EP2040267A1 - Electric cable resisting the propagation of an electric arc - Google Patents

Abstract

The cable (1) has layers (3, 4, 5) respectively formed of mica ribbon, polyimide ribbon, and PTFE ribbon rolls. The layer (3) surrounds an electrical conductor (2). The ratio of linear density of PTFE to the sum of linear densities of polymeric binder e.g. silicone resin, and polyimide binder is greater than or equal to 2, 4, 6, and 12 when a conductor section is between 0.1 and 0.2 square mm, between 0. 2 and 0.6 square mm, equal to 0.6 square mm, and less than/equal to 3 square mm, respectively. A fluorinated ethylene-propylene copolymer covering covers a layer of polyimide ribbon. The mica ribbon is a Cablosam 366 20-80(RTM: mica ribbon). The layer from the mica roll is thermally processed at temperature of 400 degree Celsius.

Description

The present invention relates to an electric cable, and typically but not exclusively applies to electrical cables used in aeronautics, for example on board aircraft.

This type of electric cable must satisfy many criteria necessary for its use in aeronautics, especially when it is placed in the conditions of a fire.

For example, a safety criterion is to allow the electric cable to continue to operate at high temperatures of the order of 1100 ° C for a minimum period of time, generally of the order of 5 to 15 minutes, without fusion of its electrical conductor, neither spreading the fire, nor resisting vibrations and splashing of water or extinguishing fluids, while ensuring the electrical continuity of the circuits and maintaining a minimum insulation resistance in the flame , generally of the order of 10,000 ohms.

Other criteria may also be taken into account, such as the weight and diameter of said cable which must not be excessive, the maximum temperature of use in permanent service, which must be the highest possible, generally of the order of 260 ° C for at least 20,000 hours, and the markability of said cable to allow its identification.

A newer criterion requires proper operation of the electrical safety cable when assembled with other electrical cables to form a harness.

The document FR 2,573,910 discloses an electrical cable for aeronautics comprising an electrical conductor surrounded by a first layer consisting of two windings of a mica ribbon.

This first layer is covered with a second layer of thermostable polymer which may consist for example of a tape of polytetrafluoroethylene (PTFE), or a polyimide resin.

Finally, this second layer is covered with an intermediate layer of glass fibers, as well as an outer layer of the same nature as the second layer.

However, even if this electrical cable of the prior art meets the security criteria stated above, it does not correctly meet another safety criterion which is the resistance to the propagation of electric arc according to NF EN standards. 3475-604 (dry arc propagation resistance evaluation method) and EN 2346-005 (standard defining the minimum performance of an aeronautical electrical cable resistant to fire and to the propagation of the arc electric).

This safety criterion makes it possible to guarantee a sufficient resistance of the insulation of said cable in order to avoid the triggering and the propagation of electric arcs between the electric cables on the one hand and / or between electric cables and a conductive structure of somewhere else.

The technical problem to be solved, by the object of the present invention, is to propose an electric cable making it possible to avoid the problems of the state of the art, in particular by offering resistance to the propagation of an electric arc satisfying the requirements of the present invention. the EN 2346-005 standard for the NF EN 3475-604 arc propagation test, while maintaining optimum flame and flame performance properties according to NF EN 3475-408 and EN 3475-417 .

• un conducteur électrique entouré par une première couche comprenant au moins un enroulement d'un ruban de mica, ledit ruban de mica étant composé de particules de mica déposées par l'intermédiaire d'un liant polymérique sur un support,
• une deuxième couche comprenant au moins un enroulement d'un ruban de polyimide, et
• une troisième couche comprenant au moins un enroulement d'un ruban de polytétrafluoroéthylène (PTFE),

The solution of the technical problem lies, according to the present invention, in that the electric cable comprises:

• an electrical conductor surrounded by a first layer comprising at least one winding of a mica ribbon, said mica ribbon being composed of mica particles deposited via a polymeric binder on a support,
• a second layer comprising at least one winding of a polyimide tape, and
• a third layer comprising at least one winding of a tape of polytetrafluoroethylene (PTFE),

the first layer being heat-treated at a temperature of at least 400 ° C, and

• o R est supérieur ou égal à 2 lorsque la section du conducteur électrique est au plus égale à 0,2 mm², de préférence comprise entre 0,1 et 0,2 mm²,
• o R est supérieur ou égal à 4 lorsque la section du conducteur électrique est strictement supérieure à 0,2 mm² et strictement inférieure à 0,6 mm²,
• o R est supérieur ou égal à 6 lorsque la section du conducteur électrique est égale à 0,6 mm²,
• o R est supérieur ou égal à 12 lorsque la section du conducteur électrique est strictement supérieure à 0,6 mm², de préférence d'au plus 3 mm².

