Requirements for fire-resistant wiring in Russia and foreign countries. An attempt at analysis.

Requirements for fire-resistant wiring in Russia and foreign countries. An attempt at analysis.

Requirements for fire-resistant wiring in Russia and foreign countries. Attempt at analysis

Federal Law 123-FZ «Technical Regulations on Fire Safety Requirements», new sets of rules and state standards on fire safety have been in force for over a year. A number of new standards containing fundamentally new fire safety requirements for cable products have been launched. For example, over the past year, the use of relatively new products — fire-resistant cables of the «ng-FRLS» or «ng-FRHF» design types — has become a common necessity. Today it has become obvious that there are some problems associated with the use of these cables. These problems are mainly due to the novelty of the regulatory requirements themselves.

This article attempts to conduct a comparative analysis of the requirements for fire-resistant wiring in Russia and in some foreign countries.

REGULATORY REQUIREMENTS FOR FIRE-RESISTANT CABLE SYSTEMS IN RUSSIA 

In accordance with GOST R 53315, the primary application area of ​​cables «ng-FRLS» and «ng-FRHF» is the installation of power supply circuits for electrical receivers of fire protection systems, operating rooms and resuscitation and anesthesia equipment of hospitals and inpatient facilities, as well as other electrical receivers that must remain operational in fire conditions. The following distinctive features of these types of cables can be highlighted:

• the possibility of use in group installations (taking into account the volume of flammable mass per 1 m of cable line);
• maintaining operability for at least 180 minutes when exposed to a gas burner flame;
• low toxicity of combustion products;
• low smoke emission;
• low corrosive activity of combustion products.

«ng-FRHF» cables should differ from «ng-FRLS» cables by even lower smoke emission and even lower corrosive activity of combustion products.

The general requirements of the GOST R 53315 standard for certification tests of «ng-FRLS» and «ng*FRHF» cables are given in Table 1. It should be noted that today there are only two mandatory cable parameters that are checked during certification tests: the combustion spread limit and the fire resistance limit. As stated in [5], GOST R 53315 was compiled for the future and, therefore, the remaining indicators will be included in the list of mandatory ones as the laboratory base of testing centers is ready. Thus, today, during certification tests of cables, the PKA, PTPM and PD indicators are not checked, and this should be remembered.

In accordance with GOST R 53315, cables «ng-FRLS» and «ng-FRHF» must withstand tests for non-propagation of combustion under conditions of group installation according to category A, i.e. when the volume of flammable mass of 1 m of cables reaches 7 l. These are the most «severe» installation conditions. The test method complies with GOST R IEC 60332. Vertically fixed cable samples are exposed to a gas burner flame for 40 minutes. After the gas burner is switched off, the length of the charred part of the cables should not exceed 2.5 m.

In accordance with GOST R 53315, cables «ng-FRLS» and «ng-FRHF» must remain operational for at least 180 minutes when exposed to a gas burner flame. These tests are carried out on a stand in accordance with GOST R IEC 60331-11. The cable sample is installed in a horizontal position during testing. To prevent deformation, one end of the sample is firmly fixed with special clamps, and the other can move freely on the support so as not to interfere with possible elongation of the sample due to temperature exposure. The cable is considered to have passed the tests successfully if after 180 minutes it retained its ability to transmit electric current.

Tests according to GOST R 53315 do not allow to fully judge the behavior of a cable product as part of a cable line under real fire conditions, when the cable is rigidly attached to the cable support structure and when the temperature affecting the cable and the cable support structure can be much higher than 750° C. Therefore, a fundamentally new standard was developed — GOST R 53316, according to which the cable line, and not an individual cable, should be tested. Test equipment and test conditions correspond to GOST 30247.0, when a so-called standard temperature regime is created inside a special furnace — the temperature increases according to a certain law and can reach more than 1100° C. A cable line, for example, a metal box or tray with cables laid in it, is placed and securely fastened inside the furnace, the cables are rigidly fixed to the cable support system. An equivalent load in the form of metal rods and chains is also applied to the cable support system. In this way, the impact of a fire on a real cable line is simulated. A cable line is considered to have passed the tests successfully if, over a specified period of time, the cables in its composition retained the ability to transmit electric power or signals. More detailed information on the methods of testing various types of cables for fire resistance can be found in the relevant standards specified in Table 1.

