Intrinsic safety marking

Classification and marking of intrinsically safe apparatus concerning explosion groups (I, IIA, IIB, IIC) and temperature classes (T1-T6) are organized in the same way as for, e.g., flameproof enclosure - d (see Tables 4.1 and 4.2, Chapter 4, and Chapter 5 for marking). Some special features, however, which are of importance for type of protection intrinsic safety - i only, will be explained in the following. 

For each interconnection of an intrinsically safe circuit with an associated apparatus a proof of intrinsic safety - i shall be performed on the basis of the safety-related maximum values. As far as the simple standard design given in this example (Fig. 6.218) is concerned, it is assumed that the associated apparatus only - the Ex i-isolator - can feed electrical power into the intrinsically safe circuit. Therefore the safety-related parameters are marked as out-parameters with the subscript o. On the other side, the intrinsically safe apparatus may consume electrical power only. Its values are marked as in-parameters, subscript i. ... 

A system specified to such a degree is intrinsically safe with a maximum of 32 field devices and two terminations. The proof of intrinsic safety is thereby reduced to a list of the apparatus installed and their certificate. The marking includes FISCO and the type of apparatus, e.g. power supply, field device, termination. 

Externally, the only difference between this Ex i-isolator (Fig. 6.207) with galvanic isolation and a safety barrier is its increased size. The complex electronics and, of course, the components limiting current and voltage are hidden inside. In this example, a trip amplifier for DIN rail mounting is shown, which is suitable for operating temperature sensors in intrinsically safe circuits. The marking is ... 

Ordinary electrical equipment cannot be installed in zone 0, even when it is flameproof protected. However, many chemical and oil-processing plants are entirely dependent upon instrumentation and data transmission for their safe operation. Therefore, very low-power instrumentation and data-transmission circuits can be used in special circumstances, but the equipment must be intrinsically safe, and used in conjunction with a safety barrier installed outside the hazardous area. Intrinsically safe equipment must be marked Ex ia or Ex s , specially certified for use in zone 0. 

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