Cable designed to function electrically during fire emergencies.
One of the original industrial cable tray fire tests in the United States was developed by the Philadelphia Electric Company in the 1960s. It consisted of a vertical galvanized steel cable tray loaded with cables. The flame source was a folded piece of burlap cloth soaked in transformer oil. Failure mode was the propagation of flame to the top of the tray. The test was not reliably reproducible because of the flame source variables. The Institute of Electrical and Electronics Engineers (IEEE) released Standard 383 for Class IE Electric Cables, Field Splices and Connections for Nuclear Power Generating Stations in 1974. This standard included the burlap/oil flame source, but also introduced a 254 mm (10 inch) wide ribbon burner that used natural gas or propane. The flame source was very reliable and produced a uniform flame temperature of about 815[degrees]C (1,500[degrees]F) and a heat flux of 70,000 btu/hr. This was quickly adopted as a standard. Over time, modifications to the vertical cable tray fire test included Canadian Standards Association FT-4 (IEEE 1202) which increased the tray loading of cables, reduced flame propagation allowed, and provided more flame impingement on the cables. The International Electrotechnical Commission (IEC) standards include IEC 60332-1, 2 and 3. These tests cover single conductors, single cables and a variety of cable tray loading levels.
The performance criteria for all of the above tests are restriction of vertical flame propagation. The flame sources and test temperatures are generally similar.
Early circuit integrity testing
In the early 1970s, Dekoron began receiving customer requests for restricted flame propagation, plus continued electrical function of cables during a fire (circuit integrity). Standard PVC insulated and jacketed cables, and XLPO insulation with Hypalon jackets, provided very good control of propagation, but had electrical life of onlyl-5 minutes in an IEEE 383 Fire Test. Dekoron initially developed a cable that had three very thick PVC jackets with a fiberglass tape between jackets. This type of construction could last as long as 10 minutes in the IEEE 383 fire test; however, the additional barriers only delayed the ultimate electrical failure.
Review of circuit integrity cable fire test standards
The 1974 IEEE 383 Vertical Cable Tray Fire Test Standard was chosen by Dekoron for its development test standard. The test was reproducible, the flame temperature was 815[degrees]C (which at the time seemed more than adequate), and had a 70,000 btu/hr, heat flux. Dekoron was able to electrically monitor five cables during each test. The test was severe enough to quickly eliminate marginal performing insulations and jackets. Dekoron applied a 240 Vac potential between conductors with an electrical failure indicated by a light bulb. The initial goal was 15 minutes of electrical life and flame propagation restricted to 1,830 mm, the IEEE standard.
The IEC introduced Standard 331 (Fire Resisting Characteristics of Electrical Cables) in 1970. This test was designed specifically for circuit integrity (electrical life) testing of cables. The original 331 configuration tested a single horizontal cable in a burner 610 mm (24 inches) long. The burner was directly under the cable sample. Test time was specified at three hours with a flame temperature of 750[degrees]C (1,380[degrees]F). The heat flux was not specified and the mode of failure was three amp fuses with a voltage potential of the cable rating. This standard is now designated IEC 60331 and has a number of editions including optical fiber cables.
The British Standard 6387 is also specifically designed for circuit integrity testing. The procedure uses the IEC 60331 test burner. The standard test time is 20 minutes at 950[degrees]C. There are options for three hours at 950[degrees]C, for a water sprinkler during the test, and for a mechanical shock test at the end of flame application. The shock and water simulate cable tray movement during a fire and sprinkler activation. This is a test that creates actual fire emergency conditions.
Underwriters Laboratories Standard 1709 "Rapid Rise Fire Tests of Protection Materials for Structural Steel" is designed to test the insulation used to protect the structural steel in high rise buildings. This test standard specifies a closed furnace and reaches a temperature of 1,100[degrees]C (2,000[degrees]F) within five minutes. The heat flux is 65,000 btu/ft.2-hr. Several refineries in the United States have begun investigating this test as a possible standard for refinery hydrocarbon pool fires where flame temperatures of 1,100[degrees]C are typical. This test is reproducible because the closed furnace controls flame and uniformity of temperature over the cable. This test requires at least 27% nickel copper conductors. Copper melts at 1,078[degrees]C which is less than the test temperature. Pure nickel melts at 1,454[degrees]C.
