The Third Edition of UL 1950, the Standard for Information Technology Equipment, became mandatory for new equipment to be NRTL-listed under the ITE category in the United States on April 1, 2000. It replaced both the second edition of UL 1950 covering ITE and UL 1459 Third Edition, the standard for safety of telephone equipment (see sidebar).
Originally published in July 1995 and with two revisions, it is based on the international standard IEC 950:1991 and its amendments. How does UL 1950 Third Edition compare to the European Union’s EN 60950 interpretation of IEC 950?
Although UL 1950 Third Edition and CSA C22.2 No. 950-95 are derived from the same standard and have many requirements in common, they also have some significant differences. Rather than present a detailed comparison of the standards, let’s look at some of the similarities and address the major differences as they relate to ITE and telecom equipment, particularly that sold in both North America and the EU. Because UL 1950 Third Edition and CSA C22.2 No. 950-95 exist as a binational standard for the United States and Canada, statements in this article pertaining to UL 1950 also apply to the CSA equivalent unless otherwise stated.
What Are the Similarities?
Two areas are fundamentally the same for equipment sold and installed in the United States or Europe:
Electrical Insulation
In the area of electrical insulation requirements, the UL and CENELEC standards are virtually identical to the IEC text with minor exceptions. Uniform requirements exist for creepage and clearance (electrical spacings measured over a surface and through air, respectively), insulation thickness, transformer construction, dielectric strength, leakage current, temperature rise, and overload/overcurrent protection, especially as they apply to power supplies. This has resulted in the availability of off-the-shelf linear and switching power supplies certified for use in ITE and other equipment by U.S. and European agencies.
Manufacturers of medium- to high-volume equipment often can build a single version for North American and European markets—a procedure unheard of in the not-too-distant past. This is possible only if no national deviations for components have been taken by the manufacturer; that is, all components comply with appropriate North American and European standards.
Enclosure Requirements
The other major similarity between the UL and CENELEC versions lies in the area of enclosure requirements. Three types of enclosures can be defined under IEC 950 and its variants: electrical, fire, and mechanical. Each type of enclosure has its own set of requirements although enclosures for most equipment must meet all three sets of criteria. These criteria include construction and testing, particularly where enclosures are composed partially or completely of plastics.
Access to electrical and mechanical hazards is defined in terms of the capability to contact them, through openings or operator-removable panels, with the IEC-articulated test probe. The maximum sizes for top and side openings applicable to both electrical and fire enclosures also are specified.
Other probes such as the test pin, the unarticulated test probe, and the telecom probe for RJ11 and RJ45 telecom jacks define accessibility to hazards. In addition to hazards from electric shock and moving parts, enclosures must protect operators from electrical parts carrying sufficient power to cause a burn if bridged by a conductive part, even though the voltage is not in the range considered to be a shock hazard.
Fire enclosures also are evaluated under both standards. The operative assumption behind the compliance rationale is that a fire will occur within the enclosure, notwithstanding all precautions prescribed by the standard and implemented by the manufacturer to prevent such an occurrence.
In addition to the top- and side-opening criteria, the size and configuration for bottom openings and baffles must be restricted although the standard makes clear the preference for excluding them. With some specific exceptions, the restrictions apply to all bottom openings through which flaming particles may fall and ignite the supporting surface.
Tests for enclosures, including impact, force, flammability, mold stress, and stability, are identical for both standards. The influence of older UL standards for small-scale testing and application of plastics used not only for enclosures but also for electrical insulation can be seen in the IEC standard and its variants.
The fact that telecommunications equipment is covered under both UL and EN standards also could be mentioned as a similarity. However, the UL standard has more stringent requirements for telecom network-connected equipment.
What Are the Differences?
Deviations From IEC 950
Both the UL (UL 1950) and CENELEC (EN 60950) versions of IEC 950 incorporate a number of deviations from the original IEC text. In the case of EN 60950, the revisions are of two basic, straightforward types:
Deviations from the IEC text in UL 1950 are much more numerous, detailed, and in the case of telecommunications equipment, difficult to implement than those in EN 60950. Like the common modifications and special national conditions in EN 60950 that reflect more localized issues than the IEC standard, the deviations in the UL document allow for conditions unique in North America.
Differences between the North American and European deviations usually can be attributed to differences in the areas of infrastructure, electrical installation codes, and component requirements. Though sometimes interrelated, these are addressed separately.
Infrastructure
Clause 6 of IEC 950 and its variants outline safety requirements for equipment connected to the telecommunications network. Common to the CENELEC and UL standards are requirements for accessibility to, isolation from, and interconnection between telecom network circuits classified according to voltage as TNV-1, TNV-2, or TNV-3 and other circuits within the equipment. The degree of protection distinguishes the UL from the CENELEC standard.
Besides the accessibility, isolation, and interconnection criteria specified in EN 60950 to protect equipment and users from normal telecom voltages, Subclause 6.6 of UL 1950 requires additional shock and fire protection from overvoltages that can occur on telecom network lines due to external influences. These overvoltages generally are caused by one of the following transient conditions:
Also unique to UL 1950 are requirements for limiting acoustic pressure measured from a handset or headset earpiece. Separate limits are prescribed for short-duration impulses and long-duration disturbances that may be caused by ringing, dialing, or a fault condition.
These conditions are a consequence of routing power transmission lines and telephone lines on the same poles, as has been the practice in the United States for many years. In most of Europe, power and telecom transmission lines are installed below ground and are not routed together.
Differences in supply voltage and frequency really are not a factor in the differences between the United States and European versions. There is a consistent treatment of electrical insulation issues in IEC 950 that readily translates into both markets.
