Explosive Atmospheres – Part 28: Protection of equipment and transmission systems using optical radiation
|Publication Date:||5 December 2012|
This standard explains the potential ignition hazard from equipment using optical radiation intended for use in explosive gas and combustible dust atmospheres. It covers EPLs Ma, Mb, Ga, Gb, Gc, Da, Db and Dc. It also covers equipment, which itself is located outside but its emitted optical radiation enters such atmospheres. It describes precautions and requirements to be taken when using optical radiation transmitting equipment in explosive gas and combustible dust atmospheres. It also outlines a test method, which can be used to verify a beam is not ignition capable under selected test conditions, if the optical limit values cannot be guaranteed by assessment or beam strength measurement.
This standard contains requirements for optical radiation in the wavelength range from 380 nm to 10 μm. It covers the following ignition mechanisms:
• optical radiation is absorbed by surfaces or par ticles, causing them to heat up and, under certain circumstances, this may allow them to attain a temperature which will ignite a surrounding explosive atmosphere;
• direct laser induced breakdown of the gas at the focus of a strong beam, producing plasma and a shock wave both eventually acting as the ignition source. These processes can be supported by a solid material close to the breakdown point.
NOTE 1 See items a) and d) of the introduction.
This standard applies to optical fibre equipment and optical equipment, including LED and laser equipment, other than as detailed below:
• Non-array indicator LEDs used for example to show equipment stuatus or backlight function.
• Luminaries involving light sources as follows.
- LED light sources for Gc and Dc applications.
- light sources, other than LED, that are continuous and divergent for all EPL applications.
• Optical radiation source for Gc and Dc applications which comply with Class I limits in accordance with us Code of Federal Regulations, 21 CFR Part 1040.
• Optical radiation sources for Mb, Gb or Gc, and Db or Dc applications which comply with Class 1 limits in accordance with IEC 60825-1.
NOTE 2 Class I limit evaluations in accordance with US Code of Federal Regulations, 21 CFR Part 1040 are based on normal operating conditions. Class 1 limit evaluations in accordance with IEC 60825-1 are based on normal operating and single fault conditions.
This standard does not cover ignition by ultraviolet radiation and by absorption of the radiation in the explosive mixture itself. Explosive absorbers or absorbers that contain their own oxidizer as well as catalytic absorbers are also outside the scope of this standard.
This standard specifies requirements for equipment intended for use under atmospheric conditions.
This standard supplements and modifies the general requirements of ANSI/ISA-IEC 60079-0. Where a requirement of this standard conflicts with a requirement of ANSI/ISA 60079-0, the requirement of this standard will take precedence.
NOTE Although one should be aware of ignition mechanism b) and c) explained in the introduction, they are not addressed in this standard due to the very special situation with ultraviolet radiation and with the absorption properties of most gases (see Annex B).
NOTE Safety requirements to reduce human exposure hazards from fibre optic communication systems are found in IEC 60825-2.