A persistent failure is present regardless of the force that is applied. Most faults in underground electrical cables are usually persistent, due to mechanical damage to the cable, but sometimes they can also be transient, due to lightning.[1]​.

asymmetric failure

An asymmetric or unbalanced fault does not affect each of the phases equally. The most common types of asymmetric failures and their causes are:.

Line-to-line fault: A short circuit between lines, due to air ionization or physical contact between lines, such as due to a broken insulator. Approximately 5% to 10% of transmission line failures are asymmetric line-to-line failures.[2]​.

Line-to-ground fault: A short circuit between the line and the ground, most often due to physical contact, such as due to lightning or other types of storm damage. Approximately 65% ​​to 70% of transmission line faults are asymmetric line-to-ground faults.[2]​.

Double Line to Ground Fault: Two lines come into contact with the ground (and each other), also, most of the time, due to storm damage. Approximately 15% to 20% of transmission line faults are asymmetric double-line-to-ground faults.[2]​.

Symmetrical fault

A symmetrical or balanced fault affects each of the phases equally. Approximately 5% of transmission line faults are symmetrical faults.[3]​ These faults, compared to asymmetrical faults, are rare. Two types of symmetrical faults are line-to-line-to-line (LLL) and line-to-line-to-ground (LLLT). Symmetrical failures account for 2 to 5% of all system failures. However, even though the system remains balanced, they can cause very severe damage to the equipment.

frank failure

One extreme is where the fault has zero impedance, giving the maximum of the presumed short circuit current. In theory, all conductors are designed to be connected to the ground as for a metallic conductor. This is known as "frank failure." It would be unusual in a well-designed electrical system to have a metal-to-ground short, however, unfortunately, these types of failures can happen. In one type of transmission line protection, a “frank fault” is deliberately introduced to speed up the operation of the protection devices.

ground fault

A ground fault is any fault that allows the unintentional connection of electrical circuit conductors with the ground. Such faults can cause unpleasant circulating currents or can provide dangerous voltage power to the equipment case. Some special electrical power distribution systems may be designed to tolerate a single ground fault and continue operating. In such a case, wiring codes may require a permanent insulation monitor to raise an alarm, so that the cause of the ground fault can be identified and remedied. If a second ground fault appears in such a system, it can result in overcurrent or component failure. Even in systems that are normally grounded to limit surges, some applications require a RCD or similar device to detect ground faults.

Realistic failures

In reality, the resistance at a fault can be from almost zero to quite high relative to the load resistance. If compared to the zero impedance case, where the energy is zero, a large amount of energy can be consumed in the fault. Also, arcs are highly nonlinear, so a simple resistor is not a good example. To do a good analysis, all possible cases must be considered.[4]​.

arc fault

When the system voltage is high enough, an electrical arc can form between the electrical system conductors and the ground. Compared to normal system operating levels, such arcing may have a relatively high impedance and may be difficult to detect by simple overcurrent protection. For example, an arc of several hundred amperes on a circuit that normally carries a thousand amperes may not trip overcurrent circuit breakers, but it can do enormous damage to current-carrying tubes or cables before developing into a complete short circuit. General purpose, industrial and commercial power systems have additional protective devices to detect relatively small, but unwanted currents leaking to ground. In a residential electrical installation, current electrical regulations require arc flash detectors in the installation of electrical circuits. This way, small arcs can be detected before they cause damage or a fire.

Symmetrical faults can be analyzed using the same methods used for any other phenomenon in electrical power systems and, in fact, there are a large number of Software tools to carry out this type of analysis automatically (see Power Flow). However, there is another method that is more precise and is usually more instructive.

First, some assumptions are made schematically. All electrical generators in the system are assumed to be in phase and operating at the system's rated "Voltage (electricity)" voltage. Electric motors can also be considered generators, because when a failure occurs, they typically supply power rather than extract it. Then, the voltages and currents are calculated for this reference case.

Next, the fault location is considered to be supplied with a negative voltage source, with the same voltage as the reference case, while all other sources are set to zero. This method is based on the Superposition Principle.

To obtain a more accurate result, these calculations must be carried out separately and for three different time intervals:.

The first is the sub transient and is associated with the largest currents.

The transient state is located between the sub-transient state and the stationary state.

The steady state occurs once all transients have been established.

