Principles, Standards and Implementation

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Safety Strategy

Risk Estimation

This is one of the most fundamental aspects of risk assessment. There are many ways of tackling this subject and the following pages illustrate the basic principles.

Any machinery that has potential for hazardous situations presents a risk of a hazardous event (i.e. of harm). The greater the amount of risk, the more important it becomes to do something about it. At one hazard the risk could be so small that we can tolerate and accept it but at another hazard the risk could be so large that we need to go to extreme measures to protect against it. Therefore in order to make a decision on “if and what to do about the risk,” we need to be able to quantify it.

Risk is often thought of solely in terms of the severity of injury at an accident. Both the severity of potential harm AND the probability of its occurrence have to be taken into account in order to estimate the amount of risk present.

The suggestion for risk estimation given on the following pages is not advocated as the definitive method as individual circumstances may dictate a different approach. IT IS INTENDED ONLY AS A GENERAL GUIDELINE TO ENCOURAGE A METHODICAL AND DOCUMENTED STRUCTURE.

The point system used has not been calibrated for any particular type of application therefore is not necessarily suitable for any specific application. ISO TR (Technical Report) 14121-2 “Risk assessment – Practical guidance and examples of methods” provides practical guidance and some shows different methods for quantification of risk.

The following factors are taken into account:

• THE SEVERITY OF POTENTIAL INJURY.
  
• THE PROBABILITY OF ITS OCCURRENCE.
  

The probability of occurrence includes two factors:

• FREQUENCY OF EXPOSURE.
  
• PROBABILITY OF INJURY.
  

Dealing with each factor independently we will assign values to each of these factors.

Make use of any data and expertise available to you. You are dealing with all stages of machine life, so to avoid too much complexity base your decisions on the worst case for each factor.

It is also important to retain common sense. Decisions need to take account of what is feasible, realistic and plausible. This is where a cross functional team approach is valuable.

Remember, for the purposes of this exercise you should usually not take account of any existing protective system. If this risk estimation shows that a protective system is required there are some methodologies as shown later in this chapter that can be used to determine the characteristics required.

1. Severity of potential injury

For this consideration we are presuming that the accident or incident has occurred, perhaps as a result of the hazards shown in Figure 10. Careful study of the hazard will reveal what is the most severe injury possible.

Figure 10: Potential Injury

Remember: For this consideration we are presuming that an injury is inevitable and we are only concerned with its severity. You should assume that the operator is exposed to the hazardous motion or process.

The severity of injury should be assessed as:

• FATAL: Death
  
• MAJOR: (Normally irreversible) Permanent disability, loss of sight, limb amputation, respiratory damage, etc.
  
• SERIOUS: (Normally reversible) Loss of consciousness, burns, breakages, etc.
  
• MINOR: Bruising, cuts, light abrasions, etc.
  

Each description could be assigned a points value (shown in Figure 11).

Figure 11: Points Assigned to Severity

2. Frequency of exposure

Frequency of exposure answers the question of how often is the operator or the maintenance person exposed to the hazard (Figure 12).

Figure 12: Frequency of Exposure

The frequency of exposure to hazard can be classified as:

• FREQUENT: Several times per day
  
• OCCASIONAL: Daily
  
• SELDOM: Weekly or less
  

Each description could be assigned a points value (shown in Figure 13).

Figure 13: Points Assigned to Frequency of Exposure

3. Probability of injury

You should assume that the operator is exposed to the hazardous motion or process (Figure 14).

Figure 14: How Likely

By considering the manner in which the operator is involved with the machine and other factors (speed of start up, for example) the probability of injury can be classified as:

• Unlikely
  
• Probable
  
• Possible
  
• Certain
  

Each description could be assigned a points value shown in Figure 15.

Figure 15: Points Assigned to Probability of Injury

All headings are assigned a value and they are now added together to give an initial estimate. Figure 16 shows the sum of the three components adds up to a value of 13. But we must consider a few more factors.

Figure 16: Initial Estimate

(Note: This is not based necessarily on the previous example pictures.)

The next step is to adjust the initial estimate by considering additional factors such as those shown in Table 2. Often they can only be properly considered when the machine is installed in its permanent location.

Typical Factor	Suggested Action
More than one person exposed to the hazard	Multiply the severity by the number of people
Protracted time in the danger zone without complete power isolation	If time spent per access is more than 15 minutes, add 1 point to the frequency factor.
Operator is unskilled or untrained	Add 2 points to the total.
Very long intervals (e.g., one year) between accesses. (There may be progressive and undetected failures particularly in monitoring systems.)	Add points equivalent to the maximum frequency factor.
Table 2: Additional Considerations for Risk Estimate

The results of any additional factors are then added to the previous total as shown in Figure 17.

Figure 17: Final Value with Adjustments