Is it possible for connecting a non-incendive Ex nA digital indicator to an intrinsically safe Ex i sensor in hazardous areas? Frequently, signals from pressure sensors that not have an integral digital indicator for the measured value are connected to external digital indicators. These digital indicators enable an on-site display of the measured value and, thus, a convenient and immediate reading of, for example, the level in storage vessels.
Pressure sensors for hazardous areas are frequently made with the ignition protection type ?intrinsically safe?, since this allows interventions within an active electrical circuit and less energy is necessary. However, this also means that there is insufficient energy designed for a robust measured value display. Therefore, digital indicators tend to be designed with ignition protection types that enable more current and voltage to be allowed. Frequently, digital indicators were created as ?non-incendive? (Ex nA). This ignition protection type enables power-hungry instruments to be operated in Ex areas too. However, Ex nA is limited for use in zone 2 ? so in areas where an explosive atmosphere only rarely occurs.
The proper connection of digital indicator and barrier
Non-permissible connection of an Ex i pressure sensor with isolated barrier and Ex nA digital indicator.
Every Ex i sensor requires an intrinsically safe barrier because of its voltage supply, so the interconnection is recognized as intrinsically safe and the huge benefits, such as maintenance work with no need for a fire certificate, may be used. Digital indicators are generally integrated as passive element, with or without an external power, directly in the 4 ? 20 mA current signal of a sensor. Since an Ex nA digital indicator has no Ex i performance data in its approval, it cannot therefore be used in a ?Proof intrinsic safety?. Therefore, Transparency cannot be connected within an Ex i circuit.
Correct connection of an Ex i pressure sensor with isolated barrier and Ex nA digital indicator.
There is, however, the possibility to connect an electronic indicator having an Ex i pressure sensor in zone 2. The prerequisite is that the intrinsically safe isolated barrier also possesses an Ex nA approval and should be installed in zone 2.
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Note
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