Figure 2-29.--Plug fuse.

The plunger should be checked for cleanliness,

Plug Fuses

binding, mechanical failure, and improper alignment

A plug fuse (fig. 2-29) has a piece of zinc-alloy

adjustment. The mechanism that the solenoid is to

wire mounted in a porcelain cup with a metal cover.

actuate should also be checked for proper operation.

A threaded contact base similar to a lamp socket is

The second step should be to check the energizing

provided so that the fuse can be screwed into a socket

voltage by use of a voltmeter. If this voltage is too low,

i n the fuse block. Plug fuses are used on

the result would be less current flowing through the

small-capacity circuits ranging from 3 to 30 amperes

coil and thereby a weak magnetic field. A weak

at not more than 250 volts. Some plug fuses have small

mica windows so the fusible link can be observed.

magnetic field can result in slow, ineffective

Plug fuses are not normally used in naval vessels and

operation. It could also possibly result in chatter or

are seldom used in commercial applications; however,

inoperation. If the energizing voltage is too high, it

they can be found in older buildings and houses.

will in all probability damage the solenoid by either

overheating or arcing. In either case, the voltage

Cartridge Fuses

should be reset to the proper value so further damage

or failure will not result.

A cartridge fuse (fig. 2-30) consists of a

zinc-alloy link enclosed in a fiber, plastic, ceramic, or

The solenoid should then be checked for opens,

shorts, grounds, and correct resistance with an

ohmmeter. If when you check the resistance of the

solenoid the ohmmeter indicates infinity, the solenoid

is open-circuited and should be replaced. If the

ohmmeter reads zero or less than the specified

resistance, the coil is shorted and should be replaced.

However, if the resistance of the coil is higher than

specified (but not infinity), look for a poor contact or

a damaged conductor. If the fault cannot be found or

corrected, replace the solenoid. Another check

possible with the ohmmeter is to determine if the coil

is grounded. If the coil is grounded, reinsulate the

solenoid.

PROTECTIVE DEVICES

Figure 2-29.--Plug fuse.

Most protective devices are designed to interrupt

the power to a circuit or unit under abnormal

conditions, such as short circuits, overloads, high or

low voltage, and excessive current. The most common

types of protective devices are fuses, circuit breakers,

and overload relays.

FUSES

A fuse is a protective device used to open an

electric circuit when the current flow exceeds a safe

value. Fuses are made in many styles and sizes for

different voltages and currents, but they all operate on

the same general principle. Each fuse contains a

soft-metal link that melts and opens the circuit when

overheated by excessive currents.

Figure 2-30.-Cartridge fuse.

2-19