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 An Introduction to Intrinsically Safe and Flame Proof (Explosion Proof) equipment installed in Hazardous Areas. To view these notes in pdf format please click here |
| There is an obvious risk of an explosion if a fault develops in an electrical or electronic circuit or equipment located in an area where flammable gases are present. The fault could cause heat or sparks sufficient to ignite the gas and cause a disaster. There are two systems in general use to prevent such happennings: i) Intriniscally Safe techniques or (ii) making the equipment in question Flame Proof, also known as Explosion Proof What is the difference between Intrinsically Safe and Flame Proof (Explosion Proof) equipment? Flame Proof Equipment The equipment is simply contained in a heavy protective enclosure, usually made from cast steel, Aluminium or GRP plastic. If flammable gases enter into the equipment and the heat from the sparks from the fault ignite the gases, the enclosure is strong enough to withstand the resulting fire and/or explosion within the enclosure without igniting the external atmosphere. Any system employing flameproof rated equipment must use flameproof cabling e.g. MICC cable and the cable MUST be terminated using flameproof glands and junction boxes. The flameproof equipment, cable glands and junction boxes must be approved by a recognised testing authority such as BASEEFA, KEMA or PTB certifying conformity to the European ATEX regulations.
Intrinsically Safe This approach limits the electrical energy available to the equipment by means of an intrinsically safe barrier in such a way that under no circumstance will the equipment be able to generate sufficient heat or sparks to ignite flammable gasses. The intrinsically safe barrier is located between the system control panel and the hazardous area. The intrinsically safe barrier MUST be located within the safe area. The basic circuit is shown below.  * The manufacturer of the intrinsically safe device will specify the type of intrinsically safe safety barrier required. The intrinsically safe device and the safety barrier must be approved by a recognised testing authority such as BASEEFA, KEMA or PTB certifying conformity to the European ATEX directive.
For more information about ATEX please click here Connection cabling requirements for IS Circuits The cables connecting the equipment can be of any type but you must take into account the capacitance and inductance of the IS device specified by the manufacturer and the cable itself. Generally speaking the inductance ad capacitance limits need not concern installers unless exceptionally long cable runs are likely, typically 300 metres or more. The cabling must conform to the following requirements:
There are not special requirements for junction boxes used in intrinsically safe circuits. The following types of cable can be used:
It is usually considered good practise to separate cable trays, ducts and conduits carrying intrinsically safe circuits from trays and ducts carrying any other cables. e.g. telephones and computer cables. I.E.E wiring regulations prohibit electrical services e.g. power and lighting to be carried in the same conduit. Acknowledgements and References The above notes are based on extracts from MTL application AN9003 " A users guide to Intrinsic Safety". MTL Ltd is the largest UK manufacturer of intrinsically safe zenner barriers. We express our thanks to them in the compilation of these notes. These notes do not form part of any offer or contract. The opinions based in these notes cannot take into account all the possible circumstances of a particular installation. The ultimate responsibilty for safety must inevitably remain with the end user. If there is any doubt at all, seek expert advice on the particular problem. |