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What protection technique is ‘op is’, ‘op pr’ and ‘op sh’, is optical radiation a risk in a hazardous area?

Yes, optical radiation is a risk in a hazardous area. Optical radiation can come from a variety of sources such as lamps, lasers, LEDs, optical fibres etc. these are increasingly used for communications, surveying, sensing and measurement. In many cases the optical radiation does not contain sufficient energy to cause ignition, but this is not always the case.

There are four possible ignition mechanisms for optical equipment:

  • Optical radiation is absorbed by surfaces or particles, causing them to heat up, which under certain circumstances may cause them to attain a temperature which will ignite a surrounding explosive atmosphere.
  • Thermal ignition of a gas volume, where the optical wavelength matches an absorption band of the gas or vapour.
  • Photochemical ignition due to photo dissociation of oxygen molecules by radiation in the ultraviolet wavelength range.
  • Direct laser induced breakdown of the gas or vapour at the focus of a strong beam, producing plasma and a shock wave both eventually acting as ignition source. These processes can be supported by a solid material close to the breakdown point.

To manage the risk of ignition 3 optical radiation protection techniques are available:

Inherently safe optical radiation “op is”: Inherently safe optical radiation means radiation that is incapable of supplying sufficient energy under normal or specified fault conditions to ignite a specific explosive atmosphere. The concept is that beam strength is limited to below that required for ignition.

Protected optical radiation “op pr”: This concept requires radiation to be confined inside the optical fibre or other transmission medium and is based on the assumption that there is no escape of radiation from the transmission medium.

Optical radiation interlocked with optical breakage “op sh”: This type of protection is applicable when the radiation is not inherently safe and uses an interlock to cut-off the transmission if the confinement fails. The cut off occurs on a time scale suitably shorter than the ignition delay time.


“This following information represents and opinion only of a general nature, the specifics of each individual situation must be taken into account with reference to the relevant Legislation, Codes of practice and Australian standards. Professional advice should be sort if there is any doubt.”

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