Machinery Safety 101

Scoring Severity of Injury – Hidden Probabilities

This entry is part 8 of 9 in the series Risk Assess­ment

I’ve been think­ing a lot about risk scor­ing tools and the algorithms that we use. One of the key ele­ments in risk is the Sever­ity of Injury. There are hid­den prob­ab­il­it­ies attached to the Sever­ity of Injury scores that are assigned that are not dis­cussed clearly in any of the risk assess­ment stand­ards that are com­monly in use. This all star­ted when I was chal­lenged to write an ana­lys­is of the prob­lems with the CSA Risk Scor­ing Tool that you can find in the 2014 ver­sion of CSA Z434. That tool is deeply flawed in my opin­ion, but that is not the top­ic of this post. If you want to read my ana­lys­is, you can down­load the white paper and the present­a­tion notes for my ana­lys­is from the Com­pli­ance inSight Pub­lic­a­tions page [1].

Scor­ing risk can be a tricky thing, espe­cially in the machinery sec­tor. We rarely have much in the way of real-world data to use in the ana­lys­is, and so we are left with the opin­ions of those build­ing the machine as the basis for our eval­u­ation. Sever­ity is usu­ally the first risk para­met­er to be estim­ated because it’s seen as the “easy” one – if the char­ac­ter­ist­ics of the haz­ard are well known. One aspect of sever­ity that is often missed is the prob­ab­il­ity of a cer­tain sever­ity of injury. We’re NOT talk­ing about how likely it is for someone to be injured here; we’re talk­ing about the most likely degree of injury that will occur when the per­son inter­acts with the haz­ard. Let me illus­trate this idea anoth­er way: Let’s call Sever­ity “Se”, any spe­cif­ic injury “I”, and the prob­ab­il­ity of any spe­cif­ic injury “Ps”. We can then write a short equa­tion to describe this rela­tion­ship.

Se f (I,Ps)

Since we want there to be a pos­sib­il­ity of no injury, we should prob­ably relate these para­met­ers as a product:

Se = I x Ps

Ok, so what? What this equa­tion says is: the Sever­ity (Se) of any giv­en injury (I), is the product of the spe­cif­ic type of injury and the prob­ab­il­ity of that injury. More simply yet, you could say that you should be con­sid­er­ing the most likely type of injury that you think will occur when a per­son inter­acts with the haz­ard. Con­sider this example: A work­er enters a robot­ic work cell to change the weld tips on the weld­ing gun the robot uses. This task has to be done about once every two days. The entry gate is inter­locked, and the robot was locked out before entry. The floor of the work cell has wire­ways, con­duits and pip­ing run­ning across it from the edges of the cell to the vari­ous pieces of equip­ment inside the cell, cre­at­ing uneven foot­ing and lots of slip and trip haz­ards. The work­er misses his foot­ing and falls. What can you expect for Se in this case?

We know that falls on the same level can lead to fatal­it­ies, about 600/year in the USA [2], but that these are mostly in the con­struc­tion and min­ing sec­tors rather than gen­er­al man­u­fac­tur­ing. We also know that broken bones are more likely than fatal­it­ies in falls to the same level. About a mil­lion slips and falls per year res­ult in an emer­gency room vis­it, and of these, about 5%, or 50,000, res­ult in frac­tures. Ok, so what do we do with this inform­a­tion? Let’s look at typ­ic­al sever­ity scale, this one taken from IEC 62061 [3].

Table 1 – Sever­ity (Se) clas­si­fic­a­tion [2, Table A.1]

Con­sequences Sever­ity (Se)
Irre­vers­ible: death, los­ing an eye or arm 4
Irre­vers­ible: broken limb(s), los­ing a finger(s) 3
Revers­ible: requir­ing atten­tion from a med­ic­al prac­ti­tion­er 2
Revers­ible: requir­ing first aid 1

Using Table 1, we might come up with the fol­low­ing list of pos­sible sever­it­ies of injury. This list is not exhaust­ive, so feel free to add more.

Table 2 – Poten­tial Injury Sever­it­ies

Pos­sible Injury Sever­ity (Se)
Fall on same level – Fatal­ity 4
Fall on same level – Broken wrist 3
Fall on same level – Broken col­lar­bone 3
Fall on same level – Torn rotat­or cuff 2
Fall on same level – Bruises 1
Fall on same level – Head Injury 3
Fall on same level – Head Injury 4

How do we score this using a typ­ic­al scor­ing tool? We could add each of these as line items in the risk register, and then assess the prob­ab­il­ity of each, but that will tend to cre­ate huge risk registers with many line items at very low risks. In prac­tice, we decide on what we think is the most likely degree of injury BEFORE we score the risk. This res­ults in a single line item for the haz­ard, rather than sev­en as would be the case if we scored each of these poten­tial injur­ies indi­vidu­ally.

