Chapter A General rules of electrical installation design

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G44 © Schneider Electric - all rights reserved G - Sizing and protection of conductors 7 The neutral conductor Examples Consider a three-phase circuit with a design load of 37 A to be installed using fourcore PVC insulated cable clipped to a wall, installation method C. From Figure G24, a 6 mm 2 cable with copper conductors has a current-carrying capacity of 40 A and hence is suitable if harmonics are not present in the circuit. b If 20 % third harmonic is present, then a reduction factor of 0,86 is applied and the design load becomes: 37/0.86 = 43 A. For this load a 10 mm 2 cable is necessary. b If 40 % third harmonic is present, the cable size selection is based on the neutral current which is: 37 x 0,4 x 3 = 44,4 A and a reduction factor of 0,86 is applied, leading to a design load of: 44.4/0.86 = 51.6 A. For this load a 10 mm 2 cable is suitable. b If 50 % third harmonic is present, the cable size is again selected on the basis of the neutral current, which is: 37 x 0,5 x 3 = 55,5 A .In this case the rating factor is 1 and a 16 mm 2 cable is required. 7.2 Protection of the neutral conductor (see Fig. G64 next page) Protection against overload If the neutral conductor is correctly sized (including harmonics), no specific protection of the neutral conductor is required because it is protected by the phase protection. However, in practice, if the c.s.a. of the neutral conductor is lower than the phase c.s.a, a neutral overload protection must be installed. Protection against short-circuit If the c.s.a. of the neutral conductor is lower than the c.s.a. of the phase conductor, the neutral conductor must be protected against short-circuit. If the c.s.a. of the neutral conductor is equal or greater than the c.s.a. of the phase conductor, no specific protection of the neutral conductor is required because it is protected by the phase protection. 7.3 Breaking of the neutral conductor (see Fig. G64 next page) The need to break or not the neutral conductor is related to the protection against indirect contact. In TN-C scheme The neutral conductor must not be open-circuited under any circumstances since it constitutes a PE as well as a neutral conductor. In TT, TN-S and IT schemes In the event of a fault, the circuit-breaker will open all poles, including the neutral pole, i.e. the circuit-breaker is omnipolar. The action can only be achieved with fuses in an indirect way, in which the operation of one or more fuses triggers a mechanical trip-out of all poles of an associated series-connected load-break switch. 7.4 Isolation of the neutral conductor (see Fig. G64 next page) It is considered to be the good practice that every circuit be provided with the means for its isolation. Schneider Electric - Electrical installation guide 2008
G - Sizing and protection of conductors Single-phase (Phase-Neutral) Single-phase (Phase-Phase) Three-phase four wires Sn u Sph Three-phase four wires Sn < Sph Fig. G64 : The various situations in which the neutral conductor may appear TT TN-C TN-S IT N N N N or or N 7 The neutral conductor (A) (A) or or N N N N or N N N or (A) Authorized for TT or TN-S systems if a RCD is installed at the origin of the circuit or upstream of it, and if no artificial neutral is distributed downstream of its location (B) The neutral overcurrent protection is not necessary: b If the neutral conductor is protected against short-circuits by a device placed upstream, or, b If the circuit is protected by a RCD which sensitivity is less than 15% of the neutral admissible current. Schneider Electric - Electrical installation guide 2008 N N N (B) (B) (B) G45 © Schneider Electric - all rights reserved
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Chapter A General rules of electrical installation design

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