Journeyman Electrician: Fuses & Switchgears

Fuses operate on the principle that electrical current develops heat.  By using a fuse element of a specific resistance and a low melting point, only a specified current can flow through the fuse before it melts and opens.  Fuses work in both AC and DC circuits.
Fuses are used inside appliances, in automobile electrical systems, and in home and industrial power circuits.  Most present day power distribution systems now use circuit breakers instead of fuses.
In power distribution, there are cartridge fuses and plug fuses.  Fuses are also classified by the amount of time they will allow an overcurrent condition to exist.  Those without a time delay react almost instantly to the overcurrent condition while time delay types will not open until a specific time period has elapsed with the overcurrent condition.  The time delay allows for short duration high start up current of motors for instance.
If a fuse has blown, the user should first replace it and resume operation.  If the new fuse blows, check the circuit for a problem.

Fuses Used in Power Distribution
Cartridge fuses are the most common design.  A fuse puller should be used to replace cartridge fuses.  Some cartridge fuses are renewable and can be restored by replacing their fusible elements.  Clip clamps are used to insure good contact between the fuse terminals of cartridge fuses and the fuse clips in the fuse box.
If the ends of a cartridge fuse become warmer than normal, the fuse clips should be tightened.  If the spring tension on a cartridge fuse clip is weak, the fuse clips will become warm.  Discoloration at one end of a fuse indicates poor contact with the panel.
Older homes may have boxes that take plug fuses which screw into a threaded base that resembles a light socket.  Some use rejection base adapters that take type S fuses which have a different base which prevents installation of fuses with the wrong rating.  These are also called fusestat fuses.

Circuit Breakers
Like fuses, circuit breakers may also operate on the principle that electrical current develops heat.  In the case of the breaker though, a switch responds to the heat and is opened automatically when the preset current limit is exceeded.  There may also be an electromagnetic actuator within the breaker.  Unlike a fuse, circuit breakers can be reset to resume operation.
The circuit breaker is wired only into the ungrounded conductor.  When a breaker trips, it may be due to a short circuit or other fault in either the grounded or the ungrounded conductor.
A GFCI breaker is one that has additional protection from ground faults.  If the current flowing from the ungrounded line to the grounded line is unbalanced, the device opens. This situation only occurs when there is a short circuit to ground, which could cause equipment damage or injury.
If a circuit breaker is mounted vertically, the on position must be up.

Switchgear
The assembly of devices used for interruption, control, and metering of power is called switchgear.  It may be located at the power generation source, at a distribution point such as a power station, or in or near a house or commercial building.
At a generating plant or substation, the monitoring and control functions are sophisticated and typically used not only circuit breakers, switches, and meters but in communication equipment and remote sensing and control circuits.
At the service entrance for a building, containing the loads will typically be a meter with its feeder conductors and the panels containing overcurrent protection devices for the main service and all the branch circuits.  The conductors to the service entrance are called the service drop. 
In some commercial buildings, particularly those receiving high voltage service, these devices may be enclosed in a separate building.
The main purpose of lower voltage switchgear is protection, isolation, and control of power distributed within the building.