Machinery Safety 101

Risk Factors

Severity

There are two key fac­tors that need to be under­stood when assess­ing risk: Severity and Probability (or Likelihood).

Severity seems to be fairly well understood—most peo­ple can fairly eas­ily imag­ine what reach­ing into a spin­ning blade might do to the hand doing the reach­ing. There is a prob­lem that arises when there is an insuf­fi­cient under­stand­ing of the haz­ard, but that’s the sub­ject for another post.

Probability

Probability or like­li­hood is used to describe the chance that an injury or a haz­ardous sit­u­a­tion will occur. Probability is used when numeric data is avail­able and prob­a­bil­ity can be cal­cu­lated, while like­li­hood is used when the assess­ment is sub­jec­tive. The prob­a­bil­ity fac­tor is often bro­ken down fur­ther into three sub-​​factors as seen in Figure 3 below [1]:

There is No Reality, only Perception…

Whether you use prob­a­bil­ity or like­li­hood in your assess­ment, there is a fun­da­men­tal prob­lem with people’s per­cep­tion of these fac­tors. People have a dif­fi­cult time appre­ci­at­ing the mean­ing of prob­a­bil­ity. Probability is a key fac­tor in deter­min­ing the degree of risk from any haz­ard, yet when fig­ures like “1 in 1000″ or “1 x 10^–5 occur­rences per year” are dis­cussed, it’s hard for peo­ple to truly grasp what these num­bers mean. When prob­a­bil­ity is dis­cussed as a rate, a fig­ure like “1 x 10^–5 occur­rences per year” can make the chance of an occur­rence seem incon­ceiv­ably dis­tant, and there­fore less of a con­cern. Likewise, when more sub­jec­tive scales are used it can be dif­fi­cult to really under­stand what “likely” or “rarely” actu­ally mean. Consequently, even in cases where the sever­ity may be very high, the risk related to a par­tic­u­lar haz­ard may be neglected if the prob­a­bil­ity is deemed low.

To see the other side, con­sider people’s atti­tude when it comes to win­ning a lot­tery. Most peo­ple will agree that “Someone will win” and the infin­i­tes­i­mal prob­a­bil­ity of win­ning is seen as sig­nif­i­cant.  The same odds given in rela­tion­ship to a neg­a­tive risk might be seen as ‘infin­i­tes­i­mally small’, and there­fore negligible.

For exam­ple, con­sider the deci­sions made by the Tokyo Electric Power Corporation (TEPCO) when they con­structed the Fukushima Dai Ichi nuclear power plant. TEPCO engi­neers and sci­en­tists assessed the site in the 1960’s and decided that a 10 meter tsunami was a real­is­tic pos­si­bil­ity at the site. They decided to build the reac­tors, tur­bines and backup gen­er­a­tors 10 meters above the sur­round­ing sea level, then located the sys­tem crit­i­cal con­densers in the sea­ward yard of the plant at a level below 10 meters. To pro­tect that crit­i­cal equip­ment they built a 5.7 meter high sea­wall, almost 50% shorter than the pre­dicted height for a tsunami! While I don’t know what ratio­nale they used to sup­port this design deci­sion, it is clear that the plant would have taken sig­nif­i­cant dam­age from even a rel­a­tively mild tsunami. The 11-​​Mar-​​11 tsunami topped the high­est pre­dic­tion by nearly 5 meters, result­ing in a Level 7 nuclear acci­dent and decades for recov­ery. TEPCO exec­u­tives have repeat­edly stated that the con­di­tions lead­ing to the acci­dent were “incon­ceiv­able”, and yet redun­dancy was built into the sys­tems for just this type of event, and some plan­ning for tsunami effects were put into the design. Clearly was nei­ther unimag­in­able or incon­ceiv­able, just underestimated.

