The phrase “INTRINSICALLY SAFE” is one possible method to ensure that products are suitable for use in hazardous (explosive) areas. Some examples of alternate methods are “EXPLOSION PROOF”, “FLAME PROOF” “POWDER FILLED”.
Intrinsic Safety vs. Explosion-Proof
The most popular two methods are different approaches to prevent a malfunction of electrical equipment from initiating an explosion through ignition of gases present in the surrounding atmosphere.
Explosion-Proof devices are designed so that if an explosion does occur inside the enclosure, it will be contained within the enclosure and not ignite surrounding gases. This containment is done through careful design of the enclosure so that the resulting explosion is not only contained, but the resulting heat is also dissipated through the large surface of the flanges or threads of the enclosure. Consequently, if the integrity of the enclosure is compromised, there is a risk of an explosion. This can happen because of a scratch across the flange face, or incomplete tightening of the cover. The net result is that Explosion Proof protection has a higher level of required maintenance than an Intrinsically Safe system.
Intrinsically Safe devices, through proper electronic design, are not capable of causing an explosion because the energy levels are kept at a low enough level. Due to the limited energy levels available in intrinsically safe devices, wiring methods not normally permitted in hazardous (classified) locations, can be used in their installation. For example in a Division 1 location installation normally requires threaded rigid metal conduit. However, per 504.20, intrinsically safe circuits may be installed using any method allowed by the NEC© for unclassified locations. (taken from this IAEA magazine article)
For these reasons, Gizmo Engineering prefers the Intrinsically Safe approach.
Division, class and group
The hazardous location ratings are divided into Class, Division, and Group to indicate the existence of flammable gases or vapors, ignitable dust, fibers, or filings under normal or unlikely conditions of operation. The concept was developed for safe operation in hazardous areas, particularly around petro-chemicals, flammable gases and dusts. Only a Nationally Recognized Testing Lab (NRTL) such as CSA or UL for example, can approve I.S. products.
- Class l Areas in which flammable gases or vapors may be present in sufficient quantities to be explosive or ignitable.
- Class ll Areas made hazardous by the presence of combustible dust.
- Class lll Areas in which there are easily ignitable fibers or flyings present, due to the type of material being handled, stored or processed-but in which such fibers or flyings are not likely to be in suspension in the air in quantities sufficient to produce ignitable mixtures.
Division 1 – Normal Situation: A hazard is present in the everyday normal production operation or during frequent repair and/or maintenance activity.
Division 2 – Abnormal Situation: Potentially hazardous material is expected to be safely confined within closed containers or closed systems, and will be present in the atmosphere only through accidental rupture, breakage, or abnormal operation.
Group A, B, C & D Gases and vapors in Class l locations are classified into four groups, by the code A, B, C, and D. These materials are grouped according to the ignition temperature of the substance, its explosion pressure and other flammability characteristics.
Examples: A= hydrogen, B=acetylene, C=ethylene, D=propane
Groups E, F & G apply to Combustible dusts.
Intrinsic safety certification requires the production site is subject to regular audits to conform to a range of compliance measures. These facility audits on a regular basis ensure adequate inspection, testing and acceptance techniques are in use in the manufacturing process. During an audit, production records, product documentation, and product samples are audited to verify continued compliance. These efforts demonstrate that not only are the products in compliance, but manufactured repeatedly in compliance with approval standards.
Area Classifications are generally created by the Authority Having Jurisdiction (usually local building inspector or Fire Marshall, or in Canada, the Hydro Authority, or in some cases, an insurance company, like Factory Mutual will come in and Classify areas within a facility). In Oil Patches, almost EVERYONE knows the HazLoc Classification rules and how to apply them. But in some places, this knowledge is lacking, or there are no building inspectors. There are entire Appalachian counties in SE Ohio where building departments and building inspectors simply don’t exist. A similar situation exists, I am sure in quite a few locations around the country,
The document “NFPA 497 Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas.” details how the classification is determined. (account creation required)
A flash point above 100 F (about 37 C) makes the material a combustible, not a flammable. An Auto Ignition Temperature greater than the T Code (T6 = 85 C, T5 = 100 C, T4 = 135 C, T3 = 200 C) makes an installation in a HazLoc safe. Gizmo Engineering alarms have a T code of T4. For more information on intrinsically safe design services please visit WCL corp. who specializes in Hazardous Location compliance.