WO1999048113A1

WO1999048113A1 - Cables

Info

Publication number: WO1999048113A1
Application number: PCT/GB1999/000743
Authority: WO
Inventors: Gregor Reid; Mark Froggatt; David Kemp
Original assignee: Draka U.K. Limited
Priority date: 1998-03-13
Filing date: 1999-03-12
Publication date: 1999-09-23
Also published as: AU2740499A; GB2335304A; GB9805442D0

Abstract

An electric cable (11) comprises a copper conductor (10) surrounded by insulating layers (14 and 15) and an outer sheath (16). One of the insulating layers (14 or 15) incorporates a chemical blowing agent which evolves a gas or gases at elevated temperatures. One or more of the layers (14, 15 and 16) is of a material that supports combustion. In the event of a fire, the chemical blowing agent is activated to give off gases which cause the cable to swell to form a bulge (7). The bulge (7) provides a physical barrier to flame spread and heat propagation along the cable.

Description

CABLES

The present invention relates to cables .

Cables, for example, electric power cables, and cables that transport electrical signals or data, and optical fibres, are required to link parts of buildings. They are often laid in cavities or conduits which give free passage through walls, floors and ceilings. The materials that are used as electrical insulation m electric cables or as protective coatings for optical fibres are often of a type that supports combustion. During a fire a problem is caused if the insulation or coating of the cable is set alight. The fire can be quickly propagated along the cable through the conduits, passing under or through walls and thus spreading the fire. This will prevent containment of the fire resulting in rapid flame spread leading to increased damage to the building and greater endangerment to life. It is particularly important to reduce flame spread m areas of high airflow, for example ventilation shafts and plenums.

According to the present invention a cable comprises at least one conductor, and a layer or layers of material surrounding the conductor, the material including a material that evolves gas when the temperature is raised above normal ambient temperature to a temperature such as is experienced m the event of a fire. The gas or gases evolved at elevated temperatures may retard flame spread by one or more of the following actions: by causing an increase m the cable volume which creates a physical barrier to flame spread, by causing the cable sheath to become discontinuous thus reducing the fuel for flame spread, or by introducing a non-flammable gas which displaces oxygen from around the cable and thus suffocates the flames.

In the case where the cable is an electric cable, the conductor is an electric conductor and the layer or layers of material will include electrical insulation. In the case where the cable is an optical cable the conductor is an optical fibre or bundle of optical fibres and the layers of material will include a coating to protect the optical fibre.

Preferably the material that evolves gas at elevated temperatures is a chemical blowing agent.

A chemical blowing agent is a material that produces a gas or gases, by its chemical decomposition, forming a cellular structure m a polymeric matrix. The decomposition process is usually exothermic, irreversible and takes place at elevated temperature, but m the case of certain chemical blowing agents, gas or gases are released by an endothermic reaction between two components. Examples of chemical blowing agents that evolve a gas or gases at suitable elevated temperatures include: sodium bicarbonate, azodicarbonamide, dmitrosopentamethylenetetramine, 4,4' -oxy-bis- (benzenesulphonylhydrazide) , trihydrazinotπaz ne, modified azodicarbonamide, N,N dimethyl N, N dinitrosoterephthalamide or 5-phenyl tetrazole .

Preferably, the chemical blowing agent evolves a gas or gases that are non-flammable and/or non-toxic.

Preferably the chemical blowing agent decomposes at a temperature above 180°C.

Examples of the gases that are evolved by suitable chemical blowing agents are nitrogen, carbon dioxide, water (in the form of steam) and ammonia or a combination of some or all of the aforementioned gases.

Preferably, the chemical blowing agent will have a gas evolution of between 50 - 500 millilitres at normal atmospheric pressure per gram.

The material that evolves gas may be located within the insulation material, between the conductor and the layer of insulation, between further discrete layers of insulation within the insulation layer, or between the insulating layer and an external, sheath layer.

