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 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 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 in an active electrical circuit and less energy is necessary. However, this also means that there’s insufficient energy available for a robust measured value display. Subsequently, digital indicators are often designed 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 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, in order that the interconnection is considered 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 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 utilized in a ?Proof of intrinsic safety?. Therefore, Healthy can’t 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 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
Should you need any help with selecting a suitable solution for your pressure measurement requirement, your contact person will gladly be of assistance.

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