessive current through the overload strip causes it to bend more than the compensating strip

Thermal Circuit Breakers

-used primarily for protection against overcurrent -is a bimetallic strip -if the current becomes excessive for a prolonged period of time, the bimetallic strip becomes hot and bends because of different expansion rates of the two metals triping the latch and opens the contacts

Magnetic Circuit Breaker

-used whenever instantaneous action is required to open a faulted circuit.

Overload

A current that is in excess of normal current but confined to normal conducting paths. -typically one to six times the normalcurrent. -It is usually associated with the normal startup of motors or inrush current to a transformer.

Short Circuit

A current that is in excess of normal current but not confined to normal conducting paths. -can be several hundred times greater than normal current levels

Current Limiting

A fuse is said to be current limiting if it operates quickly under short circuit conditions, typically in less than one half of a cycle. -associated with short circuit conditions, not overloads. -By opening quickly, the instantaneous peak short circuit current is limited to a much smaller value

Low Voltage Fuses

A fuse with a voltage rating of 600 volts or less.

Advantages and Disadvantages of Circuit Breakers as Compared to Fuses

A. Advantages - Breakers can be reset after trip and need not be replaced normally. - Breakers can be used as a switching device. B. Disadvantages - Cost - Size - Handling and Weight

Interrupting Rating

The maximum short circuit current that a fuse can safely interrupt without rupturing. -also called SCCR (short circuit current rating)

Voltage Rating

The voltage rating of a fuse is the maximum circuit voltage in which the fuse can be safely applied. -If the voltage rating is exceeded, re-strike is possible across the open fuse element.

What are the two basic types of current-limiting fuses?

single element and dual element fuses

Overcurrent

An excess current resulting from either a short circuit or an overload

Selective Coordination

If distribution systems protective devices are selective, only the protective device nearest the faulted circuit will open. Devices upstream remain intact, supplying other feeders. -For a system to be selective, a minimum ratio (specified by the fuse manufacturer) must be maintained between upstream and downstream continuous current ratings.

Continuous Current Rating

The continuous current rating on the fuse is the current that the fuse is intended to normally carry. -the fuse will carry this current indefinitely.