Table 1. GOST R requirements for testing cables «ng-FRLS» and «ng-FRHF»

It should be noted that domestic standards do not impose any specific requirements on cable products and cable-supporting structures for the time they maintain operability during tests according to GOST R 53316. The time of exposure to the standard temperature regime should be established by the developer (manufacturer) of the cable line. The obvious gaps in domestic standards include the lack of clear requirements and provisions on how exactly lines should be implemented in fire protection systems, including a clear gradation of which circuits fire-resistant cable lines should be used.

Table 2. Comparison of some cable testing standards

Russia is moving towards harmonizing its standards with international and, first of all, European standards. Table 2 provides information on cable testing standards in some European countries. Distinctive features of some of the standards are listed in Table 3.

Table 3. Distinctive features of some standards

NFPA STANDARDS AND FIRE RESISTANCE OF ELECTRICAL WIRING

The North American NFPA standards are widely known and respected in many countries around the world. It is safe to say that in the last few years, NFPA standards have been seriously modernized in terms of increasing the reliability of critical electrical wiring. To a large extent, the impetus for this was the recommendations of the NIST (National Institute of Standards and Technology). On October 26, 2005, the building and fire safety laboratory of this institute published a report that reflected the results of the analysis of the tragedy that occurred in the World Trade Center on September 11, 2001. In addition to the research results, the report provided recommendations aimed at increasing the safety of buildings and their occupants. The following recommendations were of particular importance for the cable industry:

• buildings should have a higher fire resistance limit, for this it is necessary to focus on improving the technical base;
• new methods need to be developed to improve the fire resistance of building structures, including the development and testing of new fire protection coatings and technologies, as well as active fire protection systems;
• building evacuation and emergency response needs to be improved, including fire lifts and emergency communication systems;
• the provisions and practice of applying the requirements of regulatory documents for the design, construction, maintenance and operation of all buildings need to be improved.

These recommendations had a direct impact on the development of CI-marked fire-resistant power, control, signaling and communication cables. These cables are designed to increase the survivability of security systems in buildings with large numbers of people: high-rise buildings, schools, hospitals, railway stations or transport hubs with large passenger flows. Over the next few years, corresponding adjustments were made to fire and building codes.

For example, the 2002 edition of NFPA 72 required (Section 6.9.4.3) that all portions of the electrical circuit necessary for the operation of fire alarms be protected up to the point of entry into the fire alarm zone they serve. Any of the following methods of protection were allowed:

• using a cable or cable system with a 2-hour fire resistance rating;
• using a cable enclosure with 2-hour fire resistance building structures;
• use of any other method approved by the authorized supervisory body.

In the 2007 edition of NFPA 72, the list of methods for protecting electrical wiring from the effects of hazardous fire factors was expanded (clause 6.9.10.4.2), and it became permissible to:

• use cable with the CI index (CI — cable maintains the functioning of the electrical circuit under fire conditions) with a fire resistance limit of 2 hours;
• use a cable system (electrical circuit protection system) with a fire resistance rating of 2 hours;
• enclose the cable with building structures with a fire resistance rating of 2 hours;
• use any other method approved by the competent authority having jurisdiction;
• enclose the building completely with an automatic sprinkler system constructed in accordance with NFPA 13, and install the wires or cables in metal raceways in accordance with the requirements of Article 760 of NFPA 70.

The 2010 edition of NFPA 72 has made significant changes. For example, a special separate chapter has appeared devoted to electrical wiring and communication lines, and it defines four “survivability levels” of electrical wiring:

• Level 0. Electrical wiring at this level does not require any means of ensuring survivability.
• Level 1. Wiring is classified as Level 1 if it is installed inside a building that is fully protected by an automatic sprinkler system in accordance with NFPA 13, and any cables or wires are installed in metal raceways.
• Level 2. Wiring is classified as Level 2 if it is installed in one or more of the following ways:
• the wiring is installed with fire-resistant (CI) cables with a 2-hour service life limit;
• the electrical wiring is laid in cable channels with a fire resistance limit of 2 hours (electrical circuit protection system);
• the electrical wiring is laid inside buildings or premises protected by building structures with a fire resistance limit of 2 hours.
• the electrical wiring is installed in another way that ensures a 2-hour protection limit, and this method is approved by the authorized supervisory body.
• Level 3. Electrical wiring is classified as Level 3 if it meets the requirements for Level 2 electrical wiring and is installed inside a building that is fully protected by an automatic sprinkler system in accordance with NFPA 13 standards.

