The Invocation Principle

01 / 11 / 2010

How do you know which standards govern the interpretation of a specification, such as an engineering drawing? That is the problem that this principle is intended to address.

When we had our old British BS308 standard, it was easy. You were asked to put a note on the drawing along the lines of ‘DRAWN TO BS 308’. With the new British Standard, BS 8888, we suggest a similar note, such as ‘CONFORMS TO BS 8888’, and the Americans require that drawings produced to the American standard show a reference to that standard, ‘ASME Y14.5-2009’.

ISO have never had a formal method of indicating that drawings or specifications conform to ISO standards. One of the key ISO standards is ISO 8015, which defines the Envelope Requirement and how size tolerances are interpreted. ISO 8015 itself contains the requirement to mark specifications ‘TOLERANCING ISO 8015’ if they are to be interpreted according to its rules. As a result, this marking has become the way in which many organisations indicate that they are working to the ISO system, and is even now a requirement of BS 8888.

Arguably, specifications which are to be interpreted according to the ISO system should list all the other applicable ISO standards on the specification as well, or at least list them in a document referenced by the specification. For most organisations, this would be a lengthy list, including references to standards such as ISO 1101, ISO 2768, ISO 5458, and many others. Unsurprisingly, many organisations don’t bother.

The invocation principle was intended to avoid this difficulty, by introducing a rule which stated that if you are using one part of the ISO system, you are using all of it. This seems sensible to me. This approach does not require you use bits of the ISO system that you were not using previously, it just means that if you make use of any particular part of it, such as surface texture specifications, then all the relevant ISO standards apply. You don’t have to list them all, you just have to have some means of showing that one part of the ISO system applies, such as the ‘TOLERANCING ISO 8015’ indication.

The contentious stuff starts with a second part to this rule, which states that use of any ISO GPS symbol is sufficient to indicate that you are working to the whole ISO GPS system. In other words, you don’t need to state ‘TOLERANCING ISO 8015’ or anything like that, you only need to use a geometrical tolerance, or a boxed dimension, or some other GPS symbol, and that makes it an ISO GPS document unless otherwise stated.


Reasons why:

1. In the UK, standards have a voluntary status. Their use cannot be imposed, and they work on an ‘opt in’ basis. This requirement would mean that many ISO standards would be imposed, quite possibly without the document owner being aware of this, unless they specifically ‘opted out’. A requirement which is enforced by the absence of any alternative marking is liable to lead to confusion, misunderstanding, and resentment (and bring the system into disrepute).

2. ISO do not have ownership of all the symbols which are used in ISO GPS standards. Many symbols are shared with ASME, and many national standards organisations have used them in national standards which are still current. Some symbols are even used differently in other ISO standards

This requirement would mean that if someone forgot to put ‘ASME Y14.5’ on a drawing, it would then be regarded as an ISO GPS drawing, irrespective of the intention. It means that a sketch on the back of an envelope, which happened to include a dimension with a box around it, would suddenly be subject to the rules and requirements of an ISO GPS standard. It means that civil engineering or architectural drawings with tolerances might become subject to ISO GPS rules and requirements, without the relevant professions even being consulted.

3. As many of the symbols which ISO regards as ‘GPS symbols’ are either in the public domain, or widely used in different ways, in different contexts, an attempt to impose a requirement like this without consulting as widely as possible (other ISO technical committees, professional bodies, industry organisations, etc, etc,) is simply irresponsible.

4. A further consequence is that we would then need to introduce an exemption symbol or marking, such as ‘NOT AN ISO GPS SPECIFICATION’ for cases where it didn’t apply, at which point it is starting to get a bit silly.

5. What happens in the case of a nation which is not a member of ISO? What happens if ASME introduce a rule saying ‘in the absence of any other marking, a technical specification is to be regarded as an ASME Y14.5 specification’?

The imposition of such a requirement would benefit no one, and simply lead to confusion, and quite possibly to litigation. ANSI have made their objections well known within ISO, but a number of people from other national bodies seem determined to force this through. I’m not clear on their motivation, but at least in part it seems to stem from a fear of ‘Americanisation’, and a desire to give ISO a higher status as the ‘ultimate default’. These motivations may be sincere, but they are misguided. Industry will not benefit from this, and it will actually lead to the diminishment of the reputation of ISO standards if it goes through.

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