A) Cable Selection Factors to be considered in sizing of cable conductors Conductor material Insulating material Method of installation Installed envi

Transcription

1 Cable and Wiring Date : 26 November 2010 Ir. KF Cheung

2 A) Cable Selection Factors to be considered in sizing of cable conductors Conductor material Insulating material Method of installation Installed environment Ambient temperature Thermal insulating enclosure e Adjacent cables Type of protective device Voltage drop Minimum cross-sectionalsectional area

3 Comparison between Copper Conductor and Aluminum Conductor A) Copper Conductor High degree of electrical conductivity Tough, slow to tarnish Can be jointed without any special provision to prevent electrolytic action B) Aluminum Conductor Lower price & light in weight Pliable, it can be used in solid-core cables Excellent resistance to corrosion

4 Insulating Materials

5 Bends of Non-flexible Cable The minimum internal radius bend in cables for fixing wiring are shown in the following table

6 Correction Factor for Conductors Factors which affect the ability of a cable to lose heat are: Grouping (Cg or C1) Ambient temperature (Ca or C2) Thermal insulation (Ci or C3) Semi-enclosed enclosed fuse to BS 3036 (0.725 or C4) Type of installation (Table 4A)

7 Correction Factor for Conductors A) Grouping factor (Cg) -1 IEE Table 4B1 gives correction factors to be applied to te tabulated current-carrying carrying capacities where cables or circuits are grouped. Where the horizontal clearance s between adjacent cables exceed two cable diameter (2D2), no correction factor need be applied.

8 Correction Factor for Conductors A) Grouping factor (Cg) -2 If a cable is expected to carry not more than 30% of its grouped rating, it may be ignored from the rest of the group.

9 Correction Factor for Conductors B) Correction Factor for Ambient Temperature (Ca) Correction factor for ambient temperature is shown in IEE Table 4C1. Where for semi-enclosed enclosed fuses are being used, see IEE Table 4C2. It In / Ca Typical data are shown in the following table for quick reference.

10 Correction Factor for Conductors C) Correction Factor for thermal Insulation (Ci) The value of current-carrying-capacity capacity for various sizes of conductors shown in Tables of Appendix 4 have been taken into account of cables installed in a thermally insulated wall or ceiling where one side of the cable is in contact with a thermally conductive surface. Where the cable is totally enclosed in thermal insulation, Ci=0.5 shall be used in absence of more precise information. It In / Ci Ci shall only be applied to the open and clipped direct column of respective IEE Tables.

11 Correction Factor for Conductors D) factor for Semi-enlosed enlosed fuse to BS3036 (C4) When semi-enclosed enclosed fuse is used for protecting the conductor, a derating factor of shall be applied.

12 Correction Factor for Conductors E) General Formula for Correction Factors Applied to Cable Sizing It In / Cg x Ca x Ci x C4

13 Example

14 Example Protective device : BS 3036 fuses Ambient temperature: 30 o C Cable use :PVC twin with cpc cable Cabling conditions at: 1) Bunched and clipped direct 2) Passed through totally enclosed thermal insulation area 3) One side in contact with thermally insulated ceiling 4) Passed through a boiler house where ambient temperature of 45 o C 5) Clipped direct Ignore voltage drop Select the appropriate size of cables?

15 Voltage Drop The overall voltage drop shall not exceed the value appropriate to the safe functioning of the equipment in normal service. The voltage drop in any circuit from the origin of installation to the current-using using equipment should not exceed 4% of the nominal voltage. Volt drop pre unit value in from of mv/a/m are shown on IEE tables of Appendix 4. The values are based on the circuit conductor working at the maximum permitted operating temperature and at unity power factor.

16 Voltage Drop

17 Voltage Drop

18 Voltage Drop

19 Voltage Drop Voltage drop (V.D.) can be calculated as follows: V.D. = design current (Ib) x circuit length (L) x volt drop per unit (mv/a/m)

20 Example A PVC/SWA/PVC armoured cable is to be installed from an HRC 100A fuse in a distribution board to a 3-phase 380V motor, along with 5 other cables fixed to a perforated metal cable tray where the cable sheaths will be touching, if the cable length is 125 meters and the power factor of the load is 0.89, what size of cable would be required to satisfy voltage drop if the ambient temperature t is 30 o C and the voltage drop in the 3 phase feeder cable up to the distribution board is 3.7V and the total voltage drop allowed is 4%?

21 Thermal Constraint t To protect conductor insulation against thermal damage during short circuit conditions. I 2 t = K 2 S 2 t = K 2 S 2 / I 2 t = duration in second S=cross cross-sectional sectional area in mm 2 I = effective short-circuit current in A K = 115 for copper conductor insulated with PVC

22 Thermal Constraint t Procedure To check the prospective p short-circuit current at the farthest point of the circuit from the point where the device is installed To check the operation time of the device according to the short-circuit current from the time/ current characteristic of the device To check the adiabatic line of the conductor by superimposing onto the characteristics of protective devices.

23 Cable Selection Procedure Select wiring system to be installed and type of cable Calculate the equipment current demand using Table 4A Calculate the circuit design current (Ib) and using diversity allowance. Determine the overcueent protective device (In) : type; rating Check Ib In Determine correction factors for installation Grouping (Cg) Ambient temperature (Ca) Thermal insulation (Ci Ci) Semi-enclosed enclosed fuse (C4)

24 Cable Selection Procedure Calculate the tabulated current carrying capacity of conductor: It (min) In x (1/ Cg) x (1/ Ca) x (1/ Ci) x (1/ C4) Select cable size from Appendix 4 Check Ib In Iz Calculate volt drop at the farthest point of circuit

25 Cable Selection Procedure Does it offer shock protection in accordance with table 41B1, 41B2 & 41D for Zs (max)? Check Zs Zs (max) from the tables If No : Re-select device or re-select phase conductor size Re-select cpc size Use alternative method as stated in Reg Checked by calculation Obtain Ze form supply authority Calculate R1 + R2 using Table 17A & B Determine actual Zs = Ze + (R1 + R2)

26 Cable Selection Procedure Does the type and size of cpc offer protection? Check : S {(I 2 t)} / K If No : re-select type and/ or size of cpc Check the adiabatic line of conductor against the characteristic of overcurrent protective device.

27 The End