the ratio R of the linear density of PTFE to the sum of the linear masses of the polymeric binder and the polyimide being such that:

• o R is greater than or equal to 2 when the section of the electrical conductor is at most equal to 0.2 mm ² , preferably between 0.1 and 0.2 mm ² ,
• o R is greater than or equal to 4 when the section of the electrical conductor is strictly greater than 0.2 mm ² and strictly less than 0.6 mm ² ,
• o R is greater than or equal to 6 when the section of the electrical conductor is equal to 0.6 mm ² ,
• o R is greater than or equal to 12 when the section of the electrical conductor is strictly greater than 0.6 mm ² , preferably not more than 3 mm ² .

The Applicant has surprisingly found that for a given section range of electrical conductors, a specific heat treatment of the first layer combined with a ratio R of the linear density of PTFE on the sum of the linear masses of the polymeric binder and the polyimide allows to withstand the spread of dry electric arc to more than 75%, according to standards NF EN 3475-604 and EN 2346-005.

In addition, the electrical cable advantageously retains a very good fire resistance and ensures the electrical continuity of the circuits optimally, while having a relatively small weight and diameter, to meet the criteria required in aeronautics.

In a preferred embodiment, the heat treatment of the first layer is carried out for a period t greater than 30% by the time t ₀ necessary for the degassing of the first layer, preferably said duration t is from minus 1 minute.

According to a preferred characteristic, the mica ribbon comprises at most 20% by weight of polymeric binder, preferably the mica ribbon comprises 13% by weight of polymeric binder.

As a preferred example, the polymeric binder is a silicone resin.

According to another preferred feature, the percentage of recovery of a tape of mica on itself during its winding and / or a polyimide tape on itself during its winding is at most 49%.

This rate advantageously makes it possible to guarantee an optimized ratio R and thus to improve the resistance to the propagation of electric arc by combining it with the minimum adapted quantity of PTFE.

According to another preferred feature, the second layer comprises a single winding of a polyimide tape.

According to another preferred feature, the third layer comprises at least two windings of a PTFE tape.

These preferred features advantageously make it possible to minimize the amount of polymeric binder and polyimide and consequently to increase the ratio R to improve the resistance to the propagation of electric arc of the electric cable while preserving its final weight and diameter and its properties. fire resistance.

In a particularly advantageous embodiment, the mica particles are of the phlogopite type.

Thanks to this type of particles, a better insulation resistance in the flame is obtained.

In another embodiment, the polyimide tape comprises a polyimide layer coated on each side with a fluorinated ethylene propylene copolymer (FEP) coating.

The FEP coatings make it possible to obtain adhesion between the overlaps and / or the coils respectively of the polyimide tape (s) on the one hand, and the adhesion of the second layer with the third layer on the other hand .

According to this embodiment, the second layer is heat-treated at a temperature above the melting temperature of the FEP layers.

The third layer may also be heat treated at a temperature greater than 340 ° C., thus allowing the sintering of the PTFE and the adhesion between the overlaps and / or the windings respectively of the PTFE tape (s).

Advantageously, the heat treatment of the second layer can be carried out simultaneously with the heat treatment of the third layer.

In another embodiment, the electrical cable further comprises an outer layer (surface) capable of being marked.

As a particularly advantageous example, the third layer further comprises said outer layer, the latter preferably being a PTFE tape comprising white pigments of titanium dioxide.

Another object of the present invention is an electrical harness comprising at least one electrical cable as defined above.

Preferably, the harness includes several electrical cables according to the present invention, said electrical cables forming an assembly covered with a protective sheath of mechanical protection type well known to those skilled in the art.

For example, the protective sheath comprises one or more metal braids of copper or steel.

Said protective sheath may also be covered by a braid of abrasion-resistant and non-fire-resistant textile material, for example of the aromatic polyamide type.

Other features and advantages of the present invention will appear in the light of the examples which follow with reference to the single annotated figure, said examples and figure being given for illustrative and not limiting.

The figure 1 schematically shows a structure, in perspective, of an electric cable 1 according to the present invention.

This electric cable 1 comprises an electrical conductor 2, for example copper or copper alloy coated with a layer of nickel, whose mass comprises at least 27% nickel, generally of the multi-strand type.

