The Importance of Grounding in an Intrinsically Safe System Using Zener Barriers

Zener barriers, also referred to as “shunt diode safety barriers,” are safety barriers used in intrinsic safety systems to provide protection for certain electrical devices by limiting the energy going to the hazardous area, given a fault event. They have several internal components that limit the maximum voltage, current and power to the hazardous areas to a defined and limited value.  

As the illustration shows, a Zener barrier is placed on the electric wires between the non-hazardous and hazardous areas of an intrinsically safe system. They limit power to the field device in the hazardous area and must be grounded properly in order to direct the fault current to the main referenced grounding point, thus limiting the voltage and current output of the field device to safe levels for the given environment.

In this illustration, a metal spike has been hammered through the mains cable as well as the supply cable to the barrier, resulting in a high voltage on the terminals in the non-hazardous area.

Given an out-of-control fault in the non-hazardous area, no voltage should be allowed to bypass the safety barrier to go into the hazardous area. Also, the metal enclosure of the field mounted instrument located in a hazardous location is directly connected to the main ground to prevent a ground loop between the IS barrier and the intrinsically safe device. It must be noted that the internal circuit of the intrinsically safe device is isolated from the external enclosure.

In summary, the Zener barrier earth must provide a secure, robust, low-resistance path back to the source of supply for any fault currents which flow. As was already shown, In the case of mains breakthrough, this arrangement prevents unacceptable voltage and current levels from going to the hazardous area. Installation of an intrinsically safe system to connect a safety barrier to a field device requires a precise assessment. The Code of Practice for such installations must be strictly adhered to in order to ensure that the installation is safe.

We hope you enjoyed this presentation. Please ask the Hazcon team if you have any questions about intrinsic safety barriers and how to install them properly.

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