Have you ever found yourself confused or unsure of the type of wire or cable to use in a project as it relates to local codes and safety requirements? What do the various cable safety ratings mean? What materials and characteristics make one cable less flammable than another? What is meant by halogen-free cable? Is plenum cable cheaper to run than cable in conduit? I'm going to try to answer these and hopefully many other questions about cable fire safety and application.
The building authorities (usually county or city) in your locality adopt standards and codes to which construction must conform for the overall good and safety of the community. Remember that regardless of national codes and standards, the local building authorities have the last word on what is considered acceptable building and wiring practice in your area.
For both high voltage and low voltage electrical wiring, all building authorities adopt standards from the National Electrical Code, or NEC. The NEC is a collection of requirements for electrical wiring and appliances that safeguard against electrical fire and electrocution. A committee under the supervision of the National Fire Protection Association, NFPA, creates the NEC. The NEC is but one code document among many created by the NFPA. Go to www.nfpa.org for more information. I think it's important to bring up the difference between a standard and a code. A standard is a level of performance that may be adopted as an option, but a code is a mandate imposed by some authority.
"I'll take CABLES for 50, please."
All signal cable used for computer networks, telephone, video, audio, and control applications of less than 50 volts is considered low voltage cabling. Low voltage cabling is categorized into the following five basic groups within the National Electrical Code (NEC):
||Class 2, Class 3
remote-control, signaling, and power-limited cables
||Power-limited fire protective signaling cables
||Power-limited tray cable
Fire safety ratings under the NEC are conducted according to a common group of flame retardancy tests, which makes the cable markings similar across all of these designations. The NEC's cable substitution hierarchy for fire safety is shown in Table 1. Video, audio, and low voltage control cables fall into Class 2 typically due to the available power limits set in the NEC. All computer network and telecommunications cabling falls into the CM class. CM and CL2 categories of cabling are of primary concern in the AV industry.
So, What's All This Cable Safety Rating Stuff Anyhow?
So, what does all the safety alphabet soup mean? Table 2 (on following page) is a handy applications table that will help you organize the cable marking designations mentally.
Plenum-rated cables (suffix "P") are at the top of the cable safety food chain because they are constructed of materials having very low "fire load." Fire load is the term used to describe how much fuel a given material provides a fire. A lower fire load rating means that the material is more fire resistant and produces less smoke, which accounts for most fire-related deaths. Cables obtain the plenum rating upon successfully passing UL 910, Test for Flame-Propagation and Smoke-Density Values for Electrical and Optical-Fiber Cables Used in Spaces Transporting Environmental Air. Plenum is a commonly used term today in the construction and system installation industries because, in most cases, plenum-rated cables may be installed in air handling systems (air plenums) without expensive metallic conduit. Plenum cable can cut installation costs dramatically.
Riser (suffix "R") describes cables having a lesser degree of flame retardancy than plenum, but may be used to convey signals vertically in shafts without requirement for metallic conduit. The compliant cable has a flame propagation of less than 12 feet and has a temperature of 850 degrees Fahrenheit or less at a height of 12 feet per UL 1666.
General-purpose (no suffix) cables may be used in conduit, behind walls, or other enclosed locations where the cable is protected and not in an air plenum. Commercial installations, at a minimum, must use general-purpose cables (the typical CL2 designation for coaxial video cables, for example). This type of cable must comply with UL 1581, the Vertical-Tray Flame Test. For CSA (Canadian Standards Association), the vertical flame test differs in loading (more cable in bundles), burner angle, and failure criterion.
CL2X and CL3X are the lowest rated cable and must comply with UL VW-1 Vertical-Specimen Flame Test. The cable is not marked VW-1. This rating may be used in residential dwellings.
PLTC (power-limited tray cable) complies with a 70,000 BTU/hr vertical-tray flame test. Cables of this type are marked PLTC with ink or marker tape.
Getting 'P' On Your Cable
What makes a cable into a plenum rated cable? The materials making up the insulation on the wires and jacket on the cable must be capable of withstanding a specified amount of heat for a specified amount of time without combustion or contributing significantly to the sustenance of a fire. The ideal cable will not burn at all.
The most common insulation and jacketing material used on wire and cable is Polyvinyl Chloride, PVC. PVC has many attributes that make it a great material for general-purpose wire. Unfortunately, PVC is very flammable. When PVC burns, a key byproduct is hydrochloric acid. The smoke and residues are very corrosive. While there are several versions of PVC with varying characteristics, none are able to pass the plenum test. Some versions of PVC and another group of polymers from the family of plastics called Polyolefin may attain plenum capability when combined with certain other polymers that are more fire resistant. However, maintaining the safety margins against the plenum flame test is sometimes difficult. Construction must be highly controlled and, in some instances, cable designs that pass the test one time may not pass on another trial.
The best insulation for fire resistance to date is also one of the best dielectric materials for lower loss cables…Teflon® FEP (fluorinated ethylene propylene). Teflon FEP is a registered trademark of Dupont. But, Teflon FEP is much more expensive to manufacture which explains the higher cost of plenum rated cables. The material is tougher and more difficult to extrude. This is why plenum cables are not as flexible as PVC.
