Safety Matters: Is it Safe?

Safety Matters: Is it Safe?

In my last blog, I promised to answer the question “Is it safe?”.

Safety is freedom from unacceptable risk. The word unacceptable is very important here. Obviously jumping out of a plane with a parachute is risky but for the person involved they have obviously decided it is an acceptable risk. Similarly, in all our lives we take risks from drinking hot coffee to driving cars. Some reckless people even smoke.

When you go to work your risk of dying should not be significantly raised and your employer has a duty of care to make sure you are not harmed or injured. This harm may come about while working with for instance a machine or robot.

Acceptable risk is often interpreted for a healthy adult worker as a chance of dying in a year of 1e-4/year. If the public is at risk you are deemed to have a higher duty to provide more safety and the acceptable risk is a factor of 10 lower at 1e-5/year.

A more technical discussion of the topic is given in The Safety Critical Systems Handbook section 2.1.1.

In general, your employer has 3 options and in order of priority they are

  1. Eliminate the risk
  2. Engineer a solution
  3. Warn and inform

There are many ways to eliminate a risk such as by changing the process so that a dangerous machine or chemical is not required but often they are unpalatable due to cost or other reasons. Warning and informing can be done as per the coffee cup above but is only done as a last resort (see picture of a Canadian coffee cup here).

Functional Safety is mostly about option 2, engineering a solution. For a dangerous machine, this might involve putting a sensor on the machine door so that if the door is opened the machine is stopped before your hand gets inside. In a future session, I will describe more about machine safety but for now I will just saw that typically functional safety involves 3 components 1) A sensor 2) some logic to make a decision on the sensor output and 3) an actuator to take the system to the safe state.

The combination of the above 3 elements constitutes a safety function. That safety function has 3 key properties

  1. A safety integrity level
  2. A safe state
  3. The time to achieve the safe state

While health and safety deals with the everyday safety, functional safety typically deals with the 1 in 10 year, 1 in 100 year or 1 in 1000 year accidents. Functional safety is having the confidence that a piece of equipment will carry out its safety related task when required to do so. However, it is only part of the safety spectrum which includes electrical safety, mechanical safety, intrinsic safety and many others types. 


The Video of the day is based on the Saw Stop system – see

These guys don’t make any claims for functional safety but I think it is a good illustration of a safety function – there is definitely a sensor to sense a human hand, there is definitely something to make the decision to stop the saw and thrash the motor and we can all see the actuator. The safe state is clearly to stop the saw and the time to achieve the safe state is before the saw cuts the hand.

For the next time -  What are safety integrity levels?

Note - for more on the order of priority in design see ISO 12100:2010 figure 1