In Times Past

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Browns Ferry Nuclear Power Plant, Athens, Ala.

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The winter of 1975 was pivotal for the electrical industry. Two major fires within a month reminded the industry of how fires in combustible cable insulation can have catastrophic results. Neither of the fires occurred in an installation governed by the National Electrical Code, but both called attention to need for better cable insulation. The fires resulted in significant property damage and the loss of critical facilities.

New York Telephone

With more than 8 million people, New York is the most populous U.S. city and the financial capital. Something that affects Manhattan can have an impact well beyond New York.

In 1975, we were all connected by landline telephones. Cellular and PCS technology did not exist. Manhattan’s only phone provider was New York Telephone, then a unit of AT&T. The NYTel exchange building at the corner of Second Avenue and 13th Street housed 12 telephone exchanges and toll­switching machines. Original construction was completed in 1923. By 1930, another eight stories had been added to the building.

This building’s exchanges served a 300-block area of lower Manhattan, and the exchange rooms were connected to the outside world with PVC-insulated cables. The telephone switches were electromechanical panel and Crossbar.

The fire began as an electrical fault in a subbasement cable vault. A local fire alarm signal was sounded at 12:15 a.m., when smoke was discovered on the third floor. Fifteen employees were working in the building at the time. A maintenance person on duty attempted to call the fire department, but the fire was already affecting the telephone service. The maintenance person used an outside callbox to contact the fire department at 12:25 a.m.

The first units reported heavy smoke in the building. The five-alarm fire, which burned for 19 hours, involved approximately 700 firefighters. A number of people were evaluated and treated for smoke inhalation.

The fire disrupted telephone service for more than 170,000 subscribers. It affected phone service for several police stations, hospitals, some universities and numerous business and residential customers. Telephone exchange update projects all over the United States were affected because equipment was diverted from scheduled upgrades to the emergency restoration in New York.

The epic restoration process took 4,000 workers 23 days. Since NYTel was part of AT&T, and included Western Electric and Bell Laboratories, the company was able to tap many corporate assets, including starting production of equipment and rescheduling deliveries. Crews also cleaned some of the electromechanical switches so they could be returned to service. This remains the worst fire loss in the history of U.S. telephone service even 45 years later because of the improvements that have taken place since then.

After telephone service was fully restored on March 21, 1975, another major cable fire happened the next day.

Brown’s Ferry nuclear plant

The Brown’s Ferry Nuclear Power Plant was the pride of the Tennessee Valley Authority. When completed, the plant would consist  of three 1,100-megawatt boiling water reactors. Construction of the first two units began in 1967, and the third unit began in 1968. Unit 1 went online on Aug. 1, 1974. Unit 2 went online on March 1, 1975.

The facility, in Athens, Ala., was designed so that positive pressure was maintained between an area that consisted of the control room, the cable-spreading room and the reactor building. The cable-spreading room connected to tunnels where cables were run to the two reactors. Positive pressure would serve to prevent air from the reactor building that could contain radiation from entering the control room.

Positive pressure could only be maintained if all penetrations between the cable-spreading room and the reactor building were sealed. Cables were sealed at the firewall with a poured-in polyurethane insulation. Other material was used to function as a dam for the poured-in insulating material, including polystyrene and sprayable forms of polyurethane. The insulation surfaces at the wall were covered with a fire-retardant coating.

When additional cables were run through the polyurethane seal, a wooden pole, such as a broom handle, was used to punch holes through the polyurethane. These additional holes needed to be sealed to maintain the positive pressure toward the reactors. For small leaks, a seal made of a silicone-rubber material was made. Larger holes were also sealed with polyurethane plugs, and there was no standard method to detect leaks. The methods used included smoke sources, soapy solutions and candles. Checking for leaks was routine maintenance.

On March 22, 1975, units 1 and 2 were operating normally, and the third unit was still under construction. Workers were testing for leaks between the two areas using a lit candle. A strong air flow caused the flame to be drawn into the seal, which ignited the combustible polyurethane foam. The resulting fire spread to cable trays, and the fire made it difficult to control some systems. The damage caused loss of some instrumentation and control of critical systems, including pumps needed to cool the reactor. The turbo generators were taken offline and both reactors were scrammed (shut down).

Several dry chemical portable extinguishers and a fixed carbon-dioxide extinguishing system were used to fight the fire, but it burned for about nine hours. Toward the end of the fire, water was used to extinguish it. The fire damaged 1,611 cables, 117 conduits and 26 cable trays. There was significant damage to the cable-spreading room and to the Unit 1 reactor building. Seven TVA employees were treated from smoke inhalation. At the time of the fire, Unit 1 had been operating for less than a year, and Unit 2 had been operating for less than a month.

NEC scope

Both of these fires occurred in utility installations. Telephone exchanges and power plants are outside the scope of the NEC. However, private telephone systems installed in commercial buildings may not be utility installations. There are also privately owned generating facilities at many industrial facilities. The lessons learned from these two fires also apply to areas governed by the NEC.

Telephone systems have changed dramatically in the 45 years since the NYTel fire, and the amount of wiring distributed in buildings has also greatly increased.

These fires spread quickly and caused considerable damage. The fires were able to propagate because of combustible insulation and inadequate sealing of fire walls openings with combustible insulation. Both fires caused significant financial losses because of  direct damage and something that the insurance industry calls “business interruption.” NYTel was able to reduce downtime by redeploying thousands of employees.

Another risk of a major fire to a critical facility can be the public relations impact.

Both fires drew the attention of the electrical industry, particularly testing laboratories, cable companies, insurance companies and an IEEE committee. Cable fires could result in direct damage, business interruption and smoke inhalation, which is a major concern for first responders.

Insurance companies have long required sprinkler protection for cable trays for critical systems as well as for sealing wall penetrations. Many industrial users have been reluctant to provide water-based extinguishing systems for cables or other electrical equipment. The cable companies started working with manufacturers on fire-retardant cable insulation. The industry sought insulation that was fire-retardant, low smoke producing and low toxicity. Primarily, the cables must have good electrical­-insulation characteristics.

UL developed testing methods it uses for the cable evaluation. Methods include Bunsen burner tests of individual conductor samples. Vertical flame tests are used to evaluate cables that will be installed in riser applications. The riser tests use a ribbon burner with 70,000 Btu/hour output as the ignition source and a specific arrangement of the cables in a vertical cable tray. The most stringent test that UL uses is the Steiner Tunnel Test for qualifying cables used in plenum applications.

In the 1987 edition of the NEC, requirements began to appear for different types of listed fire-resistive cables. Cable types were created for different applications and specific listing requirements depend on where the cables were to be installed. A cable’s physical orientation, the airflow over the cable and its orientation relative to other cables affect how it reacts to an ignition source.

The specific applications are plenums, risers, general use and dwelling applications. These requirements are now found in articles 725, 760, 770, 800, 805, 820, 830 and 840. Substitution tables and charts in each of these articles permit some usage of cables in different applications where the cables have the same fire characteristics.

More things are connected today than at any time in history. The lessons learned from these fires reshaped the electrical industry with this major effort to prevent a reoccurrence.

About the Author

Mark Earley

Mark Earley, P.E., is the chief electrical engineer. Retired from the National Fire Protection Association, he was secretary of the National Electrical Code Committee for 30 years.

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