2-9
Found in almost any large structure, including hospitals, heavy industry and high rise buildings, electrical cabinets present a hazard particularly well suited for carbon dioxide protection.
A typical electrical cabinet is the focal point of the electrical service for a large building or plant. Virtually all of the incoming service enters at the cab-inet and is dispersed, stepped up, stepped down or otherwise controlled at that point. An electrical cabinet may contain fuses, switches, transformers and other electrical equipment along with a large network of cable and wiring.
Downtime for the electrical cabinet means downtime for the entire facility.
Dry powder or liquid agents can damage sensitive equipment, or require meticulous clean-up causing additional delays in getting the facility back
‘‘on-line.’’ An air-dispersed gas, carbon dioxide eliminates these problems.
Electrical fault is the most common source of ignition in the electrical cabinet.
Energized equipment overheats or shorts and ignites insulation causing a
‘‘deep seated’’ type fire.
Protection of electrical cabinets can be accomplished with a total flood system. By injecting a sufficient amount of CO2 to suppress the fire and maintaining the CO2 laden atmosphere to allow a ‘‘soaking period,’’ even deep-seated insulation fires can be suppressed.
The design of this system should be in accordance with National Fire Protection Association Standard No. 12, 1989 Edition, which addresses the fire protection of electrical equipment. NFPA 12 states that a 50% tion of CO2 is required for dry electrical hazards and that a 30% concentra-tion shall be achieved within two minutes. Design concentraconcentra-tion must be achieved within seven minutes and maintained for an additional twenty min-utes.
Depending on the type of doors the cabinet has, the CO2 system could be designed as either a normal total flood or extended discharge system.
Cabinets with loose fitting doors or louver openings will have considerable CO2 leakage. Leakage from a weatherproof cabinet will be much slower.
In a reasonably ‘‘tight’’ cabinet, such as a weatherproof enclosure, the extended discharge may not be necessary as the sealed enclosure allows little leakage and the inert atmosphere will remain until the cabinet is opened and ventilated.
Hazard Description
Sources of Ignition and Types of Fires
Recommended Protection
The extended-discharge CO2 system consists of two cylinder banks and two piping arrangements. Upon receiving a signal from the detectors, one bank of cylinders which are piped to a set of nozzles give a high initial rate of dis-charge, meeting the requirements for reaching design concentration within seven minutes. The second bank of cylinders is piped to a set of smaller noz-zles which provide an extended discharge period, maintaining the inert atmosphere for the required twenty minutes.
Personnel safety is the first concern. Discharge alarms should be located in the area of the electrical cabinet to warn nearby personnel of the CO2 dis-charge. Due to the possibility of CO2 leaking from the cabinet and settling into low lying surrounding areas, all personnel should leave the immediate cabinet area until the space can be completely ventilated.
Normal leakage from the cabinet should relieve any CO2 pressure build-up.
However, in the case of an air-tight enclosure, pressure relief venting may be required.
Electrical power and any ventilation must be shut down prior to discharge.
Also, many electrical cabinets have cooling fans to draw air into or out of the enclosure. These must be shut down and an extended discharge system should be considered to allow for the spin-down time and unclosable open-ings.
Electrical cabinets may have completely open interiors or may be compart-mentalized. If the construction of the cabinet is a series of compartments, at least one CO2 nozzle and detector must be installed in each compartment.
In exceptionally large electrical cabinets, selector valves could be included in the system which could direct the discharge to only the section involved.
The authority having jurisdiction may have additional requirements.
Protection Considerations
000945
Transformers
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Transformers are found in heavy industry and may sit in the open or in vaults.
Transformers located in the open, where it is impractical to flood the room, are protected by locally applying carbon dioxide over the surfaces using the rate by area method. Transformers within a vault are treated as a surface type total flood hazard. There is a possibility that a heated transformer core in either a transformer located in a vault or in the open could produce a ‘‘deep seated’’ fire in the insulation.
Leakage of oil could be ignited by an electrical fault or insulation within the transformer could ignite due to an overheated core.
Transformers within an enclosure or vault can be protected by total flooding the enclosure with CO2 in accordance with National Fire Protection Association Standard 12, 1989 Edition, which states that a 50% tion is required for deep seated dry electrical fires and that a 30% concentra-tion shall be achieved within two minutes. The 50% design concentraconcentra-tion must be achieved within seven minutes and held for an additional twenty minute period.
The carbon dioxide system would consist of a bank of cylinders, a piping net-work and discharge nozzles, as shown in the figure below.
Hazard Description
Sources of Ignition and Types of Fires
Recommended Protection
000946
The figure below illustrates a typical transformer protected with CO2.
The CO2 system would consist of a group of cylinders, a piping arrangement and a set of discharge nozzles.
Any floor drain located under the transformer should be provided with a nor-mally closed valve which only opens by oil pressure during an oil spill.
Electrical clearances should be maintained in accordance with NFPA 12.
The room or area must be ventilated after CO2 discharge with consideration given to areas where CO2 might tend to settle.
The authority having jurisdiction may have additional requirements.
Protection Considerations
000947