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Lightweight Construction Criteria in Wiring Harnesses Current automotive wiring systems comprise up to 3500 m of insulated cables with considerable weight. Saving potentials can be identified both in alternative insulation and conductor materials and in cable design. Modern insulation materials such as Coroflex TT3 made by Coroplast, for example, potentially offer insulation weight savings of up to 20 %. In addition to this discussion, the company presents an overview of the complex requirements made on high-voltage cables in road vehicles with hybrid drive technologies and fuel cell generators. HARDWARE ATZelektronik 04I2008 Volume 3 42 On-board Power Supply

Lightweight construction criteria in wiring harnesses

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Page 1: Lightweight construction criteria in wiring harnesses

Lightweight Construction

Criteria in Wiring Harnesses

Current automotive wiring systems comprise up to 3500 m of insulated cables with considerable weight. Saving potentials can be identified both in alternative insulation and conductor materials and in cable design. Modern insulation materials such as Coroflex TT3 made by Coroplast, for example, potentially offer insulation weight savings of up to 20 %. In addition to this discussion, the company presents an overview of the complex requirements made on high-voltage cables in road vehicles with hybrid drive technologies and fuel cell generators.

HARDWARE

ATZelektronik 04I2008 Volume 342

On-board Power Supply

Page 2: Lightweight construction criteria in wiring harnesses

1 Introduction

Electric cables in road vehicles are sub-ject to a wide range of technical require-ments nowadays. The design of these ca-bles must comply with a considerablenumber of standards including the inter-rrnationally applicable ISO 6722. In addi-tion to cable geometries, requirements are also specified for the quality of theelectrics and therefore on the materialsused in vehicle cabling. On the basis of the basic requirements made in ISO6722, further OEM-specific regulations have been developed which, in some cas-es, place extended requirements on elec-tric cables in vehicles. The automotivecables – whereby the focus here is espe-cially on electrical conductor and con-ductor insulation - will be assessed below for their potential for producing weight savings.

2 Conductor Materials

Copper as a conducting material is spec-ified as E-Cu and complies with the Eu-ropean standard EN 13602 respective the DIN EN 13602. The advantages of copper can be seen in excellent electri-cal conductivity of at least 58.5 Sm/mm². Added to this are good mechanical prop-erties such as a tensile strength of atleast 200 N/mm². For reasons of tensile strength, the conductor cross-section isnowadays limited to a minimum valueof 0.35 mm². In some vehicle construc-tion areas, high mechanical properties are required. Here, the smallest conduc-tor cross-section is 0.5 mm². This meansthat a tensile strength of at least 70 N to 100 N is required for each electrical ve-hicle conductor at a normal tensile

strength of 200 N/mm². It is thereforeessential that the tensile strength re-quirement of at least 70 N is adhered tofor the discussion on weight reductionthrough the use of smaller nominalcross-sections.

Nowadays, aluminium is used as aconductor for larger cable cross-sections.In the aluminium conductor segment,pure aluminium E-Al 99.7 (EN AW-1370) in accordance with DIN EN 573-3 has pre-vailed. The low tensile strength of 70 N/mm² to 120 N/mm² and the electrical conductivity of at least 35.0 Sm/mm² therefore limit the use of aluminium to larger cable cross-sections above 10 mm².It is not only the cold-flow properties of aluminium which prevent the use of known bonding methods, as used for copper cables. Special processes for con-tacting aluminium cables are currently available in limited numbers on the mar-rrket. Table 1 compares some technicalbenchmark data for E-Cu and E-Al as con-ducting materials.

Alloys and other metallic conductingmaterials are currently in a phase of de-velopment. The intention is their use incables which do not transmit high electri-cal power, but only signals. Ohmic con-ductive resistance plays a secondary role in this case. The overriding requirement on alternative conductor materials is highmechanical tensile strength. They are in-tended for use in achieving the above-mentioned tensile strength for cable nominal cross-sections of up to 0.13 mm².Using the cross-section reductions possi-ble today of 0.5 mm² or 0.35 mm² down tofuture cross-sections of 0.22 mm², 0.17mm² and 0.13 mm², effective savings inconstruction space and weight will bepossible in automotive wiring systems. Inaddition to further requirements such as

The Author

Helmut Wichmannis Head of Development

Wires & Cables at Coro-

plast Fritz Müller GmbH

& Co. KG in Wuppertal

(Germany).

