Tech notes from Avago: Optocouplers in intrinsic safety applications
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.
Optocouplers meet intrinsic safety criteria
While Avago optocouplers meet the stringent separation distance requirements, alternative technology isolators are not able to achieve this [if they are constructed with] structural DTI at less than 20 μm. The 13 mm creepage/clearance ACNV optocouplers family, the 10 mm widebody optocouplers, and the 8 mm ACPL-Cxxx optocouplers, have insulation material classified as casting compound. This allows ACNV optocouplers to meet up to 375V (ia, ib) level of protection and ACNW/HCNW optocouplers up to 60V (ia, ib) level of protection (Table 3).
Applications
Optocouplers are used in intrinsically safety applications such as flow meters and field transmitters that require level, pressure, and temperature measurement. Optocouplers meet the safety requirements and provide reinforced insulation between field sensors and microcontroller of the control board. Figure 1 block diagram illustrates this.
An application example of optocoupler use in equipment with a work environment of explosive atmosphere is fluid pumps in sewage and petrol stations. Avago’s widebody 10 MBd low power ACNW261L, is used as isolated serial communication in the control board for sewage flow meters. The optocoupler’s DTI of 1 mm is critical as waste in the sewers moves constantly and this exposes the fluid pump to the explosive methane gas. Avago optocouplers, with the unique strength of a thick internal clearance, or distance through insulation, are able to meet the separation distance requirements defined by IEC 60079-11 in intrinsic safety applications as required by IECEx.
Avago; www.avagotech.com/optocouplers
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