Designers want a "Do Over" for the return air plenum spaces
With the breaking news on serious problems in the realm of IAQ and IEQ, many designers are wishing they could have a "do over" when it comes to cabling in the return air plenum spaces.
At a minimum, in the Twenty First Century, PLEC systems (Power, Lighting, Electronic and Communications) are essential in any commercial building. Every building's design team and owner/manager know that a building must have a relatively sophisticated infrastructure that can contribute to the IBS (Integrated Building Systems). These technologically advanced and rapidly changing systems must meet the demands of an increasingly information-driven society. Public and private office buildings, universities, schools, hospitals and lodging facilities must have safe and functional wire management systems that can accommodate accelerated demands.
Fire prevention, in terms of wire management, is fairly simple. We must comply with the writing of all applicable building codes. Interpretation of the intent of these codes is a major challenge. Building codes are, in fact, a "record of mistakes" not to be repeated. However, the codes have missed a big villain, TOXICITY. Amazing as may seem, the NEC (National Electrical Code) does not require TOXICITY testing on the low voltage or fiber optic cable that are rated for use in the return air plenum spaces.
Most codes have two aspects - the rules that are there to save lives and the rules that are there to protect property. Proper selection and placement of the correct wire management/wire distribution systems can achieve both.
The true costs of failing to invest in proper health and life safety measures include, but are not limited to:
Loss of life
Injuries to occupants and firefighters
Lost revenues, rents and business
Higher cost of insurance
Lost tax revenue
Increase in fire fighting and police costs
Impact on neighborhood property values and quality of life
Loss of jobs
Diminished value of asset
Therefore, when we evaluate and select any system, every aspect that affects people's safety must be considered. It is a prudent investment in the welfare of the community. In addition, safe buildings are more marketable and retain their value better.
Let's begin with the fact that few available systems can be designed properly and can be safe and code compliant. There is no "perfect system" that answers every building's requirements and often, combinations of systems are necessary to address user needs.
The purpose of this analysis is to make architects, engineers, owners, investors and agency heads aware of critical design issues. You should consider each system as it relates to new construction as well as the renovation of existing buildings. You should also be aware of the advantages, limitations and construction realities.
Emphasis should be on prevention or control, first of ignition, then of limiting the transmission of toxic gasses, smoke and fire. The next goal should be to limit problems during construction and tenant changes to reduce hidden costs and maintain code compliance.
Begin with some basic concepts about wire management systems and components.
All components must be tested and approved, and should have a good track record. Testing means that an accepted test method, usually ASTM or NFPA, has been utilized by a licensed laboratory that is acceptable to the state and local code officials where the component will be used. Laboratories that are acceptable everywhere in the U.S.A. are Underwriters Laboratories (UL), ETL, and NFPA. At this time, there are no formal requirements for plenum-approved cable to be retested over time to insure the test requirements for CMP (NFPA 262) can still be met. Many cable industry experts refer to this loophole in the NEC as "safe today and useless tomorrow".
The purpose of electrical conduit or electric metal raceway is:
To protect the enclosed wires and cables from mechanical injury and corrosion.
To provide a grounded metal enclosure for the wiring in order to avoid a shock hazard.
To provide a system ground path.
To protect surroundings against fire hazard as a result of overheating or arcing of enclosed conductors.
To support conductors, prevent sagging and thinning of ductile copper or aluminum which could increase resistance and cause overheating.
To prevent exposure of wire insulation to fire or elevated temperatures which could cause toxic fumes that can incapacitate or kill the building occupants.
Metal electric raceways, conduits and fittings must be galvanized. Review the National Electrical Code and Underwriters Laboratories Standards for other criteria and restrictions.
All electrical equipment must be properly grounded per National Electrical Code and NFPA 70, Article 250.
Due to human ergonomic requirements for viewing computers, scanners, printers and fax machines, locally controlled task-lighting is a must, resulting in considerable energy savings, but also resulting in higher wiring costs and more room required for power wiring.
Costs vary greatly from one system to another. First costs, long term costs, risks and value engineering as an investment should be considered.
The systems that will be briefly considered and examined are:
Cellular Floor Systems
Wired Partitions and Power Poles
Access Flooring (raised flooring)
Flat Wire (under-carpet) System
Cable Tray Systems (ceiling based)
Surface Raceways (For direct connect or for wired partition feed)
The Systems that will be firefly considered and examined are:
Poke-Thru Wiring (within the Plenum Space)
Cellular Floor Systems (within the floor slab)
Cable Tray Systems (within the Plenum Space)
Poke-Thru outlets are installed through core drilled holes through the floor slab. They are limited to a density of 1 outlet per 65 sq. Ft. of floor area. Core drilling can cause structural damage to the slab especially in concrete frame buildings where re-bars can be cut.
