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 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 example, the level in storage vessels.
Pressure sensors for hazardous areas are frequently designed 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 powerful measured value display. As a result, 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 use in zone 2 – so in areas in which an explosive atmosphere only rarely occurs.
The right connection of digital indicator and barrier
Every Ex i sensor requires an intrinsically safe barrier for its voltage supply, so that the interconnection is considered as intrinsically safe and the benefits, such as maintenance work without the need for a fire certificate, can be used. Digital indicators are frequently 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 has no Ex i performance data in its approval, it cannot therefore be used in a “Proof of intrinsic safety”. Therefore, an Ex nA indicator cannot be connected in an Ex i circuit.
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 must be installed in zone 2.
Should you need any help with choosing a suitable solution for your pressure measurement requirement, your contact person will gladly be of assistance.