6.3 Protective devices Flashcards
Regulation 11 of PUWER places an absolute duty on the employer to take effective measures to either:
prevent access to any dangerous part of machinery or to any rotating stock-bar to stop the movement of any dangerous part of machinery or rotating stock-bar before any part of a person enters a danger zone.
The hierarchy for machinery guarding, which should be applied so far as is ‘practicable’ is: 3
Fixed guards to the extent that it is practicable to do so, but where or to the extent that it is not, then … Other guards or protection devices to the extent that it is practicable to do so, but where or to the extent that it is not, then … Protection appliances.
Protection devices do not prevent access to the danger zone but stop the movement of the dangerous part before contact is made. They will normally be used in conjunction with a guard. Examples include: 4
trip devices, light curtains, pressure-sensitive mats and two-hand controls.
Protection appliances are used to
hold or manipulate a work piece at a machine while keeping the operators body clear of the danger zone. They are commonly used in conjunction with manually fed woodworking machines.
All guards and protection devices should be designed, fitted and used in accordance with the following principles: 8
be suitable for the purpose for which they are provided be of good construction, sound material and adequate strength be maintained in an efficient state, in efficient working order and in good repair not give rise to any increased risk to health or safety not be easily bypassed or disabled be situated a sufficient distance from the danger zone not unduly restrict the view of the operating cycle of the machinery, where a view is necessary allow safe access for maintenance work, restricted to the area where the work is to be carried out and, if possible, without having to dismantle the guard or protection device.
A fixed guard may be: (2 types)
enclosing guarddistance guard
a distance guard which does not completely enclose the danger zone but prevents access by virtue of its dimensions and distance from the danger zone, for example: 2
- a perimeter fence (Figure 6.22) - a tunnel guard (Figure 6.23).
Advantages and disadvantages of fixed guards
Advantages Low cost. Easy to monitor by visual inspection. Can only be defeated by intentional act. Minimal on-going maintenance – no moving parts or complex systems. May offer some protection from other hazards such as noise, dust or ejected parts and materialsDisadvantages Removal of the guard does not stop the machine. If the guard needs to be removed regularly for maintenance it is likely to be left off. May impede operators view. May impede ventilation.
The second level in the hierarchy allows a choice from a broad range of guards and protection devices. Other guards include: 4
Interlocked (moveable) guards Adjustable guards Self-adjusting guards Automatic (sweep) guards.
The second level in the hierarchy allows a choice from a broad range of guards and protection devices. Protection devices include: 4
Mechanical trip devices Active opto-electronic devices (AOED) such as light curtains Pressure-sensitive mats Two-hand controls.
Advantages and disadvantages of interlocked guards
Advantages Convenient where frequent access is required. Allow safe access when opened – machine is powered down. Disadvantages Complex systems requiring regular testing and maintenance. Components may fail in use. May be defeated (spare keys, stuck contacts etc.). Failure may not be apparent until after an incident.
Advantages and disadvantages of self-closing guards
Advantages Disadvantages Automatically adjust to accommodate the work piece. Disadvantages Only provide a partial guarding solution – still allows some access to the dangerous parts. Require regular inspection and maintenance.
Advantages and disadvantages of adjustable guards
Advantages Disadvantages Probably the only practical solution where a work piece needs to be brought into contact with a cutting tool, especially if used in conjunction with a protection appliance. Disadvantages Have to leave part of the cutting tool accessible for it to do its job. May be set up to accommodate largest job of a shift, rather than adjusted to best setting for each job. Can be removed and machine is still useable.
(a) Outline what is meant by the term ‘fixed guard’ and ‘automatic guard’ in relation to machinery safety AND identify the circumstances where each type of guard might be appropriate AND give a typical example in EACH case. 8 marks
(a) A fixed guard is a guard which is not connected in any way to the controls, motion or hazardous condition of a machine and is fixed to the machine in such a manner, for example with screws, nuts or by welding, that it can only be opened or removed by the use of special tools or by the destruction of the means of fixing. It is an appropriate method of providing protection against mechanical hazards when infrequent or no access is required to dangerous parts of a machine during its normal operation. A typical example of its use would be as a guard for a belt and pulley drive. An automatic guard is a guard connected to the machine mechanism which, when the machine is operated, pushes the operator away from the danger area. It is generally used on slow moving, long stroke machines such as certain types of press.
(b) To ensure that machine operators are adequately protected, describe the factors to be considered in the design and use of: (i) fixed guards (ii) automatic guards 6 marks 6 marks
(b) (i) With respect to the design features of a fixed guard, factors include: the material of construction, which should be sufficiently robust to withstand the rigours of the workplace and be able to contain any ejected material, but still allow sight of the process when required the method of fixing, usually requiring the use of a special tool for the guard’s removal the need to ensure that any necessary openings in guards are such that they do not allow access to the dangerous parts the need to address the possibility of the guard reverberating and exacerbating a noise problem. (b) continued … Factors to be considered in the use of fixed guards include: monitoring and supervision to ensure that the guard is not compromised safe systems of work for the carrying out of maintenance operations with the guard removed the provision of information and training for both operators and maintenance staff. (ii) For automatic guards, factors to be considered would be: the compatibility of the guard with the machine function and the convenience of its use the speed of movement of the machine since this type of guard would be inappropriate on fast moving machines the height and reach of the operator the force of movement of the guard together with the possibility that the operator might be crushed between the guard and an adjacent fixed object or structure the possibility that the guard might fail to danger the ease or difficulty with which the guard could be defeated the training that would have to be given to operating and maintenance staff.
