Navigating the Path to Compliance with the New Edition of UL 1950

The first version of UL 1950 was created to harmonize UL 478 and UL 114 into one safety standard. During the 1980s, UL 478 was the standard for electronic data processing equipment, and UL 114 was the standard for office appliances and business equipment. The development of the PC and its widespread use in the business environment created a new problem: some manufacturers were submitting PCs to UL for listing under UL 114, while others were submitting to UL 478. Hence, a new standard was developed that for a short time was referred to as UL 478 5th ed.

At about the same time, similar harmonizing efforts were under way in the international standards community. During this period, IEC 380 applied to typical office equipment, and IEC 435 applied to electronic data processing equipment. These two standards were harmonized into a new standard that became IEC 950. Recognizing the many similarities between this standard and UL's new standard (UL 478 5th ed.), UL agreed to harmonize UL 478 5th ed. with IEC 950. The new harmonized standard became the first edition of UL 1950. Canada also had a similar situation with two different safety standards being used for office equipment and electronic data equipment. Thus, the Canadian Standards Association (CSA) also developed a standard based on IEC 950 called CSA 950, which is now CSA standard C22.2 No. 950-95, 3rd. ed.

By the late 1980s, it became clear that additional integration of standards would be required to account for the convergence of computing and communications equipment. Desktop computers of this era routinely included modems, often with built-in fax and telephony capabilities. The compliance community was faced with a difficult issue: should these devices be considered computers and be covered by UL 1950 or telephones that would be covered by UL 1459, a standard for telephony equipment? This type of issue was occurring so frequently that UL proposed to merge the two standards. In the end, a U.S. and Canadian working group was formed to explore the integration of four standards: UL 1950, CSA C22.2 No. 950, UL 1459, and CSA C22.2 No. 225 (the Canadian equivalent of UL 1459). The new binational standard that resulted from this effort in 1995 was CSA 950/UL 1950 3rd ed.

The broad scope of UL 1950 3rd ed. encompasses equipment ranging from calculators and cash registers to facsimile equipment, modems, PBXs, and PCs. Because of the profound changes embodied in UL 1950 3rd ed., UL developed a phased implementation plan. Until April 1, 2000, manufacturers are allowed to seek listing to earlier editions of UL 1950 or UL 1459 and their Canadian equivalents. Beginning April 1, 2000, however, all new information technology equipment may only be listed to UL 1950 3rd ed., and UL 1459 will in effect cease to exist. Listings previously granted under earlier versions of these standards will remain valid until April 1, 2005. After 2005, however, such equipment still available for sale must be reevaluated and listed to UL 1950 3rd ed. because the prior listing will no longer be valid. Therefore, it is in a manufacturer's interest to seek listing to UL 1950 3rd ed. for equipment intended for a long market life.

When the binational working group considered integrating UL 1459 into UL 1950, it adopted an important philosophical change about protection and safety of equipment connected to the telecommunications network. The test requirements of UL 1459 were designed to ensure not only the safety of the equipment but also the protection of building wiring and coordination of protection with the primary overvoltage protector typically located outside the building or house. In UL 1459, the safety and protection coordination objectives were accomplished by subjecting the equipment to three types of tests: a 600-V, 40-A test simulating power contact with a high-voltage distribution power line; 600-V, 7-A and 600-V, 2.2-A tests simulating power induction (induced voltage and current from a distribution ground fault); and a 120-V, 25-A (or 240-V, 24-A) test simulating power cross with ac mains voltage. In each case, to ensure equipment safety, it was required that the cheesecloth wrapped around the equipment did not char. Building wiring protection was also ensured by requiring that a wiring simulator (often represented by a 1.6 A fuse) did not open during the test.

Equipment manufacturers typically complied with the UL 1459 overvoltage requirements by using specially designed fuses or polymer positive temperature coefficient (PTC) devices for overcurrent protection. The fuses or PTC devices served to interrupt the flow of current before any part of the equipment burned and charred the cheesecloth, and also before the wiring simulator opened.

As UL 1950 3rd ed. began to take shape, the binational working group decided that protection of building wiring was outside the scope of the standard; hence, the need for coordinated protection by limiting current would not be a requirement. In addition, computer industry representatives stated that by using certain construction techniques, computer equipment connected to the telecommunications network had demonstrated a good safety record with respect to telephone line overvoltages. Therefore, the working group decided that information technology equipment manufacturers should be allowed greater flexibility in deciding how to meet the equipment safety objectives relative to the tests prescribed in UL 1459. This decision led to the creation of a fairly complex-looking flowchart shown in the standard as Figure 18b, "Overvoltage flowchart." This flowchart is shown here in Figure 1.

