Just as a healthy diet includes milk, NEBS qualification is a critical part of telecom testing.
Before a new type of electronic equipment can be placed in a telecom central office (CO), it must satisfy the Network Equipment Building Systems (NEBS) requirements. The underlying philosophy of NEBS testing is similar to the concept of being a good team player: you must perform your own job correctly while ensuring that the team’s collective effort isn’t hurt by your presence.
Because of the high reliability needed in CO applications, the NEBS requirements can be as stringent as military test specifications. One phrase that expresses the expected reliability is five nines or 99.999%. If you work out the math for 0.001% of 365 days ´ 24 hours ´ 60 minutes, you find that five nines are equivalent to a maximum downtime of about five minutes per year.
The need for a reliable communications system is easily understood. In addition to business and personal conversations and data, COs handle emergency calls that literally can be a matter of life and death. On the other hand, many electronics manufacturers may not be familiar with applications that place as much emphasis on fail-safe equipment performance as do the NEBS requirements.
In the case of a CO, it’s a question of whether introducing a product may disrupt the equipment already working there. For example, will it corrupt the power supply with large transients or other types of EMI? Will it exhaust so much heat that nearby equipment becomes too hot to operate properly? Will the new equipment prove to be hazardous to both personnel and other equipment, and worst of all, will it catch fire or aid propagation of a nearby fire?
For many reasons, COs are unique fire environments. COs consume large amounts of power, which must not be disrupted if phone service is to be maintained. Sophisticated equipment worth millions of dollars sits side by side, filling literally acres of space and dissipating thousands of watts. Access is limited by the narrow aisles, the racks of equipment, and the connecting cables.
There have been very few serious CO fires in the United States. But, an eyewitness report filed two days after the Hinsdale, IL, fire of 1988 gives an idea of the level of possible disruption when one does occur.
- At least 40,000 subscribers, including local businesses, were without phone service for a few weeks.
- About 50% of the cellular phone service in the Chicago area was not working.
- MCI/Sprint long-distance and data lines were crippled.
- Emergency 911 calls were cut off. The police department stationed patrol cars a few blocks apart on the streets, and residents were told to go to the nearest car to report emergencies.
Efforts to extinguish the fire were hampered by highly toxic fumes from burning batteries and fiber-optic cables. The main switch was ruined by the fire and the water used to put it out, and the replacement switch cost several million dollars. In addition, costs were accrued by several emergency telephone vans that were moved into the area and the round-the-clock efforts of large numbers of technicians to restore service.
Many questions were asked at the time: Why was no one on duty in the CO when the fire broke out? Why was there a 30- to 40-minute delay in reporting the fire? Why was there no Halon extinguishing system in place to protect the main switch?1
The Development of NEBS
Many of the answers to these questions related to management practices. A part of the initial delay in reporting the fire was caused by human error. However, the materials and construction methods used in CO equipment also were to blame for the severity of the fire.
In 1990, the exchange carriers standards association (ECSA) sponsored a series of tests leading to publication of the ANSI T1.319-1995 standard, Fire Propagation Hazard Testing Procedures for Equipment:
In October 1995, Bellcore, known as Telcordia after March 1999, published Issue 01 of the GR-63 document NEBS™ Requirements: Physical Protection. This industry- standard equipment fire test was based on the proposed ANSI T1.319:
“GR-63-CORE identifies the minimum spatial and environmental criteria for all new telecommunications equipment systems used in a telecommunications network. Equipment suppliers know that compliance with NEBS criteria is the passkey for getting products into a CO and other network facilities. Compliance demonstrates the equipment has been thoroughly tested for safety and functional criteria for use in telecommunications networks…. Topics covered in GR-63-CORE include temperature and humidity, fire resistance, spatial and vibration criteria, airborne contaminants, acoustic noise, and illumination.”3
Of these topics, fire-resistance testing stands out because proving compliance destroys the equipment under test (EUT). The EUT is ignited by the flame from a line burner inserted into a hole bored into the EUT. The methane-fuel flow rate is slowly ramped up to a maximum of nine liters/min.
The EUT fans must be operating during the test, but it is acceptable to use faulty boards and subassemblies. One stipulation is that the EUT represents the worst-case fuel load. All the board positions that could be filled in various configurations must be filled.
Many factors comprise compliance. As an indication, Verizon specifies that the amount of flame and smoke should subside after the burner has been turned off so that 9½ minutes later the flames are extinguished with only minimal wisps of smoke visible.
“The greatest problem area [of the test] is fire escaping the confines of the system,” said Don Bennett, director of telecom compliance at Garwood Laboratories. “It can be caused by the materials being used or the draft of the fans. Removing the boards after the test has been completed may reveal an obvious area of charred material, which can quickly lead to determination of the cause of failure.
