Short Circuit Analysis: Why and How to Do It

Have you seen a sudden blast or shock wave in a power system?

Well, most of the blasts occur due to an electrical fault.

Short circuit analysis is harmful to every power system.

It can damage electrical equipment. It can also injure or harm the working personnel.

Short circuit analysis is an important part of electrical maintenance.

So, you had better do it when you add new cables, circuit breakers, etc., to the power system.

However, before that, you must know what a short circuit is.

What is a Short Circuit?

When high currents exceed the limit of protective devices, it becomes dangerous.

It occurs when two points of different voltages connect.

And the electric current travels to an unintended path of low resistance.

Owing to the low resistance, there is a high flow of current.

Contact between two points of different voltages creates the wrong path.

So, energy is rapidly released in the form of heat and magnetic fields.

This creates a shock wave, overheating, explosions, fires, or circuit damage.

Most of the time, short circuits occur due to faults.

However, at times, you can do it for testing purposes.

Image: short circuit and burnt cable

In an Alternating current system, a short circuit can occur:

• If one phase touches another phase
• T phase touches the ground
• If the phase touches neutral
• If the windings of a machine touch each other in one phase

Direct current system results in a short circuit in the following circumstances:

• When two poles touch the ground
• When there is contact between poles.

Different Faults that Occur Due to Short Circuits

Short Circuits can lead to faults within the system. They are:

Bolted fault: Sometimes, you bolt two conductors of different voltages together or to a power source.

This source of power is bolted to the ground.

This results in a bolted fault.

No fault resistance exists, so the highest possible current passes through the system.

Bolted faults are quite unusual and occur by chance only.

You can use bolted faults to speed up the operation of protective devices.

Arc fault: This fault occurs when current flows through two conductors which are not in contact.

When there is high system voltage, an arc forms between system conductors and the ground, this arc has very high resistance.

A simple overcurrent protection device cannot detect this arc. It can cause severe damage to the cables before becoming a complete short circuit.

Also, it may lead to arc blast or arc flash, leading to severe injuries.

Image: electric cord exploding

Ground fault: A wrong connection between the conductors and the earth results in this fault condition.

The current flows through a low-resistance path to the ground.

This results in dangerous circulating currents that damage the equipment at high voltage.

This path may lead to serious injuries when it passes through the human body.

Most of the power systems are designed to bear one ground fault.

However, a second fault leads to electrical components failure.

The electrical shock from the ground fault also stops the human heart from functioning.

Symmetrical faults and non-symmetrical faults:

Polyphase systems can have either symmetrical or non-symmetrical faults.

The differences between them are:

• Symmetrical fault current affects all phases equally. On the other hand, non-symmetrical will affect only a few or all phases unequally.
• You can analyze symmetrical faults. However, the non-symmetrical fault current is difficult to detect.
• Non-symmetrical faults are common, while symmetrical faults account for only 5%.

Why do Short Circuits Damage Equipment?

Sudden short circuit results in extensive equipment damage.

Two phenomena increase the risk of equipment damage during a short circuit event.

Image: overloaded circuit board

Thermal phenomenon: Short can lead to energy releases through intense heat.

This is the thermal phenomenon. This heat damages an electrical circuit in various ways:

• The melts the ends of the conductors.
• Causes damage to the insulation.
• It generates electric arcs.
• It destroys the thermal elements in the bimetal relay

Electro-dynamic phenomenon: Sometimes, the short circuit current creates high mechanical stress.

This results in physical damage to the conductor’s windings and conductors.

Also, it can result in the repulsion of contacts within conductors.

Protective Devices for Short Circuits

An overcurrent situation is dangerous for equipment and humans.

In such situations, you need to use a protective device.

The protective device detects any fault current. On detection, it trips the circuit at once.

This prevents the current from reaching its maximum limit.

These electrical protection devices have two features:

• Breaking capacity: When a high current travels, the protective device will break the circuit. The highest value of this current is the breaking capacity.
• Closing capacity: The maximum short circuit current at which the device will reach its rated voltage.

