Hierarchy of risk reduction measures

The following table gives examples of risk reduction measures, in a descending order of hierarchy. Some examples of unacceptable options have also been included.

Hazard Examples of risk reduction measures Comment on reliability of risk reduction measures
Possibility of fatal or very serious injuries from contact or entanglement with moving machinery during an
intervention to clear blockages  etc..
Safe isolation procedure Safe electrical isolation is the most reliable way of preventing an unexpected start-up, provided that there are suitable means to ensure accidental reconnection is prevented - for example breaking the 3-phase by mechanical means and  a locking-off facility.
This should be prioritised before consideration of the options detailed in BS EN 1037.
Note: Safe electrical isolation normally refers to 'high power' isolation rather than power to the control circuit, unless this has been justified as being an acceptable option in the risk assessment.
Key exchange interlocked on perimeter fencing The gate can only be opened by a key taken from the control panel of the operator's cabin, allowing removal of a second key from the gate. The control panel key, which is required to restart machine operation, is then trapped in the mechanism until the gate key is returned. The operative in the danger area is therefore in control of the risk at all times.
Note: See examples of variations on key exchange systems in locking devices page. When maintenance work is undertaken the machine isolator should be locked off because 'power interlocking' at the control panel is not practical unless a suitable safety plc system is in place to prevent unexpected start-up. The only exception for not locking off would be for releasing a jam from a position of safety with a bar etc.
Interlocked guard with zero-speed detection or time-delay unlocking. Entry into the danger area is prevented until the dangerous movement has stopped. Zero-speed detection is considered to be more reliable than braking associated with time-delay unlocking. Where whole-body access is required the risk of re­start when in the danger area is not entirely avoided. The machine isolator should be locked off.
Interlocked gate with guard locking on perimeter fencing. The gate can only be opened once the lock is released following the shut­down of the plant and opening the gate ensures that power cannot be reapplied. This has the advantage of ensuring that plant comes to a controlled stop under operator control before the gate is opened. However, the gate could be inadvertently closed allowing machinery to be started whilst someone is still in the danger area, so this operative is not in control of the risk.  The machine isolator should be locked off.  
Simple interlocked gate on perimeter fence. The operation of the machine is automatically interrupted when someone opens the gate and enters the danger area (a system of 'control interlocking'). However, as before, the gate could be inadvertently closed allowing machinery to be started with someone in the danger area, so this operative is not in control of the risk. The machine isolator should be locked off.
Photoelectric safety systems. This may be an option to prevent access to a crushing hazard. However, the stopping performance, safety integrity standards, separation between photoelectric curtain and hazard, integration into machinery control system and the likelihood of spurious tripping etc will need careful consideration. If the photoelectric curtain is muted for maintenance work the machine isolator should be locked off.
Fixed guard. Used as a close ­in guard, this is acceptable so long as the apertures underneath the guard for the passage of material comply with the reach distance standards of BS EN 13857. If guards need to be removed for maintenance work the machine isolator should be locked off.
Padlocked access gate. Reliance on personnel entering danger zone following a safe system of work, including isolating power before opening padlock. Compliance with system unlikely to be absolute so unacceptable
Moveable barrier. This does not meet the requirements of a fixed guard and is likely to be removed so unacceptable.
System of work. This relies on an operator and control systems to keep machine stationary while someone is in the hazardous zone so unacceptable (reasonably practicable to provide more appropriate safeguarding solutions).

More information on key exchange systems can be found in BS EN 1088:1995 +A2:2008 Incorporating Corrigendum January 2009 Safety of machinery —Interlocking devices associated with guards — Principles for design and selection.

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Updated 2021-11-09