Table 4. Requirements for the survivability of electrical wiring in ECS* systems

In fairness, it should be noted that NFPA 72*2010 reflects more precise requirements for the use of certain types of cable lines in fire protection systems compared to Russian standards. For example, Table 4 presents summary data on the requirements of NFPA 72-2010 for the survivability levels of cable lines of fire alarm and emergency communication systems (ECS – Emergency Communication Systems). Such an approach allows for a more flexible approach to the selection of cables and their installation methods, eliminating in advance unnecessary requirements for the fire resistance of cable lines, for example, in small buildings.

According to NFPA 72-2010, fire alarm wiring, including all circuits controlled and supplied by the fire alarm system, must be performed in accordance with Article 760 of NFPA 70 (NEC). NFPA 70 is an analogue of the domestic «Electrical Installation Code» and contains detailed requirements for the methods of performing various electrical circuits. The NFPA 70 and NFPA 72 documents are closely interrelated and abound in specific references to each other, as well as to a number of other important fundamental standards (for example, to the UL series standards — analogues of the domestic GOST and GOST R).

Table 5. Fire-resistant products classified in UL* catalogues

NFPA 70 defines «fire alarm» as the fire detection system, the warning and emergency communications system, the sprinkler system, and the control rooms. Circuits controlled and supplied by the fire alarm system include circuits that monitor and control the building's safety functions, elevator lockout and lowering circuits, smoke and fire door control circuits, and ventilation shutoff circuits. It is clearly stated that in order to ensure the required survivability indicators (including resistance to fire), electrical wiring must be installed in accordance with the published instructions of the manufacturers.

TESTING FIRE-RESISTANT ELECTRICAL WIRING IN THE USA 

Possible methods for constructing fire-resistant electrical wiring and methods for their certification tests are defined in the NFPA 70 standards. Test methods are usually described in the relevant UL series standards.

The test samples include not only the electric cable, but the entire range of devices necessary to create a complete cable line, such as cables, channels, cable trays, pipes, protective materials and also products intended for fixing cable systems on a horizontal or vertical surface. Protective materials for cables may consist of structural elements of the building, such as walls or shafts, or may simply be the cable sheath material, as is the case with cable products with the CI index.

Fire resistance of building assemblies is tested in accordance with UL 263. This test method involves erecting a building assembly (e.g. a wall) around the area of ​​electrical wiring to be protected. When testing wall building assemblies, the size of the test specimen is approximately 3 x 3 m. During a specified period of time, the average temperature of the wall surface remote from the fire shall not exceed 120° C. The temperature of individual areas on this wall surface shall not exceed 160° C, and no combustion shall be observed. Following the fire test, the building assembly shall undergo a water stream test from a hose. Water is supplied through a nozzle with a diameter of 28 mm under a pressure of 200 kPa at an approximate flow rate of 12-13 l/s. The pressure of the water jet on the surface of the test specimen is approximately 400 kPa. The structure is considered to have passed the test if the water jet does not penetrate it.

The fire resistance of cable support systems is tested in accordance with UL 1724. This method was developed for cable systems where fire protection is provided by a special barrier, which can be the outer sheath of an electrical cable raceway or, for example, a wall if the raceway is laid inside a wall. The tests simulate the operating conditions of electrical cables by using a single uninsulated stranded wire with a cross-section of approximately 8.4 mm2 (8 AWG). Thermocouples are connected to the wire every 150 mm along the length of the wire. The temperature measured by these thermocouples depends on the fire resistance characteristics of the housing (casing) inside which the copper wire is enclosed. The fire resistance index (e.g. 1 or 2 hours) is based on the ability of the housing or barrier to limit the average temperature increase along the entire length of the wire to 120 ° C and to limit the temperature increase at any point along the wire to 160 ° C. Similar limiting temperature values ​​are also included in the testing methods for building structures and barriers.

