Close Enough Is Not a Clause: Defining Equivalence Before the Substitution Is Made

There are few phrases in UK procurement with a wider gap between their brevity and their consequences than "or equivalent". It appears in three words on a schedule, a purchase order or a drawing note, and it quietly transfers an engineering judgement from the people who specified the article to the people who are trying to buy it economically. In a well-run supply chain that transfer is deliberate, documented and reviewed. In too many others it is assumed.

The result is familiar to anyone who has sat on a supplier audit: a component arrives, it looks right, it fits, it works, and it is almost — but not quite — the thing the designer had in mind. Nobody has done anything obviously wrong. And yet the traceability chain now has a joint in it, and a joint is exactly where an auditor will press.

Why the phrase persists

It is worth being fair to the clause. "Or equivalent" exists because strict single-source specification is expensive and slow. It allows a manufacturer to keep a production line running when a named article is on a long lead time, and it allows a smaller supplier to offer a commercially sensible alternative without forcing a full design change. For an SME with a modest engineering function, the alternative — a formal deviation for every substitute — is unworkable.

The difficulty is that the phrase performs two incompatible jobs at once. It is a purchasing convenience and a technical permission. Purchasing departments read it as permission to buy something similar. Engineering departments read it as permission for them to approve something similar. In practice, both proceed and nobody is named as the person who decided.

What an equivalence claim has to answer

Equivalence is not a vague property of an object. It is a relationship between two articles, in a particular application, against a defined set of requirements. That means a claim of equivalence can only be made sensibly if four things are known.

The requirement itself. Not the part number, but the function. What is this article doing, in what environment, for how long, and against which criteria? A gasket specified in a hygienic application and a gasket specified in a dry industrial enclosure are not the same problem.

The performance envelope. Which characteristics actually matter — dimensional, material, electrical, thermal, chemical, mechanical, fire performance, and the tolerances attached to each? This is where the honest work happens, and where it is often skipped in favour of a datasheet comparison.

The regulatory and declaration position. Does the substitute sit within the same declared standards, the same conformity assessment route and the same marking assumptions? A change of material can change a fire classification; a change of supplier can change which declaration of performance or conformity statement underpins the product.

The consequence of being wrong. Not every substitution carries the same weight. Aesthetic fixings and a pressure-retaining component do not deserve the same rigour, and pretending they do is how equivalence processes fall into disuse.

A four-step discipline for member firms

Members who have asked our helpline about this over the past year tend to arrive at the same working method once they have been through an audit or two. It can be summarised as define, evidence, record, review.

Define

Before the purchase order is raised, write down what equivalent means for that item. In most firms this is a single line on the requisition — for example, "substitution permitted only where the alternative meets the same dimensional standard, the same material grade, the same fire classification and the same declaration route, and is approved by the design authority." A sentence like that is not bureaucratic padding. It removes the ambiguity that the phrase currently creates.

Evidence

An equivalence claim should rest on something that can be produced later. Datasheets are a starting point, not a conclusion. For anything carrying a safety or performance function, the evidence set should include the supplier's declaration, the relevant test or certification reference, and confirmation that the manufacturing site is the one covered by that certification. Where the substitute comes from a different country of origin, that last point deserves particular attention.

Record

Every accepted substitution should generate a record that names the article replaced, the article used, the requirement set, the evidence considered, the person who approved it, and the date. This is the document that turns an uncomfortable audit conversation into a short one. It also protects the individual engineer who exercised judgement in good faith.

Review

Equivalence is not permanent. Suppliers change materials, processes and sub-tier sources, and the article that was equivalent two years ago may not be today. A periodic sweep — annually at minimum, and on any change of supplier or site — keeps the register honest.

Where the avoidable surprises sit

Three patterns account for most of the difficulty we see. The first is substitution below the drawing: a tier-two supplier substitutes a raw material and the tier-one supplier passes the finished article upstream as unchanged. The second is substitution during a shortage, approved informally on the shop floor and never routed to the design authority. The third is substitution that is technically sound but not reflected in the technical file, leaving the documentation describing a product that was not the one supplied.

Each of these is a documentation failure as much as an engineering one, and each is far cheaper to fix at the point of decision than at the point of recall, warranty claim or notified body review.

A modest proposal for the next quarter

Members looking for a practical starting point might take a single product family and trace every "or equivalent" that has been exercised against it in the last twelve months. In most cases the number will be small, the records will be partial, and the exercise will take an afternoon. That afternoon will tell you more about the maturity of your substitution controls than any procedure document.

Equivalence, properly handled, is a legitimate engineering tool and a commercial asset. Handled loosely, it is a clause that promises savings and delivers exposure. The difference is rarely a matter of technical skill. It is a matter of writing down, in advance, what the words are going to mean.

Members seeking guidance on substitution registers or equivalence evidence packs can contact the BEMSC member helpline, or raise the topic at the next regional standards forum.