A common belief among the industry is that if you use fiber optics in a Hazardous Area, you are basically playing safe.
The problem is that if you follow that belief, you could be potentially introducing an ignition source into your classified area.
As most of the standards and regulations on explosion protection, the investigation about the possible risks of the use of fiber optics in hazardous areas had its origins in a series of studies accomplished by the German PTB (Physikalisch-Technische Bundesanstalt), the mother of all certifying institutes in the 90’s.
The results of this research were made available as a report entitled as Prenormative Research on the use of Optics in Potentially EXplosive atmospheres, a.k.a. PROPEX. It’s really amazing the industry’s ability to create catchy acronyms all the time.
The PROPEX report covered investigations on the use of optical methods for sensing and communication in hazardous areas. It extended previous studies by including pulsed optical sources and applications where combustible dusts were present.
How can light become a potential ignition source? Just remember the classic experiment of primary school that required an amplifying glass, a sheet of paper and a sunny day.
The factor that it is necessary to consider is the minimum ignition radiation power, that is the amount of energy that could be transferred to the surrounding environment by a light source without turning itself into an ignition source.
It was discovered that under certain circumstances the value could be as low as 50 mW. For safety reasons a reduction factor should be considered and a 35 mW limit could be employed as a safe value.
One of the main problems that appeared in the PROPEX study was that there was no relationship between typical parameters used to analyze explosion characteristics and optical ignition sources, so MSIEG and MIE values were not as reliable parameters as they are with other kinds of ignition sources.
The study showed the necessity of developing new testing apparatus in order to measure safe radiation power levels for classified areas.
The results of the PROPEX report were the basis for the IEC 60079-28 standard, whose first issue was published in 2006, followed a couple of months later by the EN 60079-28 standard.
So how does a fiber optic turn into an ignition source?
There are 4 ways:
- The obvious one: Optical radiation can be absorbed by surfaces or particles, which are heated by light eventually reaching the ignition temperature of the surrounding atmosphere.
- The nerdy one: If the wavelength of the light source matches the absorption band of the explosive gas then thermal ignition may occur.
- This one’s tricky: Since light emission has a wide wavelength (from infrared to ultraviolet) it can cause photo dissociation of oxygen molecules present in the environment, thus generating an oxidant in what was previously considered a safe environment.
- The last one has a sci fi flavor: A direct laser beam, emitted on a potentially explosive gas can generate plasma and a shock wave. Both effects constitute an ignition source
Three protection methods were developed to asses these issues:
- Inherently safe optical radiation “op is“, which is similar to the concept of intrinsic safety.
- Protected optical radiation “op pr“, which is similar the hybrid protection method Ex de (flameproof + increased safety).
- Optical systems with interlock “op sh“, which is based on the concept of functional safety.
Now you can understand what was the meaning of those letters that started to appear in the markings of fiber optic equipment designed for use on classified areas, like this one: EX II(1)G [Ex op is Ga T4] IIC. Life is not easy…
Inherently safe applications allow the use of unprotected fiber optics, but they need an energy limiting light source, sort of a IS barrier but limiting light energy rather than electrical energy. This method allows distances up to 2000 m.
Protected optical radiation applications use protective covers for the fiber optics plus special connectors. You can reach 4000 m using this approach.
Optical systems with interlock are based on the detection of fiber optic breakage and the consequent shut off of the light emitting source. This method requires risk evaluation according to functional safety standards (IEC 61508 and IEC 61511). I haven’t found any real life application of this method.
The increasing use of fiber optics to interconnect networks and devices in modern plants, specially due to the ever increasing EMC problems, grounding issues and Industrial Ethernet distance limitations, the chances that an eventual breakage of the fiber optics could become a source of ignition are increasing.
As safety focused professionals its vital to get familiar with these concepts. Otherwise we are taking the risk of playing with fire.
Mirko Torrez Contreras is a freelance Process Automation consultant who likes to read standards. Whether this is a virtue or a defect or a simple and plain obsession is still a subject of discussion among his colleagues.