System Earthing and Protective Measures

System Earthing & Protective Earthing

Introduction to Earthing

The earth is a vast conductor at zero potential (reference). In the UK, it's called 'earth,' and in the USA, it's called 'ground.'
People are generally in contact with the earth, so voltage differences between touchable parts and the earth can cause electric shock.

Earthing Process

Earthing connects potentially charged parts to the earth, providing a path for fault currents and maintaining these parts close to earth potential.
Ideally, this prevents potential differences and allows fault current flow for protective system operation.
A direct connection is typically made between the electrical supply system and earth at the supply transformer. The neutral conductor (often the star point in a three-phase system) is connected to an earth electrode.

Types of Earthing

Protective Earthing: Protects people and equipment from electric shock.
Technical (Functional) Earthing: Serves a functional purpose in the electrical system.
- Protective earthing systems must be bounded at some points to the technical earthing system.
- Technical earthing systems must not only clear earth fault current but also provide a low impedance path to earth for high-frequency leakage (up to 30 MHz) currents and noise from switching and lightning. Parallel paths are needed for high-frequency currents.

Technical Earthing Systems

TN
TN-C
TN-S
TT
IT

TN System (IEC 364)

TN-C Systems

Combined neutral and earth wiring is used in both the supply and the installation.
Typically uses an earthed concentric system, which requires special installation conditions.

TN-C System Details

Transformer neutral is earthed.
Electrical load frames are connected to the neutral.
Insulation faults become short circuits, triggering Short-Circuit Protection Devices (SCPDs).
Fault voltage (indirect contact) is $U_o/2$ if the outgoing and return circuit impedances are equal. Disconnection is required if this voltage exceeds the safety limit, typically 50V.

TN-C System in LV Grid

Provides a return path for LV grid faults.
Ensures distributed grounding and reduces the risk of unsafe grounding for customers.
Faults in the MV network can migrate into the LV grid, causing touch voltages at LV clients.
The utility is responsible for proper grounding and customer safety during grid disturbances.
LV network faults can cause touch voltages at other LV clients, especially at the ends of branches where impedance is highest.
Outgoing cable length is limited (e.g., 300m) to manage circuit impedance.

TN-C System Advantages

Provides a return path for LV grid faults.
Ensures distributed grounding and reduces the risk of unsafe grounding for customers.

TN-C System Disadvantages

Faults in higher voltage networks may migrate into the LV grid.
The utility is responsible for proper grounding.
LV network faults can cause touch voltages at other LV clients.
Critical faults occur at the ends of branches due to high impedance.
The maximum length of outgoing cables is limited.

TN-C System Requirements and Limitations

Requires a very good earthing impedance (about 2 $Ω$ ).
Inadequate for EMC problems.
TN-C should be avoided as rank 3 harmonics flow in the PEN, preventing its use as a potential reference for communicating electronic systems.
Metal structures connected to the PEN become sources of electromagnetic disturbance.

TN-C System Diagram

Distribution network with a TN-C System installation including multiple earthing.

TN-S Systems (IEC 364)

TN-S System Overview

The electricity supply company provides an earth terminal at the incoming mains.
This earth terminal is connected to the star point (neutral) of the supply transformer's secondary winding, which is also connected to an earth electrode.
The earth conductor is often the armor and sheath of the underground supply cable.

TN-S System Details

LV cable with a grounded sheath is used.
Additional electrodes in the LV grid, preferably at each user, divert external induced (lightning) currents.
A TN-S system requires five conductors.

TN-S System Preference

Preferred for very long networks.
Suitable for loads with low natural insulation (furnaces) or large HF filters (large computers) and communication systems.

TN-S System Applications

Distribution network with a TN-S System installation.

TN-C-S System (IEC 364)

TN-C-S System Overview

The installation is TN-S, with separate neutral and protective conductors.
The supply uses a common conductor (PEN) for both neutral and earth.

TT System (IEC 364)

TT System Overview

Used when the Electricity Supply Company does not provide an earth terminal.
Common in rural installations fed by an overhead supply.
Neutral and earth (protective) conductors are separate throughout the installation, with the earth terminal connected to an earth electrode.

TT System Details

The transformer neutral is earthed.
Electrical load frames are also connected to an earth connection.
Insulation fault current is limited by earth connection impedance.
A Residual Current Device (RCD) disconnects the faulty part.
Effective earth connection is crucial; socket outlet circuits must be protected by an RCD with an operating current of 30 mA.