Table 1: Technical benchmark data for the conducting materials E-Cu and E-Al

Aluminium Copper

Standard: DIN EN 573-3 EN 13602:2002

Abbreviation: E-AL 99.5 / 99.7 Cu-ETP / Cu-ETP1

Material number: 1350 / 1370 CW 003A / 004A

Conductivity (DC, 20°C): 35 m/(Ohm x mm²) 58.5 m/(Ohm x mm²)

Tensile strength: min. 120 N/mm² min. 200 N/mm²

Density: 2.7 g/cm³ 8.9 g/cm³

ATZelektronik 04I2008 Volume 3 43

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bending stiffness and bending cycle resist-ttance, considerable importance is attrib-uted to bonding, as with the conductormaterial Al. The contacting requirements made on copper as an established con-ducting material set the standards for al-ternative conducting materials. Intensive investigations into current contacting methods using crimping, welding andsoldering are being carried out in order to guarantee electrical connections which stand up to use and which are proved towork well for their entire lifetime.

3 Insulation Materials

Up to now, PVC has been the dominatingmaterial as insulation for automotive wir-rring systems. The requirements made on insulation materials are increasing as auto-mobile performance improves. Thermal

requirements have especially increased. Nowadays, normative temperature classesassist cable set design engineers in select-tting suitable insulation materials. The de-sign engineer’s decision is now also based on temperature measurements in the vehi-iicle and the resulting temperature profiles.Table 2 lists current valid temperature class2 -es and allocates insulation materials totheir applications in vehicles. In addition to the operating temperature, attention must of course also be paid to further wide-ranging requirements such as cable bend-ing strength, abrasion resistance, chemicalresistance and compatibility with othermaterials relevant to cable sets.

4 Cable Design

Constructive properties of automotive wires and cables are determined, for ex-

ample, in ISO 6722. Electrical conductors are specified as multi-core strand con-ductors for reasons of flexibility and vi-bration resistance. The individual stranddiameter can be varied depending on the flexibility required. Nominal cable cross-sections are allocated according to ohmic conductive resistance windows. Core in-sulation diameters (core diameters) arealso allocated to nominal cable cross-sec-tions in the form of windows. Accordingto current standards, reduced insulationwall thicknesses (FLR …) are permissiblein the cable nominal cross-section rangebetween 0.35 mm² and 35 mm². Nowa-days, ultra-reduced insulation wall thick-kknesses (FLU …) are already being used forsome applications.

One of the requirements for this is the observance of mechanical require-ments in accordance with the individualapplication.

Table 2: Current valid temperature classes and allocation of insulation materials to uses within the vehicle

Tempe-

rature

Class

Temperature Range

Material Designation

UsageApplications and car compartments

only standard applications listed

special applications on inquiryOperating Range

(3,000 h)

Short Time

(240 h)Core Jacket

1 A -40°C to +85°C to +110°CY PVC Polyvinyl Chloride Usage just only in technical reasonable special cases

2Y PE Polyethylene Dielectric for data transmission cable

2 B -40°C to +100°C to +125°C

Y PVC Polyvinyl Chloride Standard cores in the passenger compartment

4Y PAPP Polyamide Insulation for fuel tank gauge units

9Y PP Polypropylene Dielectric for data transmission cable

3 C -40°C to +125°C to +150°C

9Y PP Coroflex TT3Insulation for engine wiring

2X VPE Polyethylene, cross linked

11Y PUR Polyurethane Sheath material for enginge compartment and wheelhouses

31Y TPE-S Thermoplastic Elastomer Insulation for engine compartment and wheelhouse wiring

