Safe Design Principles

There are multiple features we can build into a system from the design stage to make it safer. Key features include redundancy, separation of duties, the principle of least privilege, fail-safes, antifragility, negative feedback mechanisms, transparency and defense in depth.

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Review Questions

For each of the following examples, identify which safe design principle(s) is in practice.

  1. Many electrical appliances contain a fuse—a segment of wire in the circuit that melts if the current flowing through it gets too high. This breaks the circuit, preventing the high current from flowing and protecting the device’s user.
  2. A driver on a road makes sure they do not get too close to the vehicle in front, ensuring that they have sufficient space to decelerate to a halt, should the vehicle in front stop unexpectedly.
  3. During a flight, the cockpit is locked by default, preventing anyone from entering it without the pilot’s permission.
  4. During a flight, cabin crew members are able to communicate with the pilot from outside the cockpit. They know a code that will grant them entry to the cockpit if the pilot fails to reply to any communication within a given timeframe. This means they know how to enter if the pilot appears to have become incapacitated.
  5. The cables in modern suspension bridges normally comprise a large number of wires, meaning that, if a few of them fail, it is not enough to cause the bridge to collapse.
  6. Before going on a boat trip, someone ensures that the boat is in good condition, but also learns how to swim and wears a lifejacket while they are at sea.
  7. After a major accident at an industrial plant, a thorough investigation is conducted, and the organization changes its structure and processes to reduce the likelihood of a similar accident happening again.

1. Fail-safe

2. Loose coupling

3. Principle of least privilege

4. Transparency

5. Redundancy

6. Defense in depth

7. Antifragility

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