Electrical equipment and wiring are used in different environments, including hazardous locations, where there is always a risk of explosion due to any malfunction in the wiring or equipment. To mitigate this risk, electrical and thermal energy generated must be limited to a level below that required to ignite a specific mixture of the hazardous atmosphere. This technique of designing electrical equipment and wiring to be safe under normal or abnormal conditions is called intrinsic safety. Therefore, intrinsically safe wiring and equipment are incapable of releasing adequate thermal or electrical energy under any operating condition to cause a combustible or flammable atmospheric mixture to ignite.
Independent third party agencies such as the UL or Underwriters Laboratories, CSA or Canadian Standards Association, FM or Factory Mutual Research Corporation and the MSHA or Mine Safety and Health Administration, test and certify equipment for intrinsic safety. For use in explosive atmosphere, the agencies test and verify equipment for compliance to IEC international standards. Within the IEC 60079 series, the standard IEC60079-11 specifies the construction and testing of intrinsically safe apparatus intended for use in an explosive environment.
According to IEC60079-11, the basic principle in achieving intrinsic safety is for limiting the energy in the power circuit, preventing unusually high electric arcs, ignition sparks or high temperatures that could create ignition energy required to cause an explosion. For limiting the power or energy, designers should implement a resistor or fuse in series for limiting the current and a Zener diode in parallel for limiting the voltage.
Additionally, IEC60079-11 also requires that conductive parts of intrinsically safe circuits be separated from the conductive parts of non-intrinsically safe circuits. The separating distances have different requirements through insulation structures, and this includes clearance, separation and creepage distances. For example, casting compounds specified includes epoxy resins, while solid insulation specified include silicone and polyester film.
Apart from providing galvanic isolation, opto-couplers have internal clearances, which include DTI or distance through insulation. This is a part of the insulation and safety related specification of opto-couplers. DTI provides galvanic isolation through optical technology, forming a straight-line thickness distance between the LED emitter and the detector within the opto-coupler. The DTI of the opto-coupler meets the separation distance requirements 0 to 2 of the gas zone classification. This depends on the voltage level of protection required.
Typically, isolators with structural DTI less than 20µm cannot achieve the stringent separation distance requirements of intrinsic safety criteria. Special opto-couplers, such as the ACNV series from Avago, have a 13mm creepage/clearance, with insulation material classified as casting compound. This allows the ACNV opto-couplers to achieve up to the 375V level of protection. Similarly, ACNW/HCNW opto-couplers from Avago, with 10mm and 8mm creepage/clearance, can meet up to 60V level of protection.
Such intrinsically safe opto-couplers are routinely used in applications for measurement of level, pressure and temperature in flow meters and transmitters. Meeting safety requirements, these opto-couplers provide the reinforced insulation required between field sensors and micro-controllers on control boards. Typical examples of such applications are in the explosive atmospheres of petrol stations and sewage, where fluid pumps and flow meters are used.