An electrical circuit has different power system protective devices. However, the most common type is:

Fuse: This protective device gives you high breaking capacity.

Circuit breakage occurs even at low voltage. However, you have to replace it once it trips.

The fuse protects the electrical circuit from electro-dynamic stress.

This protective device is suitable for phase-by-phase protection.

Image: electrical fuses

Circuit breakers: Circuit breakers break the circuit automatically in a short cut-off time.

Circuit breakers separate the load from the power supply.

This will protect the electric circuit from damage.

Unlike a fuse, it protects the electric circuit against thermal and electro-dynamic effects.

Image: electricity distribution box with circuit breakers and wires

Ground Fault Interrupter: The current flows in the charged conductor in an electric circuit and returns through the neutral conductor.

Ground Fault Interrupter (GFI) detects any difference in this circuit current.

If there is a difference in circuit current, current flows stop at once.

This protective device can protect you from the shock wave.

You can use ground fault interrupters in your homes for bathrooms and kitchens.

This comes inbuilt in an electrical socket. You do not get over-current protection through a GFI.

As a result, any circuit that uses a GFI also uses fuses or circuit breakers.

Along with these protective devices, there are several other protective devices.

You can use them to 

• Find any changes in the voltage level and current
• Observe the ratio of voltage and current
• Offer protection against over-voltage and under-voltage
• Detect phase reversal and reverse current flow

What is a Short Circuit Analysis?

You can do this analysis for short circuit fault current calculations.

During an electrical fault, a certain current flows through the electrical system.

You can calculate the maximum fault current with this circuit study. However, don’t miss the contribution from generators and synchronous motors when calculating maximum fault current.

Also, take note of induction motor contributions. 

Compare these values with the equipment ratings of short circuit protection devices and installed electrical equipment.

Ensure that electrical equipment can bear the short circuit energy at each point.

Create fault currents at different locations for complete information.

Now, repeat the study. With this, design a suitable relaying system to protect the devices.

Your equipment will fail to handle a fault current in low-rating power systems.

This leads to major destruction in the power distribution system. 

For circuit study, you need power system analysis software that complies with IEEE standards.

For large systems, do circuit calculations for switchgear and relay settings. 

Short circuit analysis is important to meet several standards. 

According to NEC110, you must do short-circuit study for all electrical equipment and power systems.

This is important for NEC and NFPA labels.

For this, you need to meet the guidelines of two common standards; One is ANSI, and the other is IEC.

With ANSI standard guidelines, you can select power circuit breakers.

However, you do not get any information related to NEC110 labeling. On the other hand, the IEC 60909-3:2009 standard is more general.

Its guidelines help in performing a study of an asymmetrical circuit in a 3-phase electrical system.

You can use either of the calculation methods. You will get almost similar results.

Benefits of Short Circuit Analysis

Conducting a short circuit analysis is beneficial for your installation. 

• Firstly, it avoids unplanned outages and any disturbances in essential services.
• Secondly, it reduces the risk of fires and equipment damage.
• Thirdly, it ensures the safety of people.
• Finally, it ensures the safety and protection of the power system. This makes the system more reliable.

Software for Short Circuit Study

Some of the software programs for this are:

• ETAP
• SKM power tools
• Easy Power
• PSS

How to Conduct a Short Circuit Calculation

To apply the coordination of protective relays, a short analysis is essential.

Follow these steps for the circuit study.

Firstly, collect information related to all electrical components.

Secondly, draw a linear network of the system. The diagram must show the connections of all electrical components. You can also find cable impedance with this power system diagram.

Thirdly, select the suitable short circuit study software. Put system data as input. Do short-circuit calculations at different points. Put these results as output.

Now, organize the results. Compare the short circuit current calculations with the short circuit rating. The circuit values you compare must be from the same point. If the short circuit current is higher, mark that as dangerous.

Finally, prepare a detailed report of the study.

At last, suggest any corrections that need to be made.

Conclusion:

Don’t hesitate to do a circuit study if you doubt a short in an electric circuit.

Short circuits result in damage to wires and cables.

So, if you want high-quality cables and wires, Cloom is here to help you.

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