Intrinsically safe Foundation fieldbus H1 networks

The system method requires that the manufacturer combine its individual components into a certified system. The manufacturer must also provide a control drawing that depicts the system, as well as any constraints that would violate the certification/approval, such as maximum cable capacitance and inductance.

The early barriers used in IS fieldbus implementation limited the available bus current to about 80mA per segment. If you use 20 mADC per device as the load, that equates to four devices per segment without taking into account any losses due to barriers or cable lengths. This does not realize a cost savings in comparison to traditional 4-20 mADC IS circuits, nor does it demonstrate any of the benefits of bus technology. To increase the amount of usable bus current and simplify the IS design, an international standard for IS Foundation fieldbus systems was developed and is now known as the Fieldbus Intrinsically Safe COncept (FISCO).

The standard was based on work done by Physikalisch-Technische Bundesanstalt (PTB), the national metrology institute in Germany, and involved theoretical and experimental techniques. The first edition of the standard was published in April 2005 as IEC 60079-27. FISCO simplified the design calculations required in the traditional entity approach by requiring the testing and certification of all fieldbus components to a set of standards defined by the specification.

By certifying the devices to a standard and using cable that falls into the limits of the experimental data (See Table 6 below), the design calculations were simplified to Ohm’s law and inductance/capacitance entity parameters were no longer components in the calculations.

TABLE 6 

This also allowed FISCO power supplies to generate up to 120 mADC in the  IIC (A/B) gas group and 265 mADC in the IIB (C/D) gas group, which equates to perhaps five devices and twelve devices respectively, assuming a 20 mADC load per device and losses due to cable resistance and other factors. Figure 3 depicts a typical FISCO system.

FIGURE 3: WIRED FOR SAFETY

In a traditonal intrinsically safe system, a barrier in the safe area limits the amount of electrical energy that can enter a hazardous area. FISCO-based IS systems use a barrier which limits the segment to five devices in a IIC hazardous area.

Getting Around FISCO Drawbacks
The drawback to the typical entity and FISCO-based IS system is that the barrier is located in the safe area, which is the portion of the fieldbus trunk that sees maximum current flow and equates to a large voltage drop.

We found an alternative to FISCO in Moore-Hawke’s Routemaster entity-based IS system. Route-Master has the benefits of the system method and none of the disadvantages of the entity method. Route-Master systems are based on a split architecture that separates the barrier into two parts (See Figure 4 below).

FIGURE 4: BYPASSING FISCO

Moore-Hawke’s entity-based IS system uses a split architecture, which separates the barrier into two parts, allowing a H1 segment to have 350mA and support up to 16 devices in a IIC area, nearly three times as many as a FISCO system.

The first barrier includes a smaller resistance seen at the interface between the safe and hazardous area (high current position), and the second barrier includes a larger resistance seen at the device coupler located in the hazardous area. This design allows for a smaller voltage drop and 350 mA DC on the trunk, which equates to sixteen devices (assuming 20 mADC load per device).

The device couplers are compatible with FF-816 device entity parameters (Vmax. in = 24V, Imax. in = 250mA, Pmax. in = 1.2W) and can be implemented in Class 1 Division 1 Groups C/D with IS connections for Class 1 Division 1 Groups A/B/C/D, when they are installed per the appropriate control drawing (HCGFB-902). Since each spur has IS current-limiting resistors, each spur an independent IS loop.

This helps to simplify the entity parameter calculations by using worst case scenarios. You can calculate the capacitance of a worst-case spur, which is 120 m of the chosen cable, plus an FF816 device, calculate the L/R ratio for the chosen cable, and document these calculations noting that no current or future spur can be longer than 120 m and no other cable type can used. Each spur has electronic auto-resetting, short-circuit protection that prevents short circuits on one spur causing problems on another spur or the trunk.