Page 254 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Fuses are electrical safety devices that protect equipment and components from damage caused by overloaded
circuits. When the current flowing in a circuit exceeds the rated value of the fuse, the current conductor in the fuse
melts and opens the circuit. If the fuse is not present, or is too high a value, then it would be the circuit conductors
or components that would melt and possibly burn.
The opening of a fuse indicates a fault somewhere in the circuit, switches off the faulty circuit from the power
source and isolates it from other, unaffected circuits.
During over-current conditions the fuse interrupts the current source, limiting the energy allowed to pass. When a
circuit carrying a current is interrupted in this way, an arc is created across the break. This arc only lasts a short
time under normal circumstances but like the arc from an electric welding set, it can generate considerable heat.
The fuse has to be capable of withstanding this arc. This characteristic is particularly important during short circuit
conditions where the current can be very much higher than normal.
Fuse holders - or carriers - also have to be made so that they can carry the rated current as well as a high
overload current for a short time. They also have to be made so mat they can withstand the highest voltage they
will be subjected to. Standards also dictate the type of fuse that has to be used for different circuits.
Switches
A switch consists of a set of contacts manually operated by some form of actuator.

The actuator and contacts may be contained in a single moulded unit or more likely as a modular unit comprising
a selection of actuators and contact sets.
Moulded one-piece

• These are generally for low current use and are more likely to be found in the low voltage control system.
• Panel-mounted one-piece units are fixed to the panel using:
• Either a central nut and lock washer - note the locating spigot, or
• Clipped into a square hole.
The wires are generally connected using crimped spades although they can be soldered
Modular

These are built up using a choice of parts fitted to a panel-mounted body. The most popular size fits a 20.5 mm
panel hole. Other sizes are 16 mm and 30.5 mm. While the actual detail of assembly varies between manufacturers,
they are all similar to the following representative units.
There are three main parts:
• The actuator.
• Mounting adaptor.

• Contact elements.
Switch actuators
This is the part which will operate the switch contacts. There are several variations including some with lamp
indicators. The actuator is fixed to the panel through a hole with a large fixing nut behind the panel. A lettered facia
can be fitted between the flange on the actuator body and the panel.
• Rotary switch.
• Push-button switch.

• Key-operated switch.
• Lever switch.
Switch actions
• Momentary - where the contacts are operated only while the actuator is operated. Sometimes referred to as
spring return.

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CITS : Power - Electrician & Wireman - Lesson 93-100