Power Distribution Equipment 245
circuit protection should be planned utilizing these curves. They show the
response time required for a protective device to interrupt a circuit, when
an overload occurs.
Plug Fuses—Fuses are used in safety switches and power distribu-
tion panels. The plug fuse is a common type of fuse. Standard sizes for
this fuse are 10, 15, 20, 25, and 30 amperes at voltages of 125 volts or be-
low. These fuses have a zinc or metallic-alloy-fusible element enclosed in
a case made of an insulating material. Their most common use is in safety
switches and fuse panelboards.
Cartridge Fuses—Cartridge fuses are commonly used in power distri-
bution systems for voltages up to 600 volts. They have a zinc- or alloy-fus-
ible element, which is housed in a round fiber enclosure. One type has a
nonrenewable element, while another type has a renewable element. Cartridge
fuses may be used to protect high-current circuits, since they come in sizes
of 60,100,200,400,600, and 1000 amperes.
ime Delay FusesT —A modification of the plug or cartridge fuse is
called a time delay fuse. This type of fuse is used to delay the circuit-inter-
rupting action. It is useful where momentary high currents exist periodi-
cally, such as motor-starting currents. The fuse element melts only when
an excessive current is sustained over the time-lag period; thus, sufficientFigure 9-2. Typical operating-characteristic time curves for three different types
of fuses (Courtesy Littelfuse, Inc.)