# How does one define 'safe' in engineering?

What is safe and what is acceptable in the context of Engineering.

Specifically speaking we are in the process of designing a test rig which will pump water. What is a maximum safe temperature for the water to achieve without infringing employee safety. ie water at ${90}$ $^oC$ will burn so clearly that's not safe.

Q: How can I calculate/find in order to justify the value of the maximum safe water temperature to operate the test rig.

For something to be safe there must be no risk of injury, harm, disability or death to anyone when something is being assembled, used or in the future as a result of the something being used. Where anyone is defined as any person - whether they be employee, government or military official or member of the public.

Local laws may qualify this by setting limitations. In the English style legal system, and variations of it, everyone has an obligation of due care to not cause harm, injury, disability or death to anyone else.

Some legal jurisdictions will have codes, guidelines and best practice procedures that must be adhered to. All such codes and regulations are based on studies and investigations that have been conducted.

When engineering a system the users of the system must be considered. A safe hot water temperature for small children and people metal disabilities will be lower than a that used exclusively by adults and people without mental disabilities because small children and people with mental disabilities may not have the presence of mind to recognize dangers like adults without mental disabilities.

In the US there are hot water temperature laws. In Canada, the Canada Safety Council has information on its website.

There are other sources of information as well:

For a quick guide, the table on this website lists the time required for hot water at various temperatures to cause a scald injury.

• This may be overly semantical, but I disagree with there being "no risk." In a lot of systems, some amount of risk is inherent, but we deem them safe because we have taken all appropriate measures to mitigate the risk and caution the user to the remaining dangers. I guess that doesn't make it completely safe, but it's as safe as it will get, and I wouldn't consider that an "unsafe design." Jun 12 '15 at 12:15
• @TrevorArchibald To my mind the key here is defining 'appropriate.' We can always design things for more and more remote possibilities. In reality, we balance risk and cost - hopefully in a pretty conservative way - and decide that we are safe enough. Jun 12 '15 at 14:38
• @Ethan48 It absolutely is. Cost, functionality, risk, and application are all taken into account, and I think that's why "appropriate" is more, ahem, appropriate than "necessary." Jun 12 '15 at 15:01

A common refrain in these kind of discussions is "there is no such thing as safe, only safer." Nothing we make is perfectly safe in an absolute manner - we don't design buildings to withstand asteroid impacts and we don't design cars to survive falling off a bridge. Instead, when we say safe, we usually mean "safe enough" - the level of risk is acceptable based on an understanding of all the ways we anticipate it could fail.

The hard part of this is defining the acceptable level of risk for a given situation. In many fields, a governing body has already defined the acceptable level of risk and written a code/standard/regulation that we get to follow. This saves everyday engineers and designers from having to spend a lot of time deciding what is and isn't acceptable. For people working in a field that doesn't have such standards, or people developing the standards, a risk assessment (however formal) is required to decide what risks can be tolerated. These assessments involve many factors, including:

• What are the consequences of failure? If your water is just warm enough to cause discomfort, you may be able to tolerate that risk. If it is so hot that it could cause permanent injury, your risk tolerance should be much lower.
• How likely is a failure? If the water is in an open container that frequently splashes on people, you should keep it at a cooler temperature than if it's in an insulated tank and could only escape in a catastrophic equipment failure.
• What are the options for mitigating this risk? If using room-temperature water just means that your experiment will take a little more time, it may be prudent to just not heat the water at all. If your experiment can only be done at high temperatures, then you might try to mitigate the risk by not allowing employees near the hot water, or requiring them to wear special insulating clothing.

So getting to a specific number depends on a variety of other aspects of your design. Certainly one conservative approach would be to follow whatever regulations are in place for tap water in your jurisdiction. Keeping the water in the neighborhood of 38 degrees celsius would be one reasonable option - about what is comfortable for people to touch. Of course if your process requires employees to keep their hands in the water for prolonged periods of time, even this may be too hot.

However if the water is piped and insulated and employees will not be exposed to it directly, you could consider following standards established for steam heat or perhaps industrial process piping if they more closely match your application. In broad strokes, these alternate guidelines will allow you to keep your water at higher temperatures in exchange for building more robust piping systems with additional safety features to reduce the frequency of ruptures.

Without more specifics of your application, system, and jurisdiction, we can't conclusively offer a specific number, but hopefully this gives you some pointers for finding the right value.