Anti-removal measures for interlock devices

According to the ISO 14119 revision (2013), measures are now required to prevent the dismantling, removal, or disabling of interlock devices.
Among these measures, the use of a special screw which can only turn in one direction is recommended.

ISO 14119:2013 Safety of machinery -- Interlocking devices associated with guards -- Principles for design and selection

7  Design to minimize defeat possibilities of interlocking devices
7.2Additional measures to minimize defeat possibilities of interlocking devices
   Interlocking devices must be selected and/or installed so as to minimize defeat possibilities in a reasonably foreseeable way.
   (ｃ)Prevent disassembly or position changing of elements of the interlocking device by means of non-removable fasteners (e.g. by welding, bonding, disposable screws, rivets, etc.).

NBK's Tamper Resistant Screws could be your answer!

One Side Pan Head Machine Screws

Mount with a commercially available slotted screwdriver. Removal requires dedicated tools.

Other types of tamper resistant screws are also available.

About Tamper Resistant Screws