4 D -40°C to +150°C to +175°C

7Y ETFE Ethylentetrafluorethylene Insulation for engine and gearbox wiring

13Y TPE-E Thermoplastic Elastomer Insulation for reflector wiring (only dry side use)

2G SiR Silicone Rubber Engine compartment and batterie cable

5 E -40°C to +175°C to +200°C6Y FEP Tetrafluorethylenperfluorpropylene Insulation for engine and gearbox wiring

2G SiR Silicone Rubber Engine compartment and batterie cable

6 F -40°C to +200°C to +225°C6Y FEP Tetrafluorethylenperfluorpropylene Insulation for engine and gearbox wiring

2G SiR Silicone Rubber Engine compartment and batterie cable

7 G -40°C to +225°C to +250°C51Y PFA Perfluoralkoxy

Insulation for engine and gearbox wiring

8 H -40°C to +250°C to +275°C Insulation for lambda probe cable

HARDWARE

ATZelektronik 04I2008 Volume 344

On-board Power Supply

Page 4: Lightweight construction criteria in wiring harnesses

5 Weight-Saving Potentials for Electric Cables

In addition to the above-mentioned de-pendence on conductor material, theweight of electric cables in vehicles is di-rectly related to the volume of the insula-tion material. The insulation volume is a structural parameter and therefore di-rectly dependent on cable design. The insulation weight results from the pa-rameters of volume and specific density. The potential for weight savings depend-ing on these characteristic values is iden-tified below.

6 Influence of Insulation Material Specific Density

In Table 2, some insulation materials were allocated to their operating tem-perature ranges. Figure 1 shows examples of the specific densities of some insula-tion materials in temperature classes 2(To +105 °C) and 3 (To +125 °C). Arrowspoint to potential weight savings.– Temperature Class 2 (To +105°C): The

current insulation material PVC could be replaced by Compound CoroflexTT2 in the future. Weight savings of around 20 % can be made in this way due to the average ratio of the specific densities (1.1 / 1.4). Coroflex TT2 is notyet available as a series product.

– Temperature Class 3 (To +125°C): InTemperature Class T3, irradiated PE(XPE) and thermoplastic PP (CoroflexTT3) are in direct competition. Com-pound Coroflex TT3 is 20 % lighter.

TT3 stands for „Thermoplastically pro-duced automotive wires and cables inTemperature Class T3“. The thermoplas-tic process provides the advantage that the additional energy- and cost-intensivecross-linking using irradiation is no long-er required. The fact that irradiation isnot required results in a sustainable en-ergy saving. Furthermore, TT3 stands out due to its higher mechanical reserves atthe limits and its higher recycling capa-bility level. Automotive single cores and twisted cores with bare and tin-coated copper conductors are available. Theproduct range is rounded off by battery and generator cables with reduced insu-lation wall thickness down to a nominal conductor cross-section of 35 mm². Re-

lease of the first multi-core sheathed ca-bles, both screened and unscreened, isexpected this year. The trend towardsaluminium for larger cable cross-sectionswill not stop at the Coroflex TT3 productrange. Aluminium, as a lighter conduc-tor material, insulated with the TT3 Compound, can be provided for process-ing tests. In this combination, the light aluminium conductor material com-bined with the light TT3 insulation ma-terial is an extremely economically ad-vantageous product.

7 Influence of Insulation Volume

As an initial approximation, the volumeof conductor insulation is dependent on the parameters of electrical cable diame-ter and core insulation diameter. Due to requirements which this document willnot be going into any further, the corediameter is fixed, and therefore difficult to vary. What remains is the variable pa-rameter of conductor diameter. This isdetermined structurally by the conduc-tor composition. There are many conduc-tor compositions available on the mar-rrket. As described above, conductor com-position with circular individual cables (strands) has prevailed as a standard forreasons of flexibility, vibration resist-ance, bending cycle strength and ease of contacting, especially when crimped. If the nominal conductor cross-section isretained, there is potential for reduction

