Earthing system TN and IT, design

Earthing system performance is assessed according to five criteria listed below:

• Electric shock protection;

• Electric fire hazard protection;

• Continuity of supply;

• Surge protection;

• Electromagnetic disturbance protection.

A summary of the properties of each earthing system results in the technical comparison provided below.

All earthing systems guarantee the same protection against electric shocks, provided that they are implemented and used in accordance with standards.

In the TT system, and IT system on the first insulation fault, the strength of the current generated by this fault is respectively low or very low, and the fire hazard is small.

In event of a full fault, the strength of the current generated by the insulation fault is high in TN type systems, and considerable damage is caused. TN systems without residual current devices do not provide sufficient protection, the move to the TN-S system associated with use of RCDs is recommended. In normal operation, the TN-C system exhibits a greater fire hazard than the rest. During a short-circuit, the energies lost during these unplanned journeys increase considerably. For this reason the TN-C system is forbidden in rooms where there is an explosion or a fire hazard.

Choice of the IT system avoids all harmful consequences of the insulation fault:

• The voltage sag;

• The disturbing effects of the fault current;

• Damage to equipment;

• Opening of the faulty feeder.

Correct operation makes the second fault really unlikely.

NB: it is always an association of measures that contributes to continuity of supply: backing of up sources, UPS (uninterruptible power supply), discrimination of protection devcies, IT system, maintenance department, etc.

In all earthing systems, protection may be required. To choose this protection, you must allow for site exposure and the type and activity of the establishment.

You must then determine the number and quality of equipotential zones to implement the necessary protection devices (surge arresters, etc.) on the lines of the various incoming or outgoing electrical systems.

Remarks:

• The VT system more frequently requires use of surge arresters;

• No system does away with these measures;

• In the IT system, protection against surges due to MV faults must be achieved by a surge limiting device.

Any earthing system can be chosen:

• For all differential mode disturbances;

• For all disturbances (common or differential mode) with a frequency greater than MHz.

The TT, TN-S and IT earthing systems can thus satisfy all electromagnetic compatibility criteria. Note only that the TN-S system generates more disturbances during insulation fault duration as the fault current is higher.

On the other hand, the TN-C and TN-C-S earthing systems are not recommended; in these systems, the PEN conductor, the equipment frames and the cable shielding are run through by a permanent load unbalance current. This current generates disturbing voltage drops between the frames of sensitive equipment linked to the PEN. The presence of harmonics of a number multiple of three has considerably amplified this current in modern installations.

(1) In event of an insulation fault.

(2) All electromagnetic disturbances:

• External: fault on HV distribution network, switching surges, atmospheric surges (lightning), etc.

• Internal: insulation fault currents, harmonics on LV network.