Earthing and Grounding systems

Posted 22 Feb 2021 by Nikolay
Types of grounding systems
Types of grounding systems

Today, the three system earthing arrangements, defined by standards IEC 60364 and NF C 15.100, are TN, TT and IT systems.

In order to ensure protection of persons, equipment and continuity of operation, the conductive wires and live parts of an electrical installation are "insulated" from the earthed exposed conductive parts. Insulation involves:

  • separation by insulating materials.
  • separation by linear clearances in gases (e.g. in air) or by creepage distances along insulators (e.g. to prevent flashover on electrical switchgear).

The different earthing schemes (often referred to as the type of power system or system earthing arrangements) described characterise the method of earthing the installation downstream of the secondary winding of a MV/LV transformer and the means used for earthing the exposed conductive-parts of the LV installation supplied from it.

Identification of the types of system earthing arrangements is thus defined by 2 letters. The first letter for connection of the transformer neutral (2 possibilities):

  • T for "earthed".
  • I for "unearthed" (or "isolated").

The second letter for the type of connection of the exposed conductive parts of the installation (2 possibilities):

  • T for "directly" earthed
  • N for "connected to earthed neutral" at the origin of the installation.

The combination of these two letters gives three possible configurations: TT, TN and IT.

TT system

One point at the supply source is connected directly to earth. All exposed- and extraneous-conductive-parts are connected to a separate earth electrode at the installation. The PE connection is provided by a local ground electrode. This electrode may or may not be electrically independent of the source electrode. The two zones of influence may overlap without affecting the operation of protective devices. Protection of persons against indirect contact is thus ensured by an RCD with medium or low sensitivity.

The TT system

T = Terra = directly grounded neutral

T = Terra = each item of equipment has separate low-impedance ground connection

TT Earthing systems.

Figure 1. TT Earthing systems.

Technique for the protection of persons: the exposed conductive parts are earthed and residual current devices (RCDs) are used. The RCD causes the deenergizing of switchgear as soon as the fault current has a touch voltage greater than the safety voltage Ui.

Operating technique: interruption for the first insulation fault.

Main characteristics

  • Simplest solution to design and install. Used in installations supplied directly by the public LV distribution network.
  • Does not require continuous monitoring during operation (a periodic check on the RCDs may be necessary).
  • Protection is ensured by special devices, the residual current devices (RCD), which also prevent the risk of fire when they are set to <= 500 mA.
  • Each insulation fault results in an interruption in the supply of power, however the outage is limited to the faulty circuit by installing the RCDs in series (selective RCDs) or in parallel (circuit selection).
  • Loads or parts of the installation which, during normal operation, cause high leakage currents, require special measures to avoid nuisance tripping, i.e. supply the loads with a separation transformer or use specific RCDs.

Advantage: Only 3 conductors are required

Disadvantage: An effective system only when transformer is located a long way from consumers. Used in low-voltage networks in areas in which the substation is located at a large distance from consumers, i.e. in rural localities. Used in medium-voltage networks in conjunction with (overhead) power lines. 

TN systems

The source is earthed as for the TT system (above). In the installation, all exposed- and extraneous-conductive-parts are connected to the neutral conductor. The versions of TN systems are shown below.

TN-C system

The neutral conductor is also used as a protective conductor and is referred to as a PEN (Protective Conductor and Neutral) conductor. This system is not permitted for conductors of less than 10 mm2 or for portable equipment.

T = Terra = directly grounded neutral

N = low-impedance return conductor to transformer neutral point

C = a "combined" conductor for PE and N = PEN

TN-C Earthing systems.

Figure 2. TN-C Earthing systems.

The TN-C system requires an effective equipotential environment within the installation with dispersed earth electrodes spaced as regularly as possible since the PEN conductor is both the neutral conductor and at the same time carries phase unbalance currents as well as 3rd order harmonic currents (and their multiples).

The PEN conductor must therefore be connected to a number of earth electrodes in the installation.

Caution: In the TN-C system, the “protective conductor” function has priority over the “neutral function”. In particular, a PEN conductor must always be connected to the earthing terminal of a load and a jumper is used to connect this terminal to the neutral terminal.

Advantage: Only 4 conductors

Disadvantage: Susceptible to electromagnetic interference because harmonics are discharged via the PEN which means that loads with N conductor are additionally stressed by harmonics.

TN-S system

The TN-S system (5 wires) is obligatory for circuits with cross-sectional areas less than 10 mm2 for portable equipment. The protective conductor and the neutral conductor are separate. On underground cable systems where lead-sheathed cables exist, the protective conductor is generally the lead sheath. The use of separate PE and N conductors (5 wires) is obligatory

T = Terra = directly grounded neutral

N = low-resistance return conductor to transformer neutral point

S = separate conductors for PE and N

TN-S Earthing systems.

Figure 3. TN-S Earthing systems.

Advantage: System conforms with EMC guidelines

Disadvantage: 5 conductors

TN-C-S System

The TN-C and TN-S systems can be used in the same installation. In the TN-C-S system, the TN-C (4 wires) system must never be used downstream of the TN-S (5 wires) system, since any accidental interruption in the neutral on the upstream part would lead to an interruption in the protective conductor in the downstream part and therefore a danger.

With this system, a combined N and PE (PEN) conductor exits from the transformer, but at some point, the PEN conductor is split up into separate PE and N lines. PEN is nonetheless the correct description for this PE because the neutral can be separated out of the combined conductor at any time. Once the neutral has been separated out of the combined conductor, it cannot be connected to the PEN again, i.e. it must be a "spur line"! If a neutral conductor that had already been split from the PEN were reconnected to it, it would constitute a parallel connection with an incalculable impedance and thus also an incalculable short-circuit load. Furthermore, it can result in undesirable stray ("vagrant") currents.

TN-C-S Earthing systems.

Figure 4. TN-C-S Earthing systems.

IT Systems

No intentional connection is made between the neutral point of the supply source and earth.

I = transformer neutral point isolated or with high-impedance ground connection

T = Terra = each item of equipment has separate low-impedance ground connection

Advantage: First fault = conductive connection from phase to enclosure does not cause disconnection.

Disadvantage: An additional monitoring system for detecting the first fault shall be installed.

Used, for example, in situations in which high availability of electrical installations is essential, e.g. in hospital operating theaters, potentially explosive atmospheres.

IT Earthing systems.

Figure 5. IT Earthing systems.




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