To obtain a plenum rating, the cable must pass the Steiner Tunnel Test within UL 910. The Steiner Tunnel is a specially constructed fire chamber that positions a group of cables of the same type and about 24 feet in length into a horizontal frame within an air handling plenum. Air rushes into one end of the plenum. Gas burners supply a specific level flame under the cable bundle about 4.5 feet from the end near the air inlet. While the flame is applied for a specified period of time, the length of flame travel along the cable is monitored as well as the amount of smoke produced. At the opposite end of the tunnel, a vent shaft funnels the air and smoke past photoelectric sensors. Criteria under which the test results must comply are:
- Smoke Peak Optical Density: less than 0.50
- Smoke Average Optical Density: less than 0.15
- Maximum Flame Propagation: less than 5.0 feet from point of application
The Secret Ingredient
Most all of the wire and cable insulation made in the US depends on the addition of halogens for fire retardancy. What are halogens? Halogens are the elements in group VIIa on the periodic chart (yes, you will now use some of that obscure high school chemistry). The name is of Greek origin, meaning "salt-bearing." The naturally occurring halogens are fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). Halogens are nonmetallic and closely resemble one another. They readily form bonds among themselves and with most other elements.
While PVC contains chlorine, it is not fire resistant. PVC and polyolefin products must have concoctions of other elements added in order to achieve any degree of flame resistance. Most fire resistant compounds contain fluorine or are said to be fluorinated, such as FEP (fluorinated ethylene propylene).
I Can't Believe It's Not Halogenated!
European building authorities will not allow use of halogenated cables. When halogen-based cables burn (at whatever level they will produce smoke), the smoke is corrosive and contains poisonous gases. There is high concern about the true safety of halogen-based cables. More information on the move away from halogens in cable insulation may be obtained at: www.halogenfree.org.
While the European Union designs the safety tests for that region, there is great debate over the relevance of their position on cable flame retardancy and safety versus that in the US. All cabling sold in Europe must be halogen-free. Halogen-free polymers require other formulations of compounds in order to obtain low smoke cable products.
Ratings? We Don't Need No #@&% Ratings!
The cable rating to use on a project is, first and foremost, dictated by the local building authorities. Always check on your local codes before committing to the design. Generally speaking (for the US), general-purpose cable, like CL2, is acceptable in enclosed raceways and protected regions not used as air plenums. Cables run between floors in air spaces must be riser grade minimum or else run within metal conduit. Cables run in horizontal air plenums must be plenum-rated, or run in metal conduit.
"We're The Company Without a Blimp"
Underwriters Laboratories has become something of an icon always associated with US product safety. William Henry Merrill chartered Underwriters Laboratories, Inc. in Illinois in 1901. He set up a small lab in Chicago to test electrical devices. Over the past 100 years, UL has tested more than 12,500 different types of products and employs 5,000 people worldwide. UL is a very successful not-for-profit testing organization totally supported by fees charged to clients. UL is very independent and, certainly, the most widely recognized mark in the US and in more than 70 countries.
But, UL is not the only acknowledged safety-testing authority. There are several others with equal credibility. Next to UL in the US is ETL Testing Labs. Somewhat like B.F. Goodrich, the tire company that advertised in the 60's and 70's: "we're the other company without a blimp." ETL is an internationally recognized, fully independent testing company. The ETL mark is widely recognized as equivalent to UL. All test methods and standards used by either organization are identical. Many people are not familiar with ETL, I believe, because they attained recognition as a Nationally Recognized Testing Laboratory (NRTL) in 1989…relatively recent compared to UL.
Other testing laboratories such as TUV and Dash, Strauss, and Goodhue, Inc. are well known as testing authorities. TUV began in Europe and its services have been available in the US for many years. Recently, Inchcape Inspection and Testing Services, a US corporation, absorbed ETL Testing Laboratories and Dash, Strauss, and Goodhue. However, the ETL mark survives this acquisition.
Trends in Changing Standards
The NFPA 262 (UL 910) standard is undergoing changes. Several adjustments to the procedures and calibration of the Steiner Tunnel are anticipated to affect the qualification of some plenum cables. As more and more computer network installations take advantage of plenum communications cables in air spaces above ceilings, there is new concern by fire safety officials that, over time, the buildup of old, unused plenum cables (primarily this affects the plethora of CAT 5 type communications cable) left behind as systems change is creating additional fire load that could be cause for concern. We are likely to see NFPA 262 change in this area.
In the UK, a Steiner Tunnel is being installed to allow investigations of US testing methods with fluorinated polymers compared to the European position against halogenated cables. Eventual harmonization between the US and Europe could show that our flame test is more stringent and that halogen cables are not a bad approach as long as emissions are very low. Only time and a lot of testing will tell.
Cheap At Any Price?
There is definitely a cost reason for using plenum cable in commercial installations. I don't have specific numbers on the comparison and it would vary widely due to local building codes and labor rates. The popularity of plenum-rated cable pretty well tells the story. If you have not used plenum on a job, then talk to those that have to get real numbers for your area and situation. I can say that the incremental cost of plenum is only a fraction of the cost of labor to run standard cable within metallic conduit. However, one hidden concern might be the cost to clients for the removal of old cables should code authorities require the removal of old plenum cables with system design changes. And yet, the current pace of business change essentially requires that system designs be easier and faster to install.