Explosion protection for optical networks

The explosion hazard inherent in optical fibre networks is often underestimated, explains R. Stahl Technology Group of Waldenburg, Germany:

Cologne, Germany -- ­ Many applications even in hazardous areas require a fast LAN connection ­ for example for powerful HMI systems that are to be integrated seamlessly into a company's IT system.

Optical fibre is the usual method for networks, since it can cover greater distances than TP copper cables and resists electromagnetic forces. However, the safety of optical networks requires specific expertise so specialists in explosion protection should be consulted.

The explosion hazard inherent in light is frequently underestimated: as little as a few dozen or hundred milliwatt of light energy affecting a surface the size of a pencil-tip after a cable break, for example, can ignite an explosive gas atmosphere.

Optical signals must therefore comply with a suitable protection type when passing through hazardous areas. This requires certified transmitters and receivers of a special design.

Usually, the signals are designed inherently safe, that is, they are of protection type "Ex op is", which is achieved by limiting irradiance. Special caution is necessary because typically, switches with optical outputs transmit close to or even above the permitted level of light emissivity.

Additionally, a test certificate for switches and devices is essential for installation in zone 1.

Under certain circumstances, alternative protection types may be protected optical radiation ("Ex op pr") or a safe error detection with interruption and blocking of the optical signal ("Ex op sh"). As a rule, a safe and acceptable solution for hazardous areas must neither exceed a certain irradiance (i.e. radiation at a surface per unit area) nor allow for the release of excessive light energy in absolute terms, regardless of dispersion.