Over the years, the understanding of UL and how it applies to equipment and components has caused some confusion. Many electrical engineers understand that their product must "meet UL", but may not realize what this really may mean for their design. Products that meet UL requirements can be either "listed" or "recognized." Listing applies to an end product such as an instrument or a piece of electronic equipment, or occasionally to a component used for a specific application. Transformers are generally not UL listed; doing so would restrict the use of that transformer to a very narrow purpose. UL recognition is much more flexible. UL component recognition makes it easier for equipment containing recognized components to go through the UL listing process. By designing recognized components into equipment that is to be submitted for UL listing, considerable paperwork and testing can be avoided.
Transformer UL Standards:
UL506 Specialty Transformers -- covers the requirements for particular construction methods,
dielectric strengths and a maximum operating temperature rise for power transformers rated 10.0
KVA or less.
UL544 Medical and Dental Equipment -- covers equipment in which transformers may be used.
Requirements for transformers recognized to UL544 include a limit on leakage current, some means
of internal thermal or current fusing, or a manufacturer-specified external current fuse, and either
double insulation or an electrostatic shield between primary and secondary.
Medical Equipment. Most major world markets regulate medical equipment. In the United States, the Federal Food, Drug, and Cosmetic Act (and succeeding acts) requires that all medical devices be "safe and effective," and FDA recognizes consensus standards as a means to support a declaration of conformity (new 510(k) paradigm, "abbreviated 510(k)"). FDA lists IEC 60601 + national deviations (UL 2601-1) as a recognized consensus standard. In Europe, the Medical Devices Directive (93/42/EEC, Article 3) requires medical devices to meet the "essential requirements." Compliance is presumed by conformity to the harmonized standards in the Official Journal of the EC (93/42/EEC, Article 5). IEC 60601 + regional deviations (EN 60601) is a harmonized standard. Similarly, IEC 60601 forms the basis for national medical equipment safety standards in many countries, including Japan, Canada, Brazil, Australia, and South Korea.
IEC 60601 is a series of standards. The basic standard containing the core safety requirements for all electrical medical equipment is Medical Electrical Equipment—Part 1 General Requirements for Safety, IEC 60601-1, 2nd edition, (1988-12) + Amendment 1 (1991-11) + Amendment 2 (1995-03). There are collateral (horizontal) standards, which supplement the core requirements by providing technology-related safety requirements. The naming convention for collateral standards is IEC 60601-1-xx. Technologies addressed by collateral standards include medical systems, EMC, x-ray radiation, and programmable systems. There are also particular (vertical) standards, which supplement the core requirements by providing device-specific safety and performance requirements. The naming convention for particular safety standards is IEC 60601-2-xx. Specific devices addressed by particular standards include RF surgical devices, ECG monitors, infusion pumps, and hospital beds. There are approximately 40 Part 2 standards. Although most of these also include essential performance requirements, the trend is to move to another (-3-xx) series of standards for essential performance.
Within each national or regional market, the international standard must be considered together with any published deviations that take into account local installation codes and expectations of safety. In the United States, the national deviations to the core safety requirements are contained in the standard UL 2601-1 (1997-10). In Europe, regional deviations to the core safety requirements are contained in EN 60601-1 (1991-01) + Amendment 1 (1994-06) + Amendment 2 (1996-03) + Amendment 13 (1996-07).
For EMC, the internationally accepted standards that largely satisfy global regulations are in IEC 60601-1-2 (1993-04), and are based on CISPR 11, CISPR 14, and IEC 60801. For some equipment, the FDA reviewer guidance document for premarket notification (510(k)) submissions contains EMC recommendations that are not entirely represented by IEC 60601-1-2.
Since the United States and European markets are the largest ones for medical equipment manufacturers, only the national and regional standards relevant to those markets were cited above. The remainder of this article addresses the generic IEC 60950 and IEC 60601-1 requirements. All references are to IEC 60950, 3rd edition (1999-04) and IEC 60601-1, 2nd edition (1988-12), including all amendments. U. S. and European deviations do not amend the requirements referenced below.
UL1411 Transformers and Motors for Use in Audio, Radio and Television Type Appliances -- like
UL544, this is an equipment standard to which a transformer can be recognized. Requirements for
transformers recognized to UL 1411 are very similar to UL506. The major difference is a
requirement for the use of UL94VO listed material (flame retardant) for terminal anchoring points
and the abnormal performance testing required per end product specifications.
UL1585 Class 2 and Class 3 Transformers -- covers current-limited devices designed to protect
against the consequences of an overload. They can be either inherently or non- inherently currentlimited.
Inherently current limited transformers are constructed so that during a long-term
overload, or short-circuit, they will not produce a fire. Non-inherent current-limited transformers
contain either a current or a thermal fuse to protect them from overload, or a specified fuse
installed by the equipment manufacturer.
CSA serves the same function in Canada as UL does in the United States. Each product certified by
CSA goes through a process involving a company's application, then testing by CSA, evaluation of
test results, reporting of those results to the company, certification itself and periodic follow up,
involving visits to the company's facilities by CSA inspectors. The CSA standard for transformers is
C22.2, No. 66, and covers air-cooled, dry-type power frequency transformers (and reactors) of
1,000 KVA or less, with a primary voltage of 750 V or less.
International Standards: IEC
Although the initials UL, CSA and VDE are quite familiar, there are others that equipment
manufacturers selling in the international marketplace must know about. The first and most
important of these is IEC, for International Electrotechnical Commission. The IEC has generated
some new standards for the safety of "information technology equipment, including business
One important IEC document is IEC 950, which consolidates the requirements of the former IEC 380 (Safety of Electrically Energized Office Machines) and the former IEC 435 (Safety of Data Processing
IEC 950 is embodied in several other national and regional standards, including UL1950 (U.S.), EN
60950 (European Community), VDE 0805, Part 100 (Germany), BS I6204 (U.K.) and CSA C22.2 950
The European Community version, EN 60950, is intended to cover the entire community with some
national deviations that take into account special marking requirements, local climatic differences
and pre-existing national standards.