GRAPHICAL SYMBOLS FOR DIAGRAMS

13 November 2006
Jorgen Aagaard and Siv Velander
Jorgen Aagaard and Siv Velander

Over the last few decades, the number of complaints and questions regarding symbols for use on diagrams has increased especially from the end users, i.e. personnel employed at industrial plants, from engineers designing plants and from persons responsible for creating CAD (Computer Aided Drawing) libraries with symbols. These complaints are over the many different symbols for the same thing and the same symbols used for different things that can be found in existing symbol standards.

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On the other hand, persons working within a certain technical field the vacuum technique field or the fluid power field, for example have found the situation satisfactory because they normally do not need to use any other symbols than those developed specially for their own fields. As different technical fields increasingly overlap with eachother and the more plant installations are split between different suppliers a trend today the increased need for harmonization is recognized.

Within IEC (International Electrotechnical Commission), all work with symbols for use on diagrams is concentrated within one and the same technical committee, TC 3: Documentation and graphical symbols. IEC experts receive requests or proposals representing symbols for new products from various product committees, but all the proposals are discussed for coordination and published by TC 3.

ISO deals with more than 200 technical fields where the situation is totally different. Symbols for use on diagrams were developed by various technical committees, where most participants were mainly interested in products, not in documentation. About twenty different ISO committees have created symbols with insufficient coordination.

Decision to develop a new standard
In Geneva during December 1988, ISO and IEC decided to prepare a standard - common to all technical fields - with harmonized symbols for use on diagrams. An ISO-IEC joint working group with the task of listing all discrepancies between existing standards and proposing relevant measures was established. In all, more than 40 experts have contributed to the new standard.

ISO has now developed ISO 14617: Graphical symbols for diagrams, comprising a number of parts. Part 1 does not contain symbols but only general information and indexes. All the other parts are organised according to a classification system designed with regard to the function of the objects, independent of the technical field (except the electrotechnical field) the object is used in.

This is the only possibility to satisfy the objective of obtaining unique, consistent and non-conflicting symbols. Parts 2 to 6 contain symbols of a general nature and Parts 7 to 12 and 15 contain symbols for objects where functions are mainly implemented by mechanical means.

No major differences between the symbols in ISO 14617 and IEC 60617 exist, thanks to the work done by the joint working group.

Representation of objects by symbols
The symbols in ISO 14617 represent:
- a product in the form of hardware; for example, a valve, a motor and a torque converter or
- a function; for example, to control, to measure and to monitor.

3.1 Symbols for hardware products
The symbols in Figure 3 represent hardware products, a pump, a hydraulic motor, a heat exchanger and a pipeline. From the symbols, any engineer can deduce how the equipment operates.

3.2 Symbols for functions
Typical symbols for functions can be found in ISO 14617-6, which replaces ISO 3511-1, 2, and 4 (ISO 3511-3 contained mainly symbols for hardware objects). The basic symbol consists of a circle in which a special letter code is inscribed, while the symbol for a function, intended to be implemented by a computer is a hexagon instead.

Figure 4 shows an example. The connections between the circles represent a function, to transfer the result of one function to the next. Other symbols for functions are shown in Figure 5. This figure also shows how these symbols may be used to give supplementary information for the function of a shut-off valve.

Preparation and costs
ISO 14617: Graphical symbols for diagrams, is the result of twelve years of hard work. The total expenses incurred have been estimated between 5 to 7 million USD. Several Swedish companies have paid the costs for the standards committee work which is administered by the SIS Secretariat (Swedish Standards Institute).

Many conflicts between various interest groups had to be solved. The work started by mapping the symbols in existing ISO and IEC standards. Also the most important national standards were studied. From the comprehensive library of existing symbols, those symbols and those descriptors that were most useful were selected. To carry out that task, the rules given in ISO 81714-1: General principles for the creation of graphical symbols, were taken into account.