We need a prob­ab­il­ity scale to use in assess­ing the like­li­hood of injur­ies. At the moment, no pub­lished scor­ing tool that I know of has a scale for this, so let’s do the simple thing: Prob­ab­il­ity (Ps) will be scored from 0 – 100%, with 100% being a cer­tainty.

Going back to the second equa­tion, what we are really doing is assign­ing a prob­ab­il­ity to each of the sever­it­ies that we think exist, some­thing like this:

Table 3 – Poten­tial Injur­ies and their Prob­ab­il­it­ies

Pos­sible Injury (I) Sever­ity (Se) Prob­ab­il­ity (Ps)
Fall on same level – Fatal­ity 4  0.0075%
Fall on same level – Broken wrist 3  5%
Fall on same level – Broken col­lar­bone 3  5%
Fall on same level – Torn rotat­or cuff 2  5%
Fall on same level – Bruises 1  90%
Fall on same level – Head Injury 3 1%
Fall on same level – Head Injury 4   0.0075%
Fall on same level – Lacer­a­tions to hands 2 90%

The per­cent­ages for fatal­it­ies and frac­tures we taken roughly from [1]. Ok, so we can look at a table like this and say that cuts and bruises are the most likely types of injury in this case. We can either decide to group them for the over­all risk score, or we can score each indi­vidu­ally, res­ult­ing in adding two sep­ar­ate line items to the risk register. I’m going to use the oth­er para­met­ers from [2] for this example, and devel­op an example risk register, Table 4. In Table 4,

Se = Sever­ity

Pr = Prob­ab­il­ity of the Haz­ard­ous Event

Fr = Fre­quency and Dur­a­tion of Expos­ure

Av = Pos­sib­il­ity to Avoid or Lim­it Harm

The algorithm I am using to eval­u­ate the risk is R = Se x [Pr x (Fr + Av)] [1]. Note that where I have com­bined the two poten­tial injur­ies into one line item (Item 1 in the register), I have selec­ted the highest sever­ity of the com­bined injur­ies. The less likely sever­it­ies, and in par­tic­u­lar the fatal­it­ies, have been ignored. You can click on  Table 4 to see a lar­ger, more read­able ver­sion.

Table 4 - Example Risk Register
Table 4 – Example Risk Register

Note that I did not reduce the Se scores in the Final Risk Score, because I have not made changes to the slip/trip and fall haz­ards, only to the like­li­hood of the injury occur­ring. In all cases, we can show a sig­ni­fic­ant risk reduc­tion after mit­ig­a­tion. I’m not going to get into risk eval­u­ation (i.e., Is the risk effect­ively con­trolled?) in this par­tic­u­lar art­icle, but the fact that you can show a sig­ni­fic­ant risk reduc­tion is import­ant. There are lots of con­sid­er­a­tions in determ­in­ing if the risk has been effect­ively con­trolled.

Conclusions

Con­sid­er­a­tion of the prob­ab­il­ity of cer­tain kinds of injur­ies occur­ring must be con­sidered when estim­at­ing risk. This pro­cess is largely undoc­u­mented but nev­er­the­less occurs. When risk ana­lysts are con­sid­er­ing the sever­ity of injury from any giv­en haz­ard, this art­icle gives the read­er one pos­sible approach than could be used to select the types of injur­ies most likely to occur before scor­ing the rest of the risk para­met­ers.

References

[1] D. Nix, ‘Eval­u­ation of Prob­lems and Chal­lenges in CSA Z434-14 Annex DVA Task-Based Risk Assess­ment Meth­od­o­logy’, 2015.

[2] Nation­al Floor Safety Insti­tute (NFSI), ‘Quick Facts – Slips, Trips, and Falls’, 2015. [Online]. Avail­able: http://nfsi.org/nfsi-research/quick-facts/. [Accessed: 21- Jul- 2015].

[3] ‘Safety of machinery – Func­tion­al safety of safety-related elec­tric­al, elec­tron­ic and pro­gram­mable elec­tron­ic con­trol sys­tems. IEC 62061.’, Inter­na­tion­al Elec­tro­tech­nic­al Com­mis­sion (IEC), Geneva, 2005.

 

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