telecom: what is NEBS

NEBS (Network Equipment - Building System) is the most common set of safety, spatial and environmental design guidelines applied to telecommunications equipment in the United States. NEBS is why telephones work after an earthquake or thunderstorm! NEBS requirements are utilized all over the world for a host of commercial, utility and defense applications. Still, the most frequent application of NEBS is to design and test equipment intended for use in Central Offices.

http://telecom-info.telcordia.com/site-cgi/ido/docs2.pl?ID=095698532&page=nebs

Equipment deployed in the network needs to be safe, reliable, and easy to deploy. The NEBS standards in GR-63 and GR-1089 provide
uniform criteria for equipment design intended to reduce the cost of deployment and maintain reliability of the network.

SR-3580 groups the NEBS criteria into three functional levels (I, II, or III). Grouping the criteria into levels helps clarify the
impact of non-conformance and allows the broad range of NEBS requirements to be judiciously applied to equipment, based on the
equipment's application and impact on the operation of the network

http://telecom-info.telcordia.com/site-cgi/ido/docs.cgi?ID=196992314SEARCH&KEYWORDS=&TITLE%20=&DOCUMENT=SR-3580&DATE=&CLASS=&COUNT=1000

What is "NEBS Level 3"?
"NEBS Level 3" is a term from Bellcore special report, SR-3580.  This report details 3 distinct functional levels of NEBS compliance based on GR-63-CORE and GR-1089-CORE.  The levels allow a wide range of NEBS requirements to be applied to equipment based on application and network impact.
"NEBS Level 1" means that people and equipment hazards and network degradation are minimized.  NEBS Level 1 addresses the personnel and equipment safety requirements of GR-63-CORE and GR-1089-CORE.  This is useful for getting prototypes into a lab trial.  It's also a requirement that is imposed on Competitive Local Exchange Carriers (CLECs) in order for them to deploy equipment into an RBOC's network.  This level doesn't address operability, this is addressed in Levels 2 and 3.
"NEBS Level 2" addresses equipment operability, but in a controlled environment.  For example, equipment could be used in a data center, however, if the equipment performs non-critical operations it could be used under environmental extremes.  Note that this level is very rarely used because it is ambiguous.
"NEBS Level 3" means the equipment is in the network for the long haul.  We're talking about Carrier Class with this stringent level.  The equipment will operate under the environmental extremes found in a central office.  In a nutshell, the equipment meets all of the requirements of GR-63-CORE and GR-1089-CORE.
Telcordia has created a special NEBS page to their SuperStore providing customers with more info on the NEBS documents. Check out Telcordia NEBS Page
NOTES:  

1. Verizon does not follow SR-3580.  They use their own NEBS checklist,  NEBS Compliance Clarification Document, that details what they believe are important to their network's integrity.
2. SBC uses 2 levels as detailed in their NEBS checklist TP76200MP.
3. A combined Telecommunications Carrier Group Checklist can be found here TCG Checklist.