ASSESSING THE RISK

It is not such a nightmare to implement new safety regulations, as long as you know what you are doing. Kevin Ives of Pilz explains

All of the new regulations effecting machinery call for risk assessment to be carried out to ensure that machines are safe to use and maintain. In the case of new machinery, under The Supply of Machinery (Safety) regulations, these assessments must be carried out during design, and hazards either designed out or protected against. The regulation is a supply or trade regulation and was negotiated by a team on behalf of the DTI.

Any machine, new or old, that is placed for use in the workplace is covered by The Provision and Use of Work Equipment Regulations (PUWER). At the time that PUWER was introduced onto the statute books, another regulation, the Management of Health and Safety at Work Regulations 1992 (MHSWR) was also brought into effect. These regulations, like PUWER, were made under the 1974 Health and Safety at work Act. The key point of these regulations is to make employers aware of the risks to employees. They call for all employers to make a `suitable and sufficient assessment of the risks to the health and safety of his employees to which they are exposed at work’.

The findings must be recorded, and where the risks are unacceptable, control measures must be put into effect to reduce the risks.

Different types of machines have different levels of associated risk, and it is these levels that need to be identified to decide whether the existing control measures are effective as required by the regulations. Some useful definitions from the harmonised standards are summarised in Table 1.

There are several risk assessment methods and techniques outlined in EN 1050 Safety of Machinery: Risk Assessment. Some are based on mathematical models and require access to failure rate and reliability data. These are more suitable for assessing large process plant than relatively simple production machinery.

One method of assessment is based on the Hazard Rating Number (HRN) technique favoured by ROSPA. It is a qualitative method that seeks to become quantitatively allocating numbers to generate a `pass or fail’.

Although not intended to be a substitute for these scientifically proven systems, most engineers will be able to produce risk assessment data suitable to enable compliance with all the regulations.

The method takes into account four criteria and seeks to quantify a level of risk and qualify it, (Table 2). The criteria used to arrive at these levels are identified in Table 3 with a numerical value assigned to each choice option.

Each non trivial risk needs to be identified and assessed and if required by this assessment, control measures will need to be applied. The method relies on the judgement of the assessor. He must decide not only what constitutes a non trivial risk, but also the most appropriate choice from the lists. For this reason, the author can take no responsibility for the validity of the assessment. A number of important points must be borne in mind by the assessor. Always take into account the skill, awareness and boredom factors of the exposed person. People working on repetitive jobs may not be aware of deterioration or failures of equipment. Never conduct a risk assessment without the cooperation of operators and supervisors. They may be more aware of the hazards than the assessor. If the hazard requires measurement to decide its level (eg noise), then these must also be taken.

Hazards from machinery are mainly mechanical and include, but are not limited to, crushing, shearing, entanglement, stabbing, friction, loss of stability, drawing in and ejection of parts. They should all be examined. A check against those hazards identified in the regulations is the best approach.

There are also non mechanical hazards which will need to be assessed if relevant. These include, but are not limited to, high or low temperature, noise, electric shock, effect of chemical contact or inhalation, fire, explosion and the effects of laser light.

Details of risk assessments must be fully documented, with findings and recommendations. Any recommendations involving upgrading of control measures must be implemented with minimum possible delay. Risk assessments are never finished. If the work or the way the work equipment is used changes, then the risks must be reassessed.

If training is identified as a significant measure under any assessment, then it must be structured and continuously assessed. If setting, cleaning and maintaining of equipment presents risks other than or different from those presented to the operators, then these must be addressed separately, and if necessary specific control measures put in place.

To illustrate the use of this method of assessment, it is best to look at an example. The main hazard on a production machine might be identified as trapping between the tool post and the chuck during parts loading.

To assess this hazard on an existing machine under PUWER, ignore all safety devices already in place. This is because the assessment is a two step process. The first step is to arrive at a conceptual control measure. For this to be achieved, the hazards must be assessed in their `raw’ state. The second step is to compare this conceptual measure with the existing control measures. If the existing measures are as good as, or better than the concept, then no action is needed. But if the existing measure is compromised, then work must be done to bring it up to standard.

