The grid of electric power distribution in the United States includes main, secondary and tertiary high voltage (high tension) cables installed underground in conduits and ducts, normally accessible by manholes. In a large Metropolitan area, such as New York City, servicing millions of customers, there are numerous cables installed in cable bundles in a particular duct or conduit. Under normal operation, the cable jacket and insulation provides adequate thermal and electrical isolation to prevent arcing (electrical discharge) between cables.
High ambient temperatures and low air circulation in manholes, especially during the summer months, combined with the heat generated by the electric cables, can cause electrical breakdown of cable insulation, arcing and cable fires that rapidly transfer fire and heat to adjacent cables, causing further arcing. The result is a chain reaction that causes multiple cable failures interrupting power to all, or nearly all, cables in a manhole, duct or conduit. Damage spread over a wide area results in power outages to many people, requiring days or weeks to repair and restore service.
Additionally, the high levels of smoke and toxic products of combustion will endanger the lives of many people in adjacent areas, such as subway systems, as well as many individuals attempting to enter the affected areas to service or replace damaged cables.
Current cable wraps provide sufficient high voltage protection under normal conditions, but little, if any, fire and heat protection. Many of these wraps were tested and certified more than thirty years ago based on very low standards of fire protection and no requirements for limitations on smoke or toxic products of combustion. The more recently developed wraps continue to use standards for fire protection that do not adequately model the severe conditions in installations today. Further, they do not provide fire, high voltage and environmental resistance necessary for adequate protection in severe conditions.
There is a need for high performance, intumescent and high voltage wrap to isolate each cable under ordinary conditions and more importantly to provide the necessary isolation for cables in the event of overload, dielectric breakdown, and power surge in intense heat that could otherwise lead to a fire. The wrap isolates a cable failure and power can be switched, either by an operator or automatically by the system, to maintain continuity. Customers serviced by the failed cable, or adjacent cables, experience no disruption of service or only momentary disruption due to a switch over.
Intumescent coatings have been known for many years and have been used to provide thermal protection for many substrates including; wood and wood products, metals, fiberglass, and many types of plastics. However, an intumescent coating applied to these aforementioned substances is impractical for the fire and high voltage protection of electrical cables. Currently there is no single fire and high voltage wrap for electrical cables having all the following features:
1) Providing an effective cable wrap that substantially reduces, or eliminates fire spread along cables and heat transmission to adjacent cables.
2) Provide an effective intumescent fire-retardant coating that substantially reduces, or eliminates, smoke and toxic products of combustion from cable insulation when exposed directly or indirectly to a fire.
3) Provide an effective intumescent fire-retardant coating that is noncombustible and will prevent or eliminate ignition of cable insulation.
4) Provide a cable wrap with superior mechanical properties, including flexibility that allows easy handling, and wrapping of electrical cables of varying diameters.
5) Provide a durable and resistant coating to resist abrasion, impact, water, hydrocarbons, chemicals and other environmental factors associated with underground cable installation.
6) Provide a wrap that has high dielectric breakdown strength providing electrical insulation to 50 kVolts.
7) Provide a wrap that has very low ampacity deration, to maintain the high electric current carrying capacity of the cables.
8) Provide a cable wrap that is easily manufactured for mass production.