Electrical Installation Codes
Any national standard derived from an IEC standard must agree and, in fact, cannot conflict with the applicable installation codes for the country in which it is adopted. In the United States, most state and municipal electrical codes are based on requirements in the National Electrical Code, NFPA 70; in Canada, the Canadian Electrical Code C22.1-98 applies.
Deviations from the IEC text in UL 1950 reflect how equipment may or may not be installed rather than consideration of aspects internal to the equipment. Here are some examples:
Component Requirements
Not to diminish progress made during the last 10 years, we still, however, are a long way from worldwide harmonization of standards for equipment. Nowhere is this fact more evident than in the area of components.
Requirements for components in UL 1950 are contained in Annex P. Along with the IEC component standards listed as Normative References, UL 1950 Annex P.1 tabulates additional UL and CSA standards for components. The tables in Annex P.1 are cross-referenced to the UL 1950 subclause citing the technical requirement that can be fulfilled by compliance with the standards listed in the corresponding table entry.
These component deviations can be justified within the context of the IEC standard only because they do not contradict any of the original IEC text. UL is not authorized to make any such contradictory changes without first submitting them to IEC TC 74 for consideration and prior adoption into the IEC text.
Conclusion
Will we ever have a worldwide harmonized standard for ITE? Much progress has been made toward harmonization of standards in the last decade. UL now has several standards based on IEC publications while electrical safety standards published by CENELEC have been largely IEC-based for a number of years. Although these standards may share a common origin and will undoubtedly continue on parallel development paths, the influences of varying infrastructures, legal systems, and regulatory mechanisms probably will serve to keep them distinct from one another.
About the Author
Tom Brenner is manager of product safety at D.L.S. Conformity Assessment, a subsidiary of D.L.S. Electronic Systems, an EMC consulting and testing facility. He has worked in the field of product safety for more than 15 years, including seven years at Underwriters Laboratories and five years at TÜV Rheinland of North America, and in industry as manager of documentation and approvals at Kay-Ray/Sensall. Mr. Brenner received a B.S. in electrical engineering from the University of Evansville. D.L.S. Conformity Assessment, 1250 Peterson Dr., Wheeling, IL 60090, (847) 537-6400, e-mail: [email protected].
Sidebar
Notes on Overvoltage Testing
Overvoltage testing was introduced in UL 1459 Safety of Telephone Equipment, Second Edition in the late 1980s. The evolution of ITE and telecommunications equipment technology has resulted in a growing similarity between the two. This situation prompted UL and participating industries to combine the two sets of requirements into a single standard—UL 1950 Third Edition.
Before the advent of this edition of UL 1950, there were separate but unequal sets of requirements for equipment, such as a telephone key system and a modem which both have telecom connections. Testing for the phone system had to survive the overvoltage tests of UL 1459, while the modem only had to establish isolation from the telecom network by means of passing a dielectric voltage withstand test between telecom- and nontelecom-connected circuits.
Under the second and third editions of UL 1459, testing was the only means of compliance with overvoltage requirements. Results of testing at voltages ranging from 125 to 600 VAC and currents from 2 to 40 A at durations from 1.5 s and up determined whether a product complied.
Now that overvoltage testing is part of UL 1950, an optional method involves a combination of more stringent construction and less stringent test criteria to comply with Subclause 6.6. This option was not available under UL 1459 and is a consequence of insulation and fire-enclosure requirements that are better specified and documented in UL 1950. A separate but more equal ITE approach now can be distinguished from the telecom approach to overvoltage compliance.
Glossary
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CEN |
European Committee for Standardization—Develops and publishes (often in parallel with ISO) standards for machinery, manufacturing systems, quality systems, and other nonelectrical aspects of conformity assessment used in the EU and EFTA countries |
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CENELEC |
European Committee for Electrotechnical Standardization—Develops (usually in parallel with the IEC) and publishes electrical safety and EMC standards (ENs) used in the EU and EFTA countries |
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CSA |
Canadian Standards Association—Develops and certifies standards; like UL in the United States |
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EFTA |
European Free Trade Association—Allied with the EU, but not part of it; members: Iceland, Liechtenstein, Norway, and Switzerland |
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EN |
European Norm—European standard published by CEN or CENELEC |
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EU |
European Union—15 countries that have formed and participate in a central government for Europe: Austria, Belgium, Denmark, Finland, France, Germany, Greece, Ireland, Italy, Luxembourg, The Netherlands, Portugal, Spain, Sweden, and the United Kingdom |
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IEC |
International Electrotechnical Commission—Develops and publishes international standards for electrical, electronic, and related technologies |
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ITE |
Information Technology Equipment—Category of electrical equipment including business and office machines and telecommunications equipment |
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NEC |
National Electrical Code—Designated NFPA 79; provides the basis for most state and municipal codes covering safety of electrical installations, wiring protection, and utilization equipment in normal and classified hazardous locations |
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NFPA |
National Fire Protection Association |
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NRTL |
Nationally Recognized Testing Laboratory—Third-party organization recognized by OSHA as having the capability to conduct product safety testing and associated follow-up inspections of manufacturing locations |
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TC |
Technical Committee—IEC committee responsible for publication and periodic revision of a standard |
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TNV |
Telecommunications Network Voltage—Secondary circuit (in the meaning of ITE standards) whose accessible voltage level(s) and susceptibility to telecom network overvoltages are classified as TNV-1, TNV-2, or TNV-3 |
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UL |
Underwriters Laboratories—Develops standards and certifies equipment complying with those standards; UL is the largest of the NRTLs |
Copyright 2000 Nelson Publishing Inc.
June 2000
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