TT System Requirements

Each customer needs to install and maintain their own ground electrode.
Ground impedance at the customer should be low (Rc < 30 $Ω$ ).
RCDs are required.

TT System Advantages

Faults in the LV and MV grid do not migrate to other clients in the LV grid.
A broken neutral conductor does not affect single-phase connections but may damage three-phase equipment.
Good security: potential rise of grounded conductive parts is limited to 50 V for a fault inside the installation.
No influence of network evolution (fault loop impedance).

TT System Disadvantages

Not suitable for large customers because the disconnecting time of over-current protective devices is too long. TN provides a low impedance return path.
High over-voltages may occur between live parts and the PE conductor.
Requires less control of transferred potentials for assessing safety in case of HV fault.
Very good for communication systems due to low interference.

IT System (IEC 364)

IT System Overview

Naturally earthed by stray capacities of network cables or voluntarily by a high impedance of around 1,500 $Ω$ (impedance earthed neutral).

IT System Protection

Avoid second faults by using very fast protection (1st fault): I_d < 1 A
(2nd fault): $I_d ≈ 20 kA$

IT System Advantages

Voltage developed in the earth connection of frames is minimal, posing no risk.
Ensures continuity of service.
Suitable for loads sensitive to high fault currents.

IT System Disadvantages

If a second fault occurs, and the first is not eliminated, a short circuit appears, requiring SCPDs for protection.
Frames of relevant loads are brought to the potential developed by the fault current in their protective conductor (PE).

IT System Restrictions

Restrictions are linked to loads and networks with high earth capacitive coupling (presence of filters).

IT System Applications

Hospitals
Airport take-off runways
Arc Furnaces
Plants with continuous manufacturing processes
Laboratories
Cold storage units
Welding Machines.

Influence of MV Earthing Systems

In MV, the neutral is not distributed, and there is no protective conductor (PE) between substations or between the MV load and substation.
A phase/earth fault results in a single-phase short-circuit current limited by earth connection resistance and impedance.

IEC 364-4-442 Standards

The earthing system in an MV/LV substation must ensure the LV installation is not subjected to an earthing voltage of:
- $U_o + 250 V$ : more than 5 s
- $U_o + 1,200 V$ : less than 5 s
Devices connected to the LV network must withstand this constraint.
If R_p > 1 Ω, the voltage must be eliminated, e.g.:
- 100 V under 500 ms
- 500 V under 100 ms

MV/LV Earthing Considerations

If $Rp$ and $RB$ are connected, the fault current causes the LV network's potential to rise with respect to the earth.
If not, $Rp$ and $RN$ must be separate, regardless of the LV earthing system.
Grouping all earth connections can cause a dangerous rise in potential of LV frames.

Potential Rise Formula

$I{hMT} RT$ : the LV load frames are raised to this potential.

LV Grounding Systems

LV electrical networks may supply several types of applications.
A single earthing system may not be suitable for all applications.
Mixing various grounding systems in an electrical installation is advisable.

Technical Culture Clashes

Electrical engineers struggle with harmonics from static converters, causing temperature rises in transformers and abnormal currents in the neutral.
Electronic engineers use filters that may not withstand over-voltages and lower network insulation.
Lamp manufacturers are unaware of problems caused by energizing inrush currents and harmonics from electronic ballasts.
Computer engineers are concerned with equipotentiality of frames and conducted/radiated interference.

Mixing of Different Grounding Systems

Mixing of TN-C, TN-S, TT, and IT systems with HV/LV configurations.

LV Distribution

The most common systems are TT and TN.
The TN-C system needs carefully designed SCPD.
Not recommended in premises with communicating electronic systems due to potential variations in the PE caused by neutral currents.
RCDs are used for personnel protection (for very long cables).
The TT system is easiest to implement; insulation fault currents are smaller, reducing fire and electromagnetic disturbance risks.
IT Systems (unearthed neutral) are used when continuity of service is essential.
TN-C-S is increasingly chosen for large projects.

Actual Grounding System in Residential Areas

Using TN-C.

Hazards in Residential Areas

F1) Direct Contact: touch voltage is 220 V; no protection.
F2) Indirect Contact: equipment isolated; person subject to 220 V touch voltage; no protection.

