Intrinsically safe devices work by preventing sufficient energy from being available under normal or abnormal conditions to serve as an ignition source.
A passive barrier limits the electrical energy in the hazardous area to a level below the minimum needed to ignite a flammable atmosphere.
According to safety system expert Bill Mostia of Exida (www.exida.com), intrinsically safe equipment is approved on the basis of two methods: system and entity. In the system method, the manufacturer has the elements of the system approved as a system. The manufacturer then provides a control drawing that shows the system and the basis for the approval, including any wiring or other constraints.
"For entity approval, individual devices are certified as intrinsically safe apparatus or associated apparatus," reports Mostia. "The intrinsically safe engineering parameters are provided on a control drawing by the manufacturer and it is up to the OEM or its customer to design a safe system. This allows more flexibility in the combination of intrinsically safe system elements but requires more engineering expertise on the part of the specifier."
Intrinsic safety systems in Europe are different than those in the U.S. "In Europe, there are two varieties of intrinsic safety systems: Ex ia and Ex ib," adds Mostia. "In brief, Ex ia systems are considered safe under two independent fault conditions, while Ex ib systems are considered safe under one fault condition. Only Ex ia is allowed in Zone 0 hazardous locations."
U.S. and Canadian intrinsic safety systems have only one type that is equivalent to Ex ia. Intrinsic safety codes also allow certain devices, called simple apparatus, to be considered intrinsically safe without approval or certification. "These devices cannot generate or store more than 1.2 V, 0.1 A, 25 mW, and 20 ?J," reports Mostia. "Thermocouples, switches, resistance temperature detectors (RTDs), and light-emitting diodes fit this description. Intrinsically safe circuits must be approved for the hazardous location where they are used."
One of the primary advantages of intrinsic safety circuits is that the NEC (Article 504-20) and other codes allow use of ordinary wiring. "However, the system still must meet the requirements in Article 504, which may place additional requirements specific to the hazardous location (e.g., grounding and sealing requirements)," reminds Mostia.
While still needing a suitable enclosure for environmental conditions, an intrinsic safety circuit doesn't require heavy explosion-proof junction boxes. This means a substantial reduction in machine installation costs at the customer site. "The cost of installation in the customer's control building, however, will typically be higher due to intrinsically safe barriers, separate enclosures, and separate intrinsically safe grounding requirements," he reports. "Integrated intrinsic safety systems are available with the barrier built into the control system I/O, which can substantially reduce the control room installation cost."
New devices and technologies constantly are coming to market. Some of these may not be immediately (or ever) available in intrinsically safe formats. "Due to power limitations, fieldbus technologies may be limited to fewer intrinsically safe instruments than normal instruments on a communication drop," Mostia says.
On the other hand, some new technologies may make intrinsic safety a more attractive choice. "For example, new remote I/O products are available that are intrinsically safe and communicate via network," adds Mostia. "Using this type of I/O could clearly reduce wiring costs."
From a safety probability perspective, intrinsic safety data relates a lower risk than explosion-proof at about 10-17 vs. 10-7. "This can be a little misleading, as there has not been a reported accident in the last 50 years due to a properly installed explosion-proof enclosure," says Mostia. "However, it could be argued that intrinsically safe installations are safer because they are less prone to human error."
Installation and maintenance of intrinsic safety systems are generally easier, but typically require more expertise. The systems are easier to change because they do not require special wiring methods.
Intrinsic safety systems have the edge where they can be applied. "But they aren't the best for all applications," reminds Mostia. "If the facility has very limited Div. 1 areas, intrinsic safety is probably not the way an OEM will be expected to supply equipment. If the facilities or construction workforces are not familiar with intrinsic safety systems and their installation, explosion-proof is probably more practical."
Explosion-proof technologies are well established and familiar to U.S. technicians. Interest in intrinsic safety continues to grow, however, and intrinsic safety manufacturers report consistent growth of application.
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