Earthing System According to IEC 60364 and BS 7671

In any medium or low voltage three-phase system there are three single voltages measured between each phase and a common point called the “neutral point”. In a network, the earthing system plays a very important role. When an insulation fault occurs or a phase is accidentally earthed, the values taken by the fault currents, the touch voltages and overvoltages are closely linked to the type of neutral earthing connection. 

 A directly earthed neutral strongly limits overvoltages but it causes very high fault currents, whereas an unearthed neutral limits fault currents to very low values but encourages the occurrence of high overvoltages.  In any installation, service continuity in the event of an insulation fault is also directly related to the earthing system. 

An unearthed neutral permits service continuity during an insulation fault. Contrary to this, a directly earthed neutral, or low impedance-earthed neutral, causes tripping as soon as the first insulation fault occurs. The extent of the damage to some equipment, such as motors and generators presenting an internal insulation fault, also depends on the earthing system. 

What is Earthing? 

According to BS 7671

Connection of the exposed conductive parts of an installation to the main earthing terminal of the installation.

What is earthing system?

BS 7671 further defined

Earthing system is an electrical system consisting of a single source of electrical energy and an installation. For certain purposes, of the Regulations, types of systems are identified as follows, depending upon the relationship of the source, and of exposed conductive parts of the installation, to Earth.

The main purpose of which is to ensure safety by making the potential of all conductive parts in the system is the same with ground (zero Volts). Thus during fault there will be disconnection of supply as fast as possible. 

Type of earthing system

T = Earth (from the French word Terre)

N = Neutral

S = Separate

C = Combined

I = Isolated 

TN-S System

A TN-S earthing system is a type of electrical earthing system that is commonly used in power distribution networks. In this system, the protective earth (PE) and neutral (N) conductors are separated throughout the entire network, from the power generation source to the point of consumption.

Figure 1. TN-S Earthing System

Due to the separate earth and neutral conductors, the TN-S system provides a high degree of safety for both people and equipment. It is widely used in low voltage power distribution networks and is the recommended earthing system for most types of electrical installations.

TN-C System

A TN-C earthing system, also known as a combined PEN (protective earth and neutral) system, is a type of electrical earthing system in which the protective earth (PE) and neutral (N) conductors are combined into a single conductor throughout the network, from the power generation source to the point of consumption. One of the potential drawbacks of a TN-C system is that ground fault currents can flow through the combined PEN conductor. This can lead to potential safety hazards if the conductor is not properly sized, installed, or maintained.

Figure 2. TN-C Earthing System

The TN-C (combined PEN) earthing system has several potential advantages in certain applications. Firstly, it can be less expensive to install and maintain than other earthing systems, such as TN-S or TT systems, because a single conductor is used for both the neutral and protective earth functions. This also makes it simpler and quicker to install, especially in larger installations. Additionally, the combined PEN conductor in a TN-C system can help to reduce electromagnetic interference (EMI) from electrical equipment and machinery, improving the performance and reliability of sensitive electronic equipment. 

Finally, the TN-C system can be more efficient in its use of copper than other systems, as the combined PEN conductor can reduce the amount of copper required for the earthing system, which can be an advantage in applications where copper costs are high or where space is limited. However, it's important to note that the TN-C system also has potential drawbacks and limitations, such as the potential for ground fault currents and the need for careful design and installation to ensure safety and reliability. 

The choice of earthing system should always be based on the specific requirements of the application and the relevant local regulations and standards.

TNC-S System

A TNC-S (combined PEN and separate PE and N) earthing system combines features of the TN-C and TN-S systems to provide a hybrid solution. This means that the neutral and PE is combined from the source and then separated at the entrance of the service line. 

Figure 3. TNC-S System

This system has several potential advantages in certain applications. 

