Detail of GB/T 12706.2-2020

Introduction of GB/T 12706.2-2020

Codeofchina.com is in charge of this English translation. In case of any doubt about the English translation, the Chinese original shall be considered authoritative. GB/T 12706 consists of 4 parts under the general title Power cables with extruded insulation and their accessories for rated voltages from 1kV (Um=1.2kV) up to 35kV (Um=40.5kV): ——Part 1: Cables for rated voltage of 1kV (Um=1.2kV) and 3kV (Um=3.6kV); ——Part 2: Cables for rated voltages from 6kV (Um=7.2kV) up to 30kV (Um=36kV); ——Part 3: Cables for rated voltages of 35kV (Um= 40.5kV); ——Part 4: Test requirements on accessories for cables with rated voltages from 6kV (Um=7.2kV) up to 35kV (Um=40.5kV). This is Part 2 of GB/T 12706. This part replaces GB/T 12706.2-2008 Power cables with extruded insulation and their accessories for rated voltages from 1kV (Um=1.2kV) up to 35kV (Um=40.5kV) - Part 2: Cables for rated voltages from 6kV (Um=7.2kV) up to 30kV (Um=36kV). In addition to editorial changes, the following main technical changes have been made with respect to GB/T 12706.2-2008: ——The halogen-free flame-retardant (ST8) sheath material is added (see Table 4); ——The requirements for insulation, inner coverings and fillers, separation sheath and oversheath of halogen-free flame-retardant cables are added (see 6.1, 8.2.2, 13.3.3, 14.2 and Table 5); ——The requirements for the nominal insulation thickness of cables with rated voltages of 18/20(24)kV are added (see Tables 7 and 8); ——The requirements for extruded inner covering and lapped inner covering are modified (see 8.2; 8.1 of Edition 2008); ——The requirements for the overlap rate of copper strips are modified (see 10.2.3; 10.2.3 of Edition 2008);   ——The calculation formula for nominal thickness of lead sheath are modified (see Clause 12; 12.1 of Edition 2008); ——The requirements for armouring materials are modified (see 13.2; 13.2 of Edition 2008); ——The requirements for the diameter of thick round metal wires and those for the nominal thickness of separation sheath or inner covering under armour are added (see 13.4, Table 10 and 13.6); ——The requirements for thickness of oversheath are modified (see 14.3; 14.3 of Edition 2008); ——The method for determination of the cable conductor temperature during test is added (see 15.4 and Annex C); ——The withstand test of oversheath with semi-conductive layer is added (see 16.1 and 16.5); ——The requirements for the thickness of non-metal sheath are modified (see 17.5.3 and 19.3; 17.5.3 and 19.2 of Edition 2008); ——The measurement of the overlap rate of lapped tape and the gap rate of metal tape is added (see 17.11); ——The thickness measurement of lapped inner covering and (or) lapped bedding is added (see 17.12 and 19.26); ——The requirements for the diameter of bending test cylinder are modified (see 18.2.4; 18.1.3 of Edition 2008); ——The requirements for the measurement of thickness of lead sheath are added (see 19.4); ——Tests on mechanical properties, pressure at high temperature, behaviour at low temperature, burning behavior and water absorption of halogen-free flame-retardant (ST8) sheath material are added (see 19.6, 19.7, 19.9, 19.10, 19.16, 19.25, Tables 21 and 25); ——The flame spread test of cables is modified (see 19.16; 19.14 of Edition 2008); ——The applicable objects of the measurement of carbon black content of black PE oversheaths are modified (see 19.17.1; 19.15.1 of Edition 2008);   ——The requirements for electrical tests after installation of cables are modified (see 20.3.1; 20.2.1 of Edition 2008); ——The time under load of hot set test is deleted (see Tables 15 and 16; Tables 19 and 23 of Edition 2008); ——The requirements for electrical test on insulation are modified (see Table 17; Table 15 of Edition 2008); ——The rounding rules for specified values are added (see B.3); ——The code of polyolefin oversheath is added (see G.1.1); ——The representation method of flame retardant cable products is added (see G.1.3); ——The requirements for delivery length and marking of finished cable are added (see G.2.2 and G.2.3); ——The recommendation of ambient temperature during installation of halogen-free flame-retardant sheathed cables is added (see G.3.1). This part has been redrafted and modified adoption of IEC 60502-2:2014 Power cables with extruded insulation and their accessories for rated voltages from 1kV (Um=1.2kV) up to 30kV (Um=36kV) - Part 