Electrical Safety Earthing

The article highlights the importance of earthing with study of different low voltage earthing systems world-wide. - Divyanshu Pal, Shahrukh Khan, Dr. Rajesh Arora

Picture Courtesy: www.integralpower.com.au

The process of transferring the immediate discharge of the electrical energy directly to the earth by the help of the low resistance wire is known as the electrical earthing. The electrical earthing is done by connecting the non-current carrying part of the equipment or neutral of supply system to the ground.

Every building, equipment, power plant, substation facility included in electricity require earth grounding, either directly or through grounding system. The main objective of doing earthing in electrical network is safety.

But when the neutral for any system is not connected with the earth then it will be known as electrical system without earthing as depicted in fig 1.

Figure 1: Electrical system without earthing

Mostly, the galvanised iron is used for the earthing. The earthing provides the simple path to the leakage current and fault current in the system. The short-circuit current of the equipment passes to the earth which is assumed to have zero potential. Thus, protects the system equipment and personnel working with these equipment from damage as well as shock current as shown in fig 2.

Figure 2: Electrical system with earthing

Earthing is not likely to reduce the total magnitude of over voltages produce by lightening or switching surges, it can however mitigate the possibilities of excessive voltage stress on the phase to ground insulation of particular phase.

The system earth resistance should be such that which any fault occurs against which earthing is designed to give protection, the protective gear will operate to make the faulty main or plant harmless. In most cases, such operation involves isolation of the faulty main or plant, for example by circuit breaker or fuses.

Types of Electrical Earthing

The electrical equipment mainly consists of two non-current carrying parts. These parts are neutral of the system or frame or support structure of the electrical equipment. From the earthing of these two non-current carrying parts of the electrical system, earthing can be classified into two types: Neutral Earthing, and Equipment Earthing.

Neutral Earthing

In neutral earthing, the neutral of the system is directly connected to earth with the help of some metallic conducting wire. The neutral earthing is also called the system earthing. Such type of earthing is mostly provided to the system which has star winding. For example, the neutral earthing is provided in the generator, transformer, motor etc as shown in fig 3.

Figure 3: Neutral and equipment earthing

Equipment Earthing

Such type of earthing is provided to the electrical equipment. The non-current carrying part of the equipment like their metallic frame is connected to the earth by the help of the conducting wire as shown in fig 3. If any fault occurs in the apparatus, the short-circuit current to pass the earth by the help of wire. Thus, protect the system from damage.

Importance or purpose of earthing

  • To Protect the workers who regularly come in contact with electrical devices that might give them a shock.
  • To keep the voltage of the device constant in the healthy phase in case of single of single phase to ground fault.
  • A good grounding path which has a low impedance value ensure that faults in the electrical path are cleared quickly. If the faults stay within the system for a long time, they can pose a serious threat to the stability of the system.
  • Many modern electronic devices generate a form of ‘electrical noise’ that can cause damage to the device and reduce its efficiency, unless the device is property grounded.
  • Surge protection device function better with the of help of proper grounding.
  • Malfunctioning electric devices often leak electricity, which has the potential to start a fire if not redirected safely.

Classification of Earthing System

A low voltage (LV) distribution system may be identified according to its earthing system. These are defined using the five letters T (direct connection to earth), N (neutral), C (combined), S (separate) and I (isolated from earth). The first letter denotes how the transformer neutral (supply source) is earthed while the second letter denotes how the metal work of an installation (frame) is earthed. The third and fourth letters indicate the functions of neutral and protective conductors respectively. The electrical power network is shown in fig 4.

Figure 4: Power System Network

There are three possible configurations:

  • TN: Transformer neutral earthed, frame connected to neutral. The TN system includes three sub-systems: TN-C, TN-S and TN-C-S
  • TT: Transformer neutral earthed and frame earthed.
  • IT: Unearthed transformer neutral, earthed frame.

TN Earthing System

In a TN earthing system, the supply source (transformer neutral) is directly connected to earth with one or more conductors and all exposed conductive parts of an installation are connected to the neutral or protective earth conductor. The three sub-systems in TN earthing system are described below with their key characteristics.

TN-C Earthing System

TN-C system has the following features:

  • Neutral and protective functions are combined in a single conductor throughout the system. (PEN—Protective Earthed Neutral).
  • The supply source is directly connected to earth and all exposed conductive parts of an installation are connected to the PEN conductor as shown in fig 5.
Figure 5: TN-C Earthing System

Advantages of TN-C Earthing System

  • Earth fault loop impedance of TN-C earthing system is low.
  • It does not require earth electrode at site.
  • It is economical.

Disadvantages of the TN-C Earthing System

  • TNC earthing system is least safe as compared to other earthing systems.
  • TN-C system is less effective for Electromagnetic Compatibility (EMC) problems.
  • A fault in the LV network may cause touch voltages at other LV customers.

