Understanding the difference between the fire test methods used to establish fire resistive ratings for electrical
components is key to a good design
Electrical system designers have at their disposal several types of fire resistive systems that comply with specific Code requirements. It can be helpful to understand what differentiates them. But just knowing what to install isn't enough. Once these systems are up and running, it's also important to know how well they're going to perform.
Electrical power is essential for the operation of fire safety-related equipment like pumps, signaling equipment, elevators, alarms, and industrial process control equipment. The NEC acknowledges the importance of maintaining the circuits' functionality by requiring protection against fire, structural failure, or operational accidents in Art. 695 for fire pumps, Art. 700 for emergency systems, and Art. 760 for fire alarm systems.
These NEC sections refer to listed systems, yet UL refers to classified systems. While the terminology is different, classification complies with the definition of listed as detailed in Art. 100 of the NEC. For purposes of complying with the Code, the two should be considered equivalent.
Fire pumps. The requirements for fire pump supply circuit conductors on the load side of the final disconnecting means and overcurrent device can be found in 695.6(B) of the 2002 NEC. The Code identifies the following three methods for providing protection to the circuit. Such circuits must be:
Encased in a minimum 50 millimeters (2 inches) of concrete.
Located within an enclosure dedicated to the fire pump circuit(s) and have a minimum 1-hour fire resistive rating.
Listed electrical circuit protective systems with a minimum 1-hour fire rating.
The protection provided by these various provisions may not be equal in terms of the thermal barrier provided to the electrical circuit or the resistance of the protection to impact loads during a fire.
Fire-resistive barriers specified in the second method above include many forms of construction. It's significant to note that the entire wall assembly or floor-ceiling assembly is rated and not a component of the assembly, unlike a ceiling membrane or a layer of gypsum board. The fire test method used to establish these fire resistive ratings for building assemblies is defined in three standards: UL 263, ASTM E119, and NFPA 251. The testing method and acceptance criteria in each of these standards are essentially equal.
Each of these fire test standards limits the temperature rise and prevents flames on the surface of the assembly away from the fire for the rating period. However, they don't limit the temperature within an assembly, nor do they prevent flames within an assembly.
The electrical circuit protective systems specified in the third method are typically enclosures installed around cable raceways and their supports. These protective systems provide an insulating barrier to the electrical circuit. The fire resistive performance of the protective systems are determined by tests conducted in accordance with UL's Outline of Investigation Subject 1724, Fire Tests for Electrical Circuit Protective Systems (Photo 1). The fire exposure conditions described in each of these documents (UL 263, ASTM E119, NFPA 251, and UL Subj. 1724) are essentially the same.
The electrical circuit protective systems specified in the third method provide a thermal barrier as measured from the surface of the barrier exposed to the fire to the electrical conductor located within the barrier or wrapping.
A fire resistive assembly's ability to resist impact loads is determined by the application of a water hose stream (Photo 2). With respect to the resistance to impact loads, the performance of the 50 millimeters (2 inches) of concrete enclosure is evident.
The test standards associated with the second method require that a hose stream be applied to 1-hour rated walls after a minimum 30-minute fire exposure, but no hose stream is required to be applied to floor-ceiling assemblies. When the third method is used, the UL test procedure requires a hose stream to be applied after the rating period.
Emergency systems. Emergency electrical systems must operate properly during a fire so occupants can safely evacuate a building, according to 700.9(D)(1) of the NEC. It identifies the following six methods to provide a level of fire protection for the feeder-circuit wiring. The wiring must be:
Installed with buildings that are fully protected by an approved automatic fire suppression system.
A listed electrical circuit protective system with a minimum 1-hour rating.
Protected by a listed thermal barrier system for electrical system components.
Protected by a fire-rated assembly listed to achieve a minimum fire rating of 1 hour.
Embedded in not less than 50 millimeters (2 inches) of concrete.
A cable listed to maintain circuit integrity for not less than 1 hour when installed in accordance with the listing requirements.
Upon comparing the protection methods cited for fire pumps and emergency systems, you'll find that the three methods for fire pumps are essentially the same as the second, third, fourth, and fifth methods specified for emergency systems. Although not stated, it's assumed the level of protection provided by the third method for emergency systems is 1 hour. It's also interesting to note that the first method for emergency systems isn't specified for fire pumps because fire pumps are required to operate automatic fire suppression systems.
With respect to the sixth method for emergency systems, the electrical cables are tested in accordance with UL 2196, Standard for Tests for Fire Resistive Cables. For this test, the electrical cable system is exposed to the same fire as the other protective systems (Photo 3). After being exposed to fire, the cable system is subjected to the impact load from a water hose stream. The acceptance criterion is based on the electrical cable system's ability to maintain circuit integrity. The maintenance of circuit integrity is based on the the cable's ability to transmit current at rated voltage and low amperage during the fire and after the hose stream exposures.
