From 1993 NFPA FIRE INVESTIGATION REPORT
World Trade Center Explosion and Fire New York, New York February 26, 1993
Pages 14-18
Primary Electrical System
Consolidated Edison provides 13.8KV electricity to all of the World Trade Center buildings except for the Vista Hotel through a ground level substation located near the Barclay Street (north) entrance/exit to the underground parking garage. Prior to the fire, the peak load provided by the substation servicing the complex was 84 megawatts for all buildings (except the Vista Hotel), the Concourse level, all basement levels and the refrigeration plant. Eight sets of service conductors carry this load from the substation to the complex. Overcurrent relays that disconnect the service conductors are provided at the substation.
Service conductors entering the complex are routed through a concrete encased duct bank which is located below an entrance/exit ramp for the B1 level. Once inside the complex, the duct bank makes a vertical transition from below the entrance/exit ramp to the B1 level ceiling. Still enclosed in concrete, the ceiling-level duct bank enters the service equipment vault, called the primary distribution center (PDC). Once inside the PDC, the service conductors drop into eight switchboards which are grouped into pairs and are separated by gypsum wallboard partitions. Electrical power from the PDC is provided to Towers 1 and 2, Buildings 4 and 5, the Customs House, the Concourse level, and to all basement levels (see Figure 5) by several electrical feeders. These feeders are protected by 1200-ampere circuit breakers with protective relays to disconnect the power when a fault occurs. The feeders for Building 7 are tapped off of the Building 4 feeders; therefore, the substation relays protecting Building 4 also protect the feeders for Building 7.
The PDC-supplied electrical systems are designed so that electrical power to each building served is provided through more than one feeder. The substations for buildings in the complex are designed such that full electrical service will not be interrupted in any building even with the loss of two 13.8-volt electrical service conductors. Though some electrical feeders are routed differently, most are routed in the ceiling/floor assembly between the B1 and B2 levels. As a result of this type of installation, the feeders are encased in concrete over the distance to their respective substations throughout the complex.
For electrical design purposes, Towers 1 and 2 have been subdivided into two vertical sections, i.e., Tower 1 — north and south sides; Tower 2 — east and west sides. Each vertical section is supplied by four electrical substations; one substation in each of the mechanical equipment rooms (MER) on the 7th, 41st, 75th, and 108th floors. Thus, Tower 1 and Tower 2 each have a total of eight electrical substations.
The arrangement of substation equipment is the same in both towers; that is, each substation has four air-cooled transformers. The transformers are rated as 1500KVA, 3 phase, 13.8KV-480/277 volt. Each is provided with a 600-ampere, 15-KV primary, no-load disconnect switch on the primary side and a 2500-ampere circuit breaker on the secondary side.
Four feeders from the PDC supply electricity to each vertical section of the tower. Each feeder supplies electricity to one transformer in each substation within a vertical section. For example, Feeder A1 supplies electricity to one transformer in each substation for the north side of Tower 1. Similarly, Feeder A2 provides electricity to another transformer in the substations on the north side of Tower 1. According to this design scheme, each feeder will supply four transformers.
The main difference in electrical design between the towers is that Tower 2 has two additional feeders. These feeders supply a substation on the 43rd floor. The transformers in this substation are dedicated and provide power to a tenant area.
Emergency Electrical Systems
All buildings in the complex have numerous emergency systems requiring electrical power. These systems include, but are not limited to, emergency lighting in all exit stairways and corridors, public address systems, fire detection and alarm systems, fire pumps, at least one elevator in each elevator bank, fire fighter telephones in the stairways, the communications transmission equipment installed on the antenna on Tower 1, and normal telephone systems.
Many sources of electrical power have been provided for the emergency systems. The primary power supply for emergency systems in the complex is the normal electrical power provided by Consolidated Edison, and this power supply is backed up by several emergency generators located in different areas throughout the complex. Emergency generators installed in the Vista Hotel provide power to the emergency circuits in that building only. Similarly, emergency generators installed in Building 7 provide electrical power to emergency circuits in that building only. New York Telephone provides separate emergency generators dedicated to the normal telephone equipment, telecommunication circuits throughout the complex, and telephone switching equipment for the Manhattan exchange. To ensure that there is no interruption of telephone service during emergencies, New York Telephone also provides batteries to maintain electrical power in the event that the emergency generators fail.
[...]
In addition to the emergency generator backup, Towers 1 and 2 have been provided with an "alternate electrical system" which provides electrical power to some emergency systems. As indicated earlier, Tower 1 and Tower 2 are divided into two vertical electrical zones. Power is provided to areas in these zones by separate feeders. In the event that normal electrical power is interrupted to an area, switching gear will reroute normal electrical power from an unaffected vertical zone to the areas affected by the power outage. (See Figure 6.) The alternate electrical system will provide power to circuits for emergency lighting, fire detection systems, public address systems, fire alarm systems, fire suppression systems supervisory equipment, and communication systems. The alternate electrical system does not provide electrical power to elevator or fire pump motors. Electrical power for these high- load motors is provided by the normal electrical system or by the emergency generators.
actually I know just enough about everything to be dangerous.