If this were a new machine, then there would be no existing control measure and the assessor would be initially looking at the possibility of designing the hazard out. Only if this were not possible would he need to look at suitable control measures. Table 4 shows the criteria used. The score from this hazard indicates a high risk, therefore control measures will need to be applied. ( LO = 2 * FE = 5 * DPH = 6 * NP = 1 ) = 60 = High risk

At this point, the assessor must consider putting in place a control measure to bring this risk down to an acceptable level. This can be looked at in two ways. If the hazard can be removed or lessened, then this must be done first. If, as with most existing machines, the hazard is fixed by design or otherwise not capable of being removed or lessened, then the only action possible is to protect against it.

To protect the operator from the hazard identified above, a suitable guard must be fitted to prevent access whilst the machine is in motion and to prevent machine movements whilst the operator is loading parts.

Now the designer must look at what sort of guard could be used. Clearly, a fixed and bolted guard cannot be considered, as access is required on a regular basis, and the time taken to remove this guard would lead, conceivably, to its bypassing or removal. The choice would seem to lie between the choosing an electrosensitive or non contact system (light curtain or pressure mat), or an interlocked fast access mechanical guard.

Once the choice is made, the hazard has to be reassessed with the control measure in place. Of the four factors, it is possible to affect at least one. There is little that can be done about the number of people at risk, or the frequency of exposure (unless the choice of control measure is to include the use of an automatic loading system). The degree of harm is also difficult to change. The only factor that can be affected by the addition of an interlocked guard is the `likelihood of occurrence’.

The reassessment is shown in Table 5. The score from this hazard suggests that this solution reduces the risk to acceptable levels. (LO = 0.033 * FE = 5 * DPH = 6 * NP = 1 ) = 0.99 = Negligible. If there are control measures in place, the mechanical aspects should be compared with the conceptual requirements and the recommendations of the guarding standards. If they are in agreement, there is no action to be taken. If there are no guards, or the quality of the guards is not as required, remedial action must be taken. If it is not possible to reach a level low enough to be acceptable by physical means, then the addition of notices and special training may be the only way forward. These are acceptable measures, but should be the final items to be considered.

When the full risk assessment has been carried out on all the non trivial risks, and the control measures have been chosen and the risks reassessed to an acceptable level, then look at the assessment of the control system.

The method used to categorise the general risks is a simple technique, but is not officially backed by regulations or standards. The control system assessment however, is documented in EN 954 and similarly offers a method to enable the assessor to categorise the required system. Figure 1 is taken from the standard. Here, the control system requirement can be placed in any one of five categories. Similarly, as for the general assessment the same requirements for care in choosing the selection options is all important. Once a category has been identified the standard goes on to detail the technical requirements. Most engineers are familiar with the chart and can use it to give an estimate of performance of the safety system under fault conditions.

Once the risk category requirements for the control system have been identified, then the existing control arrangements can be checked against the assessment. The choices for the control system options are shown in Table 6.

The table suggests a control system with the performance characteristics of category 3, which calls for operational redundancy. Now, if the production machine was fitted with a guard to the standard suggested by the general assessment, it should also require an interlock system involving two guard switches operating together with cross monitoring. Taking this action would ensure that if a switch failed, then there would be no possibility of continued movement when the guard is opened (taking into account potential lack of operator awareness on repetitive jobs), and also no chance of start up during the loading stage. To achieve this level of security it is necessary to look at the complete safety related control system to determine whether the existing arrangements meet the needs of category 3.

Before looking at what is needed to achieve compliance, it should be remembered that the regulation qualifies its requirements with the phrase `in so far as is reasonably practicable’. This does not mean that production machines with several guards all the way round needs to have them all interlocked to the same level. Information from the HSE points out that it is possible to categorise the different areas of the machine into different control system categories. As an example, suppose that the machine in question had four interlocked guards and the assessment suggested that the guard switches needed duplication. The cost of compliance in this case could be impractical, especially if the company had 100 such machines. A conservative estimate of the cost of retrofitting 400 switches requiring 400 brackets, the extra wiring, labour and possible lost production, could be in excess of £200,000. Looking back at the control system risk assessment, the main factor in the decision tree requiring duplication of switches is `frequency of access’. If only guard £1 is used by the operator regularly, and the others are only opened for cleaning, maintenance and setting, the there is no reason to duplicate the switches on guards £2, £3, £4. A layout for this arrangement is shown in Figure 2. As an added safety precaution, the lesser used guards should be fastened with a catch requiring a tool or key to prevent rapid access and also carry a warning notice (warn and inform of any residual risks).

