Is it possible to connect 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 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 enables interventions in an active electrical circuit and less energy is necessary. However, this also implies that there is insufficient energy designed for a robust measured value display. Due to this fact, 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 use in zone 2 ? so in areas in which 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 for its voltage supply, in order that the interconnection is recognized as intrinsically safe and the 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 Deplorable does not have any Ex i performance data in its approval, it cannot therefore be utilized in a ?Proof intrinsic safety?. Therefore, an Ex nA indicator 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 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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