Dekoron participated in the development of a modified UL 1709 that uses cable in an open tray and in conduit. The test reached 1,100[degrees]C within five minutes, and had a heat flux of 65,000 btu/ft.2-hr. The only difference between this modified test and the UL 1709 standard was open tray vs. closed furnace.
Cable failure modes in circuit integrity testing
In 1980, Dekoron began reviewing failure modes of different cable designs and installation methods affecting electrical life. This review produced the following conclusions:
* Insulation melts and allows conductors to move and make contact (thermoplastic materials like polyethylene);
* insulation is consumed and ash becomes conductive (thermoplastic and thermoset materials like PVC and XLPE that have halogen flame retardant additives);
* insulation ash allows conductors to move and make contact (silicone designed without attention to the structural integrity of the ash);
* insulation ash is halogen poisoned (silicone insulation paired with halogen containing jackets like PVC, CPE or CSPE (Hypalon)) halogen poisoning makes the silicone conductive;
* insulation ash loses resistance with elevated temperature (silicone insulation not designed and formulated to have good electrical properties at flame test temperatures);
* barrier technology adds thermal insulation to protect cable circuits from flame and heat, delays failure point (multiple tapes or jackets, intumescent coatings on cables, tray wrapping with thermal insulating materials); and
* non-burning inorganic materials (MI or mica) yield cables that are stiff, difficult to terminate, and have limited constructions available (mineral insulated is a superior performing cable construction that is expensive, has limited length and available constructions, and can have serious failures if terminated incorrectly; moisture penetrating the end terminations can create a cable failure).
Current materials and constructions for circuit integrity cable design
Mica tape is an inorganic mineral (phyllosilicate) that has excellent dielectric properties and mechanical strength, and good thermal endurance. There are two main types of mica used in industrial applications, phlogopite and muscovite.
Muscovite has better dielectric and mechanical strength and is harder than phlogopite. Phlogopite has a lower dielectric strength, but has a higher calcinations temperature of 800[degrees]C. (Calcination is the temperature at which a thermal decomposition begins.) The muscovite has a calcination temperature of 600[degrees]C.
Cables made with mica tapes wrapped directly over the conductor typically use an extruded material like polyethylene, XLPE or EPR over the mica for physical protection during subsequent manufacturing. To improve the performance of these tapes at higher temperatures, two and even three layers are applied before the extruded polymer insulation is added. This adds substantially to the cost and manufacturing time. Ultimately, mica can not perform at the 2,000[degrees]F (1,100[degrees]C) temperatures that the petroleum industry is looking for. The E-glass binder tape supporting the mica flakes has an upper temperature limit of 593[degrees]C (1,100[degrees]F) and it can become conductive at approximately 1,010[degrees]C (1,850[degrees]F). At this temperature, the mica tape can stop performing as an electrical insulating barrier.
Tray wrap and other mechanical protection covers cable trays and provides a thermal barrier to the fire, theoretically protecting standard cable materials and constructions and allowing electrical life in a fire. These can include metal-foil covered insulation, intumescing sprays and flame retardant thermal covers. The disadvantages of these types of protection include inability to easily add or remove cables from trays, cost, long term resistance to the environment (UV exposure and moisture), ampacity derating requirements and maintenance issues.
The United States Nuclear Regulatory Commission (NRC) tested an electrical raceway tray wrap fire barrier at Southwest Research Laboratories in Austin, TX. Cable trays wrapped with the fire barrier materials were subjected to flame and the results indicated non-compliance with the NRC requirements with respect to temperature increase inside the tray. The banding materials failed and allowed openings in the tray wrap system.
Mineral insulated (MI) uses an inorganic magnesium oxide or silica powder packed around conductors and then encapsulated in a metal sheath. This product has superior performance in a fire, but has limitations in length, constructions available, cost and termination difficulty. The MI cable can fail if it is terminated incorrectly and water penetrates the end of the cable.
Dekoron material development
Dekoron's initial work focused on thermoset silicone rubber as the base insulation material. This polymer is unique because it forms a non-conductive char when subjected to flame. Commercially available silicones are very soft with little physical strength. Other wire manufacturers added glass braid and nylon over the silicone to provide physical protection during the twisting and cabling manufacturing process.
Dekoron experimented with an alloy of silicone and polyethylene. This material is crosslinked by electron beam irradiation. The resulting material has very good tensile strength and elongation and can be twisted and shielded without any additional physical protection. The cables can be installed and terminated like XLPE or PVC insulation.
