Third video in the "Risk assessment for mechanical designers" series
The previous video was about the risk graph and how to use it.
Todays video is about how to decide on the values of the four variables used in the risk graph, i.e. severity, duration of stay, probability of occurrence and the possibility of avoidance.
This assessment is always subjective. So you won't always agree with my opinion, but I would still like to show you on an example, how it could be done.
Therefore I want to take a closer look at the example from last time. You probably remember it.
Here, a lid is to placed and then locked in. The lid is put on by a machine. To simplify the locking, pressure is applied to the lid by the machine. The operator then locks the lid sideways via the brackets. There is a crushing or clipping point between the lid and the crate. There are other danger points, but we are only looking at this one now.
The first variable is the severity of the possible injury. I will use Docufy's risk graph again because it uses the same distinction in severity as the performance level. The key distinction is between reversible and irreversible injury.
So imagine what such an accident might look like in concrete terms. The operator has their finger on the front edge of the box. The lid lowers onto the finger. Force is exerted on the lid. In the worst case, the finger could be clipped if it is between the lid and the crate. This would probably cause an irreversible injury - i.e. S3. In my opinion the finger could not be healed again, the fingertip would have to be amputated. Therefore, S3 can be taken as a starting point with no risk reduction. But you cannot leave it at S3. Next you have to look at risk reduction.
To reduce the risk in terms of severity, one can now reduce the force on the finger. In the case of the machine, this was done by applying the force to push down the lid only when the lid is already in the box. Before that, only the force that is generated when the lid is lowered is effective. This could possibly reduce the severity to S2, i.e. reversible injury.
In the risk assessment it could then look like this.
But this is only a constructed example. The reality was different. What was not visible here is that the machine is operated with the thigh through a pressure plate.
Since the operator themself is operating the pressure plate, they have to keep pressing on the plate, even though they are already hurting themself considerably, since they have their finger in the danger zone. It is inconceivable that this could lead to the finger being clipped off. As soon as it hurts, they stop pressing. So the real scenario leads to S2 - severe reversible injury and not S3 - irreversible injury. Perhaps one could even justify S1.
What could happen that makes my first scenario worse? The operator could, for some reason, when they have their finger in the danger zone, lose their balance and hit the pressure plate. In that case, the speed of the machine ensures that the finger is not clipped off. One could consider this scenario separately and enter the speed of lowering the lid here as a risk reduction. You would do this especially if the speed was a particularly concern during construction. I am not doing that now and am sticking with S2. I therefore do not consider the scenario any further; it is covered by the first scenario.
So in my scenario of the operator getting their finger caught between the lid and the box, I have decided on S2 as the severity of the possible injury without risk reduction.
I would then enter finger/hand in the risk assessment, because this makes no difference to the assessment and the operator could just as easily have their whole hand in the danger zone.
The next variable is the duration of exposure to hazard. This is very clear here. The operator is permanently at the machine, i.e. A3.
And now the probability of occurrence:
Here we have the choice between:
almost impossible
unlikely to perhaps
very likely
sure, no doubt
"Almost impossible" is often used when two abnormal situations have to meet for the scenario to occur. So in my second scenario of someone reaching into the danger area and hitting the pressure plate at the same time, these two events would have to be independent of each other. In my case, however, the pain could cause the knee to bump against the pressure plate and making the accident worse.
For getting your finger between the lid and the box and squeezing there, I now have a choice between "almost impossible", "unlikely to perhaps" and "very likely" - likely doesn't exist. I would rule out "very likely" here because the danger zone is very clearly visible. No one deliberately puts their finger in there and actually the hands have to be at the sides of the box on the brackets anyway.
The second question that plays a role here is the question of whether there is a reason to stick one's finger in. The answer to this question would be "yes". If there is a small crumb of the contents on the rim, this would be conceivable as a reflex - I'll quickly push it into the box. Especially if you underestimate the speed at which the lid is lowered or, as is more likely to happen with an experienced operator, overestimate your own speed. "Almost impossible" is no longer conceivable. And on top of that, it's easy to get your finger in there. There isn't a small hole somewhere in the back of the machine where no finger has any business being. You could even get your finger on the front edge of the box if you were careless. So from my point of view, the only reasonable assessment is "unlikely to perhaps".
And at last possibility of avoidance:
M1 possible by reflex
M2: not possible
Here I would assume that it is still possible to flinch away and therefore choose M1. The possibility of avoidance depends on whether I can notice the danger approaching and on the speed with which the danger approaches. Another factor is whether I have enough space to flinch away.
So we come to R6:
The risk of getting your hand caught between the lid and the box cannot be considered negligible. It is a medium risk and special considerations must be made to reduce this risk. A serious injury S2 that may occur - the assesment can't stay that way.
Typical first thought? Let's put a guard on it!
OK, now the electrical designer comes in and determines the Performace Level. I will go into this in more detail in one of the next videos. With a reversible injury, I arrive at PL b - with an irreversible injury, however, I arrive at PL d. That's why the difference between severe reversible injury and severe irreversible injury is very important to me.
So we put a guard on it as a risk reduction. What does this change in the risk assessment? If I correctly dimension the guard and can exclude manipulation, the residual risk is 0 and the consideration for this hazard point can be closed.
But can I exclude manipulation in this case? With a lot of effort, perhaps, but it will be difficult because the workflow would be severely impeded by the guard. If this is the case, the guard only reduces the probability of occurrence to almost impossible. I always take almost impossible when extra protective devices have to be overridden to make the accident possible. This leads to R4 and we are in the "low risk" range. That's OK.
If I had started from S3, I would not get out of the "medium risk" range with the guard- I would still be at R5.
There are better solutions. Actually, I would want to reduce the severity to a minor injury or I would want the probability of occurrence to be impossible. Then I would have residual risk 0.
It is therefore necessary to ensure in another way that the operator does not have their finger between the box and the lid when the lid moves downwards. The manufacturer has found a simple solution here. The side brackets have been fitted so that they hang down vertically and make it impossible to put the lid on if they are not held to the side. This means that both hands really have to be where they are supposed to be, namely on the side of the brackets. If the finger is between the lid and the box, the lid cannot touch down on the box.
A nice constructive solution that leads to residual risk 0 for the danger zone under consideration.
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