Risk Perception

Significant results also show that bet­ter infor­ma­tion fre­quently has no effect on how risks are judged. More weight is put on risks with imme­di­ate, per­sonal results than those seen in longer time frames. Psychometric research has shown that risk per­cep­tion is highly depen­dent on intu­ition, expe­ri­en­tial think­ing, and emo­tions. The research iden­ti­fied char­ac­ter­is­tics that may be con­densed into three high order factors:

1. the degree to which a risk is understood;
2. the degree to which it evokes a feel­ing of dread; and
3. the num­ber of peo­ple exposed to the risk.

“Dread” describes a risk that elic­its vis­ceral feel­ings of impend­ing cat­a­stro­phe, ter­ror and loss of con­trol. The more a per­son dreads an activ­ity, the higher its per­ceived risk and the more that per­son wants the risk reduced [4]. Fear is clearly a stronger moti­va­tor than any degree of information.

Considering the dif­fer­ing views of those study­ing risk per­cep­tion, it’s no won­der that this is a chal­leng­ing sub­ject for safety practitioners!

Estimating Probability

Frequency and Duration

Some aspects of prob­a­bil­ity are not too dif­fi­cult to esti­mate. Consider the Frequency or Duration of Exposure fac­tor. At face value this can be stated as “X cycles per hour” or “Y hours per week”. Depending on the haz­ard, there may be more com­plex expo­sure data, like that used when con­sid­er­ing audi­ble noise expo­sure. In that case, noise is often expressed as a time-​​weighted-​​average (TWH), like “83 dB(A), 8 h TWH”, mean­ing 83 dB(A) aver­aged over 8 hours.

Estimating the prob­a­bil­ity of a haz­ardous sit­u­a­tion is usu­ally not too tough either. This could be expressed as “15 min­utes, once per day /​ shift” or “2 days, twice per year”.

Avoidance

Estimating the prob­a­bil­ity of avoid­ing an injury in any given haz­ardous sit­u­a­tion is MUCH more dif­fi­cult, since the speed of occur­rence, the abil­ity to per­ceive the haz­ard, the knowl­edge of the exposed per­son, their abil­ity to react in the sit­u­a­tion, the level of train­ing that they have, the pres­ence of com­ple­men­tary pro­tec­tive mea­sures, and many other fac­tors come into play. Depth of under­stand­ing of the haz­ard and the details of the haz­ardous sit­u­a­tion by the risk asses­sors is crit­i­cal to a sound assess­ment of the risk involved.

The Challenge

The chal­lenge for safety prac­ti­tion­ers is twofold:

1. As prac­ti­tion­ers, we must try to over­come our biases when con­duct­ing risk assess­ment work, and where we can­not over­come those biases, we must at least acknowl­edge them and the effects they may pro­duce in our work; and
2. We must try to present the risks in terms that the exposed peo­ple can under­stand, so that they can make a rea­soned choice for their own per­sonal safety.

I don’t sug­gest that this is easy, nor do I advo­cate “dumb­ing down” the infor­ma­tion! I do believe that risk infor­ma­tion can be pre­sented to non-​​technical peo­ple in ways that they can under­stand the crit­i­cal points.

Risk assess­ment tech­niques are becom­ing fun­da­men­tal in all areas of design. As safety prac­ti­tion­ers, we must be ready to con­duct risk assess­ments using sound tech­niques, be aware of our biases and be patient in com­mu­ni­cat­ing the results of our analy­sis to every­one that may be affected.

References

+DougNix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. (http://​www​.com​pli​an​cein​sight​.ca) in Kitchener, Ontario, and is Lead Author and Managing Editor of the Machinery Safety 101 blog.

Doug’s work includes teach­ing machin­ery risk assess­ment tech­niques pri­vately and through Conestoga College Institute of Technology and Advanced Learning in Kitchener, Ontario, as well as pro­vid­ing tech­ni­cal ser­vices and train­ing pro­grams to clients related to risk assess­ment, indus­trial machin­ery safety, safety-​​related con­trol sys­tem inte­gra­tion and reli­a­bil­ity, laser safety and reg­u­la­tory conformity.

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