Chapter A General rules of electrical installation design

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
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