2: Cables for rated voltages from 6kV (Um=7.2kV) up to 30kV (Um=36kV). The following main structural changes have been made with respect to IEC 60502-2:2014: ——The hanging paragraphs in 17.9 are adjusted according to the requirements of GB/T 1.1-2009 ——Tables 5 and 25 are added; ——The table numbers are adjusted in the order of references. Starting from Table 6, the correspondence between tables in this part and those in IEC 60502-2:2014 is Table 6 corresponding to Table 5, Table 7 to Table 6, Table 8 to Table 7, Table 9 to Table 8, Table 10 to Table 9, Table 11 to Table 10, Table 12 to Table 11, Table 13 to Table 12, Table 14 to Table 13, Table 15 to Table 19, Table 16 to Table 23, Table 17 to Table 15, Table 18 to Table 14, Table 19 to Table 16, Table 20 to Table 17, Table 21 to Table 20, Table 22 to Table 21, Table 23 to Table 28, and Table 24 to Table 22; ——12.1 and 12.12 are deleted; ——17.11, 17.12, 19.25 and 19.26 are added; ——Clause 21 is added; ——Annex B is deleted, and Annex G is added, so that the annex numbers are changed. Annex B and Annex C of this part correspond to Annex C and Annex G of IEC 60502-2:2014 respectively. The main technical differences of this part from IEC 60502-2:2014 and their justifications are as follows: ——The adjustment on technical differences has been made on “Normative references” of this part to meet the technical specification in China; the adjustment situations are embodied in a concentrated way in Clause 2 - "Normative references" and the specific adjustments are as follows:  IEC 60038 is replaced by GB/T 156 which is modified in relation to the international standard (see 4.1);  IEC 60811-201, IEC 60811-202, IEC 60811-203 and IEC 60811-501 are replaced by GB/T 2951.11-2008 which is identical to the international standard (see 17.5.1, 17.8, 19.2.2, 19.3.2, 19.4.2, 19.5.3, 19.6.1, 19.6.3, 19.21.1, and Tables 16, 20 and 21);  IEC 60811-401 is replaced by GB/T 2951.12-2008 which is identical to the international standard (see 19.5.2, 19.6.2, 19.7, and Tables 20 and 21);  IEC 60811-402, IEC 60811-502, IEC 60811-503 and IEC 60811-606 are replaced by GB/T 2951.13-2008 which is identical to the international standard (see 19.15.1, 19.18.1, 19.22.1, 19.25.1, and Tables 15, 23, 24 and 25);  IEC 60811-504, IEC 60811-505 and IEC 60811-506 are replaced by GB/T 2951.14-2008 which is identical to the international standard (see 19.10, and Tables 22, 23 and 25);  IEC 60811-403, IEC 60811-404 and IEC 60811-507 are replaced by GB/T 2951.21-2008 which is identical to the international standard (see 17.10.1, 19.12, 19.14.1, and Tables 15 and 16);  IEC 60811-508 and IEC 60811-509 are replaced by GB/T 2951.31-2008 which is identical to the international standard (see 19.9, 19.11 and Tables 22, 23, 24 and 25);  IEC 60811-405 and IEC 60811-409 are replaced by GB/T 2951.32-2008 which is identical to the international standard (see 19.8.1, 19.19.1, and Tables 22 and 23);  IEC 60811-605 is replaced by GB/T 2951.41-2008 which is identical to the international standard (see 19.17.1 and Table 24);  GB/T 3048.10 is added for reference (see 13.3.3 and 14.1);  IEC 60885-3 is replaced by GB/T 3048.12 which is modified in relation to the international standard (see 16.3 and 18.2.5);  IEC 60230 is replaced by GB/T 3048.13 which is modified in relation to the international standard (see 15.3, 18.2.8 and 18.3.5.1);  IEC 60228 is replaced by GB/T 3956 which is identical to the international standard (see Clause 5, 10.2.1, 16.2 and 17.4);  GB/T 6995.3 is added for reference (see G.2.3);  GB/T 7113.2 is added for reference (see 19.16.6.1 and Table 5);  GB/T 8170 is added for reference (see B.3);  GB/T 11017.2-2014 is added for reference (see 10.2.2);  GB/T 11091 is added for reference (see 10.2.3);  GB/T 12706.1 is added for reference (see Table 2, 17.11 and 17.12);  IEC 60060-1 is replaced by GB/T 16927.1 which is modified in relation to the international standard (see 15.3);  IEC 60060-3 is replaced by GB/T 16927.3 which is modified in relation to the international standard (see 20.3.1);  IEC 60287-3-1 is deleted;  GB/T 17650.1 and GB/T 17650.2 are added for reference (see 19.16 and Table 5);  GB/T 18380.11 and GB/T 18380.13 are added for reference (see 19.16.1);  IEC 60332-1-2 is replaced by GB/T 18380.12 which is identical to the international standard (see 19.16.1);  GB/T 18380.33, GB/T 18380.34, GB/T 18380.35 and GB/T 18380.36 are added for reference (see 19.16.2);  