Figure 1. UL 1950 3rd ed. overvoltage flowchart (Figure 18b within the UL document).

Even though the chart looks fairly complicated, the binational working group envisioned that manufacturers would typically follow one of the three vertical pathways to get from the "Connects to outside cable" rectangle to the "Acceptable" one. Starting from the right, these paths are as follows:

1) A "performance" path comprising testing the equipment to diamonds "Pass 1," "Pass 5," and "Pass 2, 3, 4."

2) A "construction" path comprising meeting the requirements of diamonds "Min. 26 AWG line cord," "Pass 6.3.3," and "Fire enclosure and spacings."

3) A "construction using current limiting" path comprising meeting the requirements of diamonds "100 A²-seconds limiting," "1.3 A limiting," and "Fire enclosure."

The key requirements of these three paths and a fourth option that may be adopted by some manufacturers as they seek equipment listing to UL 1950 3rd ed. will be described.

The performance path requires following the decision diamonds on the far right side of Figure 1. It involves testing the equipment to essentially the same set of requirements that are contained in UL 1459 and CSA C22.2 No. 225. These test requirements are described in Annex NAC of UL 1950 3rd ed. and summarized in Table I.

To meet the requirements of the performance path, an OEM must ensure that the equipment is "safe" per the overvoltage conditions that have been traditionally agreed to by the telecommunications industry. In addition, protection coordination with building wiring and primary overvoltage protectors is obtained because passing test 1 requires that the equipment limit fault energy to less than 100 A²-seconds under 600-V power contact conditions. We expect that many traditional telephone equipment manufacturers will follow this path because the circuit protection solutions such as fuses and PTC devices that they are currently using to meet UL 1459 are likely to meet the UL 1950 3rd ed. requirements.

The construction path requires following the three vertical diamonds in the center of Figure 1. The construction requirements were developed to provide an equivalent level of equipment safety to the performance path, though clearly not an equivalent performance or design. There are three requirements to meet in this path: supplying line cord, passing an insulation strength test, and providing an appropriate fire enclosure.

Supply minimum 26 AWG line cord. To meet this requirement, the manufacturer must either supply a telecommunications line cord with 26 AWG wire or larger, or describe the necessity of using such wire in the safety instructions. An example of such a statement is provided in Annex NAA: "Caution—To reduce the risk of fire, use only No. 26 AWG or larger telecommunications line cord." The rationale for this line cord exemption is that a cord of this size or larger will not melt through and present a shock or fire hazard under the equivalent energy contained in test condition 1 (600 V, 40 A, 1.5 seconds).

Pass section 6.3.3. Section 6.3.3 of the standard ensures that there is appropriate electrical isolation of the telecommunications network from ground. Compliance is checked by inspection and by performing an ac or dc insulation strength test at 1.5 kV between the telecommunications network voltage (TNV) circuit and unearthed parts of the equipment expected to be held during normal use (e.g., telephone handset). For parts that can be touched by a test finger or that provide connection to other equipment, a voltage of 1.0 kV is used. The test is conducted by slowly raising the voltage to the appropriate level and holding it for 60 seconds. Passing the test requires that there be no insulation breakdown and that current flow not exceed 10 mA. If surge suppressors bridge the TNV circuit insulation, they must have a minimum dc sparkover voltage equal to 1.6 times the rated voltage of the equipment (e.g., 120 or 240 V x 1.6). Surge suppressors are typically removed during the insulation strength test.

The rationale for this test comes from the possibility that the telephone line may be subject to power cross from the 120 V mains circuit. Voltages of 1.0 or 1.5 kV ensure the adequacy of the insulation under these conditions. If the equipment is grounded, surge suppressors will typically bridge the TNV circuit and ground, and therefore must be able to withstand the mains voltage with some margin. An alternative procedure that is allowed per the standard's Figure 18b (see Figure 1) is to perform test 5 shown in Table I (120 V, 25 A, 30 minutes).

Provide fire enclosure and spacings. The most critical and often most difficult element to meet in following the construction path is to provide a fire enclosure with the appropriate spacings. The spacings separate the TNV circuit from internal materials, some of which may be potentially flammable.

In the standard, a fire enclosure is defined as a structure designed to minimize the possible emission of flame, molten metal, flaming or glowing particles, or flaming drops. The enclosure must meet strict requirements for the size and spacing of any holes in the structure, depending on the materials used for the enclosure and the flammability rating of components enclosed within. The fire enclosure itself must meet certain flammability tests described in Annex A of the standard. These tests involve applying the flame from a Bunsen burner directly to the material (five applications of five seconds duration each) and ensuring that no flaming or molten materials fall from the test sample and ignite a cheesecloth indicator. To meet these requirements, fire enclosures are typically made of either metal or specially formulated flame-rated plastics.

