UL-60079-25:2015 pdf free download – STANDARD FOR SAFETY Explosive Atmospheres – Part 25: Intrinsically Safe Electrical Systems.
After determining the new maximum values of current and voltage, the intrinsic safety of the combined circuit should be checked by means of the ignition limit curves given in ANSI/ISA-60079-1 1 IEC 60079 11. taking account of the safety factor for the resistive circuit, and the new maximum permissible values of external inductance L0 and capacitance C0 should be determined. Here, however, the procedure introduced in lC-6OO-7-9-3-4 for the verification of intrinsically safe circuits with more than one associated apparatus with linear current/voltage characteristics, shows a weakness, caused by the following:
— the maximum permissible inductances are valid only for a maximum voltage of 24 V;
— the occurrence of both inductance and capacitance is not taken into account.
If proceeding on the basis of open-circuit voltages and short-circuit currents only, the safety factor obtained really decreases from the desired value of 1 .5 to approximately 1 .0 in the voltage range above 20 V. This seems to be acceptable. since the interconnection in accordance with IEC 60079-14-can only meet level of protection Ib” generally, even if all the individual apparatus conforms to level of protection Ia”. However, in the case of low voltages, the safety factor can drop considerably below the value of 1 .0. Such an approach is thus not effective with regard to safety.
If one or more active sources within one circuit have non-linear characteristics, evaluation on the basis of no- load voltages and short-circuit currents only cannot accomplish the original intention.
In practice, sources with trapezoidal shape (see Figure C.lb) are used and rectangular output characteristics (see Figure C.lc) occur often if electronic current-limiting devices are used. For such circuits, the ignition limit curves in ANSI/ISA-60079-11 JEC 60079-11 cannot be used. This standard therefore describes a method that allows the safety evaluation of the combination of networks including non-linear circuits by means of diagrams.
The procedure introduced here is applicable for Class I, Zone 1 and for equipment groups lIC and lIB Zone 21. Grpuo IIIC. It should be emphasized that an instrument for the assessment of the interconnection is being proposed here; using it for defining intrinsic safety parameters of individual circuits or apparatus makes sense only in the case of simple rectangular or linear circuits.
A battery, for example, fitted with an external current limiting resistor has no constant internal resistance. Likewise, the source voltage changes as a function of the degree of charge. In order to study the behaviour of such practical circuits, they are represented by their simpler equivalent circuits that should obviously not be less capable of causing ignition than the actual circuit. In the above case of a battery, one would take the maximum open circuit as U0 and the external resistance as R as in Figure C.la. This equivalent circuit voltage has a linear characteristic.
Non-linear circuits can also be reduced, usually to the two basic types shown in Figures C.lb and C.lc. The source with trapezoidal characteristic (Figure C.lb) consists of a voltage source, a resistance and additional voltage limiting components (for example, zener diodes) at the output terminals. The rectangular characteristic of Figure C.lc has the current limited by an electronic current regulator.
If one considers the output power of the different networks, it becomes obvious that different ignition limit values should apply, since the igniting spark is also a load and its matching to the source feeding it should be taken into account. The maximum available power from the source shown in Figure C.