GB/T 19666 is added for reference (see G.1.3);  JB/T 8137 (all parts) is added for reference (see G.2.4.1);  IEC 60183 is replaced by JB/T 8996 which is not equivalent to the international standard (see 4.1);  IEC 61034-2 is added for reference (see 19.16.3);  The non-normative references IEC 60853 and IEC 60986 are included in the Bibliography; ——In order to meet the technical requirements of China, the term of “fictitious value” is deleted (see Clause 3); ——In order to meet the technical requirements of China, the requirements for the nominal insulation thickness of cables with rated voltages of 18/20(24)kV are added (see Tables 7 and 8); ——In order to meet the technical requirements of China, the thickness requirements (see 8.2.3 and 8.2.4) and technical requirements (see 17.5, 17.12, 19.3 and 19.26) for extruded and lapped inner coverings are modified; ——In order to meet the technical requirements of China, the thickness test requirements of extruded inner coverings are added (see 17.5 and 19.3); ——In order to clarify the requirements of copper strip materials for cables, the requirements of copper strip raw materials are added (see 10.2.3); ——To ensure the quality of extruded separation sheath and oversheath, the requirements for spark test of extruded separation sheath and oversheath are added (see 13.3.3 and 14.1); ——In order to meet the technical requirements of China, the requirements for the diameter of thick round metal wire and those for the nominal thickness of separation sheath or inner covering under armour are added (see 13.4, Table 10 and 13.6); ——In order to meet the technical requirements of China, halogen-free flame-retardant sheath materials (see Tables 4 and 14.2) and corresponding requirements and tests (see 19.16, 19.25, and Tables 5 and 24) are added;   ——In order to meet the technical requirements of China, the requirements for measurement of overlap rate and gap rate of lapped materials (see 17.11) and the thickness measurement of lapped inner covering and (or) lapped bedding (see 17.12 and 19.26) are added; ——In order to improve the domestic burning requirements for power cables, the flame spread test of cables is modified (see 19.16); ——In order to improve the domestic technical requirements for power cables, the rounding rules for specified values (see B.3) the supplementary items for cable products (see Clause 21 and Annex G) are added; The following editorial changes have been made in this part: ——In order to adapt to China's technical standard system, the standard name is changed to Power cables with extruded insulation and their accessories for rated voltages from 1kV (Um=1.2kV) up to 35kV (Um=40.5kV) - Part 2: Cables for rated voltages from 6kV (Um=7.2kV) up to 30kV (Um=36kV); ——Annex B - “Tabulated continuous current ratings for cables having extruded insulation and a rated voltage from 3.6/6kV up to 18/30kV”, an informative annex to IEC 60502-2:2014, is deleted. This part was proposed by the China Electrical Equipment Industrial Association. This part is under the jurisdiction of SAC/TC 213 National Technical Committee on Wires and Cables of Standardization Administration of China. The previous editions of this part are as follows: ——GB 12706.2-1991, GB/T 12706.2-2002 and GB/T 12706.2-2008; ——GB 12706.1-1991; ——GB 12706.3-1991. Power cables with extruded insulation and their accessories for rated voltages from 1kV (Um=1.2kV) up to 35kV (Um=40.5kV) - Part 2: Cables for rated voltages from 6kV (Um=7.2kV) up to 30kV (Um=36kV) 1 Scope This part of GB/T 12706 specifies the construction, dimensions and test requirements of power cables with extruded insulation for rated voltages from 6kV to 30kV. This part is applicable to the power cables with extruded insulation for rated voltages from 6kV to 30kV for fixed installations such as distribution networks or industrial installations. It is also applicable to cables with longitudinal watertightness and their tests. When determining applications, it is recommended that the possible risk of radial water ingress is considered. Cables for special installation and service conditions are not included, for example cables for overhead networks, the mining industry, nuclear power plants (in and around the containment area) nor for submarine use or shipboard application. