A Program for Successful Testing of Network Equipment Building Systems

Testing in accordance with network equipment building system (NEBS) requirements can be an intimidating experience. This is especially true for a novice compliance engineer or a start-up company. The environmental and electrical testing involved is comprehensive and rigorous. However, with a basic understanding of the test regimen and with testing procedures well prepared, NEBS compliance can be achieved with a minimum of problems or delays. This overview addresses potential problems that can arise and shows how to avoid them on the way to NEBS compliance.

Preparing for Testing

The first step in preparing for a successful NEBS compliance program is to review the test requirements as defined in Telcordia Technologies's standards GR-1089, "Electromagnetic Compatibility and Electrical Safety—Generic Criteria for Network Telecommunications Equipment," and GR-63, "Network Equipment Building System (NEBS) Requirements: Physical Protection." Each of these documents contains absolute requirements, objectives, and conditional requirements (taken together, called R&Os) which are itemized in checklist tables found in the specifications. These tables help determine which tests are
required by making the compliance engineer answer critical questions such as those pertaining to identification of the type of equipment as defined in GR-1089, the fire test requirements of GR-63, and other critical test parameters. By way of illustration, Table I is a portion of the GR-1089 R&O table.

Once the R&Os checklist is complete, the compliance engineer can begin to develop the GR-1089 and GR-63 test procedures, or approach a test laboratory to develop them, for review by the regional Bell operating company (RBOC) or other service provider. The procedures will require a review of the equipment under test (EUT) in order to determine its worst-case configuration. The worst-case configuration for one test is not necessarily the worst-case configuration for all tests, so a thorough understanding of each test setup is crucial.

If enough EUTs are available, it is advisable to make a system for each scenario available for parallel testing in order to minimize time spent reconfiguring systems for each required test. Two configurations at least are recommended: a fully loaded and functioning system for operational testing, and a unit with operating fans and nonfunctional cards to constitute a full fuel load for the fire test.

GR-1089 Test Regimen

Preparation for each test group is important. For GR-1089 testing, all EMI and transient protection devices—EMI gaskets, filtering, connectors, and so forth—must be properly installed and checked prior to the start of any test procedure. The proper operation of the unit must be defined, and testing personnel must understand what it is. This is critical for determining the pass-fail criteria for the immunity and transient tests. In addition, all ports must be properly terminated and functioning.

ESD. ESD tests require that the EUT be operating during application of as many as 40 discharges at all physically accessible locations. This calls for a clear understanding of the physical configuration of the test device in operation. If the unit has a panel or door that is normally closed during operation, the door should be closed for this portion of the test. ESD test points should be detailed in the test procedure. They can be quite numerous. All test points need to be checked for compliance to Section 2 of GR-1089.

The unit being tested also must operate after the application of discharges to areas that are accessible during maintenance and repair. This specification requires testing with the doors open. Again, the test points need to be clearly defined in the test procedure and referenced in a maintenance manual or user guide. There is no requirement to test areas that are not accessible during normal operation or maintenance. If the EUT fails to meet any of the ESD requirements, compliance with GR-1089 can be achieved by the placement of warning tags on the unit.

RE and RI. For the radiated tests, the unit must be functioning in worst-case conditions (maximum processing volume), with the cable lengths as specified in the manual. The length of the cable typically is not known, because the equipment manufacturer does not install the central office cable. Therefore, the cables are run above and around the EUT, with support height sufficient to allow a line-of-sight path between the antenna and the cables. Bell Atlantic, for example, requires the cables to be at least 2 ft above the EUT. To meet the cable length requirement, approximately 25–30 ft of cable is needed for radiated tests. Radiated emission testing is required to be performed in an ANSI C63.4–compliant site such as a semianechoic chamber or an open-area test site (OATS).

