To achieve intrinsic safety, all products that are installed within such an area should be designed so that they cannot store sufficient energy to create a spark that may ignite the hazardous material being stored or processed. In the UK this requirement is tested by an independent third party, for example the Electrical Equipment Certification Service.
The requirement also applies to any cabling entering hazardous areas, regardless if it's for a zone of detectors or a sounder circuit. A barrier (galvanic or Zener) must therefore be used in conjunction with IS products to limit the energy entering the area.
Figure 1, over, shows a typical connection method for point detection into a hazardous area. But what is a hazardous area?
What constitutes a hazardous area can be difficult for the installation company to determine – close evaluation and consultation on the site must take place to identify potential hazards. Many industrial processes create hazardous environments, such as chemical plants, paint factories and others that involve chemical mixing. IEC 60079-10 defines hazardous areas into three classifications:
- Zone 0, where flammable atmospheres are present continuously or more than 1000 hours annually;
- Zone 1, where flammable atmospheres are present intermittently or for between ten and 1000 hours annually;
- Zone 2, where flammable atmospheres are present abnormally ie less than ten hours annually.
Products that have been independently certified are given a classification rating that states in which zone the product can be used. It should be noted that products designed for Zone 0 can be used in all the other zones safely.
To add confusion to this already complex system, the current IEC standards are being replaced by the ATEX directive, which will become mandatory as of 1 July 2003. This directive changes the classification for the zones into categories. It also changes the bias to prevent explosive atmospheres (EN 1127-1) or, if this is not possible, then the risk should be controlled to prevent ignition or control the effects of an explosion.
The reclassification of the zones under the ATEX directive is shown in table 1, below. Figure 2, right, compares the IEC's classification system with the proposed ATEX category approach.
What to install
All equipment installed within a hazardous area must be certified by a third party to ensure that it will not cause any sparks and potentially ignite the hazardous material. It must be remembered that the point of installing fire detection within the hazardous area is to give early warning of the possible dangers of an explosion by detecting smoke and heat.
The additional approval for IS-certified equipment that must be sought by installers is EN 54 Parts 5 and 7 for heat and smoke performance respectively. Within the UK the main independent approval body for testing to the European standards is the Loss Prevention Certification Board.
Installation in hazardous areas is not a simple case of selecting good quality third party approved equipment and installing the products. A number of safety precautions must be considered, especially in Zone 0/Category 1, where no tools should be used that could cause a spark.
In Zone 1/Category 2 and Zone 2/Category 3, tools that create a single spark can be used as they typically do not have sufficient energy to ignite gas within the area. Tools that produce a continuous stream of sparks, such as electric drills, should not be used in any of the zones; even multimeters should be rated for IS use and have the correct classification.
The last area which is often forgotten is static electricity, as this can produce high voltage discharges back to an earth connection, even in the 20 000 V range. Every precaution should be taken to prevent static build-up, such as wrist or ankle straps that can provide a safe connection to the earth potential.
Barrier grief
The type of barriers used for installations can be critical, especially when it comes to testing.
The Zener style barrier requires a connection to a dedicated earth, either back to the main power earth point or to a separate earth rod with less than 1 Ohm resistance. The earth cable must have a conductor of 4 mm² minimum and be clearly labelled so that this connection cannot be accidentally disconnected during future work within the building; if this connection is removed from the barrier the site is put at risk.
Generally a galvanic isolator provides a safer installation. Although such isolators have a higher initial cost, any cost associated with installing the earth connection is removed, along with the potential risk of the earth being disconnected for testing purposes or even accidentally.
Source
Electrical and Mechanical Contractor
Postscript
Neil Woodward is with Hochiki Europe.
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