1. The TNC-S system provides a separate protective earth (PE) conductor in addition to the combined PEN conductor, which can help to reduce the potential hazards associated with ground fault currents in the PEN conductor. This can improve safety and reliability in electrical installations. 
2. The TNC-S system can be more cost-effective than a TN-S system in some applications, as it combines the benefits of the PEN and separate earth and neutral conductors while minimizing the amount of copper required. 
3. The TNC-S system can provide a low-impedance path to earth and reduce the risk of overvoltages and electrical faults, similar to the TN-S system. Overall, the TNC-S earthing system is a versatile solution that can provide a balance between safety, reliability, and cost-effectiveness in certain electrical installations.

TT System

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. The TT system can be more reliable and safer than other earthing systems, as each installation has its own dedicated earth connection, which minimizes the risk of faults and electrical shocks. 

Also, this system can be suitable for use in areas with high soil resistivity or poor soil conditions, where other earthing systems may not be effective. The system can also be more flexible and adaptable to changing conditions, as it allows for the use of different types of earth electrodes, such as rods, plates, or grids, depending on the site conditions. his system can provide a low-impedance path to earth and reduce the risk of overvoltages and electrical faults, which can improve the overall performance and reliability of electrical installations.

Figure 4. TT Earthing System

However, the TT system may require more installation time and cost than other earthing systems, and proper maintenance is critical to ensure the continued effectiveness and safety of the system. As with any earthing system, the choice of earthing system should be based on the specific requirements of the application and the relevant local regulations and standards.

Specific Characteristics of TT System:

• 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

IT System

A system having no direct connection between live parts and Earth, the exposed conductive parts of the electrical installation are being earthed. IT earthing is a type of electrical earthing system that is commonly used in critical applications, such as in medical and data center environments. This system has several potential advantages in certain applications. Firstly, the IT system provides a high level of electrical safety and fault tolerance, as it uses an isolated power supply that is not directly connected to earth. This means that any single fault or ground fault will not cause an immediate interruption of power, which can be critical in applications where power continuity is essential. 

Figure 5. IT Earthing System

This system can be suitable for use in applications where electrical disturbances or high fault currents could cause damage to sensitive electronic equipment, as it can help to reduce electromagnetic interference and protect against electrical transients. Finally, the IT system can be more flexible and adaptable to changing conditions, as it can accommodate different types of fault currents and insulation levels. However, the IT system may require more complex design, installation, and maintenance than other earthing systems, and may be less suitable for general-purpose applications. As with any earthing system, the choice of earthing system should be based on the specific requirements of the application and the relevant local regulations and standards.

Specific Characteristics of IT System:

• Switching upon occurrence of a double fault is usually generated by phase-to-phase fault protective devices (circuit-breakers, fuses, etc.). 
• If the short-circuit current is not large enough to activate protection against phase-to-phase faults, notably if the loads are far away, protection should be ensured by residual current devices (RCDs). 
• It is not advisable to distribute the neutral. 
• It is compulsory to install an overvoltage limiter between the MV/LV transformer neutral point and earth. If the neutral is not accessible, the overvoltage limiter is installed between a phase and earth. It runs off external overvoltages, transmitted by the transformer, to the earth and protects the low voltage network from a voltage increase due to flashover between the transformer’s medium voltage and low voltage windings. 
• A group of individually earthed loads must be protected by an RCD.

Comparison of different earthing systems in low voltage

The three earthing systems are different in the way they operate and afford protection. Each has its advantages and disadvantages which we shall now consider.

Unearthed or impedance-earthed neutral (IT system)

Operating technique: 

• Permanent insulation monitoring.
• First insulation fault indication.
• Compulsory fault location and clearance.
• Switching if two insulation faults occur at the same time (double fault)

Technique for protecting persons: 

• Interconnection and earthing of exposed conductive parts.
• First fault monitoring by a permanent insulation monitor 
• Switching upon occurrence of the second fault by overcurrent protective devices (circuit-breakers or fuses).

