Erection of earthing arrangements (TNC, TN-S, TNC-S, TT)

Earthing of low voltage networks

The earthing arrangements (TNC, TN-S, TNC-S, TT) of low voltage networks is largely determined by the Low Voltage Supplies. However, if the incoming supplies are at 11kV and the transformers are in the ownership of the user, the LV supplies may be earthed in a less conventional way using a high impedance. This arrangement is not allowed for public supplies.

However, it is a useful system when it is more important to maintain supplies than it is to clear the first earth fault.

EXAMPLE: An emergency lighting scheme for the evacuation of personnel from a hazardous area could use a high impedance system if it were considered less dangerous to maintain supplies after a first earth fault than to disconnect the light completely. The Channel Tunnel could be such a case.

Even in these circumstances the original earth fault should be corrected as quickly as possible.

The more conventional earthing arrangements are:

• TN-C where the earth and neutral are combined (PEN) and
• TN-S where they are separated (5 wire) or
• TN-C- S.

The latter is very common as it allows the single-phase loads to be supplied by phase and neutral with a completely separate earth system connecting together all the exposed conductive parts before connecting them to the PEN conductor via a main earthing terminal which is also connected to the neutral terminal.

Figure 1 – Earthing concepts

For protective conductors of the same material as the phase conductor the cross-sectional area shall be the same size as the phase conductor up to 16 mm².

For conductors which are not of the same material the cross-sectional area shall be adjusted in the ratios of the factor k from Table 43A in BS 7671. The k factor takes into account the resistivity, temperature coefficient and heat capacity of the conductor materials and of the initial and final temperatures.

TN-S system

In a TN-S system (Figure 1), the Neutral and Protective conductors must remain distinct throughout the system, and the source is solidly grounded.

A TN-S system possesses a specific drawback: if the protective conductor becomes an open circuit, there is no notice of a failure, potentially leaving installations unknowingly without an earth connection. In the occurrence of an earth fault, all exposed conductive components within a consumer installation may attain a dangerous voltage.

Earth fault protection devices will remain inoperative due to the absence of current flow to the ground.

See Figure 2.

Figure 2 – TN-S system

TN-C system

In a TN-C system (Figure 2), the neutral and protective functions must be integrated into a single conductor (PEN) throughout the entire system. It is advisable to have multiple connections to earth along the PEN conductor, with the source being securely earthed.

The necessity for multiple earth connections arises from the potential for the neutral to become an open circuit. In such instances, exposed conductive parts may elevate to line-to-earth voltage in single-phase systems, and may reach values up to line-to-earth voltage in three-phase systems, contingent upon the extent of load unbalance.

See Figure 3.

Figure 3 – TN-C system

TN-C-S system

In a TN-C-S system (Figure 3), the neutral and protective functions must be integrated into a single conductor (PEN) from the solidly earthed source to the consumer’s intake. It is advisable to establish many connections to the ground along the PEN conductor.

In the consumer’s installation, the neutral and protective conductors must be maintained separately.

The multiple earthing of the CNE conductor guarantees that, in the event of an open circuit, exposed conductive parts remain grounded; consequently, the supply voltage between the installation line and neutral conductors is significantly diminished, leading to unacceptable voltage fluctuations for consumers.

Figure 3 – TN-C-S system

TT installation system

Lastly there is the TT system which uses mother earth as part of the earth return.

The neutral and the earthed parts are only connected together via an electrode system back to the source earth (and neutral). To check that conventional systems are satisfactory, i.e. that the protection operates on the occurrence of an earth fault, it is necessary to calculate the earth fault loop impedance (Z_s) and ensure that the fault current through it will cause the protection to operate.

This is quite a tedious process, involving as it does the calculation of the impedances afforded not only by the earth return but also by:

1. The phase conductor
2. Supply transformer
3. Supply network
4. Any neutral impedance.

A TT system (Figure 4) features a solidly earthed source; the exposed conductive elements of the consumer’s installation are grounded via an installation earth electrode that is electrically independent of the source earth.

