Defensive design

Paper cutting machine with two buttons required to be pressed simultaneously to cut the paper. A defensive design to prevent the operator from cutting his own hands.

Defensive design is the practice of planning for contingencies in the design stage of a project or undertaking. Essentially, it is the practice of anticipating all possible ways that an end-user could misuse a device, and designing the device so as to make such misuse impossible, or to minimize the negative consequences. For example, if it is important that a plug is inserted into a socket in a particular orientation, the socket and plug should be designed so that it is physically impossible to insert the plug incorrectly. Power sockets are often keyed in such a manner, to prevent the transposition of live and neutral.

Defensive design in software engineering is called defensive programming. Murphy's law is a well-known statement of the need for defensive design. It is considered the opposite approach to design by contract.

Examples

• Many electrical connectors apply this principle by being asymmetric.

See also

• Fail-safe
• Idiot proof
• Inherent safety
• Poka-yoke
• Usability testing