Human safety depends on reliable switching, so choosing the right switch for the job is an essential part of machine design. It is the purpose of this booklet to provide advice for owners and designers who make machines safe by reliably interlocking electrical circuits.

Table of Contents

Status and Obligations Under the HSE Act 1992
Introduction
Scope of the book

1. Use of Switches

1.1 Switch choice how specifications aid selection
1.2 Switch type normally open and normally closed
1.3 Switch description the preference for safety

2. Types of Switches and Applications

2.1 Limit switch with cam
2.2 Snap action with over travel
2.3 Monitoring normally open and normally closed switches together
2.4 Emergency stop controls
2.5 Microswitches
2.6 Magnetic switches
     2.6.1 Electromagnet
     2.6.2 Permanent magnet
     2.6.3 Solenoid bolt
2.7 Proximity switches
     2.7.1 Inductive proximity switches
     2.7.2 Capacitive proximity switches
     2.7.3 Restricting access of the target
2.8 Optical detection systems
     2.8.1 Infra-red light detection
     2.8.2 Retroreflective sensor
     2.8.3 Specular sensor
     2.8.4 Through-beam sensor
2.9 Actuator-operated switches

3. Trapped Key Interlock

4. Lockout

4.1 Lockout procedures
4.2 Power failure release

5. Braking Electric Motors

5.1 Mechanical braking
5.2 Electrical braking
5.3 Electrical plugging
5.4 Direct current injection
5.5 Capacitor braking

6. Programmable Electronic Systems

6.1 Interference
6.2 Software

7. Interlocking Appropriate to the Hazard

References and Acknowledgements
Glossary