Mot Poke-Thru outlets will allow smoke penetration in a fire event. Large amounts of smoke can quickly penetrate the slab to incapacitate the occupants of the floor above. Ironically Poke-Thrus are fire rated for 1 to 4 hours but prevention of smoke penetration is not a requirement! If you have a Pile-Thru outlet below your desk make sure it is "smoke proofed."
Cellular Floor Systems - is perhaps the most desirable of all the infrastructure distribution systems available for the "office of the future".
These systems differ from other wire management systems in that they are often part of, and integrated into, the structure of the building. The cellular system is normally part of a "composite design," using steel deck, welded studs and electrical cells to form a complete structural/electrical system in one operation.
The cellular floor system design accounts for its low overall cost. The need to have independent cells for power, telephone, data and life-safety wiring (smoke detectors, fire alarm, emergency lighting and emergency generation) is a very important advantage of this system. The electrical systems have their own space - they do not share space with other ducts, pipes, lights, vents or sprinklers; therefore, wiring in cellular floor systems is not subject to abuse by other trades. Wire security, shielding and wire integrity are not compromised.
In the event of a fire in the workplace, it is critical that the designer understand the significant value of the placement and control of the fuel load from materials placed in the cellular floor systems. These systems offer a substantial barrier to fire hazard and the spread of toxic gasses and smoke.
The most popular design of cabling infrastructure during the past 30 years has been the installation of CMP (plenum rated) communication cables in the overhead return air plenum spaces. There are no fuel loads or fill limits on the cabling in the overhead return air plenum space. This scenario has created a huge and unexpected potential danger for the workplace.
New building designs utilizing cellular floor systems are a MUST to prevent a repeat of the safety and health fiasco that developed during the past three decades.
Only companies with complete U.L. system approvals and NEC compliance, as well as a proven track record, should be considered.
Systems with "open top trenches" allow wire lay-in from the floor, a labor saving factor. Also, capacity vs. pull-thru feeders is greatly increased.
All cellular floor systems with preset outlet boxes and trench headers require spray fireproofing on the underside of the slabs. Therefore, lightweight concrete offers no cost advantage. Bar joists are not U.L. approved to support most conventional cellular floor systems.
Shallow cellular systems can be used for rehabbing existing buildings, if the structure can take the additional load. New fill materials, i.e., self-leveling gypsum that cures in one or two days and meets all fire requirements, may soon be available.
Wire capacity of cellular systems is a major consideration in today's electronic age. Three-inch deep cellular systems provide optimum wiring space and spanning capabilities, with activations as close as 2' X 2'.
Since the A.I.S.C. (American Institute of Steel Construction) Code requires a specific spacing for deck weld-down or shear studs (not to exceed 18"), cellular deck systems that exceed this dimension are in violation of requirements for composite members and structural engineers should examine this very carefully. Significant reductions in composite beam strength can occur if this spacing is exceeded, even by only a few inches.
If cells are adjacent and separated only by a vertical web member, possible cross-overs may occur at open butt joints in the deck, creating a serious safety hazard. Cell separation is mandated by U.L. #209 and also by the NEC. Only systems that maintain cell separation with concrete across the joint should be considered. Currently, only 3" deep cellular systems can comply.
All outlet boxes should have proper grommets at the entry holes to avoid accidental stripping of wires and a potential hazard. (See U.L. 209 and NEC.) Inspect samples carefully for wire exposure to raw metal edges.
Cellular floor systems perform best when abandoned wire is removed from the system. Good wire management and good housekeeping are closely related.
The cellular system is a totally grounded system since it is welded to the steel frame. The use of plastic covers on the floor outlets will eliminate static shock on hand contact -- an important consideration for office occupants.
Communication systems often require R.F. shielding and wire security that cellular systems provide.
Cellular floor systems provide the best overall benefits of any wiring distribution system, including low cost, flexibility, capacity and wire management, at a cost that is competitive with Poke-Thru wiring.
Cellular floor and underfloor duct distribution systems must be designed as part of the office building structure. Cellular floor consists of structural steel decking, which is an integral part of composite floor slab in steel frame construction. A metal channel, called a trench header, connects the individual cells and is installed flush with the finished floor slab. The degree of flexibility with cellular floor depends on the module spacing of the presets and cells. Because the preset inserts are easy to activate and de-activate, cellular decking offers some of the lowest relocation costs.
Several major changes are on the horizon. These changes will have a huge impact on building design. Indoor air quality and indoor environmental quality demands are becoming an irresistible force for the building owner. Tenant or occupant safety and health issues can no longer be ignored. The liability associated with these hazards may make a building financially untenable. Toxic gasses, hazmat (heavy metals), asbestos, and accumulated fuel loads all add up to a top priority concern for the architect, engineers, and building owners and managers. We must consider all of the safety and health factors associated with the infrastructure in the office of the future. Remember: Safety is too important to ignore.