TT Earthing System: Detailed Explanation, Diagrams

TT earthing system (or TT system): a distribution system in which one live part of a power source is earthed, exposed-conductive-parts of an electrical installation are connected to earth electrodes of the electrical installation electrically independent of earth electrodes of the power source by protective earthing conductors (PE) [defined in the IEC 60364-1].

BS7671 provides the following definition: a system having one point of the source of energy directly earthed, the exposed-conductive-parts of the installation being connected to earth electrodes electrically independent of the earth electrodes of the source.

As a summary, the TT system has one live part directly earthed at the power source. The exposed-conductive-parts of the electrical installation are connected to earth electrodes electrically independent of the earth electrode of the power source.

The Meaning of the Letters T, T

The letter codes used for designations of the types of system earthing have the following meanings.

The first letter determines presence or absence of earthing of live parts of the power source:

  • T – one live part of the power source is earthed.

Additional earthing PEN, PEM, PEL conductors and protective earthing conductor (PE) in the electrical distribution network (if any) may be provided.

The second letter specifies the earthing of exposed-conductive-parts of the electrical installation or electrical connection presence between the exposed-conductive-parts and the earthed live part of the power source:

  • Т – the exposed-conductive-parts are earthed irrespective of presence or absence of the earthing of any live part of the power source.

TT Earthing System Explained

In the TT type of system earthing (see figure 1), one of the live parts of the power supply, usually the neutral of the transformer, is earthed. The exposed-conductive-parts of the electrical installation of a building are also earthed. To carry out their protective earthing, an earthing arrangement is used, the earth electrode of which must be electrically independent from the power supply earthing arrangement.

With the TT type of system earthing, the protective conductors of the electrical installation of a building do not have the same electrical connection to the earthed neutral of the power supply as in the TN-S, TN-C-S systems.

When connecting the electrical installation of a building to the existing electrical distribution network, it can be performed with the TT type of system earthing. However, in densely built-up urban areas it is very difficult to make electrically independent earth electrodes. Electrically independent earth electrodes are not possible if the transformer substation is integrated into the building. For this reason, TN-S and TN-C-S systems are used in cities.

TT system 3-phase, 4-wire
Diagram 1. TT system 3-phase, 4-wire

The TT earthing system, in comparison to the TN-S, TN-C-S and TN-C systems, has low earth-fault currents. Therefore, in the electrical installations of buildings which comply with the TT type of system earthing, automatic disconnection of supply can only be realised by means of residual current devices.

Diagram 1 shows:

When connecting the electrical installation of a building to the existing electrical distribution network, it is possible to perform the electrical distribution system of the system they have formed with the TT type of system earthing. However, in some cases, it is very difficult, if not impossible, to perform the TT earthing system in an existing or planned electrical distribution system.

In the TT type of system earthing, exposed-conductive-parts of class I electrical equipment are connected to the earthing arrangement of the electrical installation of a building. The earth electrode of this earthing arrangement must be electrically independent from the earth electrode of the earthing arrangement of the power supply forming part of the low-voltage electrical distribution network. In urban areas with dense buildings and developed infrastructure it is very difficult, if not impossible, to install electrically independent earth electrodes.

Indeed, the earth electrodes of power supplies earthing arrangements in existing electricity distribution networks have electrical connections performed by means of PEN conductors of underground cable or overhead distribution lines, with multiple earth electrodes of earthing arrangements of the electrical installations of buildings, which correspond to the TN-C-S and TN-C type of system earthing.

The repeatedly “multiplied” earth electrodes of power supplies earthing arrangements actually “cover” the entire urban area. For electrical installations of a building located within the existing dense urban area, it is therefore only possible to speak conventionally about the implementation of the TT type of system earthing in the newly installed electrical installation of a building.

In TT systems with more than one power source, the disconnection of a power source may result in the disconnection of the neutral conductor from the earthing arrangement. It may therefore be necessary to establish a protective earthing conductor at another power source, or the system becomes an IT system.

NOTE. The TT multiple source distribution system see 444.4.6.2 of IEC 60364-4-44.

Examples of TT Earthing System Diagrams

Diagrams of the TT earthing systems are shown below.

