Control Engineering Online Update for September 9, 2005

Intrinsic safety is the only acceptable means of power for Zone 0, and is thus the only protection method suitable for all hazardous areas. Since users want to simplify purchases, spares stocking, engineering, and maintenance practices, it makes sense to standardize on intrinsic safety for all areas of the plant.

FISCO (fieldbus intrinsically safe concept) and FNICO (fieldbus non-incendive concept) evolved from a traditional entity concept for point-to-point wiring of devices to a bus-oriented concept permitting several devices to be multi-dropped on the same wire sharing a single safety barrier or limiter. Within limits, the inductance and capacitance of devices and cables need not be calculated for the FISCO/FNICO models, resulting in easier engineering and the ability to use longer wires. In addition, unlike a traditional entity barrier with a linear output characteristic, a FISCO safety barrier is trapezoidal, resulting in more power, which allows for more devices and longer wires.

There are several FISCO-conforming isolated intrinsic safety barriers to chose from for use in Exia IIC areas. These safety barriers can either be mounted on a DIN-rail in a panel in the wire marshalling room, or in a suitable enclosure with cable glands closer to the field. Because safety barriers have a limited voltage output, thereby reducing wire distance due to voltage drop caused by instrument power consumption, it is often necessary to mount the barriers in an enclosure closer to the field.

Power and communications
The IEC 61158-2 physical layer for fieldbus permits power and communication signals on the same wire, provided the power supply contains a proper impedance circuit. The inductance may be provided by a separate power supply impedance module or built into the safety barrier.

The main wire that runs the distance from the cable marshalling room to the field is called the trunk. The cable sections that connect the field instruments to the trunk are called spurs. One safety barrier plan puts power and communication on the same pair of trunk wires. A high capacity Ex e-certified power supply impedance module provides several hundred milliamps to power safety barriers as well as field instruments connected to the barriers (see graphic below).

High power trunk combining power an d signals

When a high powered trunk is used, care must be taken to ensure that the trunk cable has sufficient cross-section to carry the supply current the distance from the cable marshalling room to the barriers without causing an excessive voltage drop. At the same time, the cable insulation material must have stable electrical characteristics to meet the fieldbus requirements not only at the time of installation, but throughout the life of the system. For example, polyethylene has proven to be a suitable insulation material for communication cables in general. Additional cost of such cable and the high current power supply impedance must be taken into account.

Above all else, it should be noted that the real purpose of the inductor in the power supply impedance is to prevent a regular dc power supply from short-circuiting the communication.

Due to the trunk being high power, it does not fall within the limits of intrinsic safety. Instead, the trunk is Ex e (increased safety)—restricted to use in Zone 1. This can be overcome by field mounting barriers in Ex em-rated enclosures, limiting the power on the spurs and achieving intrinsic safety for the field instruments that can be connected in Zone 0. The total cost of barriers plus their Ex e enclosures must be taken into account. Installation and maintenance personnel must also be familiar with increased safety installation and wiring practices, such as labeling and protection mechanisms to prevent inadvertent tampering on the energized trunk.

The IEC 61158-2 physical layer for fieldbus also permits power and communication signals on separate wires, eliminating the need for a separate power supply impedance module. Although power and communication on the same wire is ideal for low-power instruments such as transmitters and control valve positioners, it is not the best option when more power is required, such as for powering safety barriers or solenoid valves.

Dedicated power wires require a second pair of wires. However, this can be low-cost PVC insulated cable and does not have to meet the electrical characteristics required for fieldbus cable. Since regular dc power and regular cable can be used, it is possible to tap power from the dc supply normally found in plants. If ac line power is available in the safe area, the dc power supply can be mounted next to the barriers.

On the safe area side of the barriers, communication and power share the same pair of wires, suitable for fieldbus instrumentation (see graphic below).

Separate cables for power and signal in the safe area, shared in the hazardous area

Using this separated plan, the installation becomes an Ex ia IIC throughout the hazardous areas, and regular in the non-classified areas. There is no need to fall back to Ex e or Ex ib, or down to IIB in order to get economy. This same process and the same barrier works for both Foundation Fieldbus H1 and Profibus PA.

Live wire
One of the advantages of intrinsic safety is that it is possible to work on the wires and devices while energized. This makes maintenance easier, as there is no need to power off before working on the wires, nor any need to manage hot work permits and “sniff” for gas before the job can be done. This is particularly important for fieldbus where many devices are connected on the same pair of wires.

When using an Ex e trunk, it is not permitted to do any live work on the trunk. However, when using an Ex ia trunk, live working is permitted on the trunk.

Long distance
FISCO permits the hazardous area segment to be up to 1,000 meters in total length—a distance that is not limited by device and cable capacitance as long as devices are FISCO-certified and cable is used within specified boundaries. One limitation, however, is imposed by voltage drop along the wires as a result of the current drawn from the devices on the bus. To maximize the cable distance for the instruments, use devices with low-power consumption. Transmitters and valve positioners with power consumption as low as 12 mA are available. Secondly, use safety barriers with a high voltage output to give the greatest possible margin for voltage drop between the barrier and device. FISCO barriers with 14 V dc operating output is available, giving a 5 V dc margin for drop along the wire since devices can operate down to 9 V dc.

The longer the FISCO wires supported by the barrier type, the further the wires can run into the hazardous area, permitting the barriers to be mounted in the non-classified area, avoiding Zone 1 and Zone 2 barrier location. This simplifies installation and service.

Barriers that have no repeater mean the fieldbus will consist of only a single network segment, restricting the distance to a max of only 120 meters from barrier to device. One such cable may not reach many devices if they are dispersed. This type of barrier therefore has multiple outputs and may be more expensive. Spurs with one device each are connected to the barrier. These spurs need no terminators since they are 120 meters or less.

The primary purpose of the terminator in fieldbus is as a shunt to convert transmitted current from the sender to a voltage that is received by all devices.

This article is provided by Smar, a supplier of process control instrumentation. For more information, visit www.smar.com .