ISA TR12.2 : 1995

1 Introduction
2 The intrinsic safety protection technique
3 Intrinsically safe systems
4 Intrinsically safe system control drawings
5 Entity concept
6 Entity parameters
7 Entity concept control drawings
8 Assessing the intrinsic safety of simple loops
9 Loop operation considerations
10 Special considerations for multi-channel associated
   apparatus loops
11 Assessing the intrinsic safety of complex loops
12 Intrinsically safe apparatus that adds energy to the
   loop
13 Considerations for intrinsically safe bus systems
14 Conclusion
15 References
Figures
1 - Example of a typical control drawing
2 - Control drawing for an intrinsically safe apparatus
     examined under the entity concept
3 - Control drawing for an associated apparatus examined
     under the entity concept
4 - Two-wire loop with neither conductor connected to
     ground using two single-channel associated apparatus
5 - Two-wire loop with neither conductor connected to
     ground using one dual-channel associated apparatus
6 - Control drawing for two-channel associated apparatus
     with Vt and lt parameters
7 - Complex loop with two intrinsically safe apparatus and
     three two-channel associated apparatus
8 - Control drawing for three two-channel associated
     apparatus with Vt and lt
9 - Control drawing for intrinsically safe apparatus with
     multiple circuits
10 - Control drawing for intrinsically safe apparatus
11 - Control drawing for hand-held communicator
12 - Typical intrinsically safe bus configuration

ISA-TR12.2-1995, Intrinsically Safe System Assessment Using the Entity Concept, explains how to evaluate intrinsically safe systems with the entity concept, a method that offers flexibility when selecting and combining equipment for hazardous locations where explosive atmospheres might occur. This technical report details how intrinsically safe apparatus and related equipment are tested independently and assigned specific parameters, such as maximum voltage, current, capacitance and inductance, which help determine if their combination will limit energy to safe levels under normal and fault conditions. This approach simplifies system design by allowing the mixing and matching of components based on these parameters rather than relying solely on pre-approved system configurations. It covers simple loops, multi-channel devices, complex loops and bus systems, providing examples and control drawing formats to guide parameter comparison and installation practices. Additionally, this report addresses special cases like devices that add energy to the loop and highlights considerations for maintaining safety in dynamic or interchangeable setups.