Area Classification on Network Selection Matters

Unlike IS, FISCO Is Based on Experimental Measurements of the Equipment Placed in Hazardous Gas Environment Where Energy Is Increased Until an Explosion Occurs

By Ian Verhappen

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Last year this column discussed different types of network topologies. Now let's take a look at the impact of area classification on network selection.

Area classification is predominantly a field/continuous process issue, although there are parts of a factory where it is also a consideration. If you use an explosion-proof enclosure as your method of protection, then there are no limits on your equipment — except for parts such as antennas that extend outside the enclosure. You have to select the right enclosure, taking into consideration heating or heat dissipation for the electronics; select and properly install the appropriate seals; and implement the correct hot work practices and procedures. You then should be good to go.

On the other hand, if you want to be able to perform live work on your system, then a variety of options are available. Let's start with fieldbus alternatives.

The original live-work alternative, Intrinsic Safety (IS), relies on the entity concept, meaning all the components need to be considered as a single entity. Unfortunately, in the case of fieldbuses with multiple drops on a single network, the number of components for which the concept needs to be verified expands exponentially (power supply connection to device A, power supply connection to device B, device A to device B, etc.). In addition, IS has an approximately 80 mA current limit that severely limits the number of nodes/field devices.

As a result, most process fieldbus systems support the fieldbus intrinsically safe concept (FISCO). Unlike IS, FISCO is based on experimental measurements of the equipment placed in the hazardous gas environment, where the energy is increased until an explosion occurs. Appropriate safety factors then are applied and the results submitted to the IEC. The IEC 60079-27 ed2.0 2008 standard describes the limitations and documentation requirements for these sorts of installations.

All FISCO installations follow the star/tree/chickenfoot configuration with a trunk and multiple spurs, although all the spurs need not be located at the far end of the trunk. You can have one barrier at 50% of the trunk length, another at 80%, and two others at the far end with the terminator (remember, a trunk is defined by the terminators at either of its ends). Because FISCO is limited to outlet voltages of about 12–13 V and currents of 120–360 mA depending on the gas group for which it is designed, many FISCO systems can be used as repeaters to extend the network. Repeaters normally are installed in series, but they can be installed in parallel to increase the number of devices connected to a single fieldbus interface port.

The third commonly used alternative is the High Energy Trunk (HET), also known as the Fieldbus Barrier concept. HET, as the name implies, uses much higher voltages and current levels than FISCO, but the trunk is not live workable because it is an Ex 'e' classified installation. This higher voltage and current make fully loaded networks at long distances possible—the trade-off for not being able to do live work on the trunk and for having active electronics (transformers and IS circuitry) in the barriers themselves. The barriers are mounted in the field, compared to the FISCO units that normally reside in the interface room, but they normally are designed for the field, so this shouldn't be an issue. This year, the Fieldbus Foundation plans release of a specification for what they call Active Device Couplers.

Because all the options provide both power and communications over the same cable, providing the same capabilities of power and signal via one cable becomes more desirable and common with Ethernet products, as well via Power over Ethernet (PoE), which has extra benefits in an industrial setting where cable installation costs are significant.

The majority of industrial Ethernet suppliers provide one or more products with PoE capability, and a number of them supply the PoE intermediaries to inject power at other than the end device. In addition, there are options available for installations of Ethernet in classified areas with intrinsically safe isolators. There is not yet a standard in place for IS Ethernet, so these products are not all interoperable.

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