So straightforward and reliable as to be impervious to misuse or error; safeguarded against accidents, mistakes, carelessness, or incompetence.

Adapted so as to be safe for, resistant to, or unlikely to harm children.

= foolproof, adj., especially of equipment.

to fail safe: of a mechanical or electrical device or machine, aircraft, etc., to revert, in the event of failure or breakdown, to a condition…