An asymmetric fault breaks the underlying assumptions used in three-phase power, namely that the load (electricity) is balanced across all three phases. Consequently, it is impossible to directly use tools such as the one-line diagram, where only one phase is considered. However, due to the linearity of electrical energy systems, it is common to consider the resulting voltages "Voltage (electricity)") and electrical current as a superposition of symmetrical components, to which three-phase analysis can be applied.

In the symmetric component method, the electrical power system is considered as a superposition of three components:

A positive sequence component, in which the phases are in the same order as the original, i.e. a-b-c.

A negative sequence component, in which the phases are in the opposite order as the original, i.e. a-c-b.

A zero sequence component, which is not actually a three-phase system, but instead the three phases are in phase with each other. To determine the currents resulting from an asymmetric fault, you must first know the zero, positive, and negative per-unit (pu) sequence impedances of the transmission lines, generators, and transformers involved. Three separate circuits are built with these impedances. The individual circuits are connected together and follow a specific order. An order that depends on the type of fault that has been studied (this can be found in most power systems textbooks). Once the sequence circuits are correctly connected, the network can be analyzed using classic circuit analysis techniques. The results of the solution are voltages and currents that exist as symmetrical components. These results are then transformed back into phase values ​​by using matrix A.

For the selection of protective devices such as fuses and circuit breakers, it is necessary to perform an analysis of the presumed short-circuit current. If a circuit is well protected, the fault current should be high enough for the protective device to activate in the shortest time possible. On the other hand, the protective device must be able to withstand the fault current and eliminate any arcing as a result of the fault without being destroyed or maintaining the arc for a significant period of time.

It depends on what type of grounding system is used, as well as the type of supply to the facility, the grounding system and its proximity to the supply that the magnitude of the fault currents can differ greatly. For example, for a UK 230V, 60A TN-S and US 120V/240V domestic supply, fault currents can be a few thousand amps. Large low voltage networks with multiple sources can have fault levels of 300,000 amperes. A heavy-duty grounding system can limit line ground fault current to only 5 amps. Before selecting protective devices, the suspected fault current must be reliably measured at the origin of the installation and at the farthest point of each circuit, and such information must be used correctly for the application of the circuits.

Overhead power lines are easier to diagnose, as the problem is usually obvious. For example, when a tree falls on the other side of the line or a utility pole breaks and drivers find themselves on the ground.

Locating faults in a cable system can be done with the circuit de-energized or, in other cases, with the circuit operating. Fault location techniques can be broadly divided into terminal methods and tracing methods. Terminal methods use voltages and currents measured at the ends of the cable, and layout methods require inspection along the length of the cable. Terminal methods can be used to locate the general area of ​​the fault, so that tracing on a long or buried cable can be accelerated.[5]​.

In very simple wiring systems, the location of the fault is often found through inspection of the cables. In complex wiring systems, such as aircraft wiring, where cables may be hidden, faults are located with a Time Domain Reflectometer.[6]​ The time domain reflectometer sends a pulse down the cable and then analyzes the reflected pulse to identify faults in the electrical cable.

For submarine cables intended for telegraph service, sensitive galvanometers were used to measure fault currents. By testing both ends of a defective cable, the location of the fault could be defined within a few miles so that the cable could be recovered and repaired. Murray and Varley bridges were two types of connections used to locate faults in cables.

Sometimes an insulation fault in a power cable cannot be observed when voltages are very low. From a series of "strong vibration" tests, a high-energy, high-voltage pulse is applied to the cable. The fault can be located by listening to the discharge sound at the fault. Although this type of test contributes to damage to the cable, it is still useful, since, in any case, once the location of the fault is located, it will have to be re-isolated.[7]​.

In a high-resistance grounded distribution system, a feeder pipe may develop a ground fault, yet the system continues to operate. The faulty but energized feeder can be found with a Current Transformer with a ring core that collects all the phase wires in the circuit. Only the circuit containing a ground fault will show an unbalanced net current. To facilitate the detection of the ground fault current, the system ground resistance can be switched between two values, so that the fault current pulses.

The presumed fault current in large batteries, such as deep cycle batteries used in autonomous power systems, is provided by the manufacturer.

In Australia, where this information is not provided, the presumed fault current in amperes “should be considered 6 times the rated capacity of the battery at C 120 Ah”, according to AS 4086 part 2 (appendix H).

• - Electrical safety").

• - Defect (technology)&action=edit&redlink=1 "Defect (technology) (not yet written)").

