Emerson Exchange 365

As always for 4-20 mA loops you need to make sure the supply voltage provided is sufficient. This often requires some engineering, particularly if there is more than one product in the loop. If the loop operates connected directly to 24 VDC from the bulk power supply you usually have plenty of margin to operate. However, if the loop operates on 2-wire loop power from the DCS/PLC AI card then you need to be more cautious and engineer it. The DCS/PLC AI card may only provide maybe 15 V - check the specification of your AI card. If there is an intrinsic safety barrier involved the voltage will also be limited. The DLC 3010 needs 12 V to operate. The 751 needs 1 V. Total 13 V. With 15 V from the AI card there is only 2 V margin for drop across the wires. Allowing for 23 mA loop current during alarm (fault) condition you can only have 86 ohm LOOP resistance (i.e. 43 ohm in each wire) so distance may be limited if wires are of a thin gauge. Note that this is too little for HART communication. To enable HART communication you need at least 230 ohm loop resistance which means 5.3 V is required. That is, 12+1+5.3 =18.3V has to be provided by the AI card or directly from DC bulk power for the HART communication to work. The same calculations apply for anything that goes into the loop such as signal conditioners, recorders, other indicators, and wireless adapters etc.