Fire Properties of Cables

Standards relating to fire properties of cables

IEC, BS standards

This is an area of increasing public and legislative concern, and therefore of increasing interest to engineers. There have been major advances in the fire performance of cables in recent years, and table below lists some of the relevant standards.

Standard	Description
IEC 60331	Fire resisting characteristics of electric cables.
IEC 60332	Tests on electric cables under fire conditions. Test methods and flame propagation of power and control/communication cables.Note the identical EN60332 and equivalent national standard BSEN60332 supersede EN50265 and BS 4066.
IEC 60754	Test of gases evolved during combustion of electric cables.
IEC 61034	Measurement of smoke density of cables burning under defined conditions. Identical EN61034 and national equivalent BSEN 61034 supersede EN50268 and BS7622.
BS 6387	Performance requirements for cable required to maintain circuit integrity under fire conditions.
BS 6724	Electric cables. Thermosetting insulated, armoured cables for voltages of 600/1000 V and 1900/3300 V, having low emission of smoke and corrosive gases when affected by fire.
BS 7211	Electric cables. Thermosetting insulated, non-armoured cables for voltages up to and including 450/750 V, for electric power, lighting and internal wiring, and having low emission of smoke and corrosive gases when affected by fire.
BS 7835	Specification for cables with cross-linked polyethylene or ethylene propylene rubber insulation for rated voltages from 3800/6600 V up to 19 000/33 000 V having low emission of smoke and corrosive gases when affected by fire.
EN 50267	Common test methods for cables under fire conditions. Tests on gases evolved during combustion of materials from cables. Apparatus. BSEN50267 is identical and supersedes BS6425. Similarly French standard NF C 20-454 is superseded.

Toxic and corrosive gases

It is recognized that conventional flame retardant cables having sheathing based upon PVC type materials evolve considerable quantities of halon acid gases such as hydrogen chloride upon burning.

Such materials are not therefore suitable for use in confined spaces where the public are likely to travel, and moreover the fire in the ENEL power station at La Spezia in 1967 showed that in certain circumstances PVC cables will burn completely and contribute to the spread of a fire.

Materials have now been developed for cable oversheaths and bedding which are normally free of halogen based compounds. They consist of a mixture of inorganic filler such as aluminium hydroxide and polymers such as ethylene vinyl acetate, acrylates and ethylene propylene rubbers.

Cables manufactured with such materials are known as ‘Low Smoke and Fume’ (LSF) and have acid gas evolution less than 0.5% in comparison to 25–30% for PVC compounds.

IEC 60754-1 specifies a method of determining the amount of halogen acid gas, other than hydrofluoric acid, evolved during combustion of halogen based compounds. The method essentially measures the existence of halogen acid greater than 0.5%, the accuracy limit for the test.

Therefore cables tested having less than the 0.5% limit are generally termed ‘zero halogen’ or ‘low smoke zero halogen’ (LS0H).

Smoke emission

Normal cable sheathing compounds also give off dense smoke when burned and this is of particular concern in underground transport system installations. The generation of large amounts of smoke obscures vision and reduces the ease with which the fire brigade is able to bring members of the public to safety in the event of a fire. LSF cables therefore play an important part in reducing this danger to a minimum.

London Underground Limited (LUL) have developed a test of practical significance which has been designed to measure the density of smoke emission from cables and it has now been adopted by British and IEC Standards. This defines the standard absorbance produced across the opposite faces of a test cubicle and is popularly known as the 3 m cube test.

Paris Metro (RATP) adopts the French Standard UTE C20-452 on smoke emission which determines under experimental conditions the specific optical density of smoke produced by burning material. This slightly different approach is generally known as the NBS smoke chamber test.

Oxygen index and temperature index

‘Oxygen index’ is the minimum concentration of oxygen in an oxygen/nitrogen mixture in which the material will burn. As air contains approximately 21% oxygen it is stated that a material with an oxygen index greater than about 26% will be self extinguishing. In general, a particular oxygen index value offers no guarantee of resistance to the spread of flames.
In practice materials having identical oxygen indices may have widely different burning properties especially if base polymers or additives are of different types.

The ‘temperature index’ of a material is the minimum temperature at which the material supports combustion in air containing 21% oxygen when tested under controlled conditions. The test is useful for the comparison of similar materials but no correlation with flammability under other fire conditions is implied.
Oxygen and temperature indices are to some extent inter-related.

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5 key factors to the correct cable selection and application

Cable selection and application

It is essential to know cable construction, characteristics, and ratings to understand problems related to cable systems. However, to correctly select a cable system and assure its satisfactory operation, additional knowledge is required. This knowledge may consist of service conditions, type of load served, mode of operation and maintenance, and the like.

The key to the successful operation of a cable system is to select the most suitable cable for the application, make a correct installation, and perform the required maintenance.

In this technical article, discussion is based on the correct cable selection and application for power distribution and utilization.

Cable selection can be based upon the following five key factors:

1. Cable installation
2. Cable construction
3. Cable operation (voltage and current)
4. Cable size
5. Shielding requirements

1. Cable installation

Cables can be used for outdoor or indoor installations depending upon the distribution system and the load served.

