Inherent safety

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An inherently safe system is a variety of a certain system that cannot be made to cause harm – obviously the best arrangement safety-wise, but not always possible.

Examples of common inherently safe systems:

The overflow tube in baths and kitchen sinks. If the valve sticks open, rather than causing an overflow and damage, the tank spills into an overflow.
In an elevator the cable supporting the car keeps spring-loaded brakes open. If the cable breaks, the brakes grab rails, and the car does not fall.
The pilot light sensor in most gas furnaces. When the pilot light is off, the sensor cools down and a mechanical arrangement such as a bimetallic switch disengages the gas valve, so that the house cannot fill with unburned gas.
Railroad semaphores are designed so that the horizontal position signifies the "Stop" (danger) position. If the controlling mechanism fails and the arm is free to fall under gravity, it will fall to the "Stop" position, regardless of the condition of the line ahead.

In practice, most systems could be rendered safer, except that users would be unwilling to bear the cost or inconvenience. An inherently safe airplane is desirable, but not economically practical, on account of users' focus on high speeds and low cost.

Inherent fail-safes are common in medical equipment, traffic and railway signals, nuclear reactors, communications equipment, and safety equipment.