Said electrical conductor 2 is surrounded by a first layer 3, said first layer 3 comprising at least one winding of a mica ribbon, preferably a single winding of a mica ribbon.

The mica ribbon is typically composed of particles (or flakes) of mica deposited via a polymeric binder on a glass fiber type support generally woven but may be nonwoven.

The mica may be of the muscovite or phlogopite type, and for example, the polymeric binder may be of the silicone resin, polyimide, polyamide-imide type or any other type of thermostable polymer.

Then, the first layer 3 is surrounded by a second layer 4, said second layer 4 comprising at least one winding of a polyimide tape, preferably a single winding of a polyimide tape.

Finally, the second layer 4 is surrounded by a third layer 5, said third layer 5 comprising at least one winding of a PTFE tape, preferably the PTFE tape being free of pigments.

The outer (superficial) layer of the third layer 5 may advantageously comprise a pigmented PTFE layer, the pigment being, for example, titanium dioxide, in order to allow UV laser marking of the surface of this outer layer.

Typically, the successive windings of the ribbons are in the opposite direction to avoid mishandling during the manufacture of said cable.

Preferably, the recovery rate of each mica tape on itself and each polyimide tape on itself is at most 49% (recovery coefficient Kr of at most 0.49).

This recovery ratio advantageously makes it possible to guarantee a ratio R (linear density of PTFE on the sum of the linear masses of polymeric binder and polyimide) optimized and adapted to the section of the electrical conductor (electrical core), or in other words to limit the linear masses of the first and second layers, and thus improves the resistance to the propagation of electric arc of the electric cable.

In the manufacture of the electric cable according to the present invention, the laying of the second and third layers may comprise a heat treatment step.

After laying (or rubbing) the first layer, the electrical conductor thus isolated is heat-treated in an oven at a temperature of at least 400 ° C. This is the thermal degradation step of the mica ribbon, including its polymeric binder.

For example, this heat treatment is performed for a time t greater than at least 30% to the time t ₀ required for degassing said ribbon.

The time t ₀ required for degassing is generally determined experimentally and degassing is typically carried out at a temperature of about 340 ° C.

More particularly, t ₀ is determined from the moment when the layers deposited above the layer to be degassed no longer "blister" under the effect of gases released when the upper layers are heat-treated ("firing"). at a temperature of at least 340 ° C.

Thus, the degassing makes it possible to limit the residual volatile compounds in the first layer, these compounds being able to create insulation defects during subsequent stages of heat treatment, such as, for example, the heat treatment of the second and third layers.

Moreover, in a particularly advantageous manner, this heat treatment also makes it possible to facilitate obtaining a sufficient resistance (greater than 75%) to the electric arc propagation of the electric cable when the temperature is at least 400 ° C. .

By way of nonlimiting example, an electrical conductor with a section of 0.6 mm ² , insulated with a first layer comprising a single winding of a mica ribbon is passed through an oven 8 meters long at six zones. of heating of identical length, the six heating zones respectively having the following successive temperatures: 340 ° C - 400 ° C - 400 ° C - 450 ° C - 450 ° C - 450 ° C.

The time required for the degassing of the mica ribbon is typically 40 seconds (t ₀ ), which is a rate of passage in the oven 8 meters in length of 12 meters per minute.

By taking at least 30% of t ₀ , at least a time t of about 1 minute is obtained, ie a rate of passage in the oven of 8 meters per minute.

Thus, for one minute (t) in the oven described above, the mica ribbon reaches at least the temperature of 400 ° C.

With a passage in said oven for 40 seconds (t ₀ ), the mica tape can reach a temperature of the order of 340 ° C.

After the laying (or rubbing) of the second layer, when the polyimide tape comprises a layer of polyimide coated on each of these faces with a layer of a fluorinated ethylene propylene copolymer (FEP), the electrical conductor thus isolated may be heat-treated in an oven at a temperature above the melting temperature of the outer layers of FEP of the polyimide tape.

Typically, this melting temperature is greater than 260 ° C. This is the step of heat sealing the second layer.

After the laying (or rubbing) of the third layer, the electrical conductor thus isolated can be heat-treated in an oven at a temperature of temperature above the melting temperature of PTFE, ie at a temperature of 342 ° C to sinter PTFE.

In a particularly preferred manner, the steps of rubannage of the second and third layers are carried out one after the other and are followed by a single step of heat treatment of the second and third layers at a temperature greater than 340 ° C. more preferably 342 ° C.