In accordance with UL 1724, a water jet test must be performed after the fire test is completed. The barrier or sheath has passed the test if the water jet does not penetrate it and the conductor does not become visible.

If the fire barrier is provided by the cable jacket itself (CI cables), then the test method described in UL 2196 is used. In this case, the cable line is laid inside a fire oven with dimensions of approximately 3 x 3.5 m. A current of 0.25 to 0.50 A is passed through the cable, the test voltage corresponds to the maximum permissible working voltage for this cable. Then the cable is exposed to fire for up to 2 hours. During this time, the lamps powered by this cable must glow. The temperature of the flame affecting the cable reaches 538 ° C after 5 minutes and 1010 ° C after 2 hours.

After the cable has successfully passed the fire test, it is de-energized. It is then removed from the furnace and immediately exposed to a stream of water. The test sample is then energized again. The cable is considered to have passed the test successfully if the lamps powered by it glow.

CLASSIFICATION OF FIRE-RESISTANT PRODUCTS IN UL CATALOGS

An undoubted advantage of UL standards is the open and accessible information about the products they classify. On the database.ul website you can find detailed information about how to implement certain systems designed to protect electrical wiring from fire.

Fire-resistant assemblies come in many different forms. It is important to note that the entire building assembly, such as a wall or floor, is evaluated, not its individual components. UL standards define fire-resistant assemblies as products in Category BXUV. This category includes fire-resistant building assemblies consisting of walls and individual components, floor slabs, roof elements, beams, and columns. These systems are published in UL Fire-Resistive Product Catalogs as assembly drawings detailing all components and assembly building materials required to achieve the assigned fire-resistance ratings.

Certified circuit protection systems typically consist of flame-resistant materials that enclose the wiring system and fasteners, or cables with their own fire-resistant components (e.g., CI cables). The performance of these protection systems is assessed under fire and water flow conditions. Such systems are classified by UL as FHIT products. They are published in UL Fire-Resistant Catalogs as assembly drawings detailing all components required for installation. FHIT products are designed to be installed as protection for special wiring systems and ensure that the integrity of the electrical circuit is maintained when exposed to external flame.

Another method of protecting electrical wiring from fire is the use of certified thermal barrier systems. These systems are typically constructed of heat- and fire-protective materials wrapped or laid around the conductors and fasteners to be protected. They prevent heat transfer above a certain temperature during fire tests. These systems are certified and classified by UL as XCLF products. They are also published in UL* fire-rated catalogs as sectional assembly drawings with detailed descriptions of all components for installation. As with FHIT products, for XCLF products, the fire resistance ratings (fire indexes) apply only to the entire protective assembly as defined in each UL design drawing. Fire resistance indices are not assigned to individual components supplied as stand-alone products and are not interchangeable between different systems.

CONCLUSIONS

It is obvious that the Russian regulatory framework for the implementation of fire-resistant electrical wiring needs serious revision. Based on examples of comparison with foreign standards, it can be assumed that the primary steps to adjust the domestic regulatory framework should be aimed at:

• detailing the requirements for fire-resistant electrical wiring in security systems in various buildings;
• defining specific methods for implementing fire-resistant electrical wiring;
• creating an open resource where assembly drawings and instructions will be officially published in the same way as is done in UL.

LIST OF SOURCES:

1. Federal Law of July 22, 2008 No. 123-FZ «Technical Regulations on Fire Safety Requirements».
2. GOST 30247.0*94 Building structures. Fire resistance test methods. General requirements.
3. GOST R 53315*2009 Cable products. Fire safety requirements.
4. GOST R 53316*2009 Electrical panels and cable lines. Maintaining operability in fire conditions. Test methods.
5. Smelkov G.I. Fire safety of electrical wiring. – M.: “CABLE”, 2009. – 328 p.
6. NFPA 70*2008 (NEC) National Electrical Code.
7. NFPA 72*2010 National Fire Alarm and Signaling Code.
8. UL 263 Fire Test of Building Construction and Materials.
9. UL 1724 Fire Tests for Electrical Circuit Protective Systems.

UL 2196 Standard for Tests of Fire Resistive Cables.

D. Yakunkin tech. Director of the company «Status-Svyaz»

Journal «Security Algorithm» No. 5, 2010