Where the material that evolves gas is formed as a separate layer it may be provided on a tape such as mica tape, or it may be incorporated into the cable by one or more of the following techniques: by surface dusting; by solution coating; or by spray coating. In some embodiments of the invention material that evolves gas at elevated temperatures is arranged in the cable that evolution of the gas causes at least a part of the insulation to become discontinuous.

In other embodiments the material that evolves gas may be located so that when the gas is evolved, at least a part of the insulating layer is maintained around the conductor, enabling electrical performance of the cable to be maintained at elevated temperature.

The chemical blowing agent may be used m conjunction with an initiator or 'kicker' such as, for example, zinc stearate, calcium stearate, zinc oxide, or other metal salts. The initiator promotes decomposition of the chemical blowing agent at the decomposition temperature of the chemical blowing agent.

The present invention will now be described with reference to the accompanying drawings m which:

FIGURE 1 shows, in diagrammatic form, a section through a building showing a cable conduit;

FIGURE 2 shows the cable conduit of figure 1 after exposure to a fire for a short period of time;

FIGURE 3 shows, on an enlarged scale, a detailed cross section of a cable according to one embodiment of the present invention; FIGURE 4 shows, on an enlarged scale, a detailed cross section of a cable according to a second embodiment of the present invention;

FIGURE 5 shows, on an enlarged scale, a detailed cross section of a cable according to a further embodiment of the present invention;

FIGURE 6 shows, on an enlarged scale, a cable conduit provided with a cable according to an embodiment of the present invention;

FIGURE 7 shows a section through a cable conduit provided with a cable according to a further embodiment of the present invention; and

FIGURE 8 shows a section through a cable conduit provided with a cable according to a further embodiment of the present invention.

Figure 1 shows a fragmentary view of a building with a cable conduit 2 containing a cable 1. The conduit leads through a wall 4 between adjoining rooms 6 and 9. A fire 3 has occurred m room 6 and is in the process of burning part of cable 1.

Figure 2 shows the same cable conduit as m figure 1 after the cable 1 has been exposed to the fire 3 for a short period. The cable insulation or coating has caught light and the flame 5 has spread along the cable. Flames 5 have travelled along the cable through 6 wall 4 via the conduit 2 and into the adjacent room 9.

Figure 3 shows a cross section of an electric cable 11 according to one embodiment of the present invention. The cable 11 has a copper conductor 10, surrounded by insulating layers 14,15. The outer insulating layer is surrounded by sheath 16. One or more of the insulating layers may be a polymeric insulating layer. One or more of the layers 14, 15, 16 is of a material that supports combustion in the event of a fire. Examples of materials suitable for use as insulating layers 14,15 are silicone rubber, polyethylene and glass/mica tape. The outer sheath layer 16 may be a zero halogen, low smoke type protective layer. In this embodiment a chemical blowing agent is incorporated into the formulation of one or both of insulation layers 14 or 15. Examples of suitable chemical blowing agents that evolve a gas or gases at elevated temperatures include : sodium bicarbonate, azodicarbonamide, dinitrosopentamethylenetetramme,

4,4' -oxy-bis- (benzenesulphonylhydrazide) , trihydrazmotriazme, modified azodicarbonamide, N,N dimethyl N, N dimtrosoterephthalamide or 5-phenyl tetrazole .

Figure 4 shows a cross section of a cable according to a further embodiment of the present invention, m which parts similar to the previous embodiment are denoted by like numbers. The chemical blowing agent in this embodiment is incorporated as a discrete layer 17 between the insulating layer 15 and the outer sheath 16. The chemical blowing agent may be impregnated into a layer of glass tape, or as a dusting/coating layer deposited on the layer 15. In other embodiments (not shown) the chemical blowing agent may be located as a discrete layer between the conductor 10 and the insulation layer 14, or between the insulation layers 14 and 15.

If the chemical blowing agent is present in the cable as an impregnated tape 17, it may be applied to the cable by a method such as winding or wrapping, rather than by extrusion. The former methods of application have the advantage that the chemical blowing agent is not heated when being applied onto the cable, and there is thus less risk that the chemical blowing agent may be activated prematurely, for example during manufacture .