The second and third layers are thus simultaneously heat treated.

By this single heat treatment step which comprises the heat-sealing step of the polyimide and the step of the sintering step of the PTFE, it ensures the adhesion of all the thicknesses of ribbons respectively of the second and third layers between them (overlaps and windings) as well as adhesion between the second and third layers.

Finally, the electrical cable may advantageously comprise an outer layer for marking, preferably UV laser marking, of the electric cable according to the present invention.

This outer layer can surround the third layer, but it can be included in the third layer as such, or in other words the outer layer is also a winding of a PTFE tape, the latter being however laser markable UV.

Typically, it is a pigmented PTFE tape, preferably comprising white titanium dioxide pigments in an amount of at most 5% by weight of said PTFE tape.

It is preferable not to exceed this value of 5%, or even to minimize it, because the presence of titanium dioxide pigments can be harmful vis-à-vis the resistance to the propagation of the electric arc.

In order to show the advantages of the electric cables according to the present invention, Tables 1a and 1b below detail various structures of electric cables whose resistance to the propagation of dry electric arc as well as that the ratio R of linear density of PTFE on the sum of the linear masses of the polymeric binder and the polyimide were studied.

Tables 1a and 1b show from top to bottom the succession of different ribbons of the first, second and third layers which constitute the electric cable (or insulated electric wire).

The first, second and third layers of the electrical cables DW24A to DW14C referenced in Tables 1a and 1b were heat treated according to the manufacturing method described above, except the first layer of the electric cable DW20A.

The details of the treatment of the first layer, the recovery coefficients Kr as well as the thicknesses of the different ribbons are also mentioned in Tables 1a and 1b.

The origin of the various constituents of Tables 1a and 1b is as follows.

The mica ribbon is a Cablosam 366 20-80 tape, marketed by the company Von Roll-Isola, with a thickness of the order of 0.1 mm.

This ribbon comprises phlogopite mica particles and a quantity of 13% by weight of polymeric binder of the silicone resin type, or in other words it comprises 17 g / m ² of silicone resin for a total mass of 130 mica ribbon. g / m ² .

The polyimide tape (or fluorinated adhesive polyimide tape) is a polyimide tape 616, marketed by DuPont de Nemours. These polyimide tapes comprise a 0.025 mm thick polyimide film coated on each of its faces with a FEP resin layer of 0.0015 to 0.0025 mm thick. The amount of polyimide is 76.5% by weight of said tape.

The non-sintered and non-marking laser PTFE PTFE ribbon and the unscented and markable PTFE white PTFE ribbon are marketed in particular by the company Plastic Omnium 3P. <b><u> Table 1a </ u></b> Electric cable DW24A DW20A DW20B DW20C DW20D Electrical conductor Strand (multi-strand) of 19 copper wires 0.12 mm in diameter each Strand (multi-strand) of 19 copper wires of 0.20 mm diameter each Electrical conductor section (mm ² ) 0.25 0.6 First layer 1 0.1 mm thick Mica tape heat treated above 400 ° C Kr = 53% - Kr = 37% Kr = 37% Kr = 37% 1 0.1 mm thick Mica tape heat treated above 400 ° C Kr = 26% - - - - 1 Mica tape 0.1 mm thick heat treated at 340 ° C - Kr = 37% - - - Second layer 1 polyimide tape 0.030 mm thick Kr = 30% Third layer 1 PTFE tape 1 UV PTFE tape 0.064 mm thick Kr = 53% 1 non-UV PTFE tape 0.076 mm thick Kr = 53% 1 non-UV PTFE tape 0.076 mm thick Kr = 53% 1 non-UV PTFE tape 0.064 mm thick Kr = 53% 1 non-UV PTFE tape 0.076 mm thick Kr = 53% 1 PTFE tape - 1 UV PTFE tape 0.064 mm thick Kr = 53% 1 UV PTFE tape 0.064 mm thick Kr = 53% 1 non-UV PTFE tape 0.064 mm Kr = 53% 1 non-UV PTFE tape 0.076 mm thick Kr = 53% 1 PTFE tape - - - 1 UV PTFE tape 0.064 mm thick Kr = 53% 1 x PTFE UV tape 0.076 mm thick Kr = 53% Weight of the electric cable (g / m) 4.9 8.9 8.9 9.9 10.8 Diameter of the electric cable (mm) 1.59 to 1.68 1.80 to 1.84 1.80 to 1.84 2.00 to 2.05 2.12 to 2.17 Electric cable DW14A DW14B DW14C Electrical conductor Strand (multi-strand) of 37 copper wires 0.25 mm in diameter each Electrical conductor section (mm ² ) 1.8 First layer 1 0.1 mm thick Mica tape heat treated above 400 ° C Kr = 49% Kr = 35% Kr = 49% 1 0.1 mm thick Mica tape heat treated above 400 ° C - Kr = 30% - 1 Mica tape 0.1 mm thick heat treated at 340 ° C - - - Second layer 1 polyimide tape 0.030 mm thick Kr = 30% Third layer 1 PTFE tape 1 non-UV PTFE tape 0.100 mm thick Kr = 53% 1 non-UV PTFE tape 0.100 mm thick Kr = 53% 1 non-UV PTFE tape 0.100 mm thick Kr = 53% 1 PTFE tape 1 x PTFE UV tape 0.076 mm thick Kr = 53% 1 non-UV PTFE tape 0.100 mm thick Kr = 53% 1 non-UV PTFE tape 0.100 mm thick Kr = 53% 1 PTFE tape - 1 x PTFE UV tape 0.076 mm thick Kr = 53% 1 x PTFE UV tape 0.076 mm thick Kr = 53% Weight of the electric cable (g / m) 23.4 28.3 26 Diameter of the electric cable (mm) 2.72 to 2.80 3.3 to 3.43 3.10 to 3.18