Effects of Sinusoidal Alternating Current

Range: 15 Hz to 100 Hz
Risk of electric shocks.

Electric Shock

Caused by current flowing through the human body.
Current depends on skin contact resistance, which varies with thickness, wetness, and resistivity.
In general:
- Current < 5mA is not dangerous.
- 10mA < Current < 20mA: dangerous due to loss of muscular control.
- Current > 50mA: can be fatal.

Human Body Resistance

$R_b$ : between two hands or between one hand and a leg, ranging from 500 Ohms to 50 K Ohms.
Example:
- If $R_b = 50 K Ohms$ , momentary contact with 600 V may not be fatal.
- $I_{body} = 600 V / 50 K Ohms = 12 mA$
- If $R_b = 500 Ohms$ and voltage is as low as 25V ac
- $I_{body} = 25 V / 500 Ohms = 50 mA$ (may be fatal).
Current is particularly dangerous when it flows in the region of the heart.

Statistical Investigation

*Statistical investigations have shown that a current may cause death if it satisfies the following equation:

$Ib = \frac{116}{\sqrt{t}}$ *where: * $Ib$ : current flow through the body in mA
* $t$ : time of current flow second
*116: an empirical constant, expressing the probability of a fatal out come
*[ IEEE transactions on industry and general application ] vol. IGA - 4, No. 5 , pages 467 to 475.

*Example: A current of 116 mA for 1 s could be fatal.

*Breaking time for RCDs 30mA (300mS), 60mA(150mS), 150mA(40mS)

*Ventricular Fibrillation is considered to be the main cause of Electrocution

Operating Principle of Earth Leakage Protection

Detection of outgoing and return current variations.
Tripping mechanism is activated when a current variation is detected.

General Specifications of RCDs

Number of poles: 2 or 4
Rated voltage: not exceeding 1000 V
Rated breaking load current
Rated breakage earth leakage fault current

Installation of RCD

General Notes

Every installation with exposed conductive parts should be protected by one or more RCDs.
If using one RCD, locate it at the starting point of the installation.
Exposed conductive parts should be connected to an earth electrode of suitable resistance.
Use RCDs with different sensitivities to protect different parts of the installation, depending on the risks involved.
Arrange for selectivity (coordination) between RCDs located at different parts of the installation.

RCD Application Example

Example of RCD placement and conductor sizing in a 220V monophase system, including different RCD ratings and circuit breakers for various loads.

Importance of Current-Operated RCDs

Essential to use current-operated residual current devices (RCDs).
Devices rated up to 500mA protect installations and sub-circuits.
Sensitive RCDs (30mA and below) provide excellent protection against electric shock and can be fitted to sub-circuits or socket outlets.

Objectives of Protective Earthing

Safety for persons.
Proper operation and long lifetime for equipment.

Earthing System Functions

Allow unwanted electrical currents to flow harmlessly to earth.
Provide low impedance paths (not just resistance) for high-energy discharges, high-frequency transients, lightning strikes, and fault currents.

Main Markets for Earthing Systems

Utility power generation, transmission, and distribution.
Lightning protection for buildings and high structures.
Private electricity distribution networks in industrial and commercial premises.
Protection of electronic equipment (e.g., computer installations, telecommunications).
Domestic housing and small commercial premises.
Locations where electrostatic potential build-up could be dangerous (e.g., oil refineries, hospitals).

Typical Earth Electrodes

Simple surface earth electrodes
Rod (vertical) electrodes
Meshed electrodes
Cable with earth electrode effect
Foundation earth electrodes

Resistance of a Grounding Point Electrode

The simplest electrode is the hemisphere.
- $R = \frac{1}{2 \pi} \int_{r}^{\infty} \frac{\rho}{x^2} dx = \frac{\rho}{2 \pi r}$
  - where $\rho$ is the earth resistivity.

Earth Surface Potential Distribution

$V_x^* = f(x)$
Around a vertical rod earth electrode with length l = 3 m, diameter d = 0.04 m

Variation of Earthing Resistance with Rod Length

Graph showing resistance variation with rod length for different soil resistivities.

Horizontal Rod Formula

*    $R = \frac{\rho}{2 \pi l} ln(\frac{2l}{t d})$

Horizontal Strip Formula

$R = \frac{\rho}{2 \pi l} Σ$

Factor B Formula

Showed values for Line name factor.

