Electrical Earthing / Grounding

Introduction

Electrical earthing / grounding is one of the most important aspects of any electrical installation. Earthing or Grounding is a technique which is as old as the use of electrical power on a commercial scale. The practice came in to existence in the early days of electricity when the systems in use were all ungrounded and that led to frequent hazardous incidents caused due to electric shocks.
The term Earthing or Grounding simply means connecting the electrical system  equipment to the ground by means of a suitable conductor. Such a ground connection provides a common return path for safe discharge of electricity to the ground.
A properly grounded electrical system serves mainly two purposes:
a) It prevents from the risk of electrical shocks to any human being coming in contact with the system.
b) It protects the connected equipment from any possible damage occurring due to leakage currents or lightening or voltage surges, by providing a safe passage to these currents to ground.
Earthing of an electrical system is achieved by inserting an electrode (plate type / rod type) in to the solid mass of earth and then connecting this electrode to the earth wire coming from the electrical equipment.

Earthing considerations, the basics

While it is easy to understand the definition of Earthing, the topic of Earthing and its application in practice is often the most misunderstood of all. It will therefore be worthwhile to explore some points about the basics of Earthing:

• The concept of Earthing is based on the fundamental assumption that – Earth potential is always considered at “zero”. i.e. Earth irrespective of where it is connected to, is always a zero potential ground mass.
• For the above reason, any electrical equipment or appliance when connected to earth (“earthed”), also turns to a zero potential.
• A good Earthing is one which gives very low resistance to the flow of current to ground.
• Therefore, it is important to see that the earth resistance is kept as low as possible.

Ungrounded System

This type of system is not in practice anymore. However, it will be worthwhile studying it beforehand. In an ungrounded system, the current flows from source to the load and returns back to source and complete the circuit. There is no point in this loop where the system is grounded. Due to absence of a ground connection, there is no way a fault current can pass out of the loop. (See fig below:)

In case in the event of any abnormal fault current, there will be occurrence of first fault by causing insulation failure at some point in the circuit. Due to ungrounded circuit, this fault will remain undetected and may lead to subsequent second fault and a possible short circuit and thus cause a severe hazard to the whole circuit and it will break down.

Neutral Grounded System

The solution to above issue of ungrounded system is provided by a Neutral Grounded system. This system avoids the risk of developing the first fault and eliminates subsequent risk of short circuiting due to second fault in the circuit.
In this type of grounding, one of the poles of the source is connected to the ground mass. This is also called “Neutral”, while the other pole that carries the charge to the load is called a “Phase”. The neutral conductor always carries the return current from the load.

In case in the event of any insulation failure in the phase conductor leading to high fault current, due to availability of a grounded conductor in the neutral line, the excess fault current will pass through to the ground. Through the ground path this current will safely return back to source to complete the circuit. Thus, through the use of grounded Neutral, the risk of equipment damage due to fault current is avoided by providing a safe passage to fault current to the ground.
It should be noted that in this particular type of system, the Neutral is always providing the return current path from load to the source. Hence for the same reason this conductor is always insulated from the ground. Due to this important function, the degree of insulation will also be same as phase conductor.

Grounded At Customer Side

In the previous section we have seen Neutral Grounding done at source side. This type of system although fine, but may not be adequate in practical situations where source is at a significant distance away from the load. In such situations, the fault current arising at fault point and travelling through the Neutral conductor would not be grounded at the nearest grounding point and hence the risk of damage to the electrical circuit would still be present.

A practical solution to this situation is by way of extending this ground reference to the customer location. All the electrical points and equipments at customer installation (that can become potential fault points in the event of failure), are connected to a common reference ground bus at customer side. All the metallic housing of the equipments are connected to this bus.
This type of earthing not only protects against insulation fault, but also protects the metallic body of the equipments from any potential electrical shocks.

Construction of a Transformer

The transformer is quiet simple in construction. As told earlier, it consists of two magnetically linked windings, wound on two separate limbs of iron. It consists of the following basic parts:

1. Core
2. Windings

CORE

Usually the core of a transformer is constructed with a material having high permeability, such as silicon steel laminations, so that the core losses such as eddy current loss and hysteresis losses are reduced. Since the steel sheets have a very high resistivity, so the current losses are greatly reduced.
A transformer core can be constructed in two ways, depending upon the arrangement of the primary and secondary windings. These two ways are:

1. Core Type construction
2. Shell Type Construction

Core type construction

If the windings are wound around the core in such a way that they surround the core ring on its outer edges, then the construction is known as the closed core type construction of the transformer core. In this type, half of the winding is wrapped around each limb of the core, and is enclosed such that no magnetic flux losses can occur and the flux leakages can be minimized.
This type of arrangement proves quiet useful for the flux circulation, such that each limb is covered by the windings and hence the flux circulates through the complete core. But during this circulations, a bit of leakages also occur.

Shell type construction

In shell type construction of the core, the windings pass through the inside of the core ring such that the core forms a shell outside the windings. This arrangement also prevents the flux leakages since both the windings are wrapped around the same center limb.