Radiated immunity testing is required to be performed in a room that will not affect the uniformity of the radiated field. Such rooms would be qualified to EN 61000-4-3 or MIL-STD-461D/E. If a semianechoic room is used, a small patch of ferrite tiles can be placed on the floor to meet the EN 61000-4-3 field uniformity requirement. This recipe is an important time-saver because it eliminates the need to disassemble and move equipment to a new test facility for immunity testing after radiated emission testing is completed. In fact, all GR-1089 testing can be performed in the semianechoic chamber if so desired.

When radiated immunity and emissions testing is conducted properly, all support equipment (except antennas) is outside the chamber in a control room. This requires cable runs of 30–40 ft for a semianechoic chamber and up to 50 ft for
an OATS.

CE and CI. For the conducted tests, each type of data and signal port needs to be defined and tested separately. The distinctions should be clearly delineated in the test procedure. Ac and dc power lines also must be tested. (Equipment on the test client's premises that is powered by alternating current will be required to meet the conducted and radiated emissions requirements of the FCC.) Analog voice-band leads have a separate CE limit.

The pass criteria for radiated and conducted immunity tests vary depending on equipment functionality within the network. Typical compliance requires that the equipment be manually reset, reset itself, or operate continuously.

Lightning and Power Cross. The lightning and power cross tests have two pass criteria: to operate after the Level 1 test without manual intervention, and to not cause a fragmentation, fire, or safety hazard for personnel as a result of the Level 2 test. These tests are based upon the level of protection against transients found in the central office. The main concern with regard to them is the cable connection as related to the designated equipment type: Does the equipment connect directly to outside cabling? If the unit does connect to external cabling (Type 1 and Type 3 equipment), then the full series of lightning and power cross tests is required. But if the cabling remains inside the building (Type 2 and Type 4 equipment), then only the intrabuilding lightning test requirement applies (along with the ac power lightning test, if applicable). Specifications for the internal protectors are necessary in order to perform the test properly. All types of circuits require testing, and, if circuit packs are powered by different sources, redundant circuit testing may be called for. Some units having both outside and intrabuilding connections may be classified as multiple equipment, for example, types 1 and 2.

Power Induction. The power induction test requires a fully functional unit that is classified as Type 1 or Type 3 equipment. The EUT is configured with up to 20,000 ft of telecommunication (external) cable to simulate line runs, and is tested for impairment when a triangular voltage waveform is injected via a transformer. The main challenge in conducting this test is determining that the test laboratory has the proper cable and transformer.

Electrical Safety. This section of the GR-1089 test regimen addresses the basic protection of personnel from electrical voltage and current hazards pertaining to telecommunication circuits and network equipment. Control of leakage currents from exposed surfaces is also investigated. A procedure for classifying various voltages found in telecommunication equipment is defined, along with accessibility requirements. Power sources with applicability for telecommunication wiring are limited, as are ring voltages. Most of the electrical safety requirements can be met by inspection. This does not replace a UL 1950 listing. Preparation for this group of tests would include a complete list of components, which speeds the evaluation. If UL 1950 is required, the list of components will be a mandatory basis for that inspection also.

Bonding and Grounding. Requirements for bonding and grounding of the EUT cover all electrically conducting paths intentionally provided for that purpose. Bonds between chassis, racks, frames, and so forth must be clean and proper. Dissimilar metals and incidental bonds are not allowed. Plating and unpainted surfaces are required. As with electrical safety, most of the requirements in this section can be met by inspection. The major testing here involves short-circuiting all embedded power supplies (converters, inverters, etc.) in order to ensure that there is no occurrence of damage to equipment, conductors, or any components in the fault current path. Preparation for this testing includes allowing for these short circuits by providing access to the requisite points.

GR-63 Test Groups

Testing for NEBS compliance with the R&O of GR-63 involves five major areas: heat dissipation, operating temperature, transportation and storage, fire, and earthquake. Preparation for each of these test groups is important.

