Intrinsic safety interconnections

Circuit capacitance and inductance, including the values of these parameters for interconnecting wiring, are integral parts of the overall analysis. It is not always possible to assure that the system will be maintained as designed with only approved intrinsically safe components and with circuits of the capacitance and inductance as originally installed. For this reason, intrinsically safe systems are used primarily at locations where there are sufficiently trained personnel to assure that the intrinsic safety of the system is always maintained. 

One of the serious problems which has faced both manufacturers and users in applying the intrinsic safety concept has been the inability to interconnect apparatus of different manufacturers and... 

In contrast to this safe but not very flexible practice the so-called entity concept has been established. Here, intrinsically safe and associated apparatus may be interconnected in an arbitrary combination as long as certain rules are observed. However, the high level of flexibility of this practice shall be verified by an arithmetical proof of intrinsic safety for the most part. In the following, this practice will be explained in detail. 

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 model for proving type of protection intrinsic safety - i with two interconnected intrinsically safe apparatus is given as follows ... 

Deviating from the method described above these are cases with several associated apparatus interconnected in one single circuit. It may then happen that two or more apparatus are active in the sense of intrinsic safety. This indicates that more than one individual apparatus may feed power into the intrinsically safe circuit. 

To prove intrinsic safety with more complex interconnections by way of calculation, it is recommended to follow PTB-Report ThEx-10 in this matter. 

The proof of intrinsic safety by way of calculation ascertains that the apparatus and their interconnection comply with the rules of intrinsic... 

A protection technique based upon the restriction of electrical energy within the apparatus and in the interconnecting wiring, exposed to an explosive atmosphere, to a level below that which can cause ignition by either sparking or heating effects. Because of the method by which intrinsic safety is achieved it is necessary that not only the electrical apparatus exposed to the explosive atmosphere, but also other (associated) electrical apparatus with which It Is Interconnected, Is suitably constructed. 

---