What is Earth Fault? It’s Causes, Effects and Protection

Earth fault is the unintended fault between live conductor and the earth. It occurs mainly due insulation breakdown of transmission line conductor.

In this article, we are going to discuss what is earth fault, causes of earth fault, effects of earth fault, and also what are the protections used to prevent earth fault?

So let’s start but before that first of all we have to understand what is  earthing and grounding? Generally people gets confused and they consider them similar. But it’s not like that. Actually earthing and grounding both are two different concepts. Here we are discussing one by one.

What is Earthing and Grounding?

Earthing and grounding both are process to connecting electrical system to the ground to protect people against electrical shock.

[ ##eye## Core balance Current Transformer ]

Earthing process means connecting the dead part of electrical system (parts that doesn’t carry current under normal conditions such as motor body, Transformer body, electrical panels, etc.) to the earth.

Whereas Grounding process means connecting the live part of electrical system (parts that carry current under normal conditions such as neutral of star connected Transformer and loads) to the earth.

Now coming to our main topic of discussion:

What is earth fault?

Earth fault is an open circuit ground fault in which any live conductor or power carrying cable detached from its place or breaks and falls on the earth surface. In other words, We can say that, ln earth fault current carrying conductor comes directly in contact with the earth surface.

So because of the direct contact between live conductor and earth surface, a huge amount of current starts flowing to earth due to conducting nature of earth.

Causes of Earth fault

The main causes of earth fault in an overhead transmission and distribution line is the failure or puncture of insulator.

Since insulators are used in overhead transmission lines to provide insulation between live conductor and metallic towers that already connected with earth surface. So if the insulation of insulator fails or in other words if insulator punctured then the fault current will flow through the live conductor and metallic tower to the earth that creates earth fault.

[ ##eye## Working, Types and Applications of Transducer ]

Also others reason of earth fault is: Sometimes overhead transmission lines breaks due to any unusual loads on line and hence falls to the ground. So in this case also live conductor gets in direct contact with ground and creates a major earth fault in the electrical system.

Whereas in case of underground PV cabling system, Earth fault may arises due to insulation dimage during installation, abrasion dimage to cable sheath, and dimage to module backsheet during installation.

Effect of Earth fault

Whenever earth fault occurs in the electrical system then during earth fault, System gets short circuited and hence huge amount of short-circuit current flows through the system. That huge current damages electrical equipment which comes in contact with loop of earth fault circuit and also it interrupts the continuity of power supply.

Due to such a large amount of current flowing to the earth, a large voltage drop occurs that affect the other loads and it may cause the burning of phase or line.

How to check Earth fault?

To identify the earth leakage fault in cables, megger is used. One terminal of the megger is connected to the live conductor and the other terminal connected to earth. If the megger indicates zero reading, it means the conductor is earthed. The same procedure is repeated for other conductors of the high voltage cables.

Difference between Earth fault and Ground fault

Earth fault vs Ground fault:

Earth fault is an open-circuit fault where power carrying cable or conductor breaks and get into contact with earth or any conductor capability material in contact with earth. In this fault, system at load side is disconnected from the source in case of radial power flow.

Whereas Ground fault is a short-circuit fault (such as L-G, L-L-G, L-L-L-G) where power carrying cable or conductor is not broken, but gets into contact with earth or any conductor capability material in contact with earth. In this fault, system at fault point experience flow of a large amount of current into ground.

Ground fault is more severe than earth fault due to flow of huge current to ground. That can dimage verious electrical equipments of power system, if fault is not cleared within specified time limits.

Earth fault protection

Well, after discussing earth fault, it’s causes and effects, one of the important question arises in our mind is.. ok, then what is the solution to minimise Earth fault. It means what are the protective equipment we can use to protect earth fault? So let’s discuss protection used to minimise earth fault in detail.

what are  earth fault protection devices?

The electrical equipments that  provide protection against earth fault is known as earth fault protection devices. These are normally Earth fault relay (EFR), Earth leakage circuit breaker (ELCB), and ground fault circuit interrupter, etc. are used to restrict the fault current. In these devices the fault current is restricted and fault is dispersed by the Restricted earth fault protection (REFP) scheme.

[ ##eye## Power factor correction techniques ]

Now first let’s discuss both earth fault protection devices such as:

• Earth fault relay (EFR)
• Earth leakage circuit breaker (ELCB)

After that we will see what is Restricted earth fault protection (REFP) scheme?

Earth fault relay (EFR)

An earth fault relay (EFR) is a protection device used in electrical installation that can sense a fault between phase and earth.

The main function of earth fault relay (EFR) is to operate the CB for leakage current to earth. RCCB also does the same function but RCCB gives protection to human body, Whereas EFR provides protection to electrical equipment from earth fault. EFR deals with high voltage and high current.

Hence we can conclude that an earth fault relay (EFR) is a earth fault monitoring device that works on the principle of Kirchhoff’s low.

Earth leakage circuit breaker (ELCB)

ELCB or earth leakage circuit breaker is a safety device used in electrical installation with high earth impedance to prevent electrical shock. In other words, we can say that ELCB is a special type of circuit breaker that is used for protection against earth leakage current. It is voltage sensing device that works on the concept of stray voltage. Whereas current sensing ELCB is also known as RCCB.

Whenever any earth leakage fault occurs in the system then a stray voltage appears across metal enclosures of electrical equipments then ELCB interrupt the circuit if dangerous voltage detected. Then it disconnect the rest of healthy system from main supply. Once when it tripped then it requires manual reset process to work again normally.

Hence simply we can assume that ELCB is a safety device whose main function is to provide protection against electrical shock. It can’t assure protection against overloading and short-circuit fault.

What is Restricted earth fault protection (REFP) scheme?

Restricted earth fault protection (REFP) scheme is very sensitive for internal earth fault for high rating power transformer, alternators, etc. This scheme is comparatively cheaper than differential protection scheme.

[ ##eye## Surge impedance loading of transmission lines ]

In this scheme, the common terminal of phase CTs are connected together with secondary of neutral CT in such a manner that secondary unbalance current of phase CTs, and secondary content of neutral CT will appose each other.

When an external fault F₁ occurs in the system then resultant of current I₁ and I₂ flowing through phase CTs and neutral CT respectively will be zero. But whenever an internal fault F₁ occurs inside the protective zone then only I₂ current will flow through secondary of neutral CT and resultant current will not be zero.

Hence earth fault relay will sense non-zero current and then it will operate the CB to protect the system with earth leakage fault. Fault current becomes 15% more than the rated current of each windings. To avoid magnetic inrush current, a stabilizing resistor is connected in series with earth fault relay.

Read More Articles:

• Theory and working of Star-Delta Starter
• Surge impedance loading (SIL) of Transmission line
• Construction and Working of RVDT
• Notch filter - Theory, circuit design and Application
• Different Parts of transformer and their functions
• Idempotent matrix and its properties
• Different types of Electrical load