Limited Energy Requirements in the NEC®
Mark W. Earley
Articles 725, 760, 770, 800, 820, and 830 are the articles of the NEC that cover limited energy systems. In the limited energy systems discussed in these articles, there is less concern that an electrical fault will cause a fire than there is with power distribution circuits. For many of these circuits, there is also less of a concern for shock hazards. The primary concerns for these articles include fire propagation along combustible insulation and through fire-rated assemblies and a concern about limited energy circuits being mingled with power circuits.
Many of the circuits discussed in these articles are control, signaling, alarm, or communications circuits. The number of these types of circuits is increasing in all types of facilities. As the number of circuits increase, the potential hazards also increase.
Cable Insulation
Two landmark fires in 1975 illustrated the problems that can exist if combustible insulation is used on cables. The first of the fires occurred in a telephone exchange in Manhattan. The fire started in the sub-basement cable vault of the 12-story exchange building and traveled vertically from floor to floor through unsealed floor penetrations. This fire affected several floors of the building and caused $90 million (1975 dollars) in damage. The fire injured over 100 fire fighters, and the resulting service interruptions affected approximately 200,000 customers. The second fire occurred a few weeks later at the Browns Ferry Nuclear Plant in Alabama. Cables penetrated fire walls that were sealed with combustible foam insulation. Workers were using candles to check the integrity of the foam seals (leaks in the foam seal would cause deflection of the flame). A candle ignited the fire which then spread into adjoining areas. The resulting blaze burned for nearly 8 hours, destroying over 1600 cables.
Although neither of these facilities was required to be built to NEC standards, these fires have had an impact on requirements in the NEC. Cables are now required to meet fire test standards, based on whether they will be installed as general wiring in a building, installed in a riser, or installed in a plenum. Openings around electrical penetrations through fire resistance–rated walls, partitions, floors, or ceilings are required to be firestopped using approved methods to maintain the fire resistance rating. These approved methods preclude the use of combustible material.
Abandoned Cables
Each of the limited energy articles requires the removal of the accessible portion of abandoned cables. Abandoned cables increase combustible loading and can create installation and maintenance problems. Excess accumulations of abandoned cables in plenums can obstruct the flow of environmental air.
Separation of Conductors
Class 1 circuits use 600-volt insulation. However, other limited energy circuits do not use 600-volt insulation. Therefore, separation of conductors is a concern that is addressed by requirements in each of the articles.
Computer local area network (LAN) circuits are considered to be Class 2 circuits and so must comply with Article 725. However, cables in computer rooms that meet the criteria of Section 645.4 do not need to meet the requirements of Article 725 if the installation meets the requirements of Article 645. Cables that leave the computer room must meet the requirements of Article 725. Article 645 installations are a unique and controlled environment. Most computer installations will not fall into the criteria of Article 645.
Structure and Numbering of Limited Energy Articles
The limited energy articles of the NEC contain some common themes. For this reason, the articles have been organized in a parallel numbering format.
Mark W. Earley, P.E., is Assistant Vice President of Electrical Engineering at NFPA, serves as secretary of the NEC® Committee, and is the chief editor of the National Electrical Code Handbook. He joined NFPA in 1986, is co-author of Electrical Installations in Hazardous Locations, and has published numerous technical articles on the fire protection of electrical equipment.