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EDN ACCESS ARTICLE: DESIGNING A NEBS-COMPLIANT POWER SYSTEM
A complete power system guarantees the problem-free operation of telecommunications equipment and that equipment's compliance with applicable standards. Designing the complete power system requires expertise in areas beyond power conversion. [HTML]

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Designing a NEBS-compliant Power System - SMH4804

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NEBS: An Historical Overview

NEBS an acronym for "Network Equipment-Building System" and is a term commonly used for a family of documents that apply to testing telecommunications equipment located in a central office. Two of the more common documents outlining the testing are the GR-63- CORE, Network Equipment-Building System Requirements: Physical Protection and the GR-1089- CORE, Electromagnetic Compatibility and Electrical Safety - Generic Criteria for Network Telecommunications Equipment.

Reliability of the telephone system is considered a national security issue, is demanded by consumers and makes good business sense. Therefore, NEBS testing is taken very seriously by both the Regional Bell Operating Companies (RBOCs) and other service providers as well as manufacturers developing equipment used for the telecommunications network. Equipment suppliers know that compliance with NEBS criteria is the key for getting products into a Central Office and other network facilities. Compliance demonstrates the equipment has been thoroughly tested for safety and functional criteria for use in telecommunications networks.

Pioneered by Telcordia, these rigorous standards - NEBS GR-63 and GR-1089 - have been industry benchmarks for more than two decades. They have been used to verify that products are truly carrier-class, installation-ready, and equipment compatible. Testing and analysis of products ranging from digital switches to hubs and routers to wireless and xDSL products are performed to insure reliability. These include an FCC-listed semi-anechoic room and Open Area Test Site for EMI; thermal chambers; seismic and vibration shaker tables; a fully pollution-controlled fire test facility; lightning and surge test facilities; and contamination laboratories.

NEBS had its genesis in the 1970s when Bell Labs began producing guidelines for equipment designers to help them focus on physical protection, electromagnetic compatibility and safety, and overall reliability. While originally intended for central office equipment, NEBS criteria are of value when designing any kind of missioncritical electronic system no matter where it is deployed. NEBS is now administered by Telcordia, the direct corporate descendent of Bell Labs and Bellcore (Bell Communications Research). Today, in the vast majority of cases, telephone companies will not deploy network equipment until it has been tested and certified to the NEBS criteria. Key NEBS categories are included in the block on the next page.

Like all telecommunications equipment, embedded computer systems in a telecommunications network are susceptible to electromagnetic interference (EMI), electrostatic discharge (ESD), power faults, and damage from lightning strikes. NEBS covers all of these electromagnetic compatibility and safety issues into account.

i. Electrostatic discharge

Noting that electrostatic discharges can cause device damage, alter software and firmware, and affect data, NEBS outlines criteria for immunity to ESD during normal operation, installation, and repair. Testing for ESD immunity involves using either contact discharge or air discharge methods. For example, during normal operations a unit must show that its test points (including panels, doors, consoles, keypads, indicator LEDs, fuses, switches, sockets, etc.) can withstand 40 air discharges (15 kV) or 20 contact discharges (8 kV). During this test, according to GR-1089, "a service-affecting response, unless within system operating limits, and manual intervention shall not occur".

ii. Electromagnetic interference

Equipment is also tested to see how well it avoids interference with other nearby equipment. For example, radiated electronic emissions from a closed-door unit must not exceed certain specified field strengths. A similar test performed with doors open. Magnetic emissions are similarly proscribed as are conducted emissions (AC and DC power and signal leads) from the unit into public utility power lines.

iii. Jolts from the blue-lightning and AC power faults While ESD and EMI represent the commonplace, everyday side of electromagnetic compatibility, power faults and lightning surges represent the arbitrary. These occurrences dictate two levels of criteria. First level compliance holds that the equipment be undamaged and able to continue operations after the fault or strike. Second level criteria state that the unit "may sustain damage, but shall not become a fire, fragmentation or electrical safety hazard". Tests include short circuit (tip to ring, tip to ground, etc.) and surge tests using a lightning surge generator on at least three units. Additional lightning surge tests exist for units that will operate on commercial AC power.

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