TN-S Earthing System

TN-S system has the following features:

  • A TN-S system has separate neutral and protective conductors throughout the system.
  • The supply source is directly connected to earth. All exposed conductive parts of an installation are connected to a protective conductor (PE) via the main earthing terminal of the installation as shown in fig 6.
Figure 6: TN-S Earthing System

Advantages of TN-S Earthing System

  • Earth fault loop impedance is low.
  • TN-S is the safest system.
  • Electromagnetic interference is low.
  • It does not require earth electrode at site.
  • TN-S earthing system could work with simple over current protection.

Disadvantages of the TN-S Earthing System

  • Low power factor (high inductance of long cable).
  • Requires extra equal potential bonding.
  • On occurrence of an insulation fault, the short-circuit current is high and may cause damage to equipment or electromagnetic disturbance.

TN-C-S Earthing System

TN-C-S earthing system has the following features:

  • Neutral and protective functions are combined in a single conductor in a part of the TN-C-S system. The supply is TN-C and the arrangement in the installation is TN-S as depicted in fig 7.
  • Use of a TN-S downstream from a TN-C.
  • All exposed conductive parts of an installation are connected to the PEN conductor via the main earthing terminal and the neutral terminal, these terminals being linked together.
Figure 7: TN-C-S Earthing System

This type of distribution is known also as protective multiple earthing and the PEN conductor is referred to as the combined neutral and earth (CNE) conductor.

The supply system PEN conductor is earthed at several points and an earth electrode may be necessary at or near a consumer’s installation.

Advantages of TN-C-S Earthing System

  • Safe system
  • Less expensive.

Disadvantages of the TN-C-S Earthing System

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.

TT Earthing System

In this system, the supply source has a direct connection to earth. All exposed conductive parts of an installation also are connected to an earth electrode that is electrically independent of the source earth as shown in fig 8.

Figure 8: TT Earthing System

The fault loop impedance is higher, and unless the electrode impedance is very low indeed.

Advantages of TT System

  • No risk of failure and suitable for premises where all AC power circuits are residual current device (RCD) protected.
  • Faults in the LV and MV grid do not migrate to other customers in the LV grid.
  • Simple earthing of the installation and the easiest to implement.

Disadvantages of the TT Earthing System

  • Each customer needs to install and maintain its own ground electrode. Safety and protection depend on the customer, thus complete reliability is not assured.
  • High over voltages may occur between all live parts and between live parts and PE conductor.
  • Possible overvoltage stress on equipment insulation of the installation.

IT System Earthing

In this system, the supply source is either connected to earth through deliberately introduced high earthing impedance (Impedance earthed IT system) or is isolated from earth All exposed conductive parts of an installation are connected to an earth electrode as shown in fig 9.

Figure 9: IT Earthing System

The conductive parts including metal body of the installations are connected to earthed through one or more local earth electrodes. These local electrodes do not have any direct connection to the source.

It is pertinent to mention here that single phase TT system shown in fig 9 is not used in India.

Advantages of IT system

The main advantages of IT system are:

  • It improves the energy availability: this is interesting for applications where a loss of electricity supply can cause a risk to people (in hospitals for example), or a financial risk (for some process in industry).
  • It can also eliminate the risks of fire or explosions in case of insulation fault, as the faulty current is very low.
  • It will increase electrical device life time, as faulty current is low, it causes less stress on the equipment.
  • Finally, it is possible to do preventive maintenance on the IT installation. Through the permanent insulation monitor device, we can detect insulation drops before they become insulation faults.

Disadvantages of IT system

  • This system experience repeated arcing grounds.
  • Insulation failure occurs during single phase to ground faults.
  • Earth fault protection for unearthed system is difficult.
  • Voltage due to lightning surges do not find path to earth.

Comparison of all Earthing Systems

Comparison of all earthing systems based on earth fault loop impedance, RCD preferred, need earth electrode at site, PE conductor cost, etc. has been carried out as mentioned in Table 1.

Brief of earthing system adopted world-wide

  • In India LT supply is generally through TN-S system. Neutral is double grounded at distribution transformer, neutral and earth run separately on distribution overhead line or cables. Additional earth electrode pits are installed at user ends for strengthening earth.
  • Most modern homes in Europe have TN-C-S earthing system. The combined neutral and earth occurs between the nearest transformer substation and the service cut-out (the fuse before the meter), separate earth and neutral cores are used in all the internal wiring.
  • In the areas of UK where underground power cabling is prevalent, the TN-S system is common.
  • In Australia, New Zealand and Israel, the TN-C-S system is in use. However, each customer is must provide a separate connection to earth via a dedicated earth electrode.
  • TN-C-S earthing system is used in the USA and Canada whereas France, Italy, Japan uses TT Earthing System.
  • TT system in suitable for rural areas because of cost.

Conclusion

From the above information it can be concluded that, if the grounding is not carried out properly, it can cause number of problems like:

  • An improper grounding results in higher potential being created in the equipment that can damage equipment and pose safety threat to working personnel.
  • It can delay in clearing of faults that will result in insufficient current flow.
  • The dangers of a fire caused by leaking electricity are increased exponentially.
  • It can cause reduction in the operational efficiency of the machine.

In addition, the choice of earthing system depends on the priority given to many aspects mentioned in table 1 by the relevant distribution company and regulatory authority of county.


Advertisements