of the conductor diameter if individualstrands can be compressed by mechani-cal processes. The final consequence is touse solid round cable as an electrical con-ductor. Figure 2 shows these three designpossibilities using the example of a nom-inal cross-section. If the installation wallthickness is retained, it is possible toachieve savings in insulation volumes of up to 10 % and reductions in core diam-eters of up to 8 % using this example. Inthe case of compressed strands, however,the decreasing cable flexibility and theconsiderably reduced bending cyclestrength must be critically discussed.This can be accounted for by the changein contact surface areas between indi-vidual strands: from point-contact in thecase of round single cores up to large-sur-rrface contact for compressed single cores,and the increased frictional resistanceunder bending moment connected withthis. The reduction in cable flexibility in the use of solid single round cores mustalso be taken into account.

8 Summary

A wide range of weight-saving potentials can be achieved through a range of compo-sitions for electric cables and their core in-nnsulation. Aluminium has already becomean alternative on the market to copper forlarge cable cross-sections. A requirementfor this is larger construction space for thehigher aluminium cable diameters. For

Figure 1: Specific densities of some insulation materials in temperature classes 2 (T0 +105 °C) and 3 (T0 +125 °C); arrows point to potential weight savings

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Page 5: Lightweight construction criteria in wiring harnesses

the average range of cross-sections in ener-rrgy-transmitting vehicle cables, aluminiumcan be considered as a conductor material.This possibility is, however, only imple-mentable in serial production if develop-ments in contacting can produce qualita-tive and commercially usable results. A wide range of alternatives in the signal ca-aable sector are currently in development and test stages. In contrast to the develop-ment situation of aluminium cables, thenew conductor materials and composition possibilities will have to prove themselvesagainst the established contacting ele-ments and processes. Modern insulationmaterials offer influences on weight-sav-vving potentials which cannot be disregard-ed. The Coroflex TT3 product range of ca-bles is already supporting the intention of reducing the weight of automotive wiring systems, and therefore fuel consumption and CO2 emissions.

9 Outlook: High-Voltage Cable Strands

Current automotive wiring system volt-ages of ≤ 60 volt with the appropriate au-tomotive wires and cables are not suitable for high-voltage systems (≤ 600 volt) as used in some mildly and all fully hybridvehicles, as well as in conventional elec-tric drives and fuel cell-driven electric ve-hicles. These high-voltage (HV) classes are already defined in the Standard ISO 6722. However, the insulation materials and wall thicknesses of high-voltage cables asdescribed in the 60 V class will remainunchanged. The specific insulation resist-ttance provided by polymer materials is at-tttributed considerable importance underpermanent use in humid climatic condi-tions and high continuous deployment temperatures from Temperature Classes T3 +125 °C up to T6 +200 °C. The demand for high environment temperatures re-

sults both from their possible use in en-gine compartments and gearboxes andfrom current warming within electricalcores in the cables during operation. TheCoroflex TT3 range of products showsgood potential use for Temperature ClassT3 +125 °C. Modern silicone materials arerecommended for higher temperature requirements, partly due to their out-standing cost-benefit ratios. Silicone has been established for generations of vehi-cles using petrol motors as a core insula-tion material for high-tension ignition cables in the motor compartment. In ad-dition to high cable flexibility, modernsilicone materials in the Coroplast sili-cone product range also offer considera-bly better mechanical resistance than earlier compounds. The increase in tear propagation resistance to the level of thermoplastics and TPEs makes this espe-cially clear. Figure 3 and Figure 4 show typi4 -cal core compositions for cables for ener-rrgy transmission and activation of variousactuators.

Figure 4: Sketch of high-voltage multi-core screened sheathed cable

Figure 3: Sketch of 35 mm2 screened high-voltage cable

Figure 2: Three design possibilities using one sample nominal cross-section – if the installation wall thickness is retained, it is possible to achieve savings in insulation volumes of up to 10 % and reductions in core diameters of up to 8 % using this example

HARDWARE

ATZelektronik 04I2008 Volume 346

On-board Power Supply

Page 6: Lightweight construction criteria in wiring harnesses