Heat Dissipation. This determination derives from a calculation based upon the heat produced by a fully loaded and functional system. For a system without fan cooling 738 W/m² is allowed, and for a fan-cooled unit, 992 W/m². Not all units meet this criterion, but the levels are only an objective rather than a requirement. Objectives for rack and for shelf equipment vary.

Operating Temperature. This test requires a fully operational unit. The system must run for eight days under various conditions of temperature and humidity. A pass-fail criterion and a verification method must be developed, typically involving a time-stamped data log. Methods of monitoring the equipment must be devised.

Some RBOCs demand that altitude testing be performed in addition to the test to establish compliance with temperature and humidity requirements. The altitude component makes the test more difficult to pass than might be supposed at first, because it requires that a large environmental and vacuum chamber be employed.

Transportation and Storage. This test group is designed to determine whether the system can survive typical shipping and handling conditions. It includes requirements for high and low nonoperational storage temperatures. In preparing for this test, the most important thing is to use the same packing method that would actually be employed in making the system ready for shipment. This includes components such as circuit packs that could be used as replacements during deployment. The test is also applicable to new circuit packs or upgrades of existing packs that are intended for shipment and replacement in situ.

The crates are mounted on a vibration table and subjected to a low-grade profile in order to simulate a typical on-off loading experience and truck ride.

Fire. The most difficult test in the GR-63 series is the fire test. Consequently, it should receive the highest consideration during preparations for NEBS compliance. This test, along with the product/electrical safety component review, should be addressed before any other requirement is taken up. All components should be checked to assure a UL 94V-0 fire rating. UL 94 specifies the rating of components. A V-0 rating for all components will not guarantee passage of the fire test, but it is a minimum requirement for fire-resistant equipment design. Some RBOCs require that there be a fire-rating database for all components.

Earthquake. The earthquake test also can be difficult to pass if relevant considerations are not addressed early in the design phase. The main element of preparation for this test is making sure that enough cable is supplied with the unit to avoid cables being pulled from support equipment during a seismic event. The method of installation of the unit in the rack and the way the rack is installed on the seismic table is critical. Discussions with the test laboratory regarding normal operation and installation are important to ensure the system is properly tested.

The office vibration test normally performed sequentially with the earthquake test is also required. The purpose of this test is to verify unit or system operation under a lower-level, longer-duration fatigue environment.

Conclusion

While NEBS compliance testing can be a long and problematic process, following the foregoing suggestions and choosing a qualified test laboratory with knowledgeable personnel can make the road smooth. Compliance can be achieved at each test milestone. The safety and fire tests ought to have top priority. Once those challenges have been met, all the other requirements will most likely be satisfied with few or no further system modifications.

Having access to an experienced testing or consulting organization that possesses knowledge of all aspects of NEBS is of great value when redesign or compromises are required. Such an organization is especially valuable early in the equipment design phase, when it can offer guidance that will prevent any need for time-consuming redesign later at the testing stage.

James Press is national EMC director for National Technical Systems (NTS; Boxborough, MA). Press is responsible for all the company's EMC technical issues and developments, including expansions of EMC test and engineering capabilities. He can be e-mailed at jimp@ntscorp.com.

Mark Betts is EMC lab manager responsible for all EMC testing at NTS's facility in Tinton Falls, NJ. Betts is certified as an EMC test witness engineer and as a technical construction file assessor. He may be reached via e-mail at markbetts@worldnet.att.net.

John Ngo is compliance manager at NTS in Fullerton, CA. Ngo has more than seven years' experience in EMC design and testing, and more than three years' experience in the product safety arena. He can be contacted via e-mail at johnn@ntscorp.com.

Mike Cantwell is EMC manager of NTS's newest facility in Plano, TX. Cantwell, who is NARTE certified and has more than 10 years of EMC testing experience, oversaw expansion of the facility into a fully compliant GR-1089 and GR-63 test laboratory. He receives e-mail at mcantwell@ntscorp.com.