A persistent failure is present regardless of the force that is applied. Most faults in underground electrical cables are usually persistent, due to mechanical damage to the cable, but sometimes they can also be transient, due to lightning.[1]​.

asymmetric failure

An asymmetric or unbalanced fault does not affect each of the phases equally. The most common types of asymmetric failures and their causes are:.

Line-to-line fault: A short circuit between lines, due to air ionization or physical contact between lines, such as due to a broken insulator. Approximately 5% to 10% of transmission line failures are asymmetric line-to-line failures.[2]​.

Line-to-ground fault: A short circuit between the line and the ground, most often due to physical contact, such as due to lightning or other types of storm damage. Approximately 65% ​​to 70% of transmission line faults are asymmetric line-to-ground faults.[2]​.

Double Line to Ground Fault: Two lines come into contact with the ground (and each other), also, most of the time, due to storm damage. Approximately 15% to 20% of transmission line faults are asymmetric double-line-to-ground faults.[2]​.

Symmetrical fault

A symmetrical or balanced fault affects each of the phases equally. Approximately 5% of transmission line faults are symmetrical faults.[3]​ These faults, compared to asymmetrical faults, are rare. Two types of symmetrical faults are line-to-line-to-line (LLL) and line-to-line-to-ground (LLLT). Symmetrical failures account for 2 to 5% of all system failures. However, even though the system remains balanced, they can cause very severe damage to the equipment.

frank failure

One extreme is where the fault has zero impedance, giving the maximum of the presumed short circuit current. In theory, all conductors are designed to be connected to the ground as for a metallic conductor. This is known as "frank failure." It would be unusual in a well-designed electrical system to have a metal-to-ground short, however, unfortunately, these types of failures can happen. In one type of transmission line protection, a “frank fault” is deliberately introduced to speed up the operation of the protection devices.

ground fault

A ground fault is any fault that allows the unintentional connection of electrical circuit conductors with the ground. Such faults can cause unpleasant circulating currents or can provide dangerous voltage power to the equipment case. Some special electrical power distribution systems may be designed to tolerate a single ground fault and continue operating. In such a case, wiring codes may require a permanent insulation monitor to raise an alarm, so that the cause of the ground fault can be identified and remedied. If a second ground fault appears in such a system, it can result in overcurrent or component failure. Even in systems that are normally grounded to limit surges, some applications require a RCD or similar device to detect ground faults.

Realistic failures

In reality, the resistance at a fault can be from almost zero to quite high relative to the load resistance. If compared to the zero impedance case, where the energy is zero, a large amount of energy can be consumed in the fault. Also, arcs are highly nonlinear, so a simple resistor is not a good example. To do a good analysis, all possible cases must be considered.[4]​.

arc fault

When the system voltage is high enough, an electrical arc can form between the electrical system conductors and the ground. Compared to normal system operating levels, such arcing may have a relatively high impedance and may be difficult to detect by simple overcurrent protection. For example, an arc of several hundred amperes on a circuit that normally carries a thousand amperes may not trip overcurrent circuit breakers, but it can do enormous damage to current-carrying tubes or cables before developing into a complete short circuit. General purpose, industrial and commercial power systems have additional protective devices to detect relatively small, but unwanted currents leaking to ground. In a residential electrical installation, current electrical regulations require arc flash detectors in the installation of electrical circuits. This way, small arcs can be detected before they cause damage or a fire.

Symmetrical faults can be analyzed using the same methods used for any other phenomenon in electrical power systems and, in fact, there are a large number of Software tools to carry out this type of analysis automatically (see Power Flow). However, there is another method that is more precise and is usually more instructive.

First, some assumptions are made schematically. All electrical generators in the system are assumed to be in phase and operating at the system's rated "Voltage (electricity)" voltage. Electric motors can also be considered generators, because when a failure occurs, they typically supply power rather than extract it. Then, the voltages and currents are calculated for this reference case.

Next, the fault location is considered to be supplied with a negative voltage source, with the same voltage as the reference case, while all other sources are set to zero. This method is based on the Superposition Principle.

To obtain a more accurate result, these calculations must be carried out separately and for three different time intervals:.

The first is the sub transient and is associated with the largest currents.

The transient state is located between the sub-transient state and the stationary state.

The steady state occurs once all transients have been established.