WINDINGS

Firstly, Arrangements of windings is also an important issue in the construction of a transformer.. The winding connected to the main AC power supply is called the primary winding, while the one connected to the load or some external circuitry is called the secondary winding.
Windings of a transformer are made up of a conducting material to allow the magnetic flux to build up and hence the current can be passes from one winding to another. These windings are wound on two separate limbs of iron to increase the magnetic flux as iron is an efficient conductor and exhibits excellent magnetic properties. These coils are also insulated from each other. Since both these coils are wound on two separate limbs and due to the distance between them, flux leakages also occur which reduce our magnetic flux density and results in a reduced magnetic coupling between the two coil windings.
To avoid this situation, the distance between the two windings is reduced, so that the flux leakages can be minimized and a strong magnetic induction can be created and sustained between the two coils. But this arrangement also does not completely eliminate all the flux problems, since the magnetic losses are still present.
In core type construction, these windings are arranged in a concentric manner on the limbs, while in a shell type core construction, the same windings are arranged in a sandwich like pattern.
Other than these two main parts, Transformer tank and Conservatory tank are also used in transformer construction:

Conservatory tank

Since some type of container is also required to keep the transformer core and windings in and to safely insulate them, so a tank is used to keep the laminating oil, to minimize its deterioration and maintain oil levels. This tank also contains some other important arrangements like a number of important sensors and a gas detecting relay as well, which acts as a gas sensor and rings an alarm if detects the presence of some unwanted gas and immediately secures your external circuitry by disconnecting the transformer.

That’s all for today, in the coming post we will throw some light on the losses of transformer. In transformer as we have seen the primary and secondary coils are magnetically linked hence the conversion of electrical energy into magnetic energy causes few energy losses which we will discuss in detail in the coming lecture.

What is a Transformer?

A Transformer is a device which transfers electric current from one circuit to another, usually by the principal of mutual induction. During this process, the frequency remains constant whereas the voltage can be increased or decreased according to the need.
This transfer of electricity occurs with the help of two coils. One of which is known as the Primary Coil, which is connected to a source of alternating current. The other is known as the Secondary Coil, and it is connected to an external circuit. This constitutes the basic structure of the transformer and is shown below:

Working principle of a transformer

Transformer works on the Principle of Faraday’s Law of Mutual Induction. This principle states that the rate of change of flux is directly proportional to the induced electromagnetic flux.
Similarly, in a transformer, when an alternating current flows through one of the coil, it creates a magnetic field around it, which constantly produces a changing magnetic flux and so, when another coil is brought near it, some of the EMF is also induced in the secondary coil as well. Since the secondary coil forms a closed loop, the EMF produces the current in it as well.
So in short, this mutual induction between the coils is responsible for transferring the electric energy.

These windings are usually made on an iron core to make the magnetic field stronger, and laminated afterwards, so that the flux does not weaken due to air, which is a perfect insulator. But still some power losses are observed such as Eddy Current losses and Hysteresis loss.

Types of transformer

Categorized on the basis of voltage build up, we primarily classify the transformers into two basic categories:

1) Step Up Transformer
2) Step Down Transformer

• STEP UP TRANSFORMER:

If we increase the number of turns in the secondary coil, such that they become greater than the number of turns in the primary coil, the induced voltage can be increased in direct relation. i.e. if the turns in the secondary are ten times the number of turns in the primary, then the induced voltage will also be ten times then the one in the primary.

• STEP DOWN TRANSFORMER:

Similarly, if the number of turns in the primary coil is more than the number of turns in the secondary coil, the voltage induced will be lesser than the original voltage.
This property of Transformer is really useful in transferring electrical energy especially over long distances. To avoid power losses, initially a step down transformer is used and at the receiving end, a step up transformer is used which builds up the voltage on the required level. Such types of Transformers are known as single phase, two windings voltage transformers.

• 3 PHASE TRANSFORMERS:

But two phase, 3 phase or higher phase transformers can also be built up, especially for commercial and industrial purposes, where the load is quiet large, three phases are mostly used. Connections of the transformer in a 3 phase are shown below:

As clear from the figure, a three phase transformer will have three primary coils and three secondary coils. The manner, in which the three windings are connected with each other, can be a delta connection or a Y connection. Both of these are shown below:

If the coils are connected in series, forming a closed loop, then the connection is known as a delta connection, but if the three windings are connected such that all have a common point then a Y-type connection is formed. It has a neutral wire at the common end point. Both of these connections are equivalent and are inter convertible from one form to another.
In the next part of this tutorial, we will have a look on the construction of transformer, which I think is very essential for an electrical engineer to know. I will explain the basics of transformer design like windings etc. and will check how to design a transformer.
So stay tuned and join our newsletter via email so that you get these amazing tutorials rite in your mailbox. Take care.

Electrical engineering

Electrical engineering (sometimes referred to as electrical and electronic engineering) is a professional engineering discipline that deals with the study and application of electricity, electronics and electromagnetism.

The field first became an identifiable occupation in the late nineteenth century with the commercialization of the electric telegraph and electrical power supply.

The field now covers a range of sub-disciplines including those that deal with power, optoelectronic, digital electronics, analog electronics, artificial intelligence, control systems, electronics, signal processing and telecommunications.

Branches of electrical engineering

• Power engineering
• Control engineering
• Electronic engineering
• Microelectronics
• Signal processing
• Telecommunications engineering
• Instrumentation engineering
• Computer engineering