Ex-pert Electrical Technologies
Dynamic elaboration on the Art & Science of Hazardous Area Protection
Optical Radiation Hazard
Optical Radiation : Ignition Risk & protection in Hazardous Areas
The industrial use of equipment, utilizing OPTICAL RADIATION , is increasing day-by-day. The most common use is in the fields of communication, surveying, measurement etc .
Some of the most familiar forms of equipment using OPTICAL RADIATION are as follows:
1. Electrical lamps
2. Lasers
3. LEDs
4. Optical Fibers
The most important field is that of communication where the high data transmission speed and immunity of transmitted signal to external interference are valued most.
In the Hazardous Area !
The optical radiation in the Hazardous Areas (potentially explosive atmospheres ) can ignite the surrounding atmosphere and may result in explosion.
HOW ?
See the following Ignition Mechanisms as listed in IEC 60079-28 by Technical Committee 31
1. Optical Radiation is absorbed by surfaces or particles and thus get heated up .Under certain (favorable) conditions this process may result in a sufficiently high temperature causing ignition of the surrounding atmosphere.
2. The WAVELENGTH of the optical radiation matches with the ABSORPTION BAND of a gas volume , initiating a THERMAL IGNITION.
3. Photochemical Ignition due to PHOTO DISSOCIATION of OXYGEN molecule by radiation in the ultraviolet wavelength range.
4. 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 may be supported by a solid material close to the breakdown point. (IEC 60079-28)
The Domain of OPTICAL RADIATION
As per IEC 60079-28 the Range described for OPTICAL RADIATION covers wavelengths from 380 nm to 10 um.
(380 nano-meters to 10 micro-meters )
PROTECTION to Prevent Ignition by OPTICAL RADIATION
The guidelines for protection are provided, in detail, by IEC 60079-28 .
Following three types of protection can be applied to prevent ignition by Optical Radiation in potentially explosive atmospheres.These types encompass the entire Optical system.
| Type | Symbol | |
| 1 | Inherently safe Optical Radiation |
op is |
| is (Inherently Safe) optical radiation means visible or infrared radiation that is incapable of supplying sufficient energy, under normal or specified fault conditions to ignite a specific explosive atmosphere. | ||
| 2 | Protected Optical Radiation |
op pr |
| pr (Protected Optical Radiation ) is CONFINED or PROTECTED inside the optical or other transmission medium such that there is no escape of radiation from the confinement. | ||
| 3 | Optical System with Interlock |
op sh |
| This Type of protection is applicable when the radiation is not [ is ] Inherently Safe with INTERLOCK CUT-OFF if the protection by the confinement fails and the radiation becomes UNCONFINED on time scales suitably shorter than the Ignition Delay Time |
How to assess Ignition Hazard related to Optical Radiation
An easy Assessment Method is given in IEC 60079-28 as Annex C . The outlines are as as follows:
Application of types of protection for optic systems based on EPLs
(EPL= Equipment Protection Level. This is a ‘measure’ of the safety built in to the equipment by the equipment manufacturer)
| Type(s) of protection | Ga | Gb | Gc |
| Inherently safe optical radiation “op is” | |||
| Safe with two faults | Yes | Yes | Yes |
| Safe with one fault | No | Yes | Yes |
| safe in normal operation | No | No | Yes |
| Protected fibre optic media with ignition capable beam “op pr” | |||
| With additional mechanical protection | No | Yes | Yes |
| Without additional mechanical protection | No | No | Yes |
| Protected fiber optic media with ignition capable beam interlocked with fiber breakage “op sh” | |||
| With additional mechanical protection | Yes | Yes | Yes |
| Without additional mechanical protection | No | Yes | Yes |
| None (unconfined, ignition capable beam) | No | No | No |
…cont.
Ex-pert Electrical Technologies 