Today, the majority of communications is transferred via optical fiber networks, powered by lasers. In automated process plants, fiber optic cables connect all sorts of computers, be it Ethernet office communication or transfer of fieldbus signals between distributed control systems and remote I/O field units.
Unfortunately, light beams could accidentally escape their cable in the event of cable damage, an open plug connector, or a faulty fiber splice. In the worst case, a tiny beam could focus on a very small surface and increase its heat above the ignition temperature of the surrounding hazardous atmosphere.
Consequently, Part 28 of IEC/EN 60079 defines three options to protect optical fiber networks. Ex op sh ‘Optical system with interlock’ prevents ignition by quickly interrupting the optical transmission in case of failure. Ex op is ‘Inherently safe optical radiation’, and limits the energy of the light beam, similar to Ex i with copper cables. ‘Protected optical radiation’ Ex op pr contains the light beam by mechanical means so it can cause no harm. This is comparable to Ex e with copper cables.