An asymmetric fault breaks the underlying assumptions used in three-phase power, namely that the load (electricity) is balanced across all three phases. Consequently, it is impossible to directly use tools such as the one-line diagram, where only one phase is considered. However, due to the linearity of electrical energy systems, it is common to consider the resulting voltages "Voltage (electricity)") and electrical current as a superposition of symmetrical components, to which three-phase analysis can be applied.

In the symmetric component method, the electrical power system is considered as a superposition of three components:

A positive sequence component, in which the phases are in the same order as the original, i.e. a-b-c.

A negative sequence component, in which the phases are in the opposite order as the original, i.e. a-c-b.

A zero sequence component, which is not actually a three-phase system, but instead the three phases are in phase with each other. To determine the currents resulting from an asymmetric fault, you must first know the zero, positive, and negative per-unit (pu) sequence impedances of the transmission lines, generators, and transformers involved. Three separate circuits are built with these impedances. The individual circuits are connected together and follow a specific order. An order that depends on the type of fault that has been studied (this can be found in most power systems textbooks). Once the sequence circuits are correctly connected, the network can be analyzed using classic circuit analysis techniques. The results of the solution are voltages and currents that exist as symmetrical components. These results are then transformed back into phase values ​​by using matrix A.

For the selection of protective devices such as fuses and circuit breakers, it is necessary to perform an analysis of the presumed short-circuit current. If a circuit is well protected, the fault current should be high enough for the protective device to activate in the shortest time possible. On the other hand, the protective device must be able to withstand the fault current and eliminate any arcing as a result of the fault without being destroyed or maintaining the arc for a significant period of time.

It depends on what type of grounding system is used, as well as the type of supply to the facility, the grounding system and its proximity to the supply that the magnitude of the fault currents can differ greatly. For example, for a UK 230V, 60A TN-S and US 120V/240V domestic supply, fault currents can be a few thousand amps. Large low voltage networks with multiple sources can have fault levels of 300,000 amperes. A heavy-duty grounding system can limit line ground fault current to only 5 amps. Before selecting protective devices, the suspected fault current must be reliably measured at the origin of the installation and at the farthest point of each circuit, and such information must be used correctly for the application of the circuits.

Overhead power lines are easier to diagnose, as the problem is usually obvious. For example, when a tree falls on the other side of the line or a utility pole breaks and drivers find themselves on the ground.

Locating faults in a cable system can be done with the circuit de-energized or, in other cases, with the circuit operating. Fault location techniques can be broadly divided into terminal methods and tracing methods. Terminal methods use voltages and currents measured at the ends of the cable, and layout methods require inspection along the length of the cable. Terminal methods can be used to locate the general area of ​​the fault, so that tracing on a long or buried cable can be accelerated.[5]​.

In very simple wiring systems, the location of the fault is often found through inspection of the cables. In complex wiring systems, such as aircraft wiring, where cables may be hidden, faults are located with a Time Domain Reflectometer.[6]​ The time domain reflectometer sends a pulse down the cable and then analyzes the reflected pulse to identify faults in the electrical cable.

For submarine cables intended for telegraph service, sensitive galvanometers were used to measure fault currents. By testing both ends of a defective cable, the location of the fault could be defined within a few miles so that the cable could be recovered and repaired. Murray and Varley bridges were two types of connections used to locate faults in cables.

Sometimes an insulation fault in a power cable cannot be observed when voltages are very low. From a series of "strong vibration" tests, a high-energy, high-voltage pulse is applied to the cable. The fault can be located by listening to the discharge sound at the fault. Although this type of test contributes to damage to the cable, it is still useful, since, in any case, once the location of the fault is located, it will have to be re-isolated.[7]​.

In a high-resistance grounded distribution system, a feeder pipe may develop a ground fault, yet the system continues to operate. The faulty but energized feeder can be found with a Current Transformer with a ring core that collects all the phase wires in the circuit. Only the circuit containing a ground fault will show an unbalanced net current. To facilitate the detection of the ground fault current, the system ground resistance can be switched between two values, so that the fault current pulses.

The presumed fault current in large batteries, such as deep cycle batteries used in autonomous power systems, is provided by the manufacturer.

In Australia, where this information is not provided, the presumed fault current in amperes “should be considered 6 times the rated capacity of the battery at C 120 Ah”, according to AS 4086 part 2 (appendix H).

• - Electrical safety").

• - Defect (technology)&action=edit&redlink=1 "Defect (technology) (not yet written)").