IEC 60079-25:2020 pdf free download – Explosive atmospheres
The majority of intrinsically safe systems are simple systems, containing a single power sourcein associated apparatus connected to a singie piece of field mounted intrinsically safeapparatus. This annex uses the combination of the temperature transmitter and the intrinsicallysafe interface shown in Annex E to illustrate the method of analysis.
The first step is to establish the safety data of the two pieces of apparatus in the circuit. Thisdata can be derived from a copy of the certificate, instructions or control drawing, which shouldbe available to the system designer. In particular, any Specific Conditions of Use should betaken into account in the system design.Precisely what information is transferred to thedescriptive system document is determined by the necessity for the system analysis to beclearly justified and for it to be relatively simple to create the particular installation drawing fromthis reference drawing.
Each of the three equivalent circuits has to be assessed for safety using the table showing thepermitted short-circuit current corresponding to the voltage and the equipment group ofiEC 60079-11.The values L,, or optionally LiR。and Chave to be established for each circuitand the most onerous value used together with its relevant equivalent circuit. The matchedpower available from each of the three equivalent circuits is also determined. The matchedpower of the combined circuit is the sum of the power available from each circuit only when thesources have the same output voltage.
Where two sources of power are connected to the same intrinsically safe circuit and theirinterconnections are not well defined by reliable interconnections as illustrated in Figure B.3,there is a possibility that the sources of power can be connected in both series and parallel. Inthese circumstances，all the possible equivalent circuits are evaluated，following both theprocedures set out.The most onerous output parameters and equivalent circuits are utilized inestablishing the integrity of the intrinsically safe system.
The apparatus can contain a power source，which results in it having output parameters, forexample from internal batteries. When this occurs，the analysis of the system includes thecombination of this power source with any power source in the associated apparatus. Such ananalysis would normally include the reversal of the interconnection because of the possiblefailure of the field wiring.
Having established the representative equivalent circuits, these circuits can be used as if therewas a single power source,and the procedure already discussed in Annex A can be used toestablish whether the system as a whole is acceptably safe.
When two or more sources of power with different output voltages are interconnected theresultant circulating current can cause additional dissipation in the regulating circuits. Wherethe circuits have conventional resistive current limiting, the additional dissipation is notconsidered to adversely affect intrinsic safety.