Electrical Fault Current and its types

31

MARCH, 2018

An electrical fault presence is an abnormal condition in our power system. It can be due to any type of equipment failures such as a transformer or electric machines, human error or environmental conditions. All these types of faults can cause heavy electrical current, interruption of electricity, equipment damage and even can result in the death of living organisms.

An electrical faulty can be illustrated as:

“The failure in an electrical system, that causes flow of electric current in abnormal manners”

The electrical system is developing in size and complexity in all segments. i.e. generation, transmission and distribution. Any type of flaw in the electrical system can cause extreme economic loss and reduces the reliability of the electrical system. In electrical power systems, protective devices can recognize faulty conditions in system and operate circuit breakers and different devices to restrain the loss of service due to a failure.

Electrical Faulty Current Types

In an electrical power system, current and voltage remain in its nominal values. Electrical fault causes deviation of those values from its nominal values or state to abnormal behaviour. The most usual and dangerous fault that occurs in the power system is a short circuit. In short circuit, heavy current flows through the circuit sidestepping the normal load.

There are mainly two types of faults in an electrical power system.

  1. Symmetrical fault
  2. Unsymmetrical fault

Symmetrical Fault

“A fault that effects all the phases equally is called an asymmetrical fault.”

In symmetrical faults, all phases are short-circuited together or often to earth. These faults are also known as Balanced fault. These types of faults are of two types.

Line to line to line ( L-L-L )

This type of fault occurs when all three phases short together due to environmental effect or insulation breakdown. Their appearance in the power system is rarely 2% – 3%. 

Line to line to line to ground( L-L-L-G )

This type of fault occurs when all three phases short together with ground wire. This type is a very severe type of fault. Their appearance in the power system is rarely 2% – 3%.

Unsymmetrical Fault

“The fault that effects all the phases unequally is known as an unsymmetrical fault.” 

Unsymmetrical faults only involve one or two phases. The phases in unsymmetrical faults get unbalanced. Thus, also known as an unbalanced fault. These type of faults are further classified into four types.

Single line to ground ( L-G ) 

It mainly occurs when any single phase line gets in contact with the ground or neutral conductor due to breakdown or environmental effect. The changes in these type of faults in an electrical power system are reasonably high.

Double line to ground ( L-L-G )

This type of fault occurs when two phases together get in contact with the ground or neutral conductor. 15% – 20% of faults in electrical systems are usually L-L-G faults.

Line to line ( L-L )

The line to line faults is when two phases contact together. These faults usually occur by conductor swinging due to the wind. The changes in these type of faults in an electrical power system are approximately 10% – 12%.

Line to line and Line to ground 

This type of fault occurs when two phases contact together and third phase contact with the ground. Such type of fault occurs due to the breakdown of conductor and insulation. Changes for such type of faults are hardly 1% – 2%.

Destructive Effects Of Fault Current

On the occurrence of these faults, heavy short circuit current flows in the circuit. This current has the following destructive effects on electrical power systems.

a) Excessive flow of current: When a fault occurs, it provides a low impedance path for the current to flow. This results in a very high current drawing from the source.

b) Electrical fire: Sometimes short circuit causes sparks due to air ionization around the conductor. This arc may further lead to a fire that may cause considerable damage to an electrical power system.

c) Instability: The instability of the power system may occur due to a short circuit that may even lead to a complete shutdown of the electric power system.

d) Equipment loss: Sometimes heavy current flow due to the short circuit may damage or completely burnt the electrical equipment. This lead to improper working of the electrical power system.

Detecting And Locating Faults

Electrical Faults in overhead transmission lines are easy to investigate. Since the problems in it will be pretty much obvious. Fault location in a cable system can either be done by de-energizing circuit or by the circuit with power.

There are two techniques use broadly for detecting electrical fault location.

  1. Terminal Method
  2. Tracer Method

The terminal method which measures current and voltage at the corner end of cables. This method only locates the general location of an electrical fault. This method can be useful for reducing fault cost expenditure of long-buried underground cables.

Tracer method which uses a time-domain reflector to locate the electric fault location. In time-domain reflector, a pulse is sent down the wire to analyze the returning reflected pulse. The mathematical calculations are done to trace the exact location of the fault. This method is usually adopted in the complex wiring system.

The Murray Loop and Varley loops are the old techniques used for locating faults in cables.

Electrical Fault Current Limiting Devices

We can reduce human error faults but we have no control over environmental changes. Tracing fault location is very tough in a complex power system. So, if we can manage to break the circuit if a fault arises, this would reduce the percentage of maximum equipment damage loss.

Most commonly used faulty current limiting devices are circuit breakers, fuses and relays.

Circuit Breaker

Circuit breakers are an automatically operating electrical switch device to protect the power system from overcurrent due to short circuit. It interrupts the current flow by tripping down when faulty current arises.

Fuse

Fuses are the most commonly used protecting device. It melts down due to heavy short circuit current when arises in the system thus break the power supply between the supply end and load.

Relay

Relays are the protecting devices which operate electromagnetically. When faulty current arises, it energizes the coil within the relay thus results in tripping down of the circuit by opening the circuit.

Transmission Lines are consist of Lightening arrestors and grounding wires to protect the system against lightning and surge voltages.

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