Grounding is an important part of any electrical system. A properly grounded system will save lives from electric shocks and also protects devices/equipment. But what exactly is Grounding? What is the need for Grounding? Are there different types of Grounding? If so, what are they? Let us explore the essentials of Grounding/Earthing.
What is Grounding?
Grounding (in US) or Earthing (in UK) is a process of connecting electrical systems, appliances and metal enclosure to ground. Here, ground refers to the physical connection to Earth which acts as a reference point and also a return path for current.
The main purpose of grounding is to provide a low resistance path for electricity to flow. We can implement the connection to ground with the help of a grounding electrode. This way, we can keep all the non-current-carrying conductors such as metal frame/housing of a computer, washer, drier, electric drill etc. at 0V potential.
A typical grounding system consists of two parts. In the first part, all the individual branch circuits consist of a wire (ground wire) that we connect to the metal frame of outlet boxes, appliances, tools, etc. All the ground wires from individual branch circuits reach the main circuit breaker panel and connect to the ground bus.
The second part of the grounding system consists of a large copper wire (known as grounding electrode conductor) that is connected to a ground rod buried in earth.
Importance of Grounding
Let us understand the importance of grounding with the help of a small example. Assume there is a large electrical appliance such as a washing machine and the hot wire touches the metal body of the machine due to an accident. If the appliance is properly grounded i.e., the metal frame of the machine is connected to the grounding bus at the main circuit breaker panel, then the following happens.
The current from the hot wire will pass through the metal body of the machine. As we connected the equipment grounding conductor, the current flows through this conductor to the main service panel instead of the neutral wire. As a result, the circuit breaker if that circuit will trip.
If it were an ungrounded system, the current from the metal body will pass through a human body who makes contact with the machine. The human body will provide an easy path for the current to flow through ground. It will lead to a severe electric shock. GFCI or Ground Fault Circuit Interrupter (in the form of an GFCI Outlet/Receptacle or GFCI Breaker) are very helpful for this. Engineers designed them specifically for detecting ground faults and opening the circuit.
Another case is buildup of static voltage due to lightning strikes. During a lightning strike, the electromagnetic pulse from the lightning will induce a voltage into the metal body of the appliance such as the washing machine. As we connected the metal body to the grounding conductor, it will take this surge voltage to the main circuit breaker panel and then transfers it to the grounding electrode.
Different Types of Grounding
We can classify grounding based on Low-Voltage and High-Voltage Networks.
Low Voltage Systems
In Low Voltage Networks i.e., the supply to residential and small industrial users, the grounding arrangement can be TN, TT or IT.
Here, the first letter indicates the connection between transformer and earth (T – direct connection to earth and I – No connection with earth). The second letter indicates the connection between the electrical equipment at the consumer and earth (T – Local earth connection and N – electricity supplier provides the earth connection).
In TN earthing system, the star point of the transformer (neutral) is connected to earth and at the consumer side, the ground wire of electrical appliances is connected with this connection. There are three types of TN Systems.
- TN-S: The ground and neutral conductors are separate and are connected at the transformer.
- TN-C: Ground and neutral conductors are the same.
- TN-C-S: Separate ground and neutral conductors but only near the consumer side.
In this type of grounding, the grounding at the transformer and at the consumer are independent i.e., the star point (neutral) at the transformer is connected to earth and a local ground electrode acts as a grounding point at the consumer. These two points have no connection.
In IT grounding, there is no grounding at the transformer but there is a local grounding using a ground electrode at the consumer.
High Voltage Systems
Power generation stations, Substations etc. form the high voltage network that are quite different from the low voltage distribution transformers and consumers. In these high-voltage systems, there are three main types of grounding.
- Ungrounded Systems
- Resistance Grounding
- Solidly Grounded Systems
In ungrounded or unearthed systems, there is no direct connection between the star point (neutral) and ground. In these systems, the ground faults have essentially no closed paths and hence their magnitude is very less. Theoretically, there is no potential between the conductors and ground but in AC systems, there is always a capacitance between conductors. Therefore, we call these systems as capacitively coupled to ground.
An important point about ungrounded systems is that even though the line-to-line ground fault currents are very low, it is very difficult to identify the line-to-ground fault.
Solidly Grounded Systems
In solidly grounded or directly grounded systems, the star point of the transformer (neutral) is connected directly to ground without any added resistance to limit the current.
You might have guessed what this type of grounding is. In resistive grounding, there is a resistor (known as Neutral Earthing Resistor) between the star point (neutral) of the transformer and the ground. This resistor limits the fault current flowing through the neutral conductor.
High Resistance Grounding
In high resistance grounding, the fault current is relatively low, about 10A or the current that is equivalent to the capacitive charging current.
Low Resistance Grounding
In low resistance grounding, the fault current is relatively high, about 50A in some areas. The fault current value varies from area to area.
Grounding forms an important part of modern electrical systems. It is a way to provide safety to both humans as well as equipment/devices from faults (ground faults and surges). In this guide, we saw what exactly is grounding, how a typical grounding system looks in a residential environment, the benefits of grounding, different types of grounding (in case of both low voltage and high voltage systems).