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

Safe operation of OR in Ex

 

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 )

 

Optical Radiation 1

 

 

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:

Optical Radiation Ignition Assessment

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

 

Ex Shed

…cont.

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This entry was posted on April 22, 2014 by in Uncategorized and tagged , , , , .