Electrical Equipment and Devices

WEEK 6: SWITCHES

Definition. Switches are electrical devices which are manually moved to either ON or OFF positions.

Classification.
1. By location
a. Indoor-type - these are general switches that can be found on control panels or operator stations
b. Outdoor-type - these are switches that can be found on power system grids or distribution systems.

2. By mounting
a. Flush-mounted - switches that need a cavity on a surface of installation
b. Surface-mounted - switches that are installed directly on surface.

3. By contruction / design
a. Open - switches which components are bare to the elements
b. Enclosed - switches which components are protected from the environment.
c. Microswitch - switches which are miniaturized, or created for a minimum space allocations. Usually, these are miniature versions of toggle and/or snap switches used for various applications especially on control panels and operator's stations.

4. By function/purpose
a. Controller - these includes the toggle/snap, tumbler-type, rotary-snap and the pushbutton switches. These switches can start or stop a load, govern, in some predetermined manner, the electric power delivered to the apparatus it is connected and disconnection from the energy source such as:

1) Emergency Stop - interrupting a current to a load
2) Power System Classes - isolation of a station equipment, load management in transmission and sectionalizing in distribution.

b. Protection - switches that guard a branch circuit or from overloads.
c. Special - to include solid-state, limit, pressure, float, flow, proximity and temperature switches for special applications.

5. By Switching mechanism
a. Leaf Spring
b. Coil Spring

Master Switches

Definition. Switches which are heavy-duty in cast-iron base construction, equipped with externally adjustable operating handle for use in cranes and other heavy material handling equipment, steel mills drives, shovels, and other variable speed drives.

Basic Parts.
1. Operating cams - phenolic-compound cams, designed for heavy-duty applciations 600 volts maximum.
2. Operating Handle - ball and rod, joy-stick, pistol grip.
3. Contact blocks/units
4. Pawl stud/spring adjustment - allow easy adjustment of handle tension to desired "feel".

Types:
1. Step-type - for desk or pedestal mounting utilized for continuous service. Consists of single-action cam follower.
2. Stepless-type -
a. Rheostat type - for use in circuits requiring stepless elements with output capacity of 100 watts. The bottom of the switch following the cams is mounted with rheostat elements.
b. Potentiometer type - for use in circuits requiring a stepless element with output capacityof 2 watts. Includes a module of potentiometer mounted on bottom of switch.
c. Induction Type - a desk mounted master switch with a differential transformer mounted on the end of the shaft. The transformer consists of a stator with three coils and a uniquely shaped rotor. As the handle position is changed, the rotor changes the amount of coupling between the couls of the stator and produces an output. In low-speed range where fineness of control is desired, a large change in handle position provides a small change in output. At high speeds where a fast response is required, a small change in handle position provides a large change in output. The primary coil has a maximum rating of about 125 volts. Any change in the input voltage will directly affect the output charactersitics.

Knife Switches

Definition. An open-type switch consisting of a movable knife blade, a pivot and a mount.

Principles.
1. Knife witches must be "dead" in open position, i.e., gravity cannot tend to close an opened switch (upside down).
2. The supply line must be on top while the load-side must be at the bottom or the pivot part of the switch.
3. When enclosed, a minimum gutter space (A) must be at least 4 inches and the wire-bending space (B) at terminals must be 6 inches minimum.

Solid State Switches

Definition. Solid-state switches are silicon-based devices which exhibit switch-like actions such as ON and OFF.

Types. There are five definitive types of solid-state switches:

Device

Structure

Symbol

Application

Rectifier

.

.

.

Thyristor

.

.

Controlled turn-on switch

Diac

.

.

.

Triac

.

.

Bi-directional controlled turn-on switch

Transistor

.

.

Controlled turn-on / turn-off switch

Study Guide