Parallel Vertical Shapes

*    $R_n = \frac{R}{1 + \lambda (n-1)}$

Electrode Materials

Steel
Galvanized steel
Steel covered by copper
High-alloy steel
Copper and copper alloys.

Earthing Conductors

Various stranding sizes for earthing conductors.

Soil Enhancement Options

Conductive Concrete: 30 to 90 ohm-meters
Bentonite: 2.5 ohm-meters
Carbon-Based Backfill Materials: 0.1 to 0.5 ohm-meters
Clay-Based Backfill Materials (GAF): 0.2 to 0.8 ohm-meters (depending on moisture content)
Marconite: 0.1 ohm-meters

Design of Earthing Cable Formula

*    $S = \frac{I^2 t}{K}$ 

    *   where:
        *   S: conductor cross-sectional area (mm2)
        *   I: rms value of the fault current (A)
        *   t: time of short circuit current duration in Sec (about 3 seconds)
        *   K: Factor depends on the limiting temperature of earthing conductor (conductor and insulation material)
        *   For copper and PVC cable, K = 115

Definition of Earthing

Connects specific parts of an electric power system with the ground, typically the equipment's conductive surface, for safety and functional purposes.

Definition of Electrical Bonding

A practice of connecting all metallic solid parts together that maybe exposed to electric faults to reduce risk for electric shock.

Electrode Ground Resistance Components

The resistance of a ground electrode has 3 basic components:
1. The ground electrode itself
2. the connection/bonding of the conductor to the ground electrode
3. the ground conductor

Resistances of Electrodes

The resistance of the ground electrode itself and the connections to the electrode is generally very low; ground rods are generally made of highly conductive/ low resistance material such as copper of copper clad.
The contact resistance of the earth to the electrode: The Bureau of Standards has shown this resistance to be almost negligible providing that the ground electrode is free from paint, grease etc. and that the ground electrode is in firm contact with the earth.
The resistance of the surrounding body of earth around the ground electrode: The ground electrode is surrounded by earth which is made up of concentric shells all having the same thickness.

Earth Resistivity Factors

The resistance of the earthing system is affected by a variety of factors:
- Soil Resistance – The composition of the soil, grain size and distribution.
- Moisture – Up to 15% water content significantly changes resistivity. Beyond that, it has little effect.
- Dissolved Salts – Pure water has very low conductivity. Salt is an electrolyte that reduces the resistance when it’s dissolved in water.
- Obstructions – Nearby concrete buildings nearby or rocks in the soil underneath the earthing system can increase its resistance.
- Current Magnitude – Long periods of exposure or higher currents flowing through the earth can dry the soil in the surrounding area and increase the system’s resistance.

General Arrangement of an Earth Electrode System at an Electrical Sub-Station

Diagram showed the earth electrode system at an electrical sub-station

Step & Touch Potential

Diagram showed the Step & Touch Potential

Step and Touch potentials formulas

Diagram showed the Step and Touch potentials

Step Potential Formula

*  $Vs = I_b(Rb + 2R_1)$ 
*  $Vs(max) = (\frac{0.116}{\sqrt{t}})(R_b +2R_1)$ 
*  $R_1 = 3 \rho_s$ 
* Maximum Permissible Step voltage 76 volt

Touch Potential Formula

* $V_T = I_f R_1 = I_1 (R_b + R_1/2)$ 
* $V_T(max) = (\frac{0.116}{\sqrt{t}})(R_b + R_1/2)$ 
*  $E_{touch} = \frac{0.116}{\sqrt{t}}(R_b = 1.5 \rho_s)$ 
* $V_T (max) = (\frac{0.116}{\sqrt{t}}) (1000) =67 Volts$

Step and Touch Potentials Formulas

*  $E_{step} = 165 + \rho_s \sqrt{t}$ 
*  $E_{touch} = 165 + 0.25 \rho_s \sqrt{t}$

*Assume $\rhos = 0.0$ and the maximum fault duration 6 sec *Therefore $E{step}$ and $E_{touch}$ must not exceed 67 volt

Measurements of the ground resistivity Formula

*The measurements of the ground resistivity Formula
* $\rho$ = 2 $\pi$ aR

Measurement of earth resistivity Formula

Diagram showed the earth resistivity Formula
soil resistivity $\rho$ =2 $\pi$ aR
where R = resistance reading

Measuring of Earthing System Resistance

Using a three-point fall-off potential method of measuring ground resistance.

