Tech notes from Avago: Optocouplers in intrinsic safety applications

February 23, 2015 // By Avago Technologies
The basic principle of IEC 60079-11 to achieve intrinsic safety is for the energy in the power circuit to be limited preventing unusually high temperatures, ignition sparks or electric arcs that create ignition energy required to cause and explosion.

The IECEx is a certification system which verifies compliance with IEC international standards relating to equipment for use in explosive atmospheres. The safety requirement for equipment under IECEx is based on the description in the international standards IEC 60079 series (Table 1). IEC 60079-11 specifies the construction and testing of intrinsically safe apparatus intended for use in an explosive atmosphere and for associated apparatus, which is intended for connection to intrinsically safe circuits which enter such atmospheres.

IEC 60079-11 Intrinsic Safety

The basic principle of IEC 60079-11 to achieve intrinsic safety is for the energy in the power circuit to be limited preventing unusually high temperatures, ignition sparks or electric arcs that create ignition energy required to cause and explosion. To limit the energy (power), a fuse or resistor in series (current limiting) and a Zener diode in parallel (voltage limiting) should be implemented.

IEC 60079-11 requires separation of conductive parts between intrinsically safe and non- intrinsically safe circuits. From Table 2 IEC 60079-11, General Safety Requirements including clearance, creepage distances and separations, there are different requirements of separation distance through insulation structures. The casting compound in column 3 refers to compounds such as epoxy resin while examples of solid insulation in column 4 are polyester film and silicone. Avago optocouplers include internal clearance, or distance through insulation (DTI), as part of the insulation and safety related specifications. Using optical technology for galvanic isolation, the DTI forms the straight-line distance thickness between the LED emitter and the detector within the optocoupler. The optocoupler’s DTI is able to meet the separation distance requirements of gas zone classification 0 to 2, depend on voltage level of protection.