Tables 2a and 2b below show the ratio R of the linear density of PTFE on the sum of the linear masses of silicone resin and polyimide as well as the resistance to the dry electric arc propagation of the various electrical cables of Tables 1a. and 1b. <b><u> Table 2a </ u></b> Electric cable DW24A DW20A DW20B DW20C DW20D Collateral damage 13% 44% 20% 16% 4% Resistance to the propagation of the electric arc 87% 56% 80% 84% 96% Report R 3.44 8.1 8.1 11.9 14.9 Electric cable DW14A DW14B DW14C Collateral damage 67% 20% 12% Resistance to the propagation of the electric arc 33% 80% 88% Report R 9.1 13 15.5

• de PTFE provenant du ou des rubans de PTFE (troisième couche),
• de liant polymérique provenant du ou des rubans de mica (première couche), et
• de polyimide provenant du ou des rubans de polyimide (deuxième couche).

The ratio R of the linear density of PTFE to the sum of the linear masses of polymeric binder and polyimide is calculated from the respective initial masses:

• PTFE from the PTFE tape (s) (third layer),
• polymeric binder from the one or more mica ribbons (first layer), and
• polyimide from the polyimide tape (s) (second layer).

The thicknesses, compositions and constructions of these ribbons as well as the recovery coefficients Kr are of course taken into account in the calculation of the R ratio.

More particularly, the mass of each of the layers of PTFE (PTFE tape), polymeric binder (mica tape), and polyimide (polyimide tape (s)) is obtained by calculating the area occupied. by each layer and multiplying it by the density of each layer, respectively.

Thus, we have the following equations:

PTFE mass:

• PTFE mass = (surfaces occupied by unsintered PTFE tape (s)) x (density = 1.62)

The mass of PTFE is calculated before the so-called "sintering" cooking operation which leads to a contraction of 25% of the radial thickness of unsintered PTFE.

Mass of polymeric binder:

• Mass of mica tape (s) = (mica tape surface area) x (density = 1.30)

• Masse de liant polymérique = (masse du ou des ruban(s) de mica) x (teneur en liant polymérique (%) du ou des rubans de mica )

The mass of polymeric binder is deduced from the mass of the mica ribbon (s) by multiplying it by the content of the mica ribbon by polymeric binder, the content indicated by the supplier.

• Mass of polymeric binder = (mass of mica tape (s)) x (polymeric binder content (%) of mica tape (s))

Mass of polyimide:

The mass of polyimide is calculated by multiplying the mass of the polyimide tape having on each side a layer of fluorinated adhesive (FEP), and multiplying this mass by the polyimide content of said tape.
$$\mathrm{Mass\; of\; polyimide}=\left(\mathrm{surface\; occupied\; by\; the\; polyimide\; tape}\right)\times \left(\mathrm{density}=1,50\right)\times \left(\mathrm{polyimide\; content}(\%)\mathrm{fluorinated\; adhesive\; polyimide\; tape}=76,5\%\right)$$