Neutral Grounding

Connecting the neutral point of a 3-phase system to earth, either directly or through a circuit element, to protect personnel and equipment.
During an earth fault, the current path is completed through the earthed neutral, and protective devices isolate the faulty conductor.

Neutral Grounding Benefits

Voltages of healthy phases do not exceed line-to-ground voltages.
High voltages due to arcing grounds are eliminated.
Protective relays can be used against earth faults.
Overvoltages due to lightning are discharged to earth.
Greater safety for personnel and equipment.
Improved service reliability.
Reduced operating and maintenance expenditures.

Methods of Neutral Grounding

Solid or effective grounding
Resistance grounding
Reactance grounding
Peterson-coil grounding

Solid Grounding

Directly connecting the neutral point to earth through a wire of negligible resistance and reactance.
The neutral point is held at earth potential under all conditions.

Solid Grounding Advantages

The neutral is effectively held at earth potential.
No arcing ground or over-voltage conditions can occur; capacitive and fault currents cancel each other.
Equipment can be insulated for phase voltage, saving costs.
Easier to protect the system from earth faults due to large fault currents.

Solid Grounding Disadvantages

The system experiences a large number of severe shocks due to frequent phase-to-earth faults, causing instability.
Heavy earth fault currents can cause burning of circuit breaker contacts.
Increased earth fault current results in greater interference in neighboring communication lines.

Solid Grounding Applications

Used for voltages up to 33 kV with a total power capacity not exceeding 5000 kVA, where circuit impedance is sufficiently high to limit earth fault current within safe limits.

Resistance Grounding

Connecting the neutral point to earth through a resistor to limit the earth fault current.
The value of R should limit the earth fault current but still allow operation of the earth fault protection system.

Resistance Grounding Advantages

The earth fault current is small, reducing interference with communication circuits.
Improves the stability of the system.

Resistance Grounding Disadvantages

The system neutral is displaced during earth faults, requiring higher insulation levels.
Costlier than the solidly grounded system.
A large amount of energy is produced in the earthing resistance during earth faults, which can be difficult to dissipate.

Resistance Grounding Applications

Used on systems operating at voltages between 2.2 kV and 33 kV with a power source capacity greater than 5000 kVA.

Reactance Grounding

Inserting a reactance between the neutral and ground to limit the earth fault current.

Reactance Grounding Disadvantages

The fault current required to operate the protective device is higher than that of resistance grounding.
High transient voltages appear under fault conditions.

Arc Suspension Grounding (Or Peterson coil )

*Adjusting inductance L of Peterson coil so that $IL = Ie$ , it is called resonant grounding, in this moment the system behaves as an ungrounded neutral system, therefore, full line voltage appears across capacitors $CR$ and $Cy$ .

Arc Suspension Grounding (Or Peterson coil ) Advantages

The Peterson coil is completely effective in preventing any damage by an arcing ground.
The Peterson coil has the advantages of ungrounded neutral system.

Arc Suspension Grounding (Or Peterson coil ) Value of L for resonant grounding

For resonant grounding, system behaves as an ungrounded neutral system and full line voltage appears across capacitors CR and Cy

Arc Suspension Grounding (Or Peterson coil ) Disadvantages

*Due to varying operational conditions, capacitance of the network changes from time to time, therefore inductance L of Peterson coil requires readjustment, lines should be transposed.

Isolated systems

Isolated systems have one big advantage. They can co ntinue operating in the presence of a single earth fault. This is because there is no return path available for the flow of earth fault current. He nce protective devices will not operate. Isolated systems also have big disadvantages. Transient, temporary and permanent overvoltage can easily occur on such systems, stressing insulation. Insulation that is applied between phase and earth must be rated based on the phase-to-phase voltage, and often for even higher voltages. Despite the name, isolated systems are not really isolated from earth. Stray capacitance will exist between conductors and the general mass of earth.

Isolated systems

Conductors themselves exhibit inductance along their length. When earth faults occur, small currents will flow using stray capacitance as a return path. Arcing behavior at the fault, combined with resonance interactions between the stray capacitance and inductance, can lead to the generation of high levels of transient overvoltage. Such systems are not widely adopted due to their disadvantages. High resistance earthed systems are becoming popular for critical applications where availability of supply is essential. They allow the system to continue operating in the presence of a single earth fault, but do not suffer from the insulation stresses associated with isolated systems