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Intrinsic safety (IS)

What is Intrinsic Safety?

Prevention is better than the cure……….

This is the basic principle of intrinsic safety. As the name implies, intrinsic safety refers to equipment and wiring that’s inherently safe. In other words, an intrinsically safe system is one with energy levels so low they cannot cause an explosion.

Intrinsic safety is a concept that was born in the mining industry in the early 19th century. At that time a lot of mining accidents were traced to sparks that were generated by electrical circuits used in mine lighting and signalling equipment etc. These mining accidents were explosions resulting from the ignition of a mine gas called Firedamp, which is now known to be composed primarily of Methane. Due to the nature of the working environment- underground mines without ventilation- the gas would become trapped inside and explode on contact with sparks. These mining explosions had larger number of fatalities than above ground explosions because of the confined area and the spreading of the fires and debris through underground passages.

To reduce the number of these accidents, it was proposed to use electrical equipment that could not generate sparks that could ignite the Firedamp. Intrinsic Safety was introduced.

Intrinsic safety

From Wikipedia, the free encyclopedia

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.

Understanding What’s Meant by “Intrinsically Safe”

Fire is a hazard in many industries. Sometimes the risk is quite evident, as when flammable gasses like hydrogen and propane are being produced or handled, but in other situations it is less obvious. In particular, dust can often be highly combustible.

A fire needs fuel, oxygen and an ignition source. Flammable gasses, vapors and dust provide the fuel, oxygen is present in most environments, and ignition can come from a spark or hot surface. Every fire is dangerous, but in more extreme cases combustion is so rapid as to cause an explosion. OSHA has many reports of devastating fires and explosions across a wide range of industries.

Preventing fire and explosions is a top priority as no business or organization wishes to be responsible for causing death and injury. Additionally, the direct financial penalties of such events – punitive fines and increased insurance premiums— can cripple a previously successful operation.

Engineers needing to install equipment in areas where fire is a risk have two options: employ explosion-proofing techniques or adopt an “intrinsically safe” design approach (and these are not mutually exclusive). This White Paper from OMEGA Engineering answers the question, “what is intrinsic safety?” Individual sections address:
Understanding Intrinsic Safety (IS)
Choosing the right pressure transducer or load cell for IS environment
Installation and risk management considerations
Using a wind tunnel for calibration
General Q&A

After reading this paper engineers and other technical specialists should understand the intrinsic safety concept and the benefits of applying it in their work. Understanding Intrinsic Safety Intrinsic Safety (IS) is an approach to the design of equipment going into hazardous areas. The idea is to reduce the available energy to a level where it is too low to cause ignition. That means preventing sparks and keeping temperatures low.

The alternatives are to design systems so oxygen is excluded (by purging with inert gas) or to isolate possible sources of ignition. This can be done either by putting equipment in enclosures strong enough to contain an explosion or by moving it outside the hazardous area. When is intrinsic safety equipment necessary? Whenever equipment is being installed in an area where combustible material is present it is essential to take steps to minimize the risk of ignition. OHSA accepts IS design as an appropriate approach, although requires that the whole system be designed accordingly. It is not sufficient to just use IS certified components.

An exception to the need for certification is made in the case of “simple apparatus”. This is the term used for very low power or passive devices that will not cause ignition. Good examples are thermocouples and RTD’s. Definitions of Hazardous Areas The National Fire Protection Association, (NFPA) publishes codes intended to minimize fire risks. NFPA 70 sets out the National Electrical Code, often referred to as the NEC. Section 500 and 505 provide definitions of hazardous areas. The reason for the duplication is that 505 is the newer version, structured to harmonize the definitions with those used outside the United States. Section 500 of the NEC defines Class 1, 2 and 3 locations. Class 1 relates to gases and vapors, Class 2 to dust, and Class 3 to fiber. Within each Class are two divisions. A Division 1 designation means the hazard can exist under normal conditions or could exist because of maintenance work or because of leakage or breakdown. Division 2 denotes a location where gases or vapors are confined and only escape due to accidental rupture or breakdown, where build-up is prevented through positive ventilation. Class III locations are those which are hazardous because of the presence of easily ignitable fibers or flyings.

Section 505 follows the same principles but uses Zones rather than Classes and Divisions. A Zone 0 location is one where “ignitable concentrations of flammable gases or vapors are present continuously … or are present for long periods of time.” A Zone 1 designation denotes that, “…ignitable concentrations … are likely to exist under normal operating conditions,” or as a result of leakage or repair operations. Designating a location as Zone 2 indicates that ignitable concentrations “are not likely to occur in normal operation, and if they do occur will exist only for a short time.” For dust, the corresponding zones are 20, 21 and 22. How Does Intrinsic Safety Equipment Work? Avoiding ignition entails minimizing both the available power and the maximum temperatures. Defining the maximum level of available power is complex, but in general terms can be considered as meaning voltage less than 29V and under 300 mA. A simpler view is to say that power must be less than 1.3 W. (Note that much instrumentation requires 24V and can often be designed to draw less than 500 mA; sufficient to meet IS certification in many situations).

Six classes define temperature levels. In general, equipment meeting the T4 designation is considered intrinsically safe because temperatures will not exceed 135°C (275°F) (equipment dissipating less than 1.3 W generally stays below this temperature). What Types of Intrinsic Safety Equipment are Available? A wide range of industrial equipment, such as flashlights, cameras, gas detectors and even radios, are available in intrinsically safe forms. In terms of instrumentation the biggest need is for pressure and weight measurement. Temperature measurement generally meets the “simple apparatus” rule although temperature transmitters may be needed to send thermocouple signals over longer distances.

 A Users Guide to Intrinsic Safety