SAFE CIRCUITS FOR HAZARDOUS AREAS

By R.J. Redding CEng., F.I.E.E. Reproduction from Electronics Weekly No. 343 29 March 1967 with kind permission of the publishers - The Certificated Engineer Nov 1967

Barriers combat fire and explosion dangers

Fire and explosion risks from inflammable vapours at chemical plants and refineries are a major consideration in the design of electrical and electronic equipment for these plants. The circuitry presents an obvious hazard against which precautions must be vigorously applied.

This problem was encountered in coal-mining at the turn of the century and a number of methods have been employed to minimise the hazards. One of these is "flameproofing" in which the electrical equipment is contained in heavy metal cases built to withstand an explosion (BS 229).

Another method is "Intrinsically Safe Design" which means that the electrical energy in the circuit is kept to such a low level that any sparking that could occur IS Innocuous. This latter method has seen the basis for the design of much electronic Instrumentation equipment for the monitoring and control of the process in chemical plants and oil refineries.

The rapid increase in the range and complexity of this equipment has, however, created problems in the certifying and approval of such systems and a fresh approach to the problem was described in IEE Paper No. 5146 C which was read at Savoy Place on February 21, 1967.

The paper described a barrier that greatly simplifies the design of completely safe systems.

When equipment is to be used in hazardous locations the practice has been to submit a set of equipment for tests III the appropriate gas hazard by the Ministry of Power.

More difficult

On the basis of the result, a certificate might be issued by the Ministry of Power or the Factory Department of the Ministry of Labour detailing the equipment and how It may be used. Whilst this procedure works admirably for such items as telephones and bell systems, it becomes more difficult for a range of, say, process control equipment.

Many changes in the design of the equipment or additions to the circuit may invalidate the certificate.

Many industrial control schemes employ items supplied by a number of manufacturers and these may be coupled together with common equipment such as a multipoint recorder or data logger.

The procedure of testing and certifying a system as a whole is hardly practicable and such safety problems have slowed down the application of electronics with the increasing use of digital computers to control some fresh approach to the problem becomes necessary.

One approach is the barrier method. This looks at the problem from a different angle. The equipment used in large control schemes is, in the main, located in the control room (which is usually a safe area) and only a few items, such as measuring devices and control signal circuits, need to be in the danger area.

The problem is, therefore, to prevent the power level in the circuits leaving the control room, exceeding a certain value, and this is achieved by a "Barrier ". This consists of a network of resistors and zener diodes chosen to have no effect on the normal signal circuits.

If, however, the voltage rises to a chosen value, the diodes conduct and limit the amount of power that could enter the dangerous area. The important point is that the barrier can be designed to cater to the worst possible fault condition, namely, a full application of the power mains. No others need to be considered.

Not relevant

If barriers are made the final terminal block of the control panel, then the apparatus used in the panel is not relevant in fact, the system is protected against mistakes in maintenance and servicing. Because the amount of power entering the danger area is closely defined, the certification of the items which may be used in the dangerous area is greatly simplified and, in many cases, is obvious by inspection.

The barrier unit has been carefully designed for extreme reliability, using the highest-grade components in duplicate. After testing, the network of components is encapsulated in epoxy resin and takes the form of a terminal block. The screws for mounting the block also form the earth connection.

A number of different networks become necessary to suit the various signal levels which are in use at present. The first type has a voltage rating of approximately 10 volts and is made in both positive and negative versions so that a pair can be used when an "earth free" circuit is required.

Further types

For low voltage circuits, as occur in resistance thermometer temperature measurements, another is available in which a series resistance has been made very stable so that the presence of the barrier causes a negligible effect on the accuracy of temperature measurement.

Further types are projected for use with the two-wire transducer systems which operate at fixed voltages such as 24 and 36 volts DC. Barriers for AC are also possible but a need has not yet arisen. In addition, the unit has potential use for protecting personnel from shocks as well as guarding against explosive atmospheres.

The unit was first described four years ago at a Symposium of the lEE and its evolution is a good example of co-operative action within the industry.

For maximum benefit and to gain official approval it was recognised that the units must be available to industry at large - rather than a monopoly of one company, although it is the subject of patents - with production closely controlled to ensure quality, economic quantities and correct usage, Original work was later developed by L. C. Towle, of George Kent Limited, and tests were carried out by the ERA and ICI Ltd.

The design of the initial unit was agreed by a committee of interested parties representing the users, the Inspectorate and the instrument makers.

The development of the range of units is now being co-ordinated within the British Industrial Measuring and Control Apparatus Manufacturers' Association.

They are, however, only being distributed by suppliers who are able to ensure adequate technical assistance in their application. This is particularly important in the design of large supervisory .. and control systems for chemical plants and utilities where equipment from a number of manufacturers has to be integrated.

In the past, the range of items of equipment permitted has been limited to those specified on a certificate.

Whilst the barrier method can remove this limitation as far as the "safe area" equipment is concerned, it is no substitute for expertise in the technical design of the system, and the signal level, power supply, earthing, etc. need careful decisions.

There is also hope for overcoming export problems when conditions in a foreign country have to be met. If the permissible inductance and capacitance for a given voltage are stated (and it varies in different countries) the system can be designed accordingly.

Some countries specify an " earth free" system and this is possible by the use of a pair of barrier units. The great advantage is that, however, complex the system, one can demonstrate on paper or in the field that such stipulations have been compiled with.