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How to calculate an Intrinsically Safe loop approval

Author : Gary Friend CEng MIET, Director at AGL Automation

05 October 2020

We all know what can happen when the correct techniques are not used when interfacing into a hazardous area. Using Intrinsic Safety (Ex i based on IEC 60079-11; IEC 60079-25), the energy in the hazardous area is limited to below the ignition energy of the gas present, thereby preventing explosions.

November 2019 TPC Group chemical plant explosion – Image: CSB
November 2019 TPC Group chemical plant explosion – Image: CSB

For an explosion to occur, three things need to be present: fuel (gas/dust), oxygen, and a source of ignition (spark or heat). There are several protection techniques to prevent ignition. For instrumentation, Intrinsic Safety Ex i (electronic protection) & Flameproof Ex d (mechanical protection) are most common. Intrinsic Safety works on the principle of removing the source of ignition which can be achieved by using a Zener barrier or galvanic isolator.

A Zener barrier is a simple device where the voltage is limited by a Zener diode and the current by a resistor. The fuse is there to protect the Zener diode and the key to safety is the Intrinsically Safe earth – without it, there is no protection. If the input voltage increases above Zener diode voltage, the Zener conducts and the fuse blows. The Zener barrier has to be replaced. In addition, the Zener barrier has a volt drop across it under operating conditions so careful calculation must be done to ensure there is sufficient voltage at the field device.

Read the full article in the October issue of Hazardex by clicking here.

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