Zener safety barrier

For this application, Zener safety barriers were selected as the protective interface. Further, every circuit going into the hazardous area is connected to a separate Zener safety barrier. 

Safety barriers (or Zener barriers ) form an essential technique in the field of intrinsic safety. 

A protection against polarity reversal cannot be realized for the non-intrinsically safe side of a safety barrier without increasing its series resistance which is considered to be frequently disturbed. One solution consists of the implementation of a second fuse so that it can be replaced if the safety barrier has been operated at reversal polarity or supplied with overvoltage (see Fig. 6.205). The internal fuse protecting the Zener diodes as well as the other components shall be guarded against external access. So, safety barriers are encapsulated for the most part. 

The classic Ex i-isolator is an associated apparatus for installation in a safe area. In this case, too, Zener diodes and resistors are used for voltage and current limitation. However, the components may be rated lower, since the components inserted for galvanic isolation are able to transfer only a limited (e.g. transformers) or absolutely no electric power (e.g. optocouplers). For the maximum voltage Um, the above-mentioned is valid concerning safety barriers. 

By definition, an intrinsically safe circuit is one in which no spark or thermal effect is capable of causing ignition of a given explosive atmosphere. The intrinsic safety of the equipment in a hazardous area is assured by incorporating a Zener diode safety barrier into the control circuit such as that shown in Fig. 13.6. In normal operation, the voltage across a... 

Safety barrier types There are two types of safety barrier DC and AC type. The DC type is shown in Figs. X/3.7.1-1 and X/3.7.3-1A where it is seen that Zener diodes are in one configuration only. In this type, polarity is an important factor and if it is connected in reverse it will not function. On the other hand, to overcome the polarity problem, the AC type (double) is used, especially for thermocouples and RTDs. An AC double-type barrier is shown in Fig. X/3.7.1-3. Here it is seen that two sets of Zener diodes are in back-to-back connection to accept any polarity of connections. Because in this connection both fault current directions are accepted, it is an AC-type barrier. 

---