Intrinsically safe Foundation fieldbus H1 networks

Many engineers today find themselves questioning which field bus technology to implement in their facility, a thorny subject further complicated when the chosen fieldbus is to be implemented in a hazardous area. Boehringer Ingelheim Chemicals, Petersburg, Va., (See Figure 1 below), was presented with exactly this challenge. This article discusses what you need to know about implementing an intrinsically safe (IS) Foundation fieldbus H1 network in a classified hazardous area, and how Boehringer Ingelheim installed its IS network.

Foundation Fieldbus H1 Primer
The Foundation fieldbus specification is open, internationally accepted and supported by the Fieldbus Foundation. There are two basic flavors of Foundation fieldbus: H1 and HSE. This article will focus on H1—a 31.25 Kb/sec, interoperable, bidirectional, digital, serial, publisher/subscriber communications network connecting smart devices that support function blocks executing in the host, the smart device or a combination of the two.

Foundation fieldbus offers several possible network topologies, such as point-to-point, bus-with- spurs, daisy-chain, tree or mixed—a combination of all supported topologies (See Figure 2 below).

The trunk is the main communication pathway between devices and is typically the power supply to spurs on the segment. The segment is a section that ends in terminators and cannot exceed 1,900 m in length, including the main trunk and spurs. A spur is a branch off the trunk and can vary in length between 1 m and 120 m.

	FIGURE 1: SEEMS SERENE, BUT IS HAZARDOUS
	Production areas at the Boehringer Ingelheim Chemicals plant in Petersburg, Va., are deemed hazardous, requiring an intrinsically safe fieldbus installation.

The physical components of a standard (non-IS) Foundation fieldbus H1 network are a power supply, power conditioner, optional repeater(s), cables, junction box(es), terminators and devices. The power supply provides 19 to 26 VDC to the trunk and must be conditioned using a power conditioner either separate from or integral to the power supply. The power supply can be either bulk or dedicated; however, the power conditioner is not optional. Repeaters can be used to extend a segment, which will allow another 1,900 m. The total number of devices on any segment is 32. The total number of devices on a segment may also be limited by logical components of the fieldbus, such as the scheduled macrocycle, control-system loop configuration and the amount of data communicated to/from a device (links and VCRs).

FIGURE 2: TOPOLOGIES GALORE

Fieldbus topologies include point-to-point, bus with spurs, daisy chain, tree, or combinations thereof.

Individual shielded, twisted-pair cable defined as Type A cable in the IEC/ISA Physical Layer Standard is preferred. The maximum segment length of 1,900 m will have to be reduced if another type of cable is selected or if cable types are mixed. Table 1 summarizes the maximum lengths of different types of cables.

Junction boxes with terminal blocks are one method for connecting several devices to the segment at a single location, as well as providing an installation point for the segment terminator. Pre-manufactured junction box systems designed for Foundation fieldbus are readily available. A terminator is an impedance-matching device located at both ends of the segment that consists of a 100 Ω resistor in series with a 1 µF capacitor.