It is the assessors role to make recommendations for any remedial work required under these regulations, but it is not the aim of the regulations to be Draconian. The assessor is required to be objective, to apply the concepts of practicality, and show due diligence.

Consider the situation where the risk assessment from EN 954 shows a requirement for redundant safety circuits. It may be considered practical to defer from a two channel safety control system (but not purely on cost grounds) if the conditions are dry and clean, and the safety devices and their cables are well positioned away from potential damage, but only if backed up by regular inspection and maintenance. The regularity of these inspections will need to be specified. It is important to document all the available information and any reasoning for deferring to lower levels. This information would be invaluable if the assessor were ever called upon to show due diligence in a court of law.

Figure 1: Chart taken from the EN 954 standard offers a method to categorise systems

Figure 2 Simplified diagram for guarding situation as described below, using PILZ PNOZ 16 safety relay as the fail safe interface

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{{Table 1: Definitions from the standard

Risk assessment Risk assessment is a series of logical steps to enable the elimination and reduction of the hazards associated with Work equipment in a systematic way. Risk assessment is an essential part of the iterative process of risk reductionHazard Something with the potential to cause harmRisk The likelihood that harm from the particular hazard is realisedExposed person Any person or persons who could be affected by the hazardControl measures Those measures taken to eliminate the hazard or to minimise the likelihood of occurrence to acceptable levels. (These levels are well defined in law)}}

{{Table 2: Four criteria are used to quantify risk

Negligible Presents very little risk to health and safetyLow but significant Contains hazards requiring control measuresHigh Has potentially dangerous hazards needing urgent implementation of control measuresUnacceptable Continued operation in this state is unacceptable}}

{{Table 3: Criteria used to quantify risk

Criteria 1

Likelihood of Contact with hazard (LO)occurrence

Almost impossible possible only under extreme circumstances 0.033Highly unlikely though conceivable 1Unlikely but could occur 1.5Possible but unusual 2Even chance could happen 5Probable not surprised 8Likely only to be expected 10Certain no doubt 15

Criteria 2

Frequency of exposure to the hazard (FE)

Annually 0.5Monthly 1Weekly 1.5Daily 2.5Hourly 4Constantly 5

Criteria 3

Degree of possible harm (take into account the worst case) (DPH)

Scratch/bruise 0.1Laceration/mild ill effect 0.5Break minor bone or minor illness (temporary) 2Break major bone or major illness (temporary) 4Loss of 1 limb, eye, hearing loss (permanent) 6Loss of 2 limbs, eyes (permanent) 10Fatality 15

Criteria 4

Number of persons exposed to the hazard (NP)

1 – 2 persons 13 – 7 persons 28 – 15 persons 416 – 50 persons 850 + persons 12}}

{{Table 4: Criteria used to assess hazard

Likelihood of occurrence Chosen because it is unusual andPossible would probably only occur if the operator was distracted during the loading operationFrequency of exposure The machine cycle time is less thanConstantly 2 minutesDegree of possible harm The machine feed is hydraulic and theLoss of 1 limb pressure could crush the whole forearmNumber of persons at risk Only the operator is at risk from this1 – 2 hazard}}

{{The Sum of the value of the chosen options will identify a risk

category as:- 0 – 5Negligible. 50 – 500 HighLow but significant. 500 + Unacceptable.

The calculation is: LO * FE * DPH *NP = LEVEL OF RISK 5 – 50}}

{{Table 5: Risk reassessment

Likelihood of occurrence Almost impossible The machine can not move with the guard openFrequency of exposure ConstantlyDegree of possible harm Loss of 1 limbNumber of persons at risk 1 – 2}}

{{Table 6: Choices for the control system