Advantages: 

• System providing the best service continuity during use. 
• When an insulation fault occurs, the short-circuit current is very low.

Disadvantages: 

• Requires maintenance personnel to monitor the system during use. 
• Requires a good level of network insulation (which means that the network must be broken up if widespread, and that loads with high leakage current must be supplied by insulating transformers). 
• Tripping checks for two simultaneous faults should be carried out if possible, when the network is being designed using calculation and must be performed during commissioning using measurement. 
• Overvoltage limiters must be installed. 
• Requires all the installation’s exposed conductive parts to be equipotentially bonded; if this is not possible RCDs must be installed. 
• Avoid distributing the neutral conductor. In the IT system, it is in fact recommended not to distribute the neutral for the following reasons: 
• if the neutral conductor is distributed, a fault affecting it will eliminate the advantages attached to the IT system. 
• if the neutral is distributed, it must be protected (except for specific cases); 
• the fact of not distributing the neutral facilitates the choice of overcurrent protective devices (see section 4.4.1.3) and fault location. 
• Locating faults is difficult in widespread networks. When an insulation fault in relation to the earth occurs, the voltage of the two unaffected phases in relation to the earth takes on the value of the phase-to-phase voltage Equipment must therefore be selected with this in mind. 

Directly earthed neutral (TT system)

Operating technique:

• Switching upon occurrence of the first insulation fault.

Technique for protecting persons: 

• Earthing of exposed conductive parts combined with the compulsory use of RCDs (at least one at the head of the installation). 
• All exposed conductive parts protected by the same RCD must be connected to the same earth. 
• Simultaneously accessible exposed conductive parts must be connected to the same earth.
• Does not require permanent monitoring during use (only a periodic inspection test of the RCDs may be necessary.
• Moreover, the presence of RCDs prevents the risk of fire when their sensitivity is below or equal to 500 mA (see standard IEC 60364-4)
• Easy location of faults.
• Upon occurrence of an insulation fault, the short-circuit current is small.

Disadvantages: 

• Switching upon occurrence of the first insulation fault. 
• Use of an RCD on each outgoing feeder to obtain total selectivity. 
• Special measures must be taken for the loads or parts of the installation causing high leakage currents during normal operation in order to avoid spurious tripping (feed the loads by insulating transformers or use high threshold RCDs, compatible with the exposed conductive part earth resistance). 

Connecting the exposed conductive parts to the neutral (TNC – TNS systems) 

Operating technique:

• Switching upon occurrence of the first insulation fault.

Technique for protecting persons:

• Imperative interconnection and earthing of exposed conductive parts.
• Switching on occurrence of the first fault via an overcurrent protective device (circuit-breaker or fuse). 

Advantages:

• The TNC system may be less costly upon installation (elimination of one switchgear pole and one conductor).
• Use of overcurrent protective devices to ensure protection against indirect contact.

Disadvantages:

• Switching on occurrence of the first insulation fault. – The TNC system involves the use of fixed and rigid trunkings (see section 413.1.3.2 of standard IEC 60364-4).
• Requires earthing connections to be evenly placed in the installation so that the protective conductor remains at the same potential as the earth.
• A tripping check on occurrence of the insulation fault should be carried out, if possible, when the network is being designed using calculations, and must be performed during commissioning using measurements; this check is the only guarantee that the system operates both on commissioning and during operation, as well as after any kind of work on the network (modification, extension). 
• Passage of the protective conductor in the same trunkings as the live conductors of the corresponding circuits. 
• Often requires extra equipotential bonding. 
• Third and multiples of third harmonics circulate in the protective conductor (TNC system). 
• The fire risk is higher and, moreover, it cannot be used in places presenting a 
• fire risk (TNC system). 
• Upon occurrence of an insulation fault, the short-circuit current is high and may cause damage to equipment or electromagnetic disturbance. 

Source:

• IET
• electrical-installation.com
• BS 7671
• City and Guilds