Figure 5 shows a cross section of a cable 11 according to a further embodiment of the present invention. The chemical blowing agent in this embodiment is incorporated into the formulation of the outer sheath 16.

Figures 6 to 8 illustrate how cables according to the invention might behave when exposed to the elevated temperatures associated with a fire.

Figure 6 shows a cable conduit equipped with a cable 1 ' according to the present invention. The cable 1' has been exposed to a fire. The high temperatures have 8 activated the chemical blowing agent, causing the chemical blowing agent to evolve gas. The gas generated causes the cable coating to swell to form a bulge 7, thereby providing a physical barrier to flame spread and heat propagation along the cable 11.

When exposed to a fire, the outer surface of cable, for example the sheath layer, will become charred at area 8. The charred area will not burn any further after it has become charred, thus forming a further physical barrier at the surface of the cable that protects the underlying insulation layers 14,15 and prevents any further insulation being burnt; the charring therefore reduces the amount of fuel that can come into contact with the flames.

Figure 7 shows a cable conduit equipped with a cable 1" according to the present invention. A chemical blowing agent is included in the cable. The cable has been exposed to a fire and the elevated temperatures have caused the chemical blowing agent to evolve gas . The gas generated may make the cable coating shatter into fragments 12a, thus removing fuel from the proximity of the flames, or may crack the sheath thus making it discontinuous as illustrated in area 12b.

Figure 8 shows a cable conduit equipped with a cable 1 ' " according to a further embodiment of the present invention. The cable includes a chemical blowing agent. During a fire, the chemical blowing agent evolves a gas because of the elevated temperatures. The gas may remove oxygen from the immediate proximity of the cable sheath and replace the oxygen with a non-flammable gas such as, for example, nitrogen, carbon dioxide, water (m the form of steam), ammonia, or a combination of some or all of the aforementioned gases. This m effect 'chokes" or 'suffocates' the flames 5. The gas or gases evolved may also be flame retardant, thus having an additional fire quenching effect .

When exposed to a fire, the cable might behave in one or more of the ways illustrated in figures 6 to 8 and described in the accompanying text.

The behaviour may depend on one or more combinations of the following factors:

1) The position or location of the chemical blowing agent within the cable;

n) The materials that are used to form the components of the cable;

m) The amount or proportion of chemical blowing agent, compared with the other constituents of the cable;

IV) The temperature profile experienced by the cable within the fire (for example behaviour may depend on the rate of temperature change and whether the heating is localised or over a large area of the cable) .

Claims

10CLAIMS

1. A cable comprising at least one conductor, and a layer or layers of material surrounding the conductor, the material including a material that evolves gas when the temperature is raised above normal ambient temperature to a temperature such as is experienced in the event of a fire.

2. A cable according to claim 1 in which the material that evolves gas at elevated temperatures is a chemical blowing agent .

3. A cable according to claim 2 m which the chemical blowing agent evolves a gas or gases that are non-flammable and/or non-toxic.

4. A cable according to claim 2 of 3 in which the chemical blowing agent decomposes at a temperature above 180┬░C.

5. A cable according to claim 2, 3 or 4 m which the chemical blowing agent has a gas evolution of between 50 - 500 m llilitres at normal atmospheric pressure per gram.

6. A cable according to any of claims 1 to 5 m which the material also includes an initiator which promotes decomposition of the chemical blowing agent at the decomposition temperature of the chemical blowing agent. 11

7. A cable according to any of the preceding claims in which the cable is an electric cable, the conductor is an electric conductor and the layer or layers of material includes electrical insulation.

8. A cable according to claim 7 m which the material that evolves gas is located within the insulation material, between the conductor and the layer of insulation, between further discrete layers of insulation within the insulation layer, or between the insulating layer and an external, sheath layer.

9. A cable according to claim 7 or 8 m which the material that evolves gas at elevated temperatures is so arranged in the cable that evolution of the gas causes at least a part of the insulation to become discontinuous.

10. A cable according to any of claims 1 to 6 in which the cable is an optical cable, the conductor is an optical fibre or bundle of optical fibres and the layers of material includes a coating to protect the optical fibre.