Chapter A General rules of electrical installation design

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G46 © Schneider Electric - all rights reserved G - Sizing and protection of conductors Q9 C9 L9 B8 x1 Fig. G65 : Example of single-line diagram Q6 C6 T6 Q7 C7 B2 Circuit 9 T1 C1 Q1 Circuit 6 P = 400 kVA U = 400 V Circuit 7 ku = 1.0 ib = 250.00 A P = 147.22 kW 8 Worked example of cable calculation Worked example of cable calculation (see Fig. G65) The installation is supplied through a 1,000 kVA transformer. The process requires a high degree of supply continuity and this is provided by the installation of a 500 kVA 400 V standby generator and the adoption of a 3-phase 3-wire IT system at the main general distribution board. The remainder of the installation is isolated by a 400 kVA 400/400 V transformer. The downstream network is a TT-earthed 3-phase 4-wire system. Following the single-line diagram shown in Figure G65 below, a reproduction of the results of a computer study for the circuit C1, the circuit-breaker Q1, the circuit C6 and the circuit-breaker Q6. These studies were carried out with ECODIAL 3.3 software (a Merlin Gerin product). This is followed by the same calculations carried out by the method described in this guide. 1000 kVA 400 V 50 Hz Circuit 1 Q10 C10 L10 Q3 B4 x1 Switchboard 2 Ks = 1.00 ib = 826.8 A Q12 C12 L12 Circuit 10 ku = 1.0 ib = 160.00 A P = 94.22 kW x1 Schneider Electric - Electrical installation guide 2008 G5 C5 Q5 Circuit 12 G ku = 1.0 ib = 250.00 A P = 147.22 kW Q11 C11 L11 x1 P = 500 kVA U = 400 V Circuit 5 Switchboard 8 Ks = 1.00 ib = 490.0 A Circuit 11 Switchboard 4 Ks = 1.00 ib = 250.0 A ku = 1.0 ib = 80.00 A P = 47.11 kW
G - Sizing and protection of conductors General network characteristics Earthing system IT Neutral distributed No Voltage (V) 400 Frequency (Hz) 50 Transformer T1 Number of transformers 1 Upstream fault level (MVA) 500 Rating (kVA) 1,000 Short-circuit impedance voltage (%) 6 Resistance of MV network (mΩ) 0.0351 Reactance of MV network (mΩ) 0.351 Transformer resistance RT (mΩ) 2.293 Transformer reactance XT (mΩ) 10.333 3-phase short-circuit current Ik3 (kA) 23.3 Cable C1 Maximum load current (A) 1,374 Type of insulation PVC Conductor material Copper Ambient temperature (°C) 30 Single-core or multi-core cable Single Installation method F Number of circuits in close proximity (table G21b) 1 Other coefficient 1 Selected cross-sectional area (mm 2 ) 6 x 95 Protective conductor 1 x 120 Length (m) 5 Voltage drop ΔU (%) .122 Voltage drop ΔU total (%) .122 3-phase short-circuit current Ik3 (kA) 23 1-phase-to-earth fault current Id (kA) 17 Circuit-breaker Q1 3-ph short-circuit current Ik3 upstream of the circuit-breaker (kA) 23 Maximum load current (A) 1,374 Number of poles and protected poles 3P3D Circuit-breaker NT 16 Type H 1 – 42 kA Tripping unit type Micrologic 5 A Rated current (A) 1,600 Fig. G66 : Partial results of calculation carried out with Ecodial software (Merlin Gerin) 8 Worked example of cable calculation Calculation using software Ecodial 3.3 The same calculation using the simplified method recommended in this guide Dimensioning circuit C1 The MV/LV 1,000 kVA transformer has a rated no-load voltage of 420 V. Circuit C1 must be suitable for a current of 3 1,000 x 10 IB = = 1,374 A per phase 3 x 420 Six single-core PVC-insulated copper cables in parallel will be used for each phase. These cables will be laid on cable trays according to method F. The “k” correction factors are as follows: k 1 = 1 (see table G12, temperature = 30 °C) k 4 = 0.87 (see table G17, touching cables, 1 tray, u 3 circuits) Other correction factors are not relevant in this example. The corrected load current is: IB 1,374 I' B = = = 1,579 A k1⋅k4 0. 87 Each conductor will therefore carry 263 A. Figure G21a G23 indicates that the the c.s.a. is is 95 mm2 . Schneider Electric - Electrical installation guide 2008 Busbars B2 Maximum load current (A) 1,374 Type Standard on edge Ambient temperature (°C) 30 Dimensions (m and mm) 1 m 2x5 mm x 63 mm Material Copper 3-ph short-circuit current Ik3 (kA) 23 3-ph peak value of short-circuit current Ik (kA) 48 Resistance of busbar R (mΩ) 2.52 Reactance of busbar X (mΩ) 10.8 Circuit-breaker Q6 3-ph short-circuit current upstream of the circuit-breaker Ik3 (kA) 23 Maximum load current (A) 560 Number of poles and protected poles 3P3D Circuit-breaker NS800 Type N – 50 kA Tripping unit type Micrologic 2.0 Rated current (A) 800 Limit of discrimination (kA) Total Cable C6 Maximum load current (A) 560 Type of insulation PVC Conductor material Copper Ambient temperature (°C) 30 Single-core or multi-core cable Single Installation method F Number of circuits in close proximity (table G20) 1 Other coefficient 1 Selected cross-sectional area (mm 2 ) 1 x 300 Protective conductor 1 x 150 Length (m) 15 Voltage drop ΔU (%) .38 Voltage drop ΔU total (%) .54 3-phase short-circuit current Ik3 (kA) 20 1-phase-to-earth fault current Id (kA) 13.7 Specific sizing constraint Overloads G47 © Schneider Electric - all rights reserved
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