Encoders are used in hazardous areas where flammable liquids, vapors, gases or combustible dusts may exist. There are several methods of designing encoders for hazardous environments and all have varying degrees of Zone and Class & Division ratings. Below are the major categories.
A common protection method is intrinsic safety (Ex ia). Intrinsically Safe rated encoders can be approved for use in either IEC Zone 0 or NEC500 Class 1 Division 1. Intrinsically safe encoders are used in conjunction with an energy limiting electronic device commonly known as an IS Barrier. This device, in Zener, opto-isolated or galvanic, limits the energy allowed in the circuit such that any arcs or sparks in this encoder has insufficient energy to ignite a vapor.
A Flame proof (Ex D) design can withstand an internal explosion and provide relief of the external pressure via flame gap(s) such as the labyrinth created by threaded fittings or machined flanges. The escaping (hot) gases must sufficiently cool down along the escape path that by the time they reach the outside of the encoder enclosure not to be a source of ignition of the outside, potentially ignitable surroundings. Both optical and magnetic encoders are available in one piece designs. Flame proof encoders are approved for use in either IEC Zone 1 or NEC505 Class 1 Zone 1.
Devices utilizing encapsulation have the electronics encased in a resin type material to isolate them from the surrounding atmosphere. Encapsulted encoders are approved for use in either IEC Zone 1 or NEC505 Class 1 Zone 1.