TN system - 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 F22.

1. PE conductor must be regularly connected to earth as much as possible.

2. The PE conductor must not pass through ferromagnetic 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 F22) before being looped to the neutral terminal of the same appliance.

4. Where the conductor ≤ 6 mm² for copper or 10 mm² 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 or circuit breakers.

The foregoing list indicates the conditions to be respected in the implementation of a TN scheme for fault protection.

Fig. F22 – Implementation of the TN earthing system

Notes:

• 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.
• For a substation in which the metering is at low-voltage level, a means of isolation is required at the origin of the LV installation, and the isolation must be clearly visible.
• A PEN conductor must never be interrupted under any circumstances.

Control and protective switchgear for the several TN arrangements will be:

• 3-pole when the circuit includes a PEN conductor,
• Preferably 4-pole (3 phases + neutral) when the circuit includes a neutral with a separate PE conductor.