The spacings requirement places an additional burden on the construction. All parts of the TNV circuit must be separated from materials of flammability class V-2 or lower by 25 mm of air or a flammability barrier made from materials of class V-1 or better. In addition, parts in the TNV circuit must be separated from openings in the sides or top of the fire enclosure by at least 25 mm of air or a barrier of class V-1 or better. The flammability class rating refers to the resistance of these materials to combustion after application of a direct flame, class V-0 being the highest-rated material.

The use of fire enclosures has been well established in the computer industry as a way of mitigating potential hazards. The addition of the spacings requirement is a recognition that TNV circuits may be subject to overvoltages as high as 600 V with energies as much as 100 A²-seconds. Without any overcurrent protection in place, these fault conditions could produce arcing and internal component explosions. By requiring a fire enclosure and spacings, the standard minimizes the possibility of an unsafe condition.

This path, shown by the three diamonds on the left-hand side of Figure 1, combines current limiting and the use of a fire enclosure to ensure the safety of information technology equipment. A unique feature of this path is that compliance with section 6 may be achieved through inspection without performing any testing, thus saving a manufacturer time and money, and avoiding the risk of not passing the tests. The three diamonds require limiting fault energy, limiting current, and providing a fire enclosure.

Limit fault energy to <100 A²-seconds. This diamond establishes the requirement to limit fault energy to less than 100 A²-seconds per the 600-V, 40-A test condition 1 as described in Table I. The standard allows that circuits or components that have been listed to UL 497A or CSA C22.2 No. 226, "Secondary Protectors for Communications Circuits," may be used to meet this requirement without additional testing. The overvoltage test requirements of UL 497A and CSA C22.2 No. 226 are essentially the same as those in UL 1459.

However, an information technology equipment OEM must understand that UL 497A is not a component specification, but is in fact an equipment specification used for listing multicomponent protection modules. As described previously, if such a module is used in the equipment, this diamond can be passed without testing. UL is currently evaluating whether certain discrete circuit protection components could receive a recognition to UL 497A if they pass key elements of the specification. If UL grants this recognition, these discrete circuit protection components could also be used without testing the equipment.

Limit current to 1.3 A. Meeting the requirements of this diamond requires that the TNV circuit contains a method for limiting current to 1.3 A maximum steady state that is also compliant with UL 497A. An example cited by the standard is a 1.0-A rated fuse. Note that meeting the 1.3-A limiting specification is not automatically achieved by meeting the UL 497A requirements, an example being a 1.6-A fuse that by definition will not limit current to 1.3 A.

Provide a fire enclosure. The fire enclosure requirements are described previously in the construction path discussion. This decision diamond does not require the additional spacing conditions because current limiting is already provided for in the previous diamonds.

As stated previously, a key benefit of following this path is that performance testing is not required. There is an alternative to providing the fire enclosure, however, which can be seen by following the "No" path at the "Fire enclosure" decision diamond and moving to the "Pass 2, 3, 4" diamond. Because tests 2, 3, and 4 are also subsets of the UL 497A requirements, circuit protection modules or discrete components used to meet the "100 A²-seconds limiting" diamond should also pass these tests.

In working through the standard with equipment manufacturers and UL, we found another valid path we call the construction and test path. This path involves meeting the requirements of diamonds "Min. 26 AWG line cord," "Pass 6.3.3" or "Pass 5," and "Pass 2, 3, 4." This path addresses the safety of the equipment by testing to a subset of the overvoltage tests (tests 2, 3, 4, and 5, or section 6.3.3), and by ensuring the 100 A²-second energy-withstand capabilities of the equipment through use of the minimum 26 AWG line cord.

From an equipment design perspective, this pathway is appealing because it bypasses the potential engineering difficulties of providing a fire enclosure with spacings. In addition, it may be possible to design circuit protection components that meet tests 2, 3, 4, and 5 and that are smaller and less costly than devices that also must meet test 1. Such components would still provide some level of current limiting, though clearly less than those that meet the complete performance path.

Each of the potential paths provides a means for designing safe equipment per the overvoltage requirements of the standard, but the paths are clearly not equivalent in the equipment performance that results. By using a fire enclosure and spacings to meet the construction path, the equipment designer is essentially controlling and limiting the damage following an overvoltage event on the telecommunications line. By using circuit protection components, either for the performance path or the construction with current limiting path, the equipment designer meets the safety requirement by limiting and interrupting current. In addition, this type of protection provides additional protection coordination with the building wiring and primary overvoltage protection devices. The latter benefit is not required by the UL 1950 standard but may be desirable in some installations.