Finally, the area occupied by a layer is calculated by subtracting from the surface of the circle of diameter equal to the outer diameter (De) of said layer, the surface of the circle of diameter equal to the inner diameter (Di) of said layer according to the following formula :
$$\mathrm{Surface\; occupied\; by\; a}=\frac{\mathrm{<figure-callout\; id="4"\; label="\pi "\; filenames="imgf0001.png"\; state="\{\{state\}\}">\pi </figure-callout>}}{4}\times \left({\mathrm{Of}}^2-{\mathrm{<figure-callout\; id="2"\; label="di"\; filenames="imgf0001.png"\; state="\{\{state\}\}">di</figure-callout>}}^2\right)$$

For the first insulation layer, the inside diameter is equal to the diameter of the conductor.

The outer diameter of the layer is equal to the sum of the inside diameter and 2 times the radial thickness (ER) of the layer, namely: $$\mathrm{Of}=\mathrm{di}+2\times \mathrm{ER}\mathrm{.}$$

The radial thickness (ER) of a banded layer is given by the following equation: $$\mathrm{ER}=\frac{\mathrm{Thickness\; of\; the\; ribbon\; in\; mm}}{1-\left(\mathrm{recovery\; Kr\; of\; the\; ribbon}\%\right)}$$

By way of example, the calculation of the ratio R concerning the electric cable DW20D is detailed below, the calculation method being identical for the other types of electrical cables DW described in Tables 1a and 1b.

Various standards relating to said electrical cables DW, well known to those skilled in the art, specify the diameter of the electrical conductor according to its section, the number of conductors and the diameter of each of said son, and the degree of compaction of said son conductors.

By way of example, according to the standard EN 2346-005, the diameter of the electrical conductor of the electric cables referenced in Tables 1a and 1b is detailed in Table 3 below. <b><u> Table 3 </ u></b> Electric cable Electrical conductor section (mm ² ) Number of wires Diameter of each conductor wire (mm) Maximum diameter of the electrical conductor (mm) DW24 0.25 19 0.12 0.62 DW20 0.60 19 0.20 1.04 DW14 2.0 37 0.25 1.82

Another example of said diameter, according to standard NF EN 4434, is detailed in Table 4 below. <b><u> Table 4 </ u></b> Electric cable Electrical conductor section (mm ² ) Number of wires Diameter of each conductor wire (mm) Minimum diameter of the electrical conductor (mm) Maximum diameter of the electrical conductor (mm) DW24 0.25 19 0.12 0.555 0,585 DW20 0.60 19 0.20 0.94 0.97 DW14 2.0 37 0.25 1.69 1.73

The diameters of conductors DW24, DW20 and DW14 of Tables 1a and 1b are those respectively mentioned in the column "maximum diameter of the electrical conductor" of Table 4 according to EN 4434, said diameters being given for illustrative and not limiting. Diameter of the driver = 0.97 mm

The first layer:

Thickness of the mica ribbon = 0.100 mm Kr overlay of the mica ribbon = 37% Density of the mica ribbon = 1.30 Polymeric binder content of mica tape = 13% Inner diameter of the mica layer = 0.97 mm Circle surface diameter 0.97 mm = 0.7390 mm ² Radial thickness of mica ribbon = 0.15787 mm = 0.100 / (1-0.37) Outside diameter of the mica layer = 1.2875 mm = 0.97 + 2 x 0.1587 Circle surface diameter 1.2875 mm = 1.3018 mm ² Surface of the mica ribbon layer = 0.5629 mm ² = 1.3018 - 0.7390 Mass of the layer of mica tape = 0.731 = 0.5629 x 1, 30 Mass of polymeric binder of the mica ribbon = .0951 = 0.7319 x 0.13

The second layer:

Thickness of fluorinated adhesive polyimide tape = 0.030 mm Kr coating of fluorinated adhesive polyimide tape = 30 % Density of fluorinated adhesive polyimide tape = 1.53 Polyimide content of the polyimide tape with fluoro adhesive = 76.5% Inside diameter of the polyimide layer with fluoro adhesive = 1.2875 mm Circle surface diameter 1.2875 mm = 1.3018 mm ² Radial thickness of polyimide tape with fluoro adhesive = 0.0435 mm = 0.03048 / (1 - 0.30) Outer diameter of the polyimide layer with fluoro adhesive = 1,3745mm = 1, 2875 + 2 x 0.0435 Circle surface diameter 1.3745 mm = 1.4839 mm ² Surface of the polyimide tape layer with fluoro adhesive = 0.1821mm ² = 1.4839 - 1.3018 Mass of the polyimide tape layer with fluoro adhesive = .2786 = 0.1821 x 1.53 Polyimide mass of polyimide tape with fluoro adhesive = .2131 = 0.2786 x 0.765