Fire alarm systems. Art. 760 of the NEC references power-limited fire alarm cable and nonpower-limited fire alarm cable. These cables are Listed by UL in accordance with the requirements in UL 1424, Cables for Power-Limited Fire Alarm Circuits, and UL 1425, Cables for Nonpower Limited Fire Alarm Circuits. For both types of fire alarm cables, Art. 760 provides guidance to ensure survivability of fire alarm circuits under fire conditions. This Article states that in order to survive the fire, alarm cable shall be listed as circuit integrity (CI) cable. One method of defining CI cable is by establishing a minimum 2-hour fire resistive rating for cable when tested in accordance with UL 2196.
Art. 760 also refers to NFPA 72, National Fire Alarm Code, in Fine Print Notes (FPNs) for 760.31(F) and 760.71(G) for guidance in providing survivability. NFPA 72 includes various recommendations regarding adequate separations between the outgoing and return cables. In NFPA 72, the following methods are also identified as a means to provide protection to fire alarm circuits. The circuits must be:
Located within a 2-hour rated cable assembly.
Installed in a 2-hour rated shaft or enclosure.
Installed in a 2-hour rated stairwell in a building equipped with sprinklers in accordance with NFPA 13, Standard for the Installation of Sprinkler Systems.
Upon comparing these methods to these recommended for fire pumps and emergency systems, you'll notice that they're similar but that the fire endurance ratings have increased from one hour for fire pumps and emergency systems to two hours for fire alarm systems.
Fire tests. The fire performance requirements contained in Art. 695, 700, and 760 of the NEC require several fire tests. Except in the case of the test designed for building assemblies, they're intended to measure the ability of electrical cables to remain functional by subjecting the electrical cables and their protective enclosures to the environmental conditions that could occur during fully developed fires. The fire tests attempt to use cable samples of sufficient length so the structural performance of the electrical cables together with their enclosures and support systems are also evaluated. These fire-related tests are conducted in addition to those tests traditionally conducted to measure the performance of electrical cables during ambient conditions.
The test samples, which measure about 3 meters (approximately 10 feet), aren't limited to just the electrical cable. They consist of the complete wiring method system, including items like cable trays, conduits, protection materials, and the means used to support the systems to a horizontal or vertical mounting surface. The protection materials for the cables may consist of building assemblies like walls or shafts, or they could consist of the cable jacketing material, as is the case with CI cable.
The test methods identified in Art. 695, 700, and 760 measure the performance of building assemblies, barrier/wrap protection, and cable jacket protection.
Building assemblies — This test method anticipates the construction of a complete building assembly, such as a wall, around the safety-related electrical circuit. The test method is described in UL 263 (ASTM E119, NFPA 251) (Photo 4). For wall assemblies, the sample is typically 10 feet by 10 feet. At the rating period, the average temperature rise on the surface of the wall away from the fire is limited to 250°F with no individual temperature rise exceeding 325°F. Flaming on the surface of the wall away from the fire is prohibited.
Barrier or wrap protection — This test method is designed for cable systems where the fire protection is provided by a barrier that surrounds the electrical raceway, such as a blanket, or where the electrical raceway is placed within a building assembly, such as a wall. The test is described in ASTM E1725, Standard Test Methods for Fire Tests of Fire-Resistive Barrier Systems for Electrical System Components, and UL's Outline of Investigation Subject 1724, Fire Tests for Electrical Circuit Protective Systems.
The test method uses a single bare 8 AWG stranded copper wire in place of a standard covered conductor to simulate electrical cables. The temperatures measured by the thermocouples attached to the conductor determine the fire resistive performance of the enclosure that surrounds the copper conductor. Ratings, such as 1-hour or 2-hour, are based upon the ability of the enclosure or barrier to limit the average temperature rise along the bare copper conductor to 250°F and limit the temperature rise at any point on the conductor to 325°F.
Note the similar temperature limits included in the test methods for building assemblies and barriers for cables and where these temperatures are measured.
Cable jacket protection — This test method is used when the cable jacketing provides the fire resistive barrier. The test is described in UL 2196. When a test is conducted in accordance with Standard UL 2196, the fire resistive cable is part of the test sample in lieu of the bare copper conductor. A minimum of 3 meters of the fire-resistive cable must be routed through the test furnace. For this test, the electrical cable is exposed to the same fire as the other protective systems, but the acceptance criteria is based upon the cable's ability to remain functional.
During the fire test, the electrical cable is energized by applying an electrical load equal to the cable's rated voltage or the cable's utilization voltage under a low current ranging from 0.25A to 0.50A. The utilization voltage is defined as the cable's maximum voltage anticipated during normal usage, such as typical line voltages of 120V, 240V, and 480V.
The electrical cable is connected to monitoring lamps that are used to verify circuit integrity. Prior to entering and upon exiting the furnace, the current is continuously monitored to provide leakage current data for the cable within the test furnace. Before and after the fire exposure, the installation resistance of the jacketing material is determined.
For CI cables used in fire alarm systems, UL 2196 includes specific mounting methods for the test sample. Steel rings spaced 61 centimeters (24 inches) on center support the cables, without any supplemental barrier or wrapping material.