Most traditional telephone equipment manufacturers are likely to continue to use discrete circuit protection components following the performance path because the cost to upgrade casings and providing for spacings to meet the construction path will be prohibitive. Computer OEMs, on the other hand, will often follow the construction path because the computer cases have already been designed to meet the fire enclosure requirements. As long as the cases are large enough and the TNV circuits are designed to also meet the spacings requirements, no overcurrent protection components will be required. Some manufacturers of high-end telephony equipment (e.g., PBX and key telephone systems) are following the construction path with current limiting, often using resettable modules, because of the added reliability in case of an overvoltage fault and because they can achieve a listing by inspection without undergoing the testing.

Manufacturers of small PC/telephony devices and components are likely to choose a path that avoids the "Fire enclosure and spacings" diamond. For example, the draft MiniPCI standard for small form factor modem and network interface cards includes requirements to meet UL 1950. However, the form factor is so small that it will not be possible to meet the spacings requirement of the construction path. Thus, there seems to be a market opportunity for smaller and less expensive overcurrent protection components to help these card manufacturers meet the construction and test path.

When information technology equipment OEMs evaluate possible circuit protection solutions to UL 1950, either performance path or construction and test path, they must also consider the coordination of performance with the overvoltage protection devices used to meet FCC Part 68 lightning requirements. FCC Part 68 approval is required for all equipment that is to be connected to the U.S. telecommunications network. The FCC test requirements are shown in Table II.

Surge	Waveform (open circuit)	Peak Voltage (open circuit)	Waveform (short circuit)	Peak Current (short circuit)	Number of Hits	Test Result
Type A metallic	10/560	800	10/560	100	2	A
Type A longitudinal	10/160	1500	10/160	200	2	A
Type B metallic	9/720	1000	5/320	25	2	B
Type B longitudinal	9/720	1500	5/320	37.5	2	B
Table II. FCC Part 68 lightning test requirements. Under test result A, the product must remain safe, with the integrity of network maintained (R > 5 M), and under test result B, the product must not cause permanent opening or shorting.

Typical protection circuits for ungrounded and grounded equipment are shown in Figures 2 and 3. Examples of overcurrent protective devices include "telecom"-rated fuses and PTC devices made from either polymeric or ceramic materials. Examples of overvoltage protection devices include metal oxide varistors, thyristors, and diodes.

Figure 2. Protection scheme for ungrounded equipment.

Figure 3. Protection scheme for grounded equipment.

In 1998, the FCC added the Type B surges described in Table II to improve the robustness to lightning of telecommunications equipment. FCC requirements state that this lightning surge must not cause any opening or shorting of the equipment. For example, if a fuse is used for overcurrent protection, then it must not blow during the test surge.

Use of PTC devices for overcurrent protection provides an additional benefit because they are self-resetting. Thus, even if the information technology equipment experiences a fault, the PTC device will protect the equipment and allow it to continue operation after the fault is removed. Figure 4 shows one potential solution for resettable protection of ungrounded customer premises equipment to meet the UL 1950 performance path and FCC Part 68. Though not required by the FCC, many OEMs design their equipment to also pass the Type A surge without opening or shorting. This practice leads to better lightning-withstand performance but can create other problems in coordinating protection with downstream components, including the overvoltage device.

Typical overvoltage protection devices, such as thyristor surge suppressors, will have a "breakover" voltage (i.e., the voltage at which the device changes from highly resistive to highly conductive) of 250 to 400 V. Thus, the 600 V applied during tests 1, 2, and 3 will cause the thyristor to conduct, and current will flow through the thyristor between tip and ring. The coordination graph comparing time-to-trip of a resettable fuse and time-to-damage for a 270-V, 50-A-rated thyristor (see Figure 5) shows that under all fault current conditions, the resettable fuse trips before the thyristor is damaged. Also, the resettable fuse does not trip during FCC Type A or Type B surges, meaning protection coordination and a fully resettable protection solution are achieved.

Figure 4. Resettable protection solution for UL 1950 performance path and FCC Part 68.

Figure 5. Comparison of time-to-trip and time-to-damage curves for thyristors and resettable fuses.

Further consideration of the interaction between overcurrent and overvoltage protection devices can lead to cost savings. The thyristor in the above example has a rating of 70 A in the FCC Type A 10/560 microsecond test—30 A less than the required 100-A performance. However, as long as the resettable fuse has a minimum resistance of 3.5 , the actual current through the thyristor will be

The new UL 1950 3rd ed. standard for information technology equipment is a major step forward in standards integration. For equipment connected to telecommunications networks, OEMs have several options for meeting the requirements, including construction options using fire enclosures and spacings and performance paths incorporating circuit protection components or modules. A complete protection solution, one that meets the requirements of the standards and produces a more-reliable information technology product, can be achieved through careful coordination of the protection components.