A good understanding of local conditions, installation crews, and maintenance personnel is essential to assure that the selected cable system will operate satisfactorily! Many times cable insulation is damaged or weakened during installation by applying the incorrect pulling tensions.

Designs of conduit systems not only should minimize the number of conduit bends and distances between manholes but also should specify the pulling tensions.

The inspection personnel should ensure that installation crews do not exceed these values during installations. It is also important that correct bending radius be maintained in order to avoid unnecessary stress points. Once a correct installation is made, routine inspection, testing, and maintenance should be carried out on a regular basis to chart the gradual deterioration and upkeep of the cable system.

Cable systems are the arteries of the electric power distribution system and carry the energy required for the successful operation of a plant. Following is a brief discussion on cable installation and maintenance.

There are several types of cable systems available for carrying electrical energy in a given distribution system. The selection of a particular system may be influenced by local conditions, existing company policies, or past experience.

No set standards or established guidelines can be given for the selection of a particular system.

2. Cable construction

Selection and application of cable involves the type of cable construction needed for a particular installation. Cable construction involves conductors, cable arrangement, and insulation and finish covering.

2.1 Conductors

Conductor materials such as copper and aluminum should be given consideration with regard to workmanship, environmental conditions, and maintenance. The requirements for aluminum conductors with regard to these factors are more critical than for copper conductors.

Cable conductors should be selected based upon the class of stranding required for a particular installation.

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Balancing Thermal Performance With Other Desirable Cable Properties

Improving the thermal performance of a control cable can be a balancing act. Some of the changes to cable construction that widen the operating temperature range can compromise the cable’s electrical or mechanical properties. Silicone and cross-linked polyolefins do a good job striking that balance. Here’s why.

Wider Temperature Range

Both types of materials can dramatically widen the continuous use temperature range of control cables. A typical PVC control cable, for instance, can function in a temperature range of -40 to 90ºC. Compare that range to silicone-based cables, which work comfortably in a range of -50 to +180ºC.

The conductor material is also an important factor. For higher temperatures, a coated conductor is necessary to protect the bare copper effectively against corrosion. Tinned copper conductors should be used within a cable with a jacket made of silicone or cross-linked polyolefin.

Improved Wear And Chemical Resistance

With the most advanced cable technology, the additional thermal performance won’t affect the electrical properties of the cable at all, and any effects on mechanical properties will be minor or even advantageous. Compared to traditional PVC cables, the silicone and cross-linked polyolefin cables will exhibit:

●     Equivalent flexibility—though polyolefin cables are slightly stiffer than silicone or PVC cables

●     Improved wear resistance (for cross-linked polyolefin)

●     Improved chemical resistance

●     Equivalent flame performance

●     Halogen-free construction

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New White Paper Explores Cable Solutions For Extreme Temperatures

Control cables increasingly have to withstand temperature extremes in applications such as food and beverage machines, industrial ovens, furnaces, foundries and industrial process equipment. Extreme temperature applications can subject the cable to continuous-use temperatures as low as -50ºC and as high as 180ºC. For these environmental conditions, you have to think about cables with jacket materials other than PVC.

A Middle Ground

You could buy very expensive specialty cables that can withstand even hotter or colder temperatures, or you could try to use a more traditional PVC control cable, whose lifecycle starts to fall dramatically in hot or cold environments.

A growing class of control cables occupies a middle ground between over-engineered specialty cables and commodity PVC cables. Based on silicone or cross-linked polyolefin copolymers, these cables can take over in thermal environments that would cause PVC cables to fail prematurely. The trick is to choose a cable that balances thermal performance against other desirable cable properties.

Silicone Cables

Silicone cables are suitable for applications involving high temperature and a need for flexible wiring. They are also resistant to UV radiation, hydrolysis, oils, chemicals and plant and animal fats. For these reasons, they are commonly employed in metal processing applications, as well as in the industrial, automotive and automation industries because of their superior chemical resistance properties.

Polyolefin Cables

The second material alternative, crosslinked polyolefin, is formed from a combination of heat and high pressure—either by irradiating or chemically cross-linking the compound. These cables exhibit important electrical and mechanical properties that make them ideal for applications in electrical systems or heat treatment plants. These benefits include:

●     Increased thermal strength

●     Improved corrosion and abrasion resistance

●     Resistance to solvents, detergents and other operating fluids

●     Resistance to high temperatures

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BENEFITS OF PIN AND SLEEVE CONNECTORS OVER STANDARD TWIST LOCK

You’re likely familiar with twist lock cable connectors—they’re the National Electrical Manufacturers Association (NEMA) standard. However, there’s a lesser-known advanced cable connector that’s common in Europe, but has yet to find its place in the United States: the pin and sleeve connector.

In a nutshell, pin and sleeve connectors seal power connections and insulate power delivery from moisture, grime and chemicals. They’re designed to prevent disconnecting under load, and are often used for applications with abusive environments. Pin and sleeve devices range from metal-housed products to high impact-resistant plastic products with varying designs.

