Intrinsic safety

Not to be confused with inherent safety, the more general industrial technique of minimizing the danger of a process.

Intrinsic safety (IS) is a protection technique for safe operation of electrical equipment in hazardous areas by limiting the energy, electrical and thermal, available for ignition. In signal and control circuits that can operate with low currents and voltages, the intrinsic safety approach simplifies circuits and reduces installation cost over other protection methods. Areas with dangerous concentrations of flammable gases or dust are found in applications such as petrochemical refineries and mines. As a discipline, it is an application of inherent safety in instrumentation. High-power circuits such as electric motors or lighting cannot use intrinsic safety methods for protection.

Operating and design principles

In normal use, electrical equipment often creates tiny electric arcs (internal sparks) in switches, motor brushes, connectors, and in other places. Compact electrical equipment generates heat as well, which under some circumstances can become an ignition source.

There are multiple ways to make equipment explosion-proof, or safe for use in ex-hazardous areas. Intrinsic safety is one of a few methods available for ex-hazardous areas. Others include explosion proof enclosures, venting, oil immersion, powder and sand filling, and hermetic sealing. For handheld electronics, intrinsic safety is the only realistic method that allows a functional device to be explosion-proof. A device termed intrinsically safe is designed to be incapable of producing heat or spark sufficient to ignite an explosive atmosphere.

There are several considerations in designing intrinsically safe electronics devices: reducing or eliminating internal sparking, controlling component temperatures, and eliminating component spacing that would allow dust to short a circuit. Elimination of spark potential within components is accomplished by limiting the available energy in any given circuit and the system as a whole. Temperature, under certain fault conditions such as an internal short in a semiconductor device, becomes an issue as the temperature of a component can rise to a level that can ignite some explosive gasses, even in normal use. Safeguards, such as current limiting by resistors and fuses, must be employed to ensure that in no circumstance can a component reach a temperature that could cause autoignition of a combustible atmosphere. In the highly compact electronic devices used today PCB's often have component spacing that create the possibility of an arc between components if dust or other particulate matter works into the circuitry, thus component spacing, siting and isolation become important to the design.

The primary concept behind intrinsic safety is the restriction of available electrical and thermal energy in the system so that ignition of a hazardous atmosphere (explosive gas or dust) cannot occur. This is achieved by ensuring that only low voltages and currents enter the hazardous area, and that no significant energy storage is possible.

One of the most common methods for protection is to limit electric current by using multiple series resistors (assuming that resistors always fail open); and limit the voltage with multiple zener devices to ground (assuming diodes always fail shorted). Sometimes an alternative type of barrier known as a galvanic isolation barrier may be used. Certification standards for intrinsic safety designs, which vary by device type, generally require that the barrier not exceed approved levels of voltage and current with specified damage to limiting components.

Equipment or instrumentation for use in a hazardous area will be designed to operate with low voltage and current, and will be designed without any large capacitors or inductors that could discharge in a spark. The instrument will be connected, using approved wiring methods, back to a control panel in a non-hazardous area that contains safety barriers. The safety barriers ensure that, no matter what accidental contact occurs between the instrument circuit and outside power sources, no more than the approved voltage or current enters the hazardous area.

For example, during marine transfer operations when flammable products are transferred between the marine terminal and tanker ships or barges, two-way radio communication needs to be constantly maintained in case the transfer needs to stop for unforeseen reasons such as a spill. The United States Coast Guard requires that the two way radio must be certified as intrinsically safe.

Another example is intrinsically safe or explosion-proof mobile phones used in explosive atmospheres, such as refineries. Intrinsically safe mobile phones must meet special battery design criteria in order to achieve UL, ATEX directive, or IECEx certification for use in explosive atmospheres.

No single field device or wiring is intrinsically safe by itself (except for properly designed battery-operated, self-contained devices), but is intrinsically safe only when employed in a properly designed IS system. Such systems are usually provided with detailed instructions to ensure safe use and maintenance.

Certifying agencies

Several different agencies develop standards for intrinsic safety, and evaluate products for compliance with standards. Agencies may be run by governments or may be composed of members from insurance companies, manufacturers, and industries with an interest in safety standards. Certifying agencies allow manufacturers to affix a label or mark to identify that the equipment has been designed to the relevant product safety standards. Examples of such agencies in North America are the Factory Mutual Research Corporation, which certifies radios, Underwriters Laboratories (UL) that certifies mobile phones, and in Canada the Canadian Standards Association.[1] In the EU the standard for intrinsic safety certification is the ATEX directive, while in other countries around the world the IECEx standards are followed. To facilitate world trade, standards agencies around the world engage in harmonization activity so that intrinsically safe equipment manufactured in one country eventually might be approved for use in another without redundant, expensive testing and documentation.

See also

References

  1. ↑ "Intrinsic Safety Approvals for Radio Communications Equipment". Intrinsic Safety (IS) Approvals. American Communication Systems, Inc. Retrieved 25 Sep 2012.

Further reading

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