Design Standards and Delegation
Design standards are disfavored by economists and standards purists, but not by UL engineers. Many of the requirements in UL 1482, and other UL standards, specify design parameters, such as the minimum metal thickness for cast-iron stoves. Economists favor performance standards—for example, a "burn through" test instead of a specific metal thickness—because such standards are less likely to constrain technological innovation. However, all of UL's design standards are qualified by an "equivalency statement" that, in theory, allows for technological innovation. This reads:
A product employing materials or having forms of construction differing from those detailed in the requirements of this Standard may be examined and tested in accordance with the intent of these requirements and, if found to be substantially equivalent, may be judged to comply with the Standard.
This clause is invoked "at least once a week," according to a UL attorney, who would not disclose any details about whether or when it has been applied to woodstoves (or any other specific product).
UL uses design standards for two reasons that are often overlooked in the discussion of performance and design standards. First, design standards are much cheaper from the point of view of testing. The thickness of cast iron can easily be measured. A performance test intended to simulate "burn through" would be much more complicated and expensive. Second, design standards—at least the way in which they are used by UL—allow certain matters to be "delegated" to the manufacturer. This is particularly helpful when field data or relevant research do not provide an adequate basis for a performance standard.
Many of UL's design standards are actually codifications of generally accepted business practice. The requirements for minimum metal thicknesses do not constrain woodstove manufacturers; they choose them in the first place. Design standards of this origin appear to contradict the notion that UL's standards are an "independent" test of safety. In UL's defense, reliance on industry practice, if done critically and selectively, helps keep down the cost of testing, while ensuring that products beneath the accepted minimum are not certified.