246 Electrical Power Systems Technology
circuit protection is still provided.
ime delay fuseT are used to limit current on systems including electric
motors, which draw higher currents during their start cycle than during
normal operation. These devices allow the system to start up at a higher
than normal current; they then protect the system during normal opera-
tion without disrupting the distribution system.
Fuse Metals—The type of metal used in fuses is ordinarily an alloy
material or, possibly, aluminum. All metals have resistance, so when cur-
rent flows through metal, heat energy is produced. As the current increas-
es, more heat is produced, causing the temperature of the metal to in-
crease. When the melting point of the fuse metal is reached, the fuse will
open, causing the circuit to which it is connected to open. Metals that de-
compose rapidly are used, rather than ones that produce small metallic
globules when they melt. This reduces the likelihood of any arc-over occur-
ring after the fuse metal has melted. The current rating of fuses depends
upon the melting temperature of the fuse metal, as well as its shape, size,
and the type of enclosure used.
Low-voltage Circuit Breakers
Circuit breakers are somewhat more sophisticated overload devices
than are fuses. Although their function is the same as that of fuses, circuit
breakers are much more versatile. In three-phase systems, circuit breakers
can open all three hot lines when an overload occurs. They may also be ac-
tivated by remote-control relays. Relay systems may cause circuit breakers
to open in response to changes in frequency, voltage, current, or other cir-
cuit variables. Circuit breakers are used in industrial plants, and are usu-
ally of the low-voltage variety (less than 600 volts). They are not nearly as
complex as their high-voltage counterparts (which were discussed pre-
viously). Most low-voltage circuit breakers are housed in molded-plastic
cases that mount in metal power distribution panels. Circuit breakers are
designed so that they will automatically open when a current occurs that
exceeds the rating of the breaker. Ordinarily, the circuit breakers must be
reset manually. Most circuit breakers employ either a thermal tripping ele-
ment or a magnetic trip element. Ratings of circuit breakers extend into cur-
rent ranges that are as high as 800 to 2000 amperes.
Protective Relays
Protective relays provide an accurate and sensitive method of protect-
ing electrical equipment from short circuits and other abnormal condi-