Basic principles of intrinsic safety

An electronically stabilized power source may show an idealized voltage-current diagram according to the right side curve. The output voltage remains constant U = U l at all load conditions I Isc, and in the short-circuit-condition I = Isc, U drops down to zero. At point A, the output power is P UNL Isc, four times the maximum output power of a linear circuit at identical values for UNL and Isc. 

The output power P = 0.25 UNL Isc can be achieved by electronic stabilization (dashed line in the left side diagram) with U = 0.5 UNL and I = 0.5 Isc, defining the intrinsic safety of the circuit (the minimum ignition curves cannot be applied as a reference in this case, however, due to the nonlinearity of a stabilized power source). 

In resistive circuits, the diagram I versus U at constant power, P = U I, I = P/U, should be expected to be hyperbolically shaped, or, in double-logarithmic representation, lg I = lg P - lg U, to be a straight line. Obviously, 

As explained in Fig. 6.168, the maximum available output power fed by a linear voltage source (i.e. a constant internal resistance determines the U-I behaviour) into an external resistive load at the resistance matching condition equals 25% of the power values given above. 

In the same way, capacitive circuits (see Figs 6.163 and 6.164) show curves C versus U deviating from those expected for a constant storage energy  

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