The third layer:

Thickness of the first unsintered PTFE tape = 0.076 mm Kr overlay of the first unsintered PTFE tape = 53% Density of the first unsintered PTFE tape = 1.62 Inside diameter of the first ribbon layer Unsintered PTFE = 1.3745 mm Circle surface diameter 1,5362 mm = 1.4839 mm ² Radial thickness of the first unsintered PTFE tape = 0.11617 mm = 0.076 / (1 - 0.53) Outside diameter of the first ribbon layer Unsintered PTFE = 1.6980 mm = 1.3745 + 2 x 0.11617 Circle surface diameter 1,6980mm = 2.2643 mm ² Surface of the layer of the first unsintered PTFE tape = 0.7804 mm ² = 1.8536 - 1.4839 Mass of the layer of the first unsintered PTFE tape = 1.2643 = 0.7804 x 1.62 Thickness of second unsintered PTFE tape 0.076 mm Kr overlay of the second unsintered PTFE tape = 53% Density of second unsintered PTFE tape = 1.62 Inside diameter of the second ribbon layer Unsintered PTFE = 1.6980 mm Circle surface diameter 1.66980 mm = 2.2643 mm ² Radial thickness of the second unsintered PTFE tape = 0.11617 mm = 0.076 / (1 - 0.53) Outside diameter of the second ribbon layer Unsintered PTFE = 2.0214 mm = 1.6980 + 2x0,1617 Circle surface diameter 2.0214 mm = 3,2090 mm ² Surface of the layer of the second unsintered PTFE tape = 0.9447 mm ² = 3.2090 - 2.2643 Mass of the layer of the second unsintered PTFE tape = 1.5304 = 0.9447 x 1.62 Thickness of the third unsintered PTFE tape = 0.076 mm Kr overlay of the third unsintered PTFE tape = 53% Density of the third unsintered PTFE tape = 1.62 Inside diameter of the third ribbon layer Unsintered PTFE = 2.0214 mm Circle surface diameter 2.0214 mm = 3,2090 mm ² Radial thickness of the third unsintered PTFE tape = 0.11617 mm = 0.076 / (1 - 0.53) Outside diameter of the third ribbon layer Unsintered PTFE = 2.3448 mm = 2.0214 + 2 x 0.11617 Circle surface diameter 2,3448 mm = 4.3180 mm ² Surface of the layer of the third unsintered PTFE tape = 1.1090 mm ² = 4.3180 - 3.2090 Mass of the layer of the third unsintered PTFE tape = 1.7966 = 1.1090 x 1.62 Total mass of PTFE = 1.2643 + 1.5304 + 1.7966 = 4.5913

Ratio for the DW20D electric cable:

$$\mathrm{R}=4,5913/\left(0,0951+0,2131\right)=14,9$$

Each electrical cable of Tables 1a and 1b undergoes the dry arc propagation resistance test according to the test method of standard NF EN 3475-604.

This test makes it possible to produce, in a controlled manner, the effects of failures which are representative of what can occur in use when a wiring harness is damaged by wear so that arcing trips between the electrical cables and / or or between electrical cables and the conductive structure.

This test consisted in successively submitting 18 bundles of 7 electric cables each (of a length of 0.5 m) at 6 different intensities of short-circuit current, 3 of the 18 bundles being tested at the same intensity for the reproducibility of the test .

For each bundle of 7 cables, two electrical cables are deliberately damaged and short-circuited, making a total of 18 x 5 = 90 electrical cables for which collateral damage is measured.

To satisfy the requirement of the EN 2346-005 standard, less than 25% (collateral damage) of these 90 electrical cables must be damaged or the same as the resistance to the propagation of the arc must be at least 75% ( resistance propagation of electric arc = 100 - collateral damage).

Collateral damage is the ratio of the number of electrical cables damaged by the electric arc to the total number of electrical cables that have not been voluntarily damaged subjected to the test.

Thus, on the 90 electrical cables, it is necessary that at least 67 electrical cables resist the propagation of dry electric arc.

To do this, the collateral damage of the outer layer of the electrical cables is first visually controlled.

Then, the collateral cables of the bundle are subjected to a tensile strength test in water according to the method of the standard EN 3475-302, for a duration and at an alternating voltage value defined by the standard EN 2346- 005.