One of the rules implies that a graphical symbol for an object consisting of several elements should be composed by symbols for integral elements as far as possible. For that reason, many of the existing symbols used in some restricted fields had to be refused. Many other symbols had to be refused because several symbols for the same object already existed; many identical symbols with different meanings also were in existence.

In some cases, none of the existing symbols fitted into the system, so that a new symbol had to be created. Of course the standard, IEC 60617: Graphical symbols for diagrams, was consulted when relevant. For complex objects, the above rule cannot be applied. Instead, block symbols have been developed according to rules given in ISO 81714-1.

All symbols in the standard have been given a registration number. This number will also be maintained in revised standard editions. Equally, the examples that display how symbols can be combined possess registration numbers starting with the letter X. Modified symbols and examples in the future will receive registration numbers.

Each section of ISO 14617: Graphical symbols for diagrams, that contain symbols has been subdivided consistently into five clauses:

- clause 1, with symbols giving basic information;
- clause 2, with applications rules for the symbols in clause 1;
- clause 3, with symbols giving additional information;
- clause 4, with application rules for the symbols in clause 3;
- and clause 5, with application examples, i.e. how to construct composite symbols.

ISO 14617: Graphical symbols for diagrams and IEC 60617: Graphical symbols for diagrams form a universal library of symbols for all types of diagrams.

Future activities
Two parts, ISO 14617-13: Devices for material processing and ISO 14617-14: Devices for transport and handling of material, are under preparation and are due to be published in 2003.

Practical use of the standard will show which symbols are lacking. Organisation of the standard will be laid out in the 2001 ISO/IEC Directives. Any technical committee that finds the need for a new or revised symbol has to prepare a proposal and send this to TC10/SC10 for further review. We hope most of the gaps in the standard will be eliminated over the next five years.

At the moment, ISO Central Secretariat and ISO/TC 10/SC 10 are attempting find the best method for the release of an electronic database containing all the symbols and examples in ISO 14617.

TC 10 is also working on a standard for the preparation of diagrams, i.e. how the symbols should be applied in diagrams. This standard is expected to generally consist of rules common to all types of diagrams and several parts will contain supplementary rules for special types or application fields.

Implementation of ISO 14167
We realize that many companies have been waiting a long time for this standard to be published. Up until now, they have had great difficulties in identifying which standards require application. For those that wanted to establish their own symbol databases, it has been difficult.

The advantage of having a universal standard is perfectly obvious with the documentation of complete plants. Personnel no longer need to find the correct symbol by searching through a number of partially contradictory standards.

We feel certain that implementation of this standard will not be delayed, as is the case with a number of standards. Although, representatives from different application fields will have some difficulties accepting the new harmonized standard.

A symbol database is under development. The possibility of using the same database technique that is already in use at IEC will be investigated. Meanwhile, a CD-ROM will be produced so the standard can be easily and immediately applied. This means that the possibility for users to build databases adopted for their own CAD systems will be achieved. In this way, the standard will implemented faster.

Symbols needed by other technical committees
Now that ISO 14617 has been published, other committees working within specific fields shall have the possibility to prepare 'collective standards,' i.e. extracts from ISO 14617. When a technical committee finds that a new symbol has to be developed or an existing symbol has to be modified, the committee will prepare a draft on a special form to be sent to TC 10/SC 10.

A 'maintenance team' studies the draft and decides if it can be accepted as is or modified or if the drafted symbol has to be dealt with according to ordinary procedure. The new or modified symbol will receive a new registration number from the TC 10/SC 10 database.

Since symbols are a language-independent method for communication and companies to an ever-larger extent work globally using different CAD systems and branches, these companies will have to cooperate. ISO 14617 may be regarded as a break-through in a field formerly far too complicated.




About the authors
Jorgen Aagaard and Siv Velander, Chairman and Secretary of ISO/TC 10, Technical product documentation, SC 10, Process plant documentation and tpd-symbols.