Each of the three test methods includes a fire exposure designed to represent a fully developed fire that would be anticipated in a commercial or residential building. Temperatures in the furnace chamber for this fire exposure reach 1,000°F at 5 minutes, 1,700°F at 60 minutes, and 2,000°F at 4 hours.
The two test methods designed to measure the performance of barrier/wrap protection or cable jacket protection also include a fire exposure intended to represent a hydrocarbon pool fire, such as burning kerosene, oil, or alcohol. Temperatures in the furnace when this fire is represented reach 2,000°F at 5 minutes and maintain this temperature throughout the duration of the fire exposure, which may be as many as 4 hours.
Hose stream tests — The hose stream test has been part of the fire endurance rating system of building assemblies for many years. Its primary function is to measure an assembly's ability to withstand impact loads.
For fire pumps, the test standards associated with the second method (UL 263, ASTM E119, NFPA 251) require a hose stream test be applied to 1-hour rated walls after a minimum 30-minute fire exposure. The hose stream test isn't required to be applied to floor-ceiling assemblies. Past performance is the reason it's not applied to floor-ceiling assemblies. This acceptable performance is understandable because floor-ceiling assemblies must support a load uniformly applied over the surface of the floor.
With respect to the third method, the UL test procedure (Subject 1724) requires a hose stream test to be applied after the rating period, whereas the ASTM standard (ASTM E1725) doesn't require hose stream test performance at all.
The hose stream test specified in the second and third methods consists of water being delivered through a 1.125-inch play pipe nozzle at a pressure of 30 pounds per square inch at a flow rate of about 200 gallons per minute. The impact pressure applied to the test sample is about 58 pounds per square foot.
For acceptable performance, the water stream can't penetrate a protective barrier or wrapping and cause the electrical cable to become visible, nor is the water stream permitted to pass through a building assembly.
The hose stream test applied to cables intended for emergency systems is identical to the test applied to cables intended for connection to fire pumps.
Electrical cables tested in accordance with UL 2196 are subjected to the impact and cooling effects of a hose stream test immediately after the fire exposure. Two levels of the hose stream test are available. For CI rated cables as defined in the NEC, the water stream is applied through a 1.5 inch fog nozzle at a minimum discharge of 75 gallons per minute. The impact pressure applied on the CI cables is about 0.2 pounds per square foot.
To meet the acceptance criteria of UL 2196, the cables must remain functional during the fire exposure and after the hose stream tests. Functionality is determined by illumination of the monitoring lamps. Ratings like 1-hour or 2-hour relate to the duration of the fire exposure.
Conductor ampacity considerations. When using any type of enclosure or barrier system to provide the fire protection, you must also consider the NEC's Art. 310, Conductors for General Wiring, regarding conductor ampacity.
The use of a barrier or enclosure could reduce the allowable conductor ampacity because the barrier acts as an additional thermal jacketing to the conductor. It may be necessary to derate the conductor ampacity. The significance of this effect is dependent upon the loads being carried by the conductor and the anticipated duration of the operation of the equipment connected to the conductor.
If the UL Classified system doesn't indicate the ampacity reduction due to the electrical circuit protection system or the thermal barrier system for normal ambient conditions, then the effect of the system on the electrical conductor ampacity hasn't been investigated and should be determined in the field.
Art. 695 for fire pumps, Art. 700 for emergency systems, and Art. 760 for fire alarm systems recognize the need to maintain electrical circuit integrity during a fire so that electrical power isn't interrupted to critical circuits under fire conditions. Products under UL's Follow-Up program, such as power-limited fire alarm cable with the CI marking (HNIR), nonpower-limited fire alarm cable with the CI marking (HNHT), electrical circuit protective systems (FHIT), thermal barrier systems (XCLF), fire-resistive assemblies (BXUV), and fire-resistive cables (FHJR), provide a means to meet those Code requirements.
Berhinig is a principal engineer working for the Fire Protection Division of Underwriters Laboratories, Northbrook, Ill.
Product Directories. A summary of the rated building assemblies and barrier/wrap protection options is published in UL's Fire Resistance Directory. The summary includes the fire rating and a description of the features of interest to the authority having jurisdiction, architects, and the specifier. The Fire Resistance Directory is available at www.ul.com, under “Certifications” and then “Fire Resistive Assemblies and Systems.”
Listed fire alarm cable is located under the UL product categories Power Limited Fire Alarm Cable (HNIR) and Nonpower-Limited Fire Alarm Cable (HNHT). Fire alarm cable that also meets the requirements for CI cable is marked “CI (max voltage___).”
These as well as all UL product categories can be located by the four-letter code in parentheses following the product category title, in this case HNIR or HNHT. This four-letter code is the UL product category guide designation, also known as the CCN or category code. The guide information and Listings for these product categories can be accessed on UL's Online Certification Database at www.ul.com/database. Enter the code at the “UL Category Code/Guide Information” search. This information is also located in the UL General Information for Electrical Equipment Directory (the White Book) and the UL Electrical Construction Equipment Directory.