Switching Guide

In electrical installations a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another.

The most familiar form of switch is a manually operated electromechanical device with one or more sets of electrical contacts. Each set of contacts can be in one of two states: either 'closed' meaning the contacts are touching and electricity can flow between them, or 'open', meaning the contacts are separated.

switch1
switch2

A switch may be directly manipulated by a human as a control signal to a system, such as to control power flow in a circuit, as in a light switch, or to isolate a supply, such as a Firemans switch. Automatically-operated switches can be used to control the motions of machines, for example, to indicate that a garage door has reached its full open position or that a machine tool is in a position to accept another work piece.

Switches may be operated by process variables such as pressure, temperature, flow, current, voltage, and force, acting as sensors in a process and used to automatically control a system. For example, a thermostat is a temperature-operated switch used to control a heating process.

A switch that is operated by another electrical circuit is called a relay. Large switches may be remotely operated by a motor drive mechanism. Some switches are used to isolate electric power from a system, providing a visible point of isolation that can be pad-locked if necessary to prevent accidental operation of a machine during maintenance, or to prevent electric shock.

N/O & N/C Switches

In a push-button type switch, in which the contacts remain in one state unless actuated, the contacts can either be normally open (abbreviated "n.o." or "no") until closed by operation of the switch, or normally closed ("n.c. or "nc") and opened by the switch action. A switch with both types of contact is called a changeover switch. These may be "make-before-break" which momentarily connect both circuits, or may be "break-before-make" which interrupts one circuit before closing the other. Examples of NC Switches include Fire & Intruder Alarms, Emergency Lighting, a Fridge Light or a Walk-In wardrobe...

The following lists the common (and not so common) switches you may encounter or require:

Type of Switch Circuit Symbol Example

ON-OFF
Single Pole, Single Throw = SPST

A simple on-off switch. This type can be used to switch the power supply to a circuit.

When used with mains electricity this type of switch must be in the live wire, but it is better to use a DPST switch to isolate both live and neutral.

switch4a
switch4
SPST toggle switch

(ON)-OFF
Push-to-make = SPST Momentary

A push-to-make switch returns to its normally open (off) position when you release the button, this is shown by the brackets around ON. This is the standard doorbell switch.

switch5a
switch5
Push-to-make switch

ON-(OFF)
Push-to-break = SPST Momentary

A push-to-break switch returns to its normally closed (on) position when you release the button (N/C) such as a fridge door.

switch6a
switch6
Push-to-break switch

ON-ON
Single Pole, Double Throw = SPDT

This switch can be on in both positions, switching on a separate device in each case. It is often called a changeover switch. For example, a SPDT switch can be used to switch on a red lamp in one position and a green lamp in the other position.

A SPDT toggle switch may be used as a simple on-off switch by connecting to COM and one of the A or B terminals shown in the diagram. A and B are interchangeable so switches are usually not labelled.

switch7a
switch7

SPDT toggle switch

switch8
SPDT slide switch

Dual ON-OFF
Double Pole, Single Throw = DPST

A pair of on-off switches which operate together (shown by the dotted line in the circuit symbol).

A DPST switch is often used to switch mains electricity because it can isolate both the live and neutral connections.

switch9a
switch9
DPST rocker switch

Dual ON-ON
Double Pole, Double Throw = DPDT

A pair of on-on switches which operate together (shown by the dotted line in the circuit symbol).
A DPDT switch can be wired up as a reversing switch for a motor as shown in the diagram.

ON-OFF-ON
DPDT Centre Off

A special version of the standard SPDT switch. It has a third switching position in the centre which is off. This can be very useful for motor control because you have forward, off and reverse positions. Momentary (ON)-OFF-(ON) versions are also available where the switch returns to the central off position when released.

switch10a
switch10
DPDT slide switch
switch11
Wiring for Reversing Switch

Unusual Switches:

Micro-switch (usually SPDT = ON-ON)
Microswitches are designed to switch fully open or closed in response to small movements. They are available with levers and rollers attached.

switch12

Unique Key-switch

A key operated switch. The example shown is SPST.

switch13

Tilt Switch (SPST)

Tilt switches contain a conductive liquid and when tilted this bridges the contacts inside, closing the switch. They can be used as a sensor to detect the position of an object. Some tilt switches contain mercury which is poisonous.
switch14

Reed Switch (usually SPST)
The contacts of a reed switch are closed by bringing a small magnet near the switch. They are used in security circuits, for example to check that doors are closed. Standard reed switches are SPST (simple on-off) but SPDT (changeover) versions are also available.

Warning: reed switches have a glass body which is easily broken!
switch15

DIP Switch (DIP = Dual In-line Parallel)

This is a set of miniature SPST on-off switches, the example shown has 8 switches. The package is the same size as a standard DIL (Dual In-Line) integrated circuit.

This type of switch is used to set up circuits, e.g. setting the code of a remote control.
Dip Switch

Dimmer switches

A dimmer switch contains a solid-state circuit to allow changing the brightness by reducing the average voltage applied to the lamp.

With lighting design it is worth remembering, if a single pole, one-way switch is used, the lighting scheme has two options, ‘ON’ or ‘OFF’. Introduce a simple dimmer and the options multiply existentially... break the circuits up and introduce a multiple dimmer, and the scheme has endless alternative.

switch3


Contact Us:

Unit 28
Appin Road
Argyle Trading Estate
Wirral
CH41 9HH

Tel: 0151 342 2985
Mob: 07747 337 685

This email address is being protected from spambots. You need JavaScript enabled to view it.