“I recently determined the cause of failures on two different tests we conducted,” he continued. “Both were due to the flammability of the materials used in the EUT. The failed components were tested and found not to meet the ignition requirements. When they were replaced with compliant parts, the tests were passed.”
The Telcordia documents listed in Table 1 comprise the NEBS generic requirements.4
GR-63 Issue 02 has adopted several changes to the fire testing procedure, and these are described in the new ANSI T1.319-2002 specification. As Bob Backstrom, director of fire protection programs and services at Underwriters Laboratories (UL) and technical editor of the T1E1.8 working committee, said, “T1.319 is intended to minimize fire propagation beyond the equipment enclosure. The committee has taken into account recent changes to the size and construction of typical CO equipment in the reissue of the document. It should be noted that ultimately the acceptance of the test report is up to the authority having jurisdiction.”
Rather than having predominately frame- or rack-size equipment in a CO, today equipment generally is smaller and fits on separate shelves within a frame. This distinction is shown in Figures 1a and 1b. In both cases, the X indicates the location of the standard 6² line burner called out in GR-63 Issue 01.
Changes to ANSI T1.319 include exemption from fire testing for certain equipment containing horizontal PCBs. There also exists a reduction in the total time for which methane gas flows during the fire test and the rate of flow, depending on PCB height, an alteration of the type and positioning of ignition indicators, and the use of a simulated equipment cabinet.
Although it appears to be easier for equipment to pass the new fire testing requirements, the prevailing point of view is that some aspects of T1.319-1995 were too stringent. For example, in smaller pieces of equipment, if the line burner could not be completely inserted, the exposed holes were to be taped off and the test run with the normal methane flow rates and duration. This approach created a more intense flame over the shorter remaining length of the burner than may have been realistic.
As well as alterations to the fire test, Issue 02 of GR-63 addresses methods to ensure testing accuracy for the airborne-contaminants section. Updates include the following:
- Clarified text and supporting material.
- New and improved flowing gas test procedures.
- Revised hygroscopic dust test procedures.
- Altitude test procedures.
Summary
Meeting the various NEBS requirements is taken as a minimum equipment qualification by many telecom companies. It may not be enough that a manufacturer’s product has passed all NEBS tests. In addition, large telecom operators often include additional NEBS-related requirements.
This is a good reason to choose carefully when selecting a test lab. Although test-requirement documents are readily available, a lab that has experience in running tests to the satisfaction of many regional Bell operating companies (RBOCs) and independent telecom operators can save you time and expense. After all, you’re going to burn up thousands of dollars worth of equipment. You want the tests done once—correctly.
Design procedures that have proven to be effective in meeting the fire test requirements of GR-63 start with basic considerations. The following short list was provided by Tom Hennessy, an engineering associate at Underwriters Laboratories:
- Use of ignition-resistant materials as demonstrated by UL 94 V-0 or V-1 with an oxygen index of 28%.
- Use of ignition-resistant components as demonstrated by the needle flame test.
- Use of enclosure compartmentalization as an integral design feature, which tends to minimize flame propagation within the device.
- Use of perforated fire barriers to restrain flaming beyond the confines of the equipment.
Finally, don’t underestimate the importance of meeting each paragraph of the NEBS fire-testing requirements. Even if your equipment passes the tests, you may need to prove that it passed because of its detailed design, not because you got lucky. To this end, carriers may request documentation to support what Verizon terms a fire-risk analysis.
“The equipment supplier shall provide, upon request by the carrier, a database detailing the flammability characteristics of all equipment, materials, components, and wire and cable per GR-63-CORE. An estimate of the maximum possible fuel load of polymeric materials in the fully equipped frame or subassembly should be documented, such as structural materials, electronic and electrical components, and associated wire and cable provided or specified by the equipment supplier. This estimate should identify, in tabular format, the constituent items. The estimate shall account for at least 95% of the maximum possible fuel load. The remaining 5% should be described qualitatively in terms of the types of polymers present.”5
References
- Townson, P. A., TELECOM Digest, Vol. 8, Issue 76, May 1988.
- www-comm.itsi.disa.mil/t1/319.htm
- http://telecom-info.telcordia.com/site-cgi/ido/index.html
- “Telecommunications Carrier NEBS Compliance Checklist,” Verizon Communications, SIT.NEBS.TE.NPI.2002.010, Jan. 24, 2003, p. 7.
- Ibid, p. 18.
Acknowledgement
Thanks to Bob Backstrom of UL for his firsthand comments about telecom fire standards and help in acquiring Chief Bulat’s photograph of the Hinsdale fire.
FOR MORE INFORMATION
on ANSI documents
www.rsleads.com/304ee-220
on Telcordia documents related to NEBS
www.rsleads.com/304ee-221
Return to EE Home Page
Published by EE-Evaluation Engineering
All contents © 2003 Nelson Publishing Inc.
No reprint, distribution, or reuse in any medium is permitted
without the express written consent of the publisher.
April 2003