Regulations for the construction and examination of intrinsically safe equipment intended for use in areas in danger of gas explosion and of associated equipment for connection to intrinsically safe circuits that lead into such areas are stipulated in standard International Electrotechnical Commission (IEC) 60079-11. This standard is also valid for electric equipment or parts of electric equipment, which are found outside of the areas in danger of gas explosion or which are protected by another sort of ignition protection according to IEC 60079-0. The intrinsic safety of the electric circuits in the areas in danger of gas explosion depends on the design and construction of the electric equipment or parts of electric equipment that are found outside of the areas in danger of gas explosion. “Intrinsic safety” as a type of ignition protection is based on the limitation of electric energy within the equipment and on connecting cables, exposed to an explosive atmosphere, to a level below which ignition by spark formation or heating can no longer take place.
The term “malfunction” is of particular importance within the relevant standards. Every defect of any component, disconnection, insulation or connection between components is meant here, which is not stipulated as non-susceptible to malfunctions by IEC 60079-11 and on which the intrinsic safety of a circuit depends. Malfunctions are differentiated into countable malfunctions and non-countable malfunctions. Countable malfunctions are malfunctions that occur in parts of the electric equipment, which satisfy the construction requirements of IEC 60079-11. Other malfunctions are characterized as non-countable malfunctions.
For example, the requirement for electric equipment with the protection level “ia” resulting from the standard is that the intrinsically safe circuits of the equipment at a defined set voltage should not be able to cause ignition in undisturbed operation. In addition, these intrinsically safe circuits should not be able to cause ignition with the presence of two countable malfunctions together with those non-countable malfunctions, which result in the most unfavorable condition.
In order to avoid thermal ignition, all surfaces of components, housing and the wiring that could come into contact with explosive gas atmospheres have to be evaluated and/or tested in terms of maximum temperature. This results in upper limits for allowable power dissipation of components as a function of surface size and the maximum ambient temperature. Accordingly, it is necessary to limit the power that is maximally provided to these components.
The choice of allowable components for the limitation of current is, however, limited. Controllable semiconductor devices may only be used as current-limiting circuits connected in series in electric equipment with protection levels “ib” and “ic”. However, current limiters connected in series comprising controllable and non-controllable semiconductor devices may be used for limiting power for equipment with the protection level “ia”.
An electric safety barrier for protecting consumers and/or transmitters found in areas in danger of explosion is known from German Patent DE 36 22 268 C1 and corresponding U.S. Pat. No. 4,831,484, which is connected via cables to circuit parts found outside of the areas in danger of explosion. Two longitudinal control members connected in series are used in this safety barrier, which are controlled by four control circuits. The control voltage for the control circuits here is the sum of at least a part of the output voltage of a current-measuring member and at least one part of a voltage corresponding to the voltage drop in the longitudinal control members.
In particular, the current-measuring member is an ohmic resistor and the longitudinal control member and the control circuit are formed by transistors. The ohmic resistor is chosen here so that, at a desired limiting current, such a voltage drop is created across this resistor. The control voltage reaches a sufficiently large value—for example 0.6 volts—in order to conductively switch a transistor of the control circuit, through which, in turn, the transistor of an associated longitudinal control member is switched in a non-conductive state. The ohmic resistor not only limits a short-circuit current in a connected consumer but also sets a desired limiting current, which leads to a corresponding control of the semiconductor devices. The electric safety barrier is also not sufficient for protection level “ia” since, when there is a short-circuit of either longitudinal control members or the first longitudinal control member and the transistor of an associated control circuit, a sufficient limitation of a short-circuit current in a connected consumer is not guaranteed.
It is known from the prior art to use safety fuses for limitation of current or power. In the case of a malfunction that results in triggering and blowing a safety fuse, however, the safety fuse has to be replaced, which can lead to a bit of effort in the case of power supply devices that are not easily accessible. It is also known from the prior art to use resistors for limiting power. In order to limit the power to a value of 1 watt required to avoid thermal ignition, a resistance of at least 144 ohm is necessary at the given voltage of 12 volts. In the case of a power-adapted consumer, i.e., that the resistance of the load current is chosen to be the same as the resistance for limiting power, one quarter of this value, namely 36 ohm, is potentially sufficient. The use of a resistor for limiting power, however, has the disadvantage that undesirably high power dissipation falls take place in this resistor.