Intrinsically Safe

Intrinsically safe is a protection concept employed in potentially explosive atmospheres. Intrinsic safety relies on the electrical apparatus being designed so that it is unable to release sufficient energy, electrical or thermal, to cause an ignition of a flammable mixture. This is typically achieved through the use of barriers, either zener diode barriers or isolated barriers, that limit the amount of energy entering the hazardous area. (A hazardous area is defined as any location with combustible materials such as gas, dust or fibers that may produce an ignitable mixture.)

Since being intrinsically safe requires maintaining an energy level lower than that required to ignite a specific hazardous mixture, it is important to know what the energy allowances are. The chart below (Fig. 1) identifies some of the common hazardous mixtures and their auto-ignition temperatures. (Auto-ignition temperature is the minimum temperature at which ignition takes place.)

A complete listing of hazardous mixtures defined by group can be found in the National Fire Protection Association document NFPA 497 M.

Because all electrical circuits store energy which may be released under abnormal conditions, power levels must not be of sufficient magnitude to ignite a hazardous mixture. Abnormal conditions, or fault conditions, are considered to be circumstances in which the circuit has failed in an unsafe manner creating the risk of explosion. Intrinsically safe means fault conditions are to be considered normal and expected. Fault conditions may be due to an opening, shorting or grounding of field wiring as well as hot surfaces or the introduction of higher voltages than intended for the circuit. In general, a circuit can only be made intrinsically safe if it requires less than 1 watt of power. This is why intrinsic safety is limited to measurement and control circuits. Any circuit powered by higher energy levels require the use of other explosion protection techniques.

Requirements for an intrinsically safe system do not include reducing risk of explosion due to mechanical or electrostatic sparking, chemical action, radio waves or lightning strikes. Additional protection against such events should be implemented.