What are the types of electrical grounding systems?
Grounding electrical systems is a critical part of ensuring the safety and stability of electrical networks. The grounding system is an essential element in protecting personnel and equipment from electrical hazards, and is used to safely direct short or excess currents to the ground. The grounding system varies depending on the type of use and environment, and here we will provide an overview of what electrical grounding systems are and their types.
Types of electrical grounding systems:
- Equipment Grounding: It is used to connect electrical equipment and devices to the ground in order to protect them and direct short currents to the ground.
- Service Grounding: Related to connecting the electrical system in general to the ground, and is usually done through a conductor known as the main grounding conductor.
- Branch Circuit Grounding: It is used to connect the individual branch of an electrical circuit to the ground, and this is done through the grounding conductor in electrical cables and outlets.
- Grounding for Lightning Protection: It aims to direct lightning currents to the ground, protecting buildings and equipment from the effects of lightning.
- Grounding for Electronic Systems: It is used in sensitive systems such as electronic systems and computers to provide an environment free of electromagnetic interference.
- Generator Grounding:Includes connecting the generators to the ground to ensure the safe discharge of excess electrical currents.
- Electric Vehicle Grounding: In electric vehicles, chassis grounding is used to direct dangerous currents to ground in the event of an electrical fault.
Grounding of electrical systems plays a vital role in keeping people safe and ensuring the safety of equipment and property. Grounding systems must be carefully designed and implemented to ensure their effectiveness and compliance with electrical standards and regulations.
Definition of electrical grounding
The electrical grounding process is done by connecting the metal casing of electrical appliances and equipment to the ground. This is done by using a metal connection called the “protective ground connection.” (PE), as this connection ensures that the electrical voltage on the surface of the devices is balanced with the surface of the Earth.
This connection is essential to prevent electrical shocks in the event that individuals come into contact with devices carrying an electrical leak resulting from a defect in the insulation, as it ensures that the difference in voltage between these surfaces and the ground is zero.
In addition, the protective ground connection is a path for high current in the event of a fault or short circuit, activating protective devices such as circuit breakers, and ensuring the power source is disconnected from the load.
There are several ways to implement the grounding process, each of which differs in its advantages and disadvantages, and the appropriate method is chosen according to the specific application and safety requirements.
Electrical grounding systems are classified according to the international standard IEC 60364 into three groups coded with the letters TT, TN, and IT.
The first letter on the left shows the status of the ground connection with the power source (generator or transformer), and this letter is either I or T.
- T: refers to a direct connection of a point with the Earth, and is derived from the French word (Terre).
- I: Indicates that there is no connection with ground at any point or connection through a high impedance.
The second letter shows the state of the ground connection with the electrical appliances being powered (the load), and this letter is either T or N.
- T: Refers to a direct connection of the load to the ground, independent of any ground connection in the power distribution network.
- N: indicates that the load is connected to the ground through the distribution network.
TN System: In this system, one of the points, usually a star point in the three-phase system of the generator or transformer, is connected to ground. The external structure of the electrical device (the load) is connected to the ground via the ground connection of the source, which causes a short, high current to pass in the event of a defect in the insulation, which leads to the circuit breakers or fuses being activated and the current disconnecting from the load.
The wire that connects the exposed metal parts of consumer appliances is known as the protective (PE) line, and the wire that connects to the star point in a three-phase system or to the return current in a single-phase system is known as the neutral (N) line. From an economic point of view, the TN system demonstrates the cost savings of establishing a low-impedance ground connection at each consumer’s location. This system is divided into three types: TN-S system, TN-C system, and TN-C-S system.
TN-S system:
In a TN-S system, there is one grounding point at the source. The name of the system shows that there is a source-specific grounding (first letter from the left, T), and also a completely independent (separate) grounding line from the neutral line along the distance from the source to the consumer. The two points are connected together only at the source, as shown in the figure.
The TN-S system is economical because there is no need for an additional ground cable running along the entire path. Cable Sheath is often used to connect the grounding point of the consumer to the grounding point.
However, it is noted that any interruption in the medium voltage network can raise the voltage of metal objects at the consumer to dangerous values. The TN-S system is an improvement over the TN-C system in providing a path for the ground fault current.
TN-C system:
In TN-C system, the neutral (N) and protective (PE) line are combined into one line known as PEN line, and the neutral (N) line is used as ground line at the consumer, which is grounded at the source.
The TN-C system is characterized by its low cost, as prevention and neutralization are shared in one line. However, the disadvantage of this system is that a break in this common line will raise the voltage of the metal objects at the consumer to dangerous values, especially if the loads are unbalanced.
TN-C-S System:
The TN-C-S system combines the previous two systems, where the tie and protective line are separated at the entrance to the building containing the loads. PEN is used in the distance between the substation and the building entrance, and is separated into two independent lines (protection line and equalization line) at the building entrance.
The system is also referred to as PME (Protective Multiple Earthing) in Britain, where the PEN is connected to the ground at various locations to reduce the risk of a tie line break. This system is used in many modern homes in Europe.
TT system:
The TT system is the safest system as it is known as a direct earthing system. There is a complete separation between the grounding point at the source and the consumer grounding. It is used in locations containing sensitive equipment and often requires the use of ground leakage circuit breakers (RCD) to provide protection and safety.
IT system:
In an IT system, the distribution network is isolated from the ground, or connected to the ground through a high-impedance link. The load is connected to the ground through an independent grounding connection. This system is used in places that need high protection and is usually used in places such as hospitals and laboratories.
Note: RCD earth leakage circuit breakers must be used in TT system and IT system to provide protection.
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