Although Zener Barriers have largely been superseded by Galvanic Isolation interfaces, their simplicity goes back to the core principles making them a good mechanism for demonstrating the fundamentals of intrinsic safety.
The conventional diagram for a Zener Barrier only shows one Zener diode, it demonstrates the principle of the circuit but would make it only suitable for use in category 3.
In practice the Zener diode would be triplicated to give an additional 2 failsafe conditions which is the more common configuration.
Zener diodes tend to fail to short circuit making the circuit safe i.e. reduce the voltage presented at the intrinsic safety output terminals. However there is a very slight possibility that it could fail to open circuit therefore a Zener Diode is NOT failsafe, making this circuit EPL 'c' and only suitable for category 3 installation.
In reality unless designed specifically for a lower EPL standard intrinsically safe interface will have 3 diodes and therefore safe with up to 2 failures giving it an EPL 'a' i.e. Category 1.
Note: Although rare, using 2 diodes would give it an EPL 'b' i.e. category 2.
To simplify the diagrams where one Zener diode is shown it can be assumed this would be triplicated (unless otherwise detailed). Most commercial barriers are designed for Ex ia.
Within the Zener barrier the resistor limits the current to the field (Ohms law - Zener Voltage/Resistance). Should a return signal be required then it is often used in conjunction with a diode return channel, the rectifier diode does not contribute any current to the field circuit but allows a voltage input.
For illustration show here are the more common 'positive' Zener barriers i.e. 0V line is connected to the intrinsic safety earth.
Negative voltage versions exist for positive polarised Zener barriers can be used where the supply voltage is negative with respect to earth, although these are rare.
AC barriers are used with alternating voltage signals e.g. frequency sensors & microphones. The voltage limitation is achieved by using back-to-back Zener diodes within the barrier, usually in a dual channel configuration.
As the AC barrier configuration allows one channel to be positive with respective to earth and the other negative the resulting voltage to the field could twice that of each channel which could present a problem for field equipment. Therefore, an alternative configuration is the 'AC Star barrier which limits the voltage across any combination of terminals.
Each of the barrier types would have a working voltage appropriate for the application and a higher safety voltage Uo. The resistor defines the current rating and in most cases is limited to the current allowed for IIC gasses to cover every possibility although some will have a higher current (only allowed in IIB) particularly those designed for digital output.
Historically polarised barriers were designed for standard voltages 24vdc and 12v with Uo of 28v and 15v (or thereabouts) respectively.
AC barriers tend to be a lower voltage as they are primarily used for signals rather than power as the voltage has to be considered as double each channel e.g. with a double channel 12vac barrier (Uo 15v) one channel positive 15v the other negative giving a combined Uo of 30v.
Refer to Safety Parameters for more information.