INTRINSIC SAFETY:
Comms and power on two wires

Using the Hawke Route-Master Series 100 products means that, for the first time, the cost savings of fieldbus can be realised in hazardous area installations. Dr Romilly Bowden explains

After many years of waiting, users of industrial process control systems are now able to specify proven and standardised fieldbus technology for their field instruments. Buses conveying both communication signal and power on the same pair of wires are among the most popular. An international standard physical layer for these is specified by IEC61158-2, followed by Foundation Fieldbus, Profibus PA and WorldFIP.

A major influence on the economy of fieldbus installations is the number of field devices which can be connected on a single bus segment and hence to a single controller input/output (I/O) channel. In particular applications, users may choose to use only a small number of devices on a segment, in order to achieve faster scan cycles or for reasons of system integrity. But in most cases, economic considerations lead to the desire to connect as many devices as possible onto a single cable and I/O channel.

The IEC specification allows for up to 32 devices connected to the single pair of wires forming a bus segment, but anticipates only up to twelve devices when these are bus-powered. There may be limitations depending on the design of the host system, but in practice it is the need to supply power to all the devices which limits the possible number of bus-powered devices on one segment. DC voltage drop in the fieldbus cable reduces the supply voltage to the most remote devices, and the situation is much worse in Intrinsically Safe (IS) systems, where the power supply and isolator include current-limiting resistors which contribute significantly to the voltage drop.

When the IEC standard was in development, it was expected that each field device would need about 10mA at 9V; unfortunately, in reality, most of today's bus-powered instruments take up to 20mA or even more, making the voltage drop problem more severe. Until now, only three or four typical devices have been possible on an IS segment. The expected economy of fieldbus has been significantly impaired by this problem.

Hazardous areas

Intrinsic Safety is a widely-used technique to ensure the safety of plant when instruments are located in hazardous areas where explosive gases may be present. The basis of the technique is to ensure that the energy which could be released in the event of an instrument fault (for example a failed component or a short circuit) is limited to below that which would cause ignition of the most combustible mixture of the gases actually or potentially present. This is achieved by limiting the voltage and current supplied to the device, together with restrictions on the stored energy in the system which could be released by a fault.

For industrial process applications, Gas Group IIC is the most easily ignited, IIB next, and IIA the least easily ignited. Group IIC contains only hydrogen and acetylene, Group IIB contains such gases as ethylene, and group IIA such as propane. The energy limits which must be applied to an instrumentation system depends on which group of gases may be present.

It is the practice of most users to specify Gas Group IIC certified instruments, so as to cover all requirements with a single range of products, and most instrument manufacturers design to that requirement. For traditional analogue instruments, this was not a problem. Even for this most stringent gas group, the power available under IS rules is quite adequate to power a single 4-20mA device. However, for a multidrop fieldbus, the restriction of power is a serious limitation. For Gas Group IIC applications, only about 80mA is available from a 19V power supply. Unfortunately, designers of loop-powered instruments have generally been unable to reduce the power consumption of their devices to the recommended 10mA. As a result, it has been possible to connect only three or four devices to a single bus segment.

Hawke's Route-Master Series 100 fieldbus products use a new technique to greatly increase the number of devices which can be connected to an intrinsically safe fieldbus segment. The technique is based on the understanding that many process plants do not require the most stringent Gas Group IIC certification, and even those that do need IIC in some areas may have areas close by where IIB certification would be sufficient. The Hawke products take advantage of the much higher power available in IIB systems. They can be used in any application where the gases concerned do not include hydrogen or acetylene (the IIC gases).

Wider applications

Equally importantly, they can be used in applications where the main trunk can be located in IIB areas, with the field devices mounted up to 120m away in a group IIC area. In this technique, the Fieldbus power supply feeds a number of isolators, located in the safe area, which in turn feed limited power to Gas Group IIB trunk cables. Connected to each trunk, field-mounted passive energy-limiting junction boxes ('device couplers') feed a number of spurs to individual field devices.

The power supply (which can be dual redundant) supports up to eight isolators, each of which provides two independently-protected trunk connections for a single fieldbus segment, with an isolated host system communication port. If both trunks are used, the segment terminators are mounted at the remote end of each trunk - the power feed and host connection are then in the centre of the segment. Distributing the connected devices over two trunk cables in this way may allow twice as many devices to be connected to a single host port. The field-mounted junction boxes are supplied in 4-spur and 8-spur variants. The main trunk is also passed straight through.

As well as the energy-limiting resistor, each spur connection includes a fuse which protects the main trunk against short circuits in the spur or a connected device. A linkable terminator is included for convenience, eliminating the need for an additional external component. Critical components are infallible and/or duplicated for very high system integrity.

Running the main trunk as Gas Group IIB, with additional power limiting to Group IIC level in the junction boxes, has four major advantages compared with using a conventional Group IIC barrier:

• The lower series resistors in the isolator greatly reduce the DC voltage drop to the trunk, allowing higher current and therefore more connected devices.
• The lower series resistors also cause less attenuation of the communication signal, allowing two trunks to be connected back-to-back as a single segment.
• The higher energy storage allowed for group IIB means that the capacitance of the trunk cable is no longer a limiting factor on cable length.
• The additional power limitation to each spur allows the use of existing field devices, designed to IIC rules (1.2W maximum power). The energy-limiting series resistors in the power supply and isolator cause about 3dB attenuation of the fieldbus signal between field devices and host. This attenuation acts twice, between devices on the two connected trunks, if the dual-trunk option is used, but this is easily accommodated within the 10.5dB attenuation allowed by the standard, allowing up to 4.5dB loss due to attenuation in the cable. Up to 2km of cable is possible, when high-quality cable is used. Conventional IIC IS barriers cannot be used back-to-back in this way, since the total attenuation through two such barriers would be too high to permit reliable communication.

Connected devices

The practical result of this approach is that the number of connected devices and trunk cable length are limited only by the DC voltage drop and the overall IEC 1900m limit (though even this may well be exceeded with best quality cable). The diagram (above left) shows the number of devices which could be used, assuming an average 20mA per device and all devices connected at the far end of a 1.0mm conductor trunk cable, using worst case values for power supply output voltage (18.0V) and resistors (26Ohms and 56Ohms in isolator and junction box respectively). The performance of a conventional Group IIC IS barrier, in the same conditions, is shown for comparison.

If some of the devices are connected part way along the trunk, the DC voltage drop due to cable resistance is reduced, and even more devices may be possible - Ohm's Law applies, so the calculation is quite easy. Remember, too, that the Hawke system allows the use of two trunks as parts of the same fieldbus segment. Using this option, the number of devices per controller I/O channel can be further doubled (subject to the overall cable and spur length limits, and any limit imposed by the host system).

• Hawke
  Email c177@industrialnetworking.co.uk
  

PROFILE: Dr Romilly Bowden

Dr romilly bowden BSc, ARCS, PhD (ARCS is Associate of the Royal College of Science, and not the Royal College of Surgeons! - Ed) began his education at St. Christopher School, Letchworth, going on to gain a BSc in Physics at Imperial College, London. He followed this with a PhD at Manchester University, working in Radio Astronomy at the Jodrell Bank Experimental Station. His employment has included seven years with Ferranti in computer design, and 26 years with Rosemount and Fisher-Rosemount in the design and application of digital electronics and computers to industrial measurement and control, first in research and development, and later providing technical support to the company and its customers as European Software Consultant. Since retirement in 1998, Dr Bowden has maintained a technical 'HART and Fieldbus' website (at http://www.romilly.co.uk), and continues part-time consulting on industrial communication technology.