Connection of an Ex nA digital indicator to an Ex i sensor

Is it possible for connecting a non-incendive Ex nA digital indicator to an intrinsically safe Ex i sensor in hazardous areas? Very often, signals from pressure sensors that not have a built-in 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 instance, the level in storage vessels.
Pressure sensors for hazardous areas are generally designed with the ignition protection type ?intrinsically safe?, since this allows interventions within an active electrical circuit and less energy is needed. However, this also means that there is insufficient energy available for a powerful measured value display. Subsequently, Truly are often 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 in which an explosive atmosphere only rarely occurs.
Expensive 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 for its voltage supply, so the interconnection is recognized as intrinsically safe and the benefits, such as for example maintenance work with no need for a fire certificate, can be utilized. Digital indicators are frequently integrated as passive element, with or lacking any external power supply, 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 utilized in a ?Proof of intrinsic safety?. Therefore, an Ex nA indicator cannot be connected in an Ex i circuit.
Correct connection of an Ex i pressure sensor with isolated barrier and Ex nA digital indicator.
There is, however, the chance to connect a digital indicator with 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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