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 linked 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 particular level in storage vessels.
Pressure sensors for hazardous areas are frequently made with the ignition protection type ?intrinsically safe?, since this allows interventions in an active electrical circuit and less energy is needed. However, this also means that there is insufficient energy available for a robust measured value display. Because of this, digital indicators tend to be made with ignition protection types that enable more current and voltage to be allowed. Frequently, digital indicators are designed 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 used in zone 2 ? so in areas in which 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 benefits, such as maintenance work without the need for a fire certificate, can be used. Digital indicators are generally 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 does not have any Ex i performance data in its approval, it cannot therefore be used in a ?Proof intrinsic safety?. Therefore, an Ex nA indicator cannot be connected in an Ex i circuit.
Liberal of an Ex i pressure sensor with isolated barrier and Ex nA digital indicator.
There is, however, the chance 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 must be installed in zone 2.
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Note
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