Zener barriers and isolators are two different devices that provide safety by limiting the energy in the field. An explosive atmosphere is a mixture of flammable substances (like gas, vapors, mists, or combustibles in a pulverulent state) with air or combustive agent under certain atmospheric conditions in which, upon ignition, combustion spreads to the flammable mixture. For an explosive atmosphere to exist, a flammable or combustible substance must be in a certain concentration; if this concentration is too high or too low, a combustion reaction, or even no reaction may happen. Since a hazardous mixture may be present under normal plant conditions or occasionally an explosion danger can be prevented by using three main protection techniques: containment (explosion-proof), segregation (encapsulation) and prevention (intrinsic safety).
Intrinsic safety is therefore a protection technique intended for the safe operation of electrical equipment in hazardous areas by limiting the energy available for ignition. For signals and circuits that can operate at low currents and voltages, the use of intrinsic safety simplifies system architectures and reduces installation costs compared to other protection methods. European standards harmonized to ATEX European Directive 2014/34/EU are the basis for defining design and construction requirements for intrinsically safe equipment and systems.
Measurement loops devices
Considering different devices, equipment, and interfaces, in intrinsically safe applications it is important to prefer a system approach. Zener barriers are passive devices that divert fault energy to ground via Zener diodes. Differently, isolators are active devices that use transformers or opto-couplers to block fault energy and to provide isolation between circuits in hazardous area and those in safe area. Devices used in intrinsically safe applications must comply to standards and to the worst use case. In addition, the use of an isolator or a barrier is not a sufficient condition for achieving a safe state. The measurement system or loop must be verified or certified in its entirety, including field elements (thermocouples, resistance thermometers, switches, resistors, semiconductors), their safety parameters, intrinsically safe devices (transmitters, valves, proximity sensors, LEDs, modems, etc.), wiring, temperatures, and potential use with other certified systems (Ex, SIL, TUV) or surge protectors. Devices should be certified to establish their "Entity parameters" prior to their installation in an intrinsically safe circuit. The “Entity” model exploits intrinsic safety concept, defining minimum voltage and current values and power levels with which devices can be certified.
Zener barriers are simple and low-cost but not at all easy to choose devices, and they were very popular in the past. Currently they are mainly used in revamping projects and OEM applications. They are based on energy detour concept and consist of a very simple network of components. This kind of protection can be mounted in safe area so that the field sensor is isolated. Barriers consist of a simple electrical circuit created by Zener diodes in series with resistors and fuses. This protection deflects an overcurrent or an overvoltage to ground connection before they can enter in classified area and cause an explosive atmosphere ignition. Zener barriers require several routine checks, and relatively high installation costs compared to isolators.
In normal operating conditions, barrier passes electrical signals, in both directions, without lessening them. When a fault voltage occurs at barrier terminals turned to safe area, resulting current is diverted to earth through a fuse and Zener diodes. During the fault transient, the open circuit voltage at the terminals to the hazardous area is limited to the Zener voltage, while the short circuit current in the hazardous area is limited by the limiting resistor. A barrier’s efficiency depends on the ground connection, which must ensure the return of the fault current in safe area preventing any substantial voltage and current increase in the hazardous area terminals. This is ensured by the use of a dedicated grounding conductor, separated from any other grounding connection. The resistance of connection between the ground of the Zener barrier and the farthest one of the reference points must be kept below 1 Ohm.
This type of unit, unlike barriers, maintains a high degree of isolation that prevents the transmission of overcurrent or surges from safe area to hazardous area. The operating system is based on the use of transformers and optical couplers ensuring segregation and isolation characteristics. When a galvanic isolator is used, earth connection is no longer necessary, ensuring that data transmission operates in a balanced manner, i.e., the signal is not altered by parasite or drift currents due to grounding. Isolators or isolated barriers are more accurate and robust, and they are based on the principle of isolating potentially dangerous energy. The difference consists of providing isolation between the circuits in the hazardous area and those in safe area, using components such as transformers, and opto-isolators, which must be compliant with intrinsic safety standards to guarantee safety against the danger of explosion. If well designed, isolation barriers do not allow the fault voltage to reach the energy limiting circuit which must be able to withstand only the voltage on the secondary winding of the transformer. Galvanic isolation allows limiting circuits to be floating to ground; therefore, both grounding and fuses are no longer required for this circuit. Safety parameters are determined in a similar way to that used for Zener barriers. Isolators are easily programmable and can be used with duplicators, redundant 1oo2 architectures, critical applications (in hazardous areas or with vibrations) and can also provide standard signals and have I/O on board.
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