How does the type of fault impact protective devices in power distribution?

Faults are abnormal conditions in power distribution systems that cause excessive current, voltage, or power flow. They can damage equipment, disrupt service, and endanger personnel. To prevent or minimize these consequences, protective devices are installed to detect and isolate faults as quickly and selectively as possible. However, not all faults are the same, and different types of faults require different types of protective devices. In this article, you will learn how the type of fault impacts the choice and operation of protective devices in power distribution.

1 Symmetrical and asymmetrical faults

A symmetrical fault is a balanced fault that affects all three phases equally. For example, a three-phase short circuit is a symmetrical fault. A symmetrical fault can be analyzed using per-unit or phasor methods, and the fault current is proportional to the pre-fault voltage and inversely proportional to the system impedance. A symmetrical fault usually requires high-speed and high-current protective devices, such as circuit breakers, fuses, or relays, to clear the fault and protect the equipment.

An asymmetrical fault is an unbalanced fault that affects only one or two phases. For example, a single-phase or a phase-to-phase short circuit is an asymmetrical fault. An asymmetrical fault can be analyzed using symmetrical components or sequence networks, and the fault current has both a steady-state and a transient component. An asymmetrical fault usually requires time-delayed and selective protective devices, such as reclosers, sectionalizers, or ground fault interrupters, to coordinate with other devices and avoid unnecessary interruptions.

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2 Shunt and series faults

A shunt fault is a fault that creates a parallel path for the current to flow. For example, a short circuit or a ground fault is a shunt fault. A shunt fault reduces the system voltage and increases the system current. A shunt fault can cause thermal and mechanical stress on the equipment, as well as voltage sags and flickers on the load side. A shunt fault requires protective devices that can sense the overcurrent and interrupt the circuit.

A series fault is a fault that creates an open circuit or a high impedance in the current path. For example, a broken conductor or a loose connection is a series fault. A series fault increases the system voltage and reduces the system current. A series fault can cause overvoltage and insulation breakdown on the equipment, as well as voltage swells and harmonics on the load side. A series fault requires protective devices that can sense the undercurrent or the overvoltage and isolate the faulty section.

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Ahmed Refaat Rashed , PMP®

Senior Project Engineer
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shunt and series faults in electrical systems: Shunt faults are like when you have a short circuit in your home. This happens when a live wire touches a ground wire or another live wire. This creates a path for electricity to flow that doesn't go through your appliances, which can cause a fire or damage your equipment. Series faults are like when you have a broken wire in your home. This means that the electricity can't flow through the wire, which can make your appliances stop working. Both shunt and series faults can be dangerous, so it's important to have protective devices in your electrical system that can detect and shut off the power in the event of a fault. This will help to prevent fires and damage to your equipment.

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3 Arcing and non-arcing faults

An arcing fault is a fault that involves an electric arc between two conductors or a conductor and the ground. For example, a flashover or a tree contact is an arcing fault. An arcing fault has a variable and nonlinear impedance that depends on the arc length, temperature, and medium. An arcing fault can generate heat, light, sound, and electromagnetic interference. An arcing fault requires protective devices that can extinguish the arc and prevent its restriking.

A non-arcing fault is a fault that does not involve an electric arc. For example, a solid or a resistive fault is a non-arcing fault. A non-arcing fault has a constant and linear impedance that depends on the fault resistance. A non-arcing fault can cause power losses and voltage drops. A non-arcing fault requires protective devices that can detect the fault resistance and clear the fault.

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Cisco Avontuur

Director of Operations
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An example of a non arcing protection device is RCD which pics up minute differences in current caused by leakage in the system. Arc-fault detection device is a circuit breaker that breaks the circuit when it detects the electric arcs that are a sign of loose connections in installation wiring. Loose connections, which can develop over time, can sometimes become hot enough to ignite house fires. An AFDD selectively distinguishes between a harmless arc (incidental to normal operation of switches, plugs, and brushed motors), and a potentially dangerous arc (that can occur, for example, in a lamp cord which has a broken conductor).

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4 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Ahmed Refaat Rashed , PMP®

Senior Project Engineer
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Here are some of the ways in which fault type can impact the distribution system: Level of damage: Different types of faults can cause different levels of damage to system equipment. For example, a bolted three-phase fault will cause a large amount of current to flow, which can damage transformers, switches, and other components. On the other hand, a single line-to-ground fault may only cause a small amount of damage, or it may even go undetected by protective relays. Safety hazards: Faults can create l Duration of power outages:

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How does the type of fault impact protective devices in power distribution?

1

2

3

4

1 Symmetrical and asymmetrical faults

2 Shunt and series faults

3 Arcing and non-arcing faults

4 Here’s what else to consider

Power Engineering

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How does the type of fault impact protective devices in power distribution?

1

2

3

4

1 Symmetrical and asymmetrical faults

2 Shunt and series faults

3 Arcing and non-arcing faults

4 Here’s what else to consider

Power Engineering

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