Applying foundation Fieldbus in a hazardous area

The IEC 1158-2 physical layer completely replaces the old 4 to 20 mA technology in hazardous and non-hazardous area. One advantage of the use of Fieldbus in an intrinsic safety application is the possibility to connect up to 4 devices to a single fieldbus barrier (while in the 4 to 20 mA technology is necessary one barrier for each device). Another important point that should be evaluated is the current consumption of each bus-powered device connected in the network; a device with lower current consumption allows longer cable distances or more devices connected in the same fieldbus intrinsic safety barrier. This paper describe the savings in control panel size, analyzes the cable length reached based on the resistance and capacitance value of the cables in conjunction with device consumption and the current supplied by the fieldbus intrinsic safety barrier. It is also shown that the rule of 4 devices per barrier write in IEC fieldbus standard can be bent, depending on the devices consumption, barriers and cables used.