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. GB/T 156 Standard voltages (GB/T 156-2017, IEC 60038:2009, MOD) GB/T 2951.11-2008 Common test methods for insulating and sheathing materials of electric and optical cables - Part 11: Methods for general application - Measurement of thickness and overall dimensions - Tests for determining the mechanical properties (IEC 60811-1-1:2001, IDT) GB/T 2951.12-2008 Common test methods for insulating and sheathing materials of electric and optical cables - Part 12: Methods for general application - Thermal ageing methods (IEC 60811-1-2:1985, IDT) GB/T 2951.13-2008 Common test methods for insulating and sheathing materials of electric and optical cables - Part 13: Methods for general application - Measurement for determining the density - Water absorption tests - Shrinkage test (IEC 60811-1-3:2001, IDT) GB/T 2951.14-2008 Common test methods for insulating and sheathing materials of electric and optical cables - Part 14: Methods for general application - Test at low temperature (IEC 60811-1-4:1985, IDT) GB/T 2951.21-2008 Common test methods for insulating and sheathing materials of electric and optical cables - Part 21: Methods specific to elastomeric compounds - Ozone resistance, hot set and mineral oil immersion tests (IEC 60811-2-1:2001, IDT) GB/T 2951.31-2008 Common test methods for insulating and sheathing materials of electric and optical cables - Part 31: Methods specific to PVC compounds - Pressure test at high temperature - Test for resistance to cracking (IEC 60811-3-1:1985, IDT) GB/T 2951.32-2008 Common test methods for insulating and sheathing materials of electric cables - Part 32: Methods specific to PVC compounds - Loss of mass test - Thermal stability test (IEC 60811-3-2:1985, IDT) GB/T 2951.41-2008 Common test methods for insulating and sheathing materials of electric and optical cables - Part 41: Methods specific to polyethylene and polypropylene compounds - Resistance to environmental stress cracking - Measurement of the melt flow index - Carbon black and/or mineral filler content measurement in polyethylene by direct combustion - Measurement of carbon black content by thermo gravimetric analysis (TGA) - Assessment of carbon black dispersion in polyethylene using a microscope (IEC 60811-4-1:2004, IDT) GB/T 3048.10 Test methods for electrical properties of electric cables and wires - Part 10: Spark test of extruded protective sheaths GB/T 3048.12 Test methods for electrical properties of electric cables and wires - Part 12: Partial discharge test (GB/T 3048.12-2007, IEC 60885-3:1988, MOD) GB/T 3048.13 Test methods for electrical properties of electric cables and wires - Part 13: Impulse voltage test (GB/T 3048.13-2007, IEC 60230:1966, IEC 60060-1:1989, MOD) GB/T 3956 Conductors of insulated cables (GB/T 3956-2008, IEC 60228:2004, IDT) GB/T 6995.3 Markings for electric cables and wires - Part 3: Identifications of cables and wires GB/T 7113.2 Flexible insulating sleeving - Part 2: Methods of test (GB/T 7113.2-2014, IEC 60684-2:2003, MOD) GB/T 8170 Rules of rounding off for numerical values & expression and judgment of limiting values GB/T 11017.2-2014 Power cables with cross-linked polyethylene insulation and their accessories for rated voltage of 110kV (Um=126kV) - Part 2: Power cables GB/T 11091 Copper strip for cables GB/T 12706.1 Power cables with extruded insulation and their accessories for rated voltages from 1kV (Um=1.2kV) up to 35kV (Um=40.5kV) - Part 1: Cables for rated voltage of 1kV (Um=1.2kV) and 3kV (Um=3.6kV) (GB/T 12706.1-2020, IEC 60502-1:2009, MOD) GB/T 16927.1 High-voltage test techniques - Part 1: General definitions and test requirements (GB/T 16927.1-2011, IEC 60060-1:2010, MOD) GB/T 16927.3 High-voltage test techniques for low-voltage equipment - Part 3: Definitions and requirements for on-site testing (GB/T 16927.3-2010, IEC 60060-3:2006, MOD) GB/T 17650.1 Test on gases evolved during combustion of materials from cables - Part 1: Determination of the amount of halogen acid gas (GB/T 17650.1-1998, IEC 60754-1:1994, IDT) GB/T 17650.2 Test on gases evolved during combustion of materials from cables - Part 2: Determination of degree of acidity of gases by measuring pH and conductivity (GB/T 17650.2-1998, IEC 60754-2:1991, IDT) GB/T 18380.11 Tests on electric and optical fibre cables under fire conditions - Part 11: Test for vertical flame propagation for a single insulated wire or cable - Apparatus (GB/T 18380.11-2008, IEC 60332-1-1:2004, IDT) GB/T 18380.12 Tests on electric and optical fibre cables under fire conditions - Part 12: Test for vertical flame propagation for a single insulated wire or cable - Procedure for 1kW pre-mixed flame (GB/T 18380.12-2008, IEC 60332-1-2:2004, IDT) GB/T 18380.13 Tests on electric and optical fibre cables under fire conditions - Part 13: Test for vertical flame propagation for a single insulated wire or cable - Procedure for determination of flaming droplets/particles (GB/T 18380.13-2008, IEC 60332-1-3:2004, IDT) GB/T 18380.33 Tests on electric and optical fibre cables under fire conditions - Part 33: Test for vertical flame spread of vertically-mounted bunched wires or cables - Category A (GB/T 18380.33-2008, IEC 60332-3-22:2000, IDT) GB/T 18380.34 Tests on electric and optical fibre cables under fire conditions - Part 34: Test for vertical flame spread of vertically-mounted bunched wires or cables - Category B (GB/T 18380.34-2008, IEC 60332-3-23:2000, IDT) GB/T 18380.35 Tests on electric and optical fibre cables under fire conditions - Part 35: Test for vertical flame spread of vertically-mounted bunched wires or cables - Category C (GB/T 18380.35-2008, IEC 60332-3-24:2000, IDT) GB/T 18380.36 Tests on electric and optical fibre cables under fire conditions - Part 36: Test for vertical flame spread of vertically-mounted bunched wires or cables - Category D (GB/T 18380.36-2008, IEC 60332-3-25:2000, IDT) GB/T 19666 Flame retardant and fire resistant wires and cables JB/T 8137 (all parts) Delivery drums for wires and cables JB/T 8996 Guide to the selection of high-voltage cables (JB/T 8996-2014, IEC 60183:1984, NEQ) IEC 60229:2007 Electric cables - Tests on extruded oversheaths with a special protective function IEC 61034-2 Measurement of smoke density of cables burning under defined conditions - Part 2: Test procedure and requirements ISO 48 Rubber, vulcanized or thermoplastic - Determination of hardness (hardness between 10 IRHD and 100 IRHD) 3 Terms and definitions For the purposes of this document, the following terms and definitions apply. 3.1 Terms concerning dimensional values 3.1.1 nominal value value by which a quantity is designated and which is often used in tables Note: Usually, in this part, nominal values give rise to values to be checked by measurements taking into account specified tolerances. 3.1.2 approximate value value which is neither guaranteed nor checked Note: It is used, for example, for the calculation of other dimensional values. 3.1.3 median value when several test results have been obtained and ordered in an increasing (or decreasing) succession, the median value is the middle value if the number of available values is odd, and the mean of the two middle values if the number is even 3.2 Definitions concerning the tests 3.2.1 routine tests tests made by the manufacturer on each manufactured length of cable to check that each length meets the specified requirements   3.2.2 sample tests tests made by the manufacturer on samples of completed cable or components taken from a completed cable, at a specified frequency, so as to verify that the finished product meets the specified requirements 3.2.3 type tests tests made before supplying, on a general commercial basis, a type of cable covered by this part, in order to demonstrate satisfactory performance characteristics to meet the intended application Note: These tests are of such a nature that, after they have been made, they need not be repeated, unless changes are made in the cable materials or design or manufacturing process which might change the performance characteristics. 3.2.4 electrical tests after installation tests made to demonstrate the integrity of the cable and its accessories as installed 4 Voltage designations and materials 4.1 Rated voltages The rated voltages U0/U (Um) of the cables considered in this part are as follows: 3.6/6 (7.2)kV, 6/6 (7.2)kV, 6/10 (12)kV, 8.7/10 (12)kV, 8.7/15 (17.5)kV, 12/20 (24)kV, 18/20 (24)kV and 18/30 (36)kV. Note: The voltages given above are the correct designations although in some countries other designations are used, e.g. 3.5/6kV, 5.8/10kV, 11.5/20kV and 17.3/30kV. In the voltage designation of cables U0/U (Um): ——U0 is the rated power frequency voltage between conductor and earth or metal screen for which the cable is designed; ——U is the rated power frequency voltage between conductors for which the cable is designed; ——Um is the maximum value of the "highest system voltage'' for which the equipment may be used (see GB/T 156).