The results of Tables 2a and 2b clearly demonstrate that the electrical cables according to the invention (DW24A, DW20B, DW20C, DW20D, DW14B, DW14C) have a resistance to electric arc propagation of at least 75%, or even at least 90% according to the prescriptions of standard NF EN 3475-604.

Identical results were also obtained with an electrical cable of identical construction to the cable DW20B, but with an electric conductor section of 0.34 mm ² (DW22) for a ratio R greater than or equal to 4.

It is thus observed that the higher temperature of the heat treatment of the first layer of electric cables according to the present invention favors obtaining a much better resistance to the propagation of electric arc.

For example, 80% of resistance to electric arc propagation is obtained for DW20B, unlike the electric cable DW20A with which 56% is obtained.

Other tests concerning the fire resistance were also carried out according to the methods of standards NF EN 3475-408 and prEN 3475-417.

It clearly appears that the electric cables according to the present invention have a fire resistance higher than the requirements of the standard EN 2346-005, namely the insulation resistance of the electric cable in the flame for 15 minutes (according to NF EN 3475- 408) or for 5 minutes (according to prEN 3475-417) must be greater than 10,000 Ohms.

For example, the NF EN 3475-408 fire resistance test carried out on the electrical cable DW20D of Table 1a gives an insulation resistance of between 64,000 and 242,000 ohms.

For example, the prEN 3475-417 fire resistance test performed on the electrical cable DW20D of Table 1a according to different harness configurations gives an insulation resistance of between 54,000 and 2,300,000 ohms.

In parallel, it follows that the heat treatment of the first layer according to the present invention is not detrimental to the fire resistance of said cable.

The present invention is not limited to the examples of electric cables which have just been described and generally relates to all electrical cables that can be envisaged from the general indications provided in the description of the invention.

Claims (13)

Electrical cable comprising: an electrical conductor surrounded by a first layer comprising at least one winding of a mica ribbon, said mica ribbon being composed of mica particles deposited by means of a polymeric binder on a support, a second layer comprising at least one winding of a polyimide ribbon, and a third layer comprising at least one winding of a tape of polytetrafluoroethylene (PTFE), the first layer being heat-treated at a temperature of at least 400 ° C, and
the ratio R of the linear density of PTFE to the sum of the linear masses of the polymeric binder and the polyimide being such that: ○ R is greater than or equal to 2 when the cross section of the electrical conductor is at most 0.2 mm ² , preferably between 0.1 and 0.2 mm ² , ○ R is greater than or equal to 4 when the section of the electrical conductor is strictly greater than 0.2 mm ² and strictly less than 0.6 mm ² , ○ R is greater than or equal to 6 when the section of the electrical conductor is equal to 0.6 mm ² , ○ R is greater than or equal to 12 when the section of the electrical conductor is strictly greater than 0.6 mm ² , preferably not more than 3 mm ² . Electrical cable according to claim 1, characterized in that the heat treatment of the first layer is carried out for a time t greater than 30% by the time t ₀ required for degassing the first layer, preferably the duration t is at least 1 minute. Electrical cable according to claim 1 or 2, characterized in that the mica ribbon comprises at most 20% by weight of polymeric binder, preferably the mica ribbon comprises 13% by weight of polymeric binder. Electrical cable according to any one of the preceding claims, characterized in that the polymeric binder is a silicone resin. Electrical cable according to any one of the preceding claims, characterized in that the percentage of covering a tape of mica on itself during its winding and / or a polyimide tape on itself during its winding is at most 49%. Electrical cable according to any one of the preceding claims, characterized in that the second layer comprises a single winding of a polyimide tape. Electrical cable according to any one of the preceding claims, characterized in that the third layer comprises at least two windings of a PTFE tape. Electrical cable according to any one of the preceding claims, characterized in that the mica particles are of the phlogopite type. Electrical cable according to any one of the preceding claims, characterized in that the polyimide tape comprises a layer of polyimide coated on each side with a fluorinated ethylene propylene copolymer (FEP) coating. Electrical cable according to claim 9, characterized in that the second layer is heat-treated at a temperature above the melting temperature of the FEP layers. Electrical cable according to any one of the preceding claims, characterized in that the third layer is heat-treated at a temperature above 340 ° C. Electrical cable according to any one of the preceding claims, characterized in that the third layer further comprises an outer layer capable of being marked. Electrical harness comprising at least one electrical cable as defined in the preceding claims.

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