Chapter A General rules of electrical installation design

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F22 © Schneider Electric - all rights reserved F - Protection against electric shock MV/LV NC100 diff. 300 mA selective S 5 Implementation of the TT system Discriminative protection at three levels (see Fig. F37) Withdrawable Masterpact or Visucompact NC100L MA intantaneous 300 mA Vigicompact NS100 Setting 1 300 mA Leakage current of the filter: 20 mA Terminal board Fig. F37 : Typical 3-level installation, showing the protection of distribution circuits in a TT-earthed system. One motor is provided with specific protection Schneider Electric - Electrical installation guide 2008 Discont. Leakage current equal to 3,5 mA per socket outlet (Information technology equipement): max 4 sockets outlets. NS100 MA
F - Protection against electric shock Three methods of calculation are commonly used: b The method of impedances, based on the trigonometric addition of the system resistances and inductive reactances b The method of composition b The conventional method, based on an assumed voltage drop and the use of prepared tables 6 Implementation of the TN system 6.1 Preliminary conditions At the design stage, the maximum permitted lengths of cable downstream of a protective circuit-breaker (or set of fuses) must be calculated, while during the installation work certain rules must be fully respected. Certain conditions must be observed, as listed below and illustrated in Figure F38. 1. PE conductor must be regularly connected to earth as much as possible. 2. The PE conductor must not pass through ferro-magnetic conduit, ducts, etc. or be mounted on steel work, since inductive and/or proximity effects can increase the effective impedance of the conductor. 3. In the case of a PEN conductor (a neutral conductor which is also used as a protective conductor), connection must be made directly to the earth terminal of an appliance (see 3 in Figure F38) before being looped to the neutral terminal of the same appliance. 4. Where the conductor y 6 mm 2 for copper or 10 mm 2 for aluminium, or where a cable is movable, the neutral and protective conductors should be separated (i.e. a TN-S system should be adopted within the installation). 5. Earth faults may be cleared by overcurrent-protection devices, i.e. by fuses and circuit-breakers. The foregoing list indicates the conditions to be respected in the implementation of a TN scheme for the protection against indirect contacts. RpnA PEN Notes: b The TN scheme requires that the LV neutral of the MV/LV transformer, the exposed conductive parts of the substation and of the installation, and the extraneous conductive parts in the substation and installation, all be earthed to a common earthing system. b For a substation in which the metering is at low-voltage, a means of isolation is required at the origin of the LV installation, and the isolation must be clearly visible. b A PEN conductor must never be interrupted under any circumstances. Control and protective switchgear for the several TN arrangements will be: v 3-pole when the circuit includes a PEN conductor, v Preferably 4-pole (3 phases + neutral) when the circuit includes a neutral with a separate PE conductor. Fig. F38 : Implementation of the TN system of earthing Schneider Electric - Electrical installation guide 2008 1 5 2 2 3 5 PE N 4 TN-C system TN-C-S system 6.2 Protection against indirect contact Methods of determining levels of short-circuit current In TN-earthed systems, a short-circuit to earth will, in principle, always provide sufficient current to operate an overcurrent device. The source and supply mains impedances are much lower than those of the installation circuits, so that any restriction in the magnitude of earth-fault currents will be mainly caused by the installation conductors (long flexible leads to appliances greatly increase the “fault-loop” impedance, with a corresponding reduction of shortcircuit current). The most recent IEC recommendations for indirect-contact protection on TN earthing systems only relates maximum allowable tripping times to the nominal system voltage (see Figure F12 in Sub-clause 3.3). 5 F23 © Schneider Electric - all rights reserved
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2 3 4 5 Chapter Q EMC guidelines Co
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Chapter A General rules of electrical installation design

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