Is it possible to connect a non-incendive Ex nA digital indicator to an intrinsically safe Ex i sensor in hazardous areas? Very often, signals from pressure sensors that do not have a built-in 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 level in storage vessels.
Pressure sensors for hazardous areas are generally made with the ignition protection type ?intrinsically safe?, since this allows interventions in an active electrical circuit and less energy is necessary. However, this also means that there’s insufficient energy available for a powerful measured value display. As a result, 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 bound for use in zone 2 ? so in areas where an explosive atmosphere only rarely occurs.
The right 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 utilized. Digital indicators are generally integrated as passive element, with or lacking any external power, 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 utilized in a ?Proof intrinsic safety?. Therefore, Instantly can’t 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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