In the TT system, the earth fault loop impedance usually increases due to the presence of both the supply’s earth electrode resistance and the consumer’s installation’s earth electrode resistance in the fault path.

Figure 4 – TT system

Earthing Study and Testing

The substation will house the circuit breakers of fuses for the main cable connections to the sub-distribution boards and motor control centres. These protective devices must discriminate with those further down the line nearer the ultimate loads. A system study must therefore establish the correct ratings of the substation equipment to discriminate with the distribution network.

Earthing of equipment should be electrically complete and confirmed mechanically sound and tight.

Figure 5 – Earthing bolt on the switchboard roof

Earthing conductors (previously termed earth leads) must be checked for compliance with the IEE Regulations, i.e. they must not be aluminium and they must be not less than 25 mm² for copper and 50 mm² for steel, unless they are protected against corrosion. These conductors are for connection to the earth electrodes.

Another important point to bring out is that the earthing conductor to the earth electrode must be clearly and permanently labelled ‘SAFETY ELECTRICAL CONNECTION – DO NOT REMOVE’ and this should be placed at the connection of conductor to the electrode.

Figure 6 – Label: SAFETY ELECTRICAL CONNECTION – DO NOT REMOVE

Fuse ratings should also be checked in relation to other fuse ratings in the supply circuit or against the settings of protective relays to assure correct sequence of operation and discrimination. Circuit charts for distribution boards should be completed and designation labels fitted to ensure safe operation of switches and isolators.

All tests should be carried out as required in BS 7671, Part 7, and an Electrical Installation Certificate given by the contractor to the person ordering the work.

Many installations now incorporate rcds and fault current operated protective devices. These also must be tested using appropriate test equipment, full details of which can be found in BS 7671 or for more elaborate apparatus in BS 7430 and Guidance Notes which are published separately and amplify the requirements in the British Standard.

The nominal voltages at present are:

• 230V + 10% and -6%
• 400V + 10% and -6%

Watch Video – Earth fault loop impedance explained

Protective equipotential bonding

The safety protocol of automated supply termination necessitates:

1. Automated disconnection upon detection of an earth fault;
2. Protective earthing (to enable this); and
3. Implementation of protective equipotential bonding to mitigate contact voltages.

In instances when automated termination of power is implemented, protective equipotential bonding is advised, irrespective of the supply system type (see to Figure 7). It is advisable to bind extraneous conductive elements to the main earthing terminal of the installation using primary equipotential bonding conductors.

These extraneous conductive parts comprise:

1. Plumbing pipes for water installation;
2. Gas installation piping;
3. Additional installation piping and ductwork;
4. Central heating and air conditioning systems;
5. Exposed metallic components of the building framework; and
6. Additional metallic components including cable trays and cable ladder racks.

When both primary gas and water pipes approach a site, a common bonding conductor may be utilized; however, this conductor must be continuous or permanently joined (by soldering or crimping) to maintain continuity. A bonding conductor may also be utilized in conjunction with other external conducting components.

The primary equipotential bonding connections to the water and gas pipelines should be established as close as feasible to their entrance points into the premises. If an insulating section or insert is present, the bonding connection must be established to the metallic pipes on the consumer’s side of the section or insert, prior to the branch pipework.

The connection should be established, if feasible, within 600 mm of the meter outlet union or at any access point to the building if the meter is located externally (see to BS 7671:2008+A1, Regulation 544.1.2).

Figure 7 – Earthing arrangement and protective conductors for consumers’ installations

Suggested Course

Mastering Power Substations: Electrical Equipment, Busbar Schemes and Relay Protection

Mastering Power Substations: Electrical Equipment, Busbar Schemes and Relay Protection

Reference: Handbook of electrical installation practice fourth edition – Eur Ing GEOFFREY STOKES