Fail-safe

What does Fail-safe mean?

Fail-safe is a design principle for the construction of complex mechanical systems or electric control units.

The fail-safe principle is targeting 2 goals:

1. avoiding errors
2. minimizing results of errors

Avoiding errors is the priority of the fail-safe principle. If an error is avoided, the results of it do not have to be minimized. Each representative of this principle acknowledges that sooner or later, every technical system always fails. At some point even highly developed fail-safe technologies fail. That is why the resultuts of errors are minimized in the second step. This is often in the interest of companies to increase the quality of their products.

In many areas the compliance to the principle is not just a desire of the customer but much more legally required. In areas like air, sea or rail traffic, they have especially strict requirements to avoid errors and minimize the results of such.

Fail-safe Principle

Two circumstances are leading up to the fail-safe principle:

1. Each technical system is subject to its own wear and tear which leads to failure of its components
2. Each technical system may be constructed faulty from the get go

If you take into account that modern mechanical engineering is just about 170 years old and almost every year ground breaking new innovative possibilities are being added, construction of fail proof machinery becomes a steadily increasing challenge. Often interactions between components or with up until now unknown interference valuables appear which makes the protection against failure increasingly difficult. Even a single power drill with a faulty interference-suppression capacitor can downgrade the radio reception of the whole neighbourhood.

A steady learning process which collects a steadily growing list of expected and potential failures has to precede the fail-safe-principle. Therefore the special challenge is that companies are required to inform one another about newly discovered interactions of old and new systems. Naturally, competitors communicate rather unwillingly about internally gained knowledge. Within the fail-safe principle there are good reasons to overcome this understandable secrecy and disclose imminent dangers of until to this point unknown interactions. Since companies often fail to do so, the legislator is regularly forced to apply new rules to demand the necessary measures.

Fail-safe Examples

A typical example for fail-safe is found in railway technology. Brakes of a train are not engaged by an electric impulse but kept open. If there is a power outage of the overhead line, the brakes of the train are automatically engaged without requiring any action of personnel.

In modern cars a typical example of fail-safe is the limp mode. If it is activated the car cannot exceed a speed of 50 km/h (30 mph) and via the display the driver is asked to immediately go to a garage. This avoids the car becoming a potential traffic obstruction and keeps damage to the own system as low as possible.

In aviation fail-safe is of utmost importance. Mistakes in aviation quickly present dangers to passengers and persons on ground. Some systems are rather simple like a secured switch. This requires opening a cover to flip the switch. Other systems are implemented much deeper in the technical structure or software.

A simple fail-safe method in the industry is two hand operation. This means after putting in a command into a keyboard a confirmation button has to be pressed at the same time as the trigger button. This is often the case for feeds of shaft adjustments or release of conveyor belts. These days even in the industrial sector a lot of tasks are automated and managed by use of sensors. Especially in production a high number of light barriers and proximity switches is responsible for letting a machine proceed to the next step only after all actuators are set in their defined position. This avoids collision of actuators or production of faulty parts.