How Fail-Safe Design Keeps Workers Safe When Things Go Wrong
When equipment fails, fail-safe design ensures that it fails safe. Fail-safe components limit the risk to personnel, damage to property, and harm to the environment.
Even if you have the most air-tight safety program, you can't guarantee that your equipment and machinery won't fail.
It could happen for a number of reasons. Safety procedures get overlooked, complacency checks in, components wear out faster than expected. The factors align to create the worst case scenario. Failure is imminent and there isn't enough time to do anything about it.
What will happen next?
If the system has been well designed, not a whole lot.
"Fail-safe" is a design and engineering principle that considers the effects of a potential failure and builds systems with that failure in mind. Instead of hoping that a failure won't happen, fail-safes aim to minimize the harm and damage when it does.
Failure Mode and Effect Analysis
A “Failure Mode and Effect Analysis” (FMEA) is often used as a starting point to identifying probable points of failure within a system. In some ways, it is similar to conducting a hazard assessment, since it is a structured speculation based on the available data that allows meaningful application of controls.
FMEA generally applies to designed systems with components that have known properties, behaviors, and interactions. As such, proper engineering calculations and methods can be used to estimate the probability of failure and add safety margins that make the failure unlikely.
That said, failures can happen even after thorough calculations and careful planning. If specific types of failure present a hazard to people, property, or the environment, designers have to take special care to ensure that when the system fails, it will fail safe. That is, it will fail in a way that mitigates the resulting damage.
This is often achieved through the introduction of a fail-safe device or process, sometimes called a "poka-yoke" mechanism.
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5 Types of Fail-Safe Components
The various fail-safe applications all serve different functions, but they have the same goal in mind: if the system fails, make sure it does so safely.
These are a few of the ways that principle has been applied.
Dead Man Switch
This morbidly-named approach is so called because the default mode is engaged (the safe mode) and a person has to actively nod an apparatus to disengage.
That might sound a bit technical, but you are likely very familiar with the dead man switch. It has been a standard feature on all push lawn mower for decades now. It's the reason you have to hold the handle for the lawn mower blades to activate. When you let go, the dead man switch shuts off the mechanism and returns it to its safe mode.
Circuit Breakers and Fuses
These are designed to prevent overcurrent or overheating conditions by interrupting a circuit, essentially cutting power to the device. They can prevent fires, equipment failures, and short circuits that could damage a device or pose an electrical hazard to a worker.
These will be familiar to any homeowner who has pulled too much current on a circuit and had the lights go dark. Circuit breaker to the rescue!
These systems default to a “closed” mode in the event of certain conditions, such as a power failure. For example, an isolation valve could close in the event of a failure, since remaining open might result in the release of a hazardous substance.
The fail-closed implement uses a spring, gravity, or some other mechanism that will close in any condition other than the one actively keeping it open. For example, a valve that only remains open because an electrical component keeps it open will automatically close in the event of a power failure or damage to the circuitry.
A version of this is also used in a lot of security systems. The idea being that the electricity being cut should activate an alarm event rather than power down the system. Otherwise, they would be simple to defeat.
In fluid chemical processing and water/steam systems, safety valves are often used to release built-up pressure. The thinking behind these is that releasing the fluid is preferable to the catastrophic failure of an explosion due to pressure buildup.
These safety valves work automatically, without the use of sensors or even a power source.
Air brakes release under positive pressure, allowing the vehicle to move freely. A failure in the braking system component will cause a pressure release, which will set the brakes and bring the vehicle to a stop.
Air brakes are commonly used on trains and tanker trucks, since a runaway for either of these would be particularly disastrous.
Plan for Success by Planning for Failure
It seems counter-intuitive to design for failure, but these safety features are both necessary and effective. Even when the fail-safe mechanism is destructive, they are an important aspect of the design. Crippling a machine, or even a whole facility, is a fair price to pay for saving a single life.
When fail-safe devices are well-designed, however, they tend to result in a benign and reversible failure. They also require a deliberate and manual process to restart the device, so that the action of the fail-safe isn't reversed until it is safe to do so.
Even with numerous safety features in place, things can still go wrong on the worksite. You can't always prevent failures, but with fail-safe components in place you can at least prevent them from becoming disasters.