TT system single-phase, 2-wire with the earthed phase conductor throughout the distribution system
Diagram 2. TT system single-phase, 2-wire with the earthed phase conductor throughout the distribution system
TT three-phase three-wire system
Diagram 3. TT system 3-phase 3-wire with earthed protective conductor and no neutral conductor throughout the system
TT system 3-phase, 4-wire (neutral included)
Diagram 4. TT system 3-phase, 4-wire with earthed protective conductor and neutral conductor throughout the distribution system
TT system 3-phase, 3-wire without the neutral conductor throughout the distribution system
Diagram 5. TT system 3-phase, 3-wire without the neutral conductor throughout the distribution system

Note to diagrams 2-5. Additional earthing of the protective conductor PE in the electrical installation may be provided.

What Types of Electrode Can Be Used for TT Earthing Systems?

  • (i) Earth rods or pipes
  • (ii) Earth tapes or wires
  • (iii) Earth plates
  • (iv) Underground structural metalwork embedded in foundations or other metalwork installed in the foundations
  • (v) Welded metal reinforcement of concrete (except pre-stressed concrete) embedded in the ground
  • (vi) Lead sheaths and other metal coverings of cables, where not precluded by Regulation 542.2.5 [2]
  • (vii) other suitable underground metalwork.

Note: Further information on earth electrodes can be found in BS 7430.

Does a TT System Need an RCD?

One or more of the following types of protective device shall be used, the former being preferred [2]:

  1. An RCD
  2. An overcurrent protective device.

NOTE 1: An appropriate overcurrent protective device may be used for fault protection provided a suitably low value of Zs is permanently and reliably assured.
NOTE 2: Where an RCD is used for fault protection the circuit should also incorporate an overcurrent protective device in accordance with Chapter 43 [2].

Where an RCD is used for fault protection, the following conditions shall be fulfilled:

  • The disconnection time shall be that required by Regulation 411.3.2.2 or 411.3.2.4 [2], and
  • RA x IΔn< 50 V

where:

RA – is the sum of the resistances of the earth electrode and the protective conductor connecting it to the exposed-conductive-parts (in ohms)

IΔn – is the rated residual operating current of the RCD.

NOTE 1: Where selectivity between RCDs is necessary refer also to Regulation 536.4.1.4 [2].
NOTE 2: Where RA is not known, it may be replaced by Zs.

What Resistance Values Are Required for a TT Earthing System?

The requirements of this regulation [411.5.3] are met if the earth fault loop impedance of the circuit protected by the RCD meets the requirements of Table 41.5 [2].

Rated residual operating current (mA)Maximum earth fault loop impedance Zs (ohms)
301667*
100500*
300167
500100
Table 41.5 – Maximum earth fault loop impedance (Zs) for non-delayed and time delayed ‘S’ Type RCDs to BS EN 61008-1 and BS EN 61009-1 for U0 of 230 V (see Regulation 411 .5.3) [2]

Table 41.5 of BS 7671:2018+A1:2020 states that 1667 ohms is the maximum earth fault loop impedance value where an RCD with a rated residual operating current of 30 mA is used.

Disconnection shall be within the times stated in Table 41.1 [2].

NOTE 1: Figures for Zs result from the application of Regulation 411.5.3(i) and (ii).
NOTE 2: The resistance of the installation earth electrode should be as low as practicable. A value exceeding 200 ohms may not be stable. Refer to Regulation 542.2.4.

NOTE 3: The type and embedded depth of an earth electrode shall be such that soil drying and freezing will not increase its resistance above the required value.

Advantages of the TT Earthing System

The TT earthing system ensures a sufficiently high level of electrical safety in the electrical installation of a building, and is therefore widely used in many countries around the world.

The advantage of the TT earthing system is that on exposed-conductive-parts of class I electrical equipment and extraneous-conductive-parts of a metal building, the electrical potential under normal conditions is always equal to the earth potential.

Sometimes the TT type of system earthing is preferred for the electrical installation of a building or that part of it which provides electricity to computing, information and control systems built on computers.

Disadvantage of the TT Earthing System

The only disadvantage of the TT earthing system is the simultaneous failure of a residual current device (RCD) and a breakdown of a phase conductor to the earthed enclosure of an electrical appliance.

In such a case, the PE protective conductors and the exposed conductive surfaces will be under potential (voltage), because the circuit-breaker of the damaged line may not trip when the phase conductor is shorted to the protective conductor, because the short-circuit current will not be sufficient. Therefore, the only protection in such a situation remains the equipotential bonding system and the installation of a two-stage residual current protection.

References

  1. IEC 60364-1
  2. BS 7671:2018+A2:2022
  3. IEC TS 62257-5-2015