Contents of GB/T 12706.2-2020

Foreword III 1 Scope 2 Normative references 3 Terms and definitions 4 Voltage designations and materials 5 Conductors 6 Insulation 7 Screening 8 Assembly of three-core cables, inner coverings and fillers 9 Metal layers for single-core and three-core cables 10 Metal screen 11 Concentric conductor 12 Lead sheath 13 Metal armour 14 Oversheath 15 Test conditions 16 Routine tests 17 Sample tests 18 Electrical type tests 19 Non-electrical type tests 20 Electrical tests after installation 21 Supplementary terms for cable products Annex A (Normative) Fictitious calculation method for determination of dimensions of protective coverings Annex B (Normative) Rounding of numbers Annex C (Informative) Determination of the cable conductor temperature Annex D (Normative) Method of measuring resistivity of semi-conducting screens Annex E (Normative) Determination of hardness of HEPR insulations Annex F (Normative) Water penetration test Annex G (Normative) Supplementary terms of cable products Bibliography Figure C.1 Typical test set-up for the reference loop and the main test loop Figure C.2 Example of an arrangement of the temperature sensors on the conductor of the reference loop Figure D.1 Measurement of the volume resistivity of the conductor screen Figure D.2 Measurement of the volume resistivity of the insulation screen Figure E.1 Test on surfaces of large radius of curvature Figure E.2 Test on surfaces of small radius of curvature Figure F.1 Schematic diagram of apparatus for water penetration test Figure G.1 Composition and arrangement sequence of product model Table 1 Recommended rated voltages U Table 2 Insulating compounds Table 3 Maximum conductor temperatures for different types of insulating compound Table 4 Maximum conductor temperatures for different types of sheathing compound Table 5 Test methods and requirements for halogen free compounds Table 6 Nominal thickness of PVC/B insulation Table 7 Nominal thickness of cross-linked polyethylene (XLPE) insulation Table 8 Nominal thickness of ethylene propylene rubber (EPR) and hard ethylene propylene rubber (HEPR) insulation Table 9 Thickness of extruded inner covering Table 10 Nominal diameter of round armour wires Table 11 Nominal thickness of armour tapes Table 12 Routine test voltages Table 13 Number of samples for sample tests Table 14 Voltages for sample tests Table 15 Test requirements for particular characteristics of various thermosetting insulating compounds Table 16 Test requirements for particular characteristics of elastomeric sheathing compound Table 17 Electrical type test requirements for insulating compounds Table 18 Impulse voltages Table 19 Non-electrical type tests Table 20 Test requirements for mechanical characteristics of cable insulation (before and after ageing) Table 21 Test requirements for mechanical characteristics of sheathing compounds (before and after ageing) Table 22 Test requirements for particular characteristics for PVC sheathing compounds Table 23 Test requirements for particular characteristics for PVC insulating compound Table 24 Test requirements for particular characteristics of PE (thermoplastic polyethylene) sheathing compounds Table 25 Test requirements for particular characteristics for halogen free sheathing compounds Table A.1 Fictitious diameter of conductor Table A.2 Increase in diameter for concentric conductors and metallic screens Table A.3 Increase in diameter for additional bedding Table B.1 Rounded digits of specified values, measured values or their calculated value Table G.1 Common cable models Table G.2 Minimum bending radius during cable installation