 

 

These male-female connections are well-suited for supplying power in a wide range of equipment such as welders, motor gen-sets, compressors, conveyors and portable tools and lighting. They’re also good for matching high-current power sources with the right equipment and integrate with switched and fused interlock receptacles in wet or corrosive environments.

Pin and sleeve connectors have plenty of other benefits that give them the edge over standard twist lock. Their rugged design provides heightened durability and a click-lock housing makes assembly fast and easy. Male plugs are surrounded by a shroud to protect the contact pins. These pins are exposed to the environment in most NEMA plugs.

In addition, pin and sleeve connectors have more configuration options than twist lock, and they’re color-coded for different amps—between 20 and 100 in the United States. NEMA twist lock sockets don’t have color coding.

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CHOOSE LOW SMOKE, HALOGEN-FREE CABLES FOR IMPROVED SAFETY

While halogen-free wires and cables have been widely used in Europe for some time, they’re now starting to gain traction in the United States. Products containing halogen—such as wires and cables, conduits, routing ducts and more—are receiving attention domestically due to the negative effects they impose on both industrial workers and machinery. And the push to reduce halogen usage is now reflected in UL and other domestic safety standards.

In the event of a fire, halogenated wires and cables give off toxic fumes that can cause serious health concerns if inhaled, not to mention they also destroy expensive electronic equipment. As industrial companies become more conscious of these problems, they’ve begun taking a closer look at the benefits of halogen-free cables.


Here’s a guide to some places where it makes sense to use halogen-free cables, and why you might want to consider them over halogenated cables in many applications.

A number of industry standards evaluate the cable fume toxicity produced during a fire. Each standard is unique because they approach the subject of determining cable fume toxicity through different evaluation parameters. They’re used to quantify smoke levels, light transmittance, levels of acid gas, concentration levels of toxic gases and halogen content.

While these standards are all different, they’re used to provide some determination concerning halogen-free or low smoke zero halogen cable requirements:

• IEC 60754-1: Details the amount of halogen acid gas measured from a specified amount of raw material. This test isn’t performed in the finished product wire or cable form, and compliance comes from not exceeding the mg/g that’s specified within the standard.

• IEC 60754-2: Shows the ph levels to determine the poisonousness of the gases during a fire. This standard approaches acidic levels that arise when halogenated components are burned and react with the moisture in the air.

• IEC 61034-2: Concerns the amount of light you can transmit while testing to determine the smoke density generated during a fire. High numbers show the effectiveness that a lighted pathway creates in a smoke-filled area.

• NES 713 Part 3: Determines the toxicity index of materials through complete combustion methods and analysis of the emitted gases. Measured in PPM, the gases must follow the highest values indicated, while concentration levels shouldn’t exceed the amounts for the 14 specified gases.

• UL 1685: This standard involves both the flame spread and fire resistance of cables, as well as methods for measuring smoke release. It establishes some pass/fail criteria, especially in the areas of peak and total smoke release. Lower numbers are desirable here, signifying the amount of smoke released.

• MIL-DTL-24643: Approaches the cumulative total contents of halogens in a cable by using X-ray fluorescence to determine amounts, giving an overall amount of concentration of halogen levels in a cable. Levels shouldn’t exceed the critical point of 0.2% under this standard specification for shipboard use.

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The basics of busway distribution systems

A distribution system is a system that distributes electrical power throughout a building. Distribution systems are used in every residential, commercial, and industrial building.…

Distribution Systems

A distribution system is a system that distributes electrical power throughout a building. Distribution systems are used in every residential, commercial, and industrial building.
Distribution systems used in commercial and industrial locations are complex.

A distribution system consists of metering devices to measure power consumption, main and branch disconnects, protective devices, switching devices to start and stop power flow, conductors, and transformers.

Power may be distributed through various switchboards, transformers, and panelboards. Good distribution systems don’t just happen. Careful engineering is required so that the distribution system safely and efficiently supplies adequate electric service to both present and possible future loads.

Feeders

A feeder is a set of conductors that originate at a main distribution center and supplies one or more secondary, or one or more branch circuit distribution centers. Three feeders are used in this example. The first feeder is used for various types of power equipment.
The second feeder supplies a group of 480 VAC motors. The third feeder is used for 120 volt lighting and receptacles.

Bus Bars

Commercial and industrial distribution systems use several methods to transport electrical energy. These methods may include heavy conductors run in trays or conduit. Once installed, cable and conduit assemblies are difficult to change. Power may also be distributed using bus bars in an enclosure. This is referred to as busway.

A bus bar is a conductor that serves as a common connection for two or more circuits. It is represented schematically by a straight line with a number of connections made to it. Standard bus bars in Siemens busway are made of aluminum or copper.

NEMA Definition

Busway is defined by the National Electrical Manufacturers Association (NEMA) as a prefabricated electrical distribution system consisting of bus bars in a protective enclosure, including straight lengths, fittings, devices, and accessories.
Busway includes bus bars, an insulating and/or support material, and a housing.

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