9.4 Control Measures for Machinery

The basic principles of operation of control methods, with advantages and limitations

These are the types of control that can be used for safe machinery operation.

• guards: fixed, interlocking, and adjustable or self-adjusting
• protective devices: two-hand controls, hold-to-run controls, sensitive protective equipment (trip devices) and emergency stop controls
• jigs, holders and push sticks
• information, instruction, training and supervision
• personal protective equipment

Guards

Machine guards are protective devices that cover moving pieces of machinery that may be dangerous to workers. Moving parts are a hazard to machine operators, and flying debris can affect anyone near a machine. For this reason, industrial machines are required to have guards that protect workers from injury while they work. Guards are the primary line of defence between workers and their machines, and it is essential that they are in place and properly secured during operation to prevent accidents and injuries.

Before removing a guard for work, establish and verify secure isolation and control all hazardous stored energy. Powering down alone does not prevent pressure release, gravity movement or unexpected restart. Refit and validate safeguards before returning to normal operation.

• Fixed guards. Simple and easy to provide, they cover moving parts and also stop particles being thrown. They come in many materials and designs. The guard opening and its distance from the dangerous part should be completely safe. They should fit closely, be adjusted properly and withstand speed, vibration and impact, and they should be fitted securely with clamps, bolts and so on.
• Interlocking guards. An interlock prevents hazardous operation with the guard open and initiates stopping when it is opened. It does not necessarily lock the guard or stop movement immediately. Where run-down would allow access before danger has ceased, suitable guard locking or another validated arrangement is needed. Closing a guard must not itself cause an unexpected restart.
• Adjustable guards. These are also permanently attached to the machine, but they have moving parts that can be adjusted by hand to suit different operating conditions. For a short period an opening can be made to allow work on the machine. Operating an adjustable guard takes a certain level of expertise to judge the right guard adjustment for the operating conditions, so companies must train operators to the level needed.
• Self-adjusting guards. These move with the material being processed and return to a protective position as it passes. They must limit access as far as practicable and be checked for the actual operation; they do not authorise hands entering dangerous parts.

Protective devices

Protective devices perform different functions to keep workers safe. They can be used on their own, as part of the operating cycle, or together with barrier guards. Several examples show how they work.

• Two-hand devices. These keep the operator's hands in a fixed position while the machine operates. Two-hand controls require the operator to keep constant pressure on both controls at all times. A two-hand trip requires the operator to press a button with each hand to start the machine, after which the hands are free.
• Hold-to-run controls. The commanded movement continues only while the control is held; release initiates stopping. Run-down time and access to danger still need assessment. One hand holding a control does not ensure the other hand or another person is safe. Suitable enabling devices can have a middle operating position, but the complete safeguarding design must be validated.
• Safety trip controls. A trip can use a variety of sensors, including pressure-sensitive body bars, trip wires or safety tripods. Their function is to deactivate a machine immediately in an emergency.
• Sensing devices. These detect when a person or object is in a hazardous area. Sensors can use light, radio frequency or a physical probe to watch the area. If a sensor detects an object, it can stop the machine from running or prevent it from starting.
• Emergency stop controls. These initiate the machine's designed emergency stopping function. They may stop hazardous movement without disconnecting every supply, and do not replace guards or isolation. They may be needed for:
  • Machine malfunction. If a machine is defective or malfunctioning, there must be a way to cut the power and stop it immediately.
  • Injury prevention. In a busy environment, if a person gets in the way of a machine, there must be a way to cut the power to it.

An emergency stop push button is used because it is quick and easy to reach. The master switch on a machine may not be somewhere easy to reach in an emergency, which makes the push button extremely important and often a key requirement in certain industries. Inspect all areas of your machines and production lines carefully to identify the danger zones. When a risk area that would benefit from an emergency stop is identified:

• make sure the emergency stop is easy for an operator to reach
• make sure operators can press the push button quickly and effectively
• make sure the emergency stop is clearly visible. Stations are commonly bright yellow with a red push button, which makes them easy to identify, and nothing should block them from view
• reset must not itself restart the machine; the complete safeguarding system and authorised restart procedure must be checked before operation
• do not use an emergency stop as the isolation point for maintenance. Securely isolate relevant supplies, prevent reconnection and control stored energy

Summary notes for Element 9 · Cheat sheet · Glossary. A study aid, not published or endorsed by NEBOSH.