System Safety Concepts, Part 1

System Safety Concepts – a Short Introduction

System Safety Concepts – Transcript

Hi everyone and welcome to the Safety Artisan, where you will find professional, pragmatic and impartial advice. Whether you want to know how safety is done or how to do it. I hope you’ll find today’s session helpful. It’s the 21st of September 2019 as I record this. Welcome to the show. So, let’s get started. Well, we’re going to talk today about System Safety concepts. What does it all mean?  We need to ask this question because it’s not obvious, as we will see.

If we look at a dictionary definition of the word safe, it’s an adjective. To be protected from or not exposed to danger or risk. Not likely to be harmed or lost. There are synonyms – protect, shield, shelter, guard and keep out of harm’s way. They’re all good words, and I think we all know what we’re talking about. However, as a definition, it’s too imprecise. We can’t objectively say whether we have achieved safety or not.

A Practical Definition of ‘Safe’

What we need is a better definition, a more practical definition. I’ve taken something from an old UK Defence standard. Forget about which standard, that’s not important. It’s just that we’re using a consistent set of definitions to work through basic safety concepts. And it’s important to do that because different standards, come from different legal systems and they have different philosophies. So, if you start mixing standards and different concepts together, that doesn’t always work.

OK so whatever you do, be consistent. That’s the key point. We’re going to use this set of definitions from the U.K. defence standard because they are consistent.

In this standard, ‘safe’ means: “Risk has been demonstrated to have been reduced to a level that is ALARP, and broadly acceptable or tolerable. And relevant prescriptive safety requirements have been met. For a system, in a given application, in a given Operating Environment.” OK, so let’s unpack that.

System Safety – Risk

So, we start with risk. We need to manage risk. We need to show that risk has been reduced to an acceptable level. As required perhaps by law, or regulation or a standard. Or just good practice in a particular industry. Whatever it is, we need to show that the risk of harm to people has been reduced. Not just any old reduction, we need to show that it’s been reduced to a particular level. Now in this standard, there are two tests for that.

And they’re both objective tests. The first one says as low as reasonably practicable. Basically, it’s asking have all reasonably practicable risk reduction measures been taken. So that’s one test. And the second test is a bit simpler. It’s basically saying reduce the absolute level of risk to something that is tolerable or acceptable. Now don’t worry too much about precisely what these things mean. The purpose for today is to note that we’ve got an objective test to say that we’ve done enough.

System Safety – Requirements

So that’s dealt with risk. Let’s move on to safety requirements. If a requirement is relevant, then we need to apply it. If it’s prescriptive, if it says you must do this, or you must do that. Then we need to meet it. There are two separate parts to this ‘Safe’ thing: we’ve got to meet requirements; and, we’ve got to manage risk. We can’t use one as an excuse for not doing the other.

So just because we reduce risk until it’s tolerable or acceptable doesn’t mean that we can ignore safety requirements. Or vice versa. So those are the two key things that we’ve got to do. But that’s not actually quite enough to get us there. Because we’ve got to define what we’re doing, with what and in what context. Well, we’re reducing the risk of a system. And the system might be a physical thing.

Defining the Scope: The System

It might be a vehicle, an aeroplane or a ship or a submarine, it might be a car or a truck. Or it might be something a bit more intangible. It might be a computer program that we’re using to make decisions that affect the safety of human beings, maybe a medical diagnosis system. Or we’re processing some scripts or prescriptions for medicine and we’ve got to get it right. We could poison somebody. So, whether it’s a tangible or an intangible system.

We need to define it. And that’s not as easy as it sounds, because if we’re applying system safety, we’re doing it because we have a complex system. It’s not a toaster. It’s something a bit more challenging. Defining the system carefully and precisely is really important and helpful. So, we define what our system is, our thing or our service. The system. What are we doing with it? What are we applying it to?

Defining the Scope: The Application

What are we using it for? Now, just to illustrate that no standard is perfect. Whoever wrote that defence standard didn’t bother to define the application. Which is kind of a major stuff-up to be honest, because that’s really important. So, let’s go back to an ordinary dictionary definition just to get an idea of what it means. By the way, I checked through the standard that I was referring to, and it does not explain in this standard.

What it means by the application. Otherwise, I would use that by preference. But if we go back to the dictionary, we see application: the act of putting something into operation. OK, so, we’re putting something to use. We’re implementing, employing it or deploying it maybe we’re utilizing it, applying it, executing it, enacting it. We’re carrying it out, putting it into operation or putting it into practice. All useful words that help us to understand.

I think we know what we’re talking about. So, we’ve got a thing or a service. Well, what are we using it for? Quite obviously, you know a car is probably going to be quite safe on the road. Put it in water and it probably isn’t safe at all. So, it’s important to use things for their proper application, to the use to which they were designed. And then, kind of harking back to what I just said, the correct operating environment.

Defining the Scope: The Operating Environment

For this system, and the application to which we will put it to. So, we’ve got a thing that we want to use for something. What’s the operating environment in which it will be safe? What’s it qualified or certified for? What’s the performance envelope that it’s been designed for? Typically, things work pretty well within the operating environment, within the envelope for which they were designed. Take them outside of that envelope and they perform not so well.

Maybe not at all. You take an aeroplane too high and the air is too thin, and it becomes uncontrollable. You take it too low and it smashes into the ground. Neither outcome is particularly good for the occupants of the aeroplane. Or whoever happens to be underneath it when it hits the ground. All of those three things:  what is the system? What are we doing with it? and where are we doing it? All those things have to be defined. Otherwise, we can’t really say that risk has been dealt with, or that safety requirements have been met.

System Safety: why Bother?

So, we’ve spent several slides just talking about what safe means, which might seem a bit over the top. But I promise you it is not, because having a solid understanding of what we’re trying to do is important in safety. Because safety is intangible. So, we need to understand what it is we’re aiming for. As some Greek bloke said, thousands of years ago: “If you don’t know to which port, you are bound, then no wind is favourable.”

It’s almost impossible to have a satisfactory Safety Program if you don’t know what you’re trying to achieve. Whereas, if you do have a precise understanding of what you’re trying to achieve, you’ve got a reasonably good chance of success. And that’s what it’s all about.

Copyright Statement

Well, I’ve quoted you some information. From a UK government web site. And I’ve done so. In accordance with the terms of its creative commons license and you can see. More information about the terms of that can be found at this page.

The Full Version is Here…

If you want more, if you want to unpack all the Major Definitions, all the system safety concepts that we’re talking about, then there’s a longer version of this video. Which you can get at my Patreon page.

I hope you enjoy it. Well that’s it for the short video, for now. Please go and have a look at the longer video to get the full picture. OK, everyone, it’s been a pleasure talking to you and I hope you found that useful. I’ll see you again soon. Goodbye.

Author: Simon Di Nucci

Hi everyone, I'm Simon and I have been a system safety engineer for over twenty years. For my full bio, please see my LinkedIn page.

Leave a Reply

Your email address will not be published. Required fields are marked *