The Dekoron material development program resulted in the evaluation of over 60 different formulations. Material chemists compounded small batches on an internal mixer and made material slabs that would be crosslinked by irradiation. Dekoron developed a test fixture in an oven with a data logging system to monitor electrical resistance vs. temperature.
Each formulation was tested for physical properties and electrical insulation resistance (IR) at temperature. Figure 1 shows a typical graph. This 14G4 is our current standard material. Insulation resistance at 482[degrees]C (900[degrees]F) is about 10E9 ohms. This falls to 10E4 ohms at 1,100[degrees]C (2,000[degrees]F). Slab thickness was 3.2 mm (0.125 inches).
[FIGURE 1 OMITTED]
Figure 2 shows this material monitored for capacitance over the test temperature range. The capacitance has a spike at about 482[degrees]C (900[degrees]F) and then gradually increases. The spikes in all the graphs occurred when the material reached an auto ignition temperature.
Additives that were tested included both muscovite and phlogopite mica
compounded into our standard silicone alloy insulation. Figure 3 shows insulation resistance of the two mica formulations in comparison to the 14G4 standard and an experimental X104-32 silicone formulation. The muscovite with the lower calcinations temperature has an order of magnitude lower IR than the other materials at 1,038[degrees]C (1,900[degrees]F).
Subsequent material development work included an additive of micro glass beads and a ceramic nanotechnology. The glass beads had early promise, but large batch compounding and production extrusion crushed the beads and negated their effect. The ceramic filler was difficult to compound and very expensive. Its physical properties were marginal, although it had superior electrical insulation resistance at elevated temperatures.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
The current state-of-the-art circuit integrity insulation is a ceramifiable silicone that uses a blend of vitreous materials which have a very broad melting range. The lowest melting components begin to melt at around 350[degrees]C. At about 800[degrees]C special ingredients in the formulation begin to "devitrify." They pass from a vitreous state to a crystalline state.
The ceramifiable additive imparts fire barrier and smoke suppression properties in composite materials by the following mechanisms:
* When heated beyond its activation temperature of around 350[degrees]C, the low melting point components within the formulation begin to melt, causing vitreous material to flow around the burning resin;
* the resulting "encapsulation" restricts the access of oxygen to combustible materials and restrains carbonaceous decomposition products from being emitted as smoke;
* at higher temperatures of around 750-850[degrees]C, components in the formulation "devitrify." They pass from a glassy state to a crystalline state;
* then the ceramifiable additive acts as a high temperature adhesive, bonding the composites together. In effect the composites then behave like a ceramic material; and
* this system retains much of the mechanical strength of the composite which would otherwise have been lost at these extreme temperatures.
Dekoron circuit integrity cable performance
Standard Dekoron circuit integrity cables pass the following tests:
* IEEE 383 70,000 BTU/hr. flame, 1,500[degrees]F, 60 minute electrical life with minimal propagation;
* IEC 331 cable maintains circuit integrity for three hours at 750[degrees]C. Cable has also passed one hour at 1,000[degrees]C and 15 minutes at 2,000[degrees]F (1,100[degrees]C);
* BS 6387 S, W, Z cable maintains circuit integrity for 20 minutes at 950[degrees]C and is then subjected to mechanical shock and water spray (sprinkler) with electrical integrity maintained;
* Modified Underwriters Laboratories 1709. Dekoron cables maintain circuit integrity in this test environment at 1,100[degrees]C for 20 minutes in both open tray and conduit. Cable modifications for this test include 27% nickel copper conductors and two layers of a fiberglass fire barrier tape.
Final cable design
Properly designed circuit integrity cables must have the following characteristics:
* A mechanically stable and non-conductive ash system that forms after exposure to fire. This ash physically keeps the conductors separated;
* a silicone insulation system that has good electrical properties at fire test temperatures;
* a non-halogenated jacket that forms a stable char during a fire to help protect the insulated conductors in the core. This type of jacket prevents the silicone from becoming conductive from halogen poisoning. The jacket must also have UV (sunlight) resistance for outdoor installations;
* construction components that will not propagate fire to other areas;
* an insulation system that is moisture resistant. This type of cable is suitable for outdoor installations in cable trays and in conduit that could contain water; and
* a cable that has an agency listing (Underwriters Laboratories, etc.) to ensure compliance with industry standards.
by Stan Stephan, Dekoron Wire and Cable
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|Date:||May 1, 2010|
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