Go to main contents pageIt is possible to combine the TN-C and TN-S layouts in the same installation. In such a case, the TN-C system must be located upstream from the TN-S system (=TN-C-S). These constructions are not recommended by current standardization. The currents which circulate in the and the raising of the potential in this conductor could affect the operation of the computing configuration. The Information Processing machines are equipped with switch-mode power supply with high leakage current.
| The TN-S neutral point connection is the only one highly recommended for powering Information Processing Machines (standard NF C15.100 - section 707 - May 1991). |
In this method for constructing a TN-S neutral point connection, the protecting conductor is connected to the neutral conductor at the origin of the installation. The same applies at the level of the output terminals of a power supply interface. As the Neutral and the P.E. wire are connected to the same output point of the source, and therefore to the same potential, the voltage measured between "neutral" and "earth" anywhere in the installation should be very close to 0 Volt.
At the secondary winding of the power supply interface, the Neutral point is connected directly to the measurement bar of the earth plate, or to the nearest bottom of excavation output. This connection is known as the "Neutral reference wire" because it defines the neutral point connection and how to construct the earth circuit of the site. See Figure 4-3.
Note: To define a neutral point connection, the output Neutral point of a power source or power supply interface must always be connected (reference) directly to the bar of the nearest earth plate, never to a ground point only.
In light of the current harmonics generated by the deforming charges linked to the switch-mode power supplies, the cross-section of the neutral conductor must be equal to or greater than that of the phase conductors. It is even recommended, for major single-phased configurations, to over-size it.
Pre-Requisites:
The TN-S point connection should be created at the output of a power supply interface. Its construction should comply with standard NF C15.100 and with the following figure.
Earth circuit diagram for a TN-S point connection