Introduction
Different circuit types have different current and voltage values. But not all of them operate consistently at the same power, voltage, and values. Due to unexpected issues, there can be a current or voltage overload or short circuits. Most people purchase circuit breakers, which is the right choice. But understanding short-circuit capacity is more important during this selection.
Today we will understand short-circuit capacity and evaluate various factors related to it. Let’s know!
What is Short-Circuit Capacity?
Short-circuit capacity is a simple but crucial term used for selecting circuit breakers and other components.
It refers to the maximum current a power device can provide during a short-circuit condition. Short-circuit capacity often represents the power of the electrical system connected to the system.
Short-circuit capacity is often represented in:
- Short-Circuit MVA (SC MVA)
- Fault Current (kA)
- Fault Power (MVA)
Do you know the purpose of understanding the short-circuit capacity? There can be several reasons.
- Select circuit breakers
- Design protection systems
- Perform arc flash studies
- Coordinate relays
- Choose busbars
- Equipment rating verification
Why Does Short-Circuit Capacity Matter?
Short-circuit capacity defines a lot of factors. Not all circuits operate under normal conditions. A few might experience faults in the circuitry and fail to perform.
Circuit Breakers can be those tools to avoid the mishap due to this failure. Short-circuit capacity gives an idea of the fault currents and helps decide on the right circuit breaker with the right capacity.
Here are a few major benefits and options.
- Prevents equipment destruction
- Improves electrical safety
- Reduces arc flash hazards
- Supports proper relay coordination
- Ensures circuit breaker interrupting capability
- Minimizes downtime
- Improves system reliability
- Meets electrical standards and codes
Types of Short Circuits
A short circuit is never of a single type. There can be different preferences and options for short circuit systems. Here are four key types with their details and options.
Three-Phase Fault
Three-Phase fault is one of the most common and severe fault types that is symmetrical. It has a few features and applications listed below.
Characteristics:
- Highest current
- Balanced fault
- Used for equipment rating
- Simplest calculations
Applications:
- Circuit breaker selection
- Generator design
- Switchgear rating
Single Line-to-Ground Fault
If you are looking for the most common fault, a single line-to-ground fault is one you should know about. It often occurs due to a single source, such as lightning and many more causes listed below.
Causes include:
- Lightning
- Insulation failure
- Tree branches
- Broken conductors
Features:
- Unbalanced fault
- Lower current than three-phase in many systems
- Depends heavily on grounding method
Line-to-Line Fault
If your wires have bare insulation and are exposed, then a line-to-line fault is the most common one. It occurs due to touching of the two phases.
The current level is always lower than the three-phase fault.
The common causes are listed below.
- Cable damage
- Equipment failure
- Mechanical accidents
Double Line-to-Ground Fault
A double line-to-ground fault occurs when the two phases simultaneously touch the ground. It produces complex current paths, unbalanced systems, and higher faults compared to unsymmetrical systems.
| Fault Type | Frequency | Current Magnitude | System Balance |
| Three Phase | Low | Highest | Balanced |
| Line-to-Line | Moderate | Medium | Unbalanced |
| Single Line-to-Ground | Highest | Medium to High | Unbalanced |
| Double Line-to-Ground | Low | Medium to High | Unbalanced |
Sources of Short Circuit Currents
There can be different options and sources of short-circuit currents. Here are a few examples of sources listed and explained.
Utility Grid
Since the utility grid is the main distribution of current and voltage, it contributes to the highest fault current values.
Characteristics of the utility grid related to the short circuits include:
- High generation capacity
- Low source impedance
- High fault levels
Moreover, it depends on how large your substation is. Usually, large substations may provide:
- 25 kA
- 40 kA
- 63 kA
- Even higher values
Generators
Generators can be a source of the fault. They even contribute to the higher current values but help maintain the system during operations.
They supply 5-8 times the rated current when causing the failure.
Motors
Motors draw significantly higher starting currents. During an assessment, they are often ignored for the fault currents, which is a wrong choice.
However, motors can cause fault currents. Large induction motors keep on rotating after a fault current due to kinetic energy stored in them.
Battery Energy Storage Systems
Compared to the generators and other systems, battery energy storage systems are less likely to interfere and cause major failures in the system. Fault currents and short circuits depend on:
- Inverter design
- Protection settings
- Grid-forming capability
Their contribution is also crucial to understand when choosing the circuit breakers for your system.
Factors Affecting Short-circuit Capacity
If you are wondering what factors could influence and change the short-circuit capacity, we have added a few factors here.
Factor #1: Source Strength
Source Strength matters a lot depending on the current production levels. For example, a utility grid contributes the highest current values compared to the generators and other systems.
If you have a large-capacity utility grid, then the fault currents would be even higher. Moreover, impedance is inversely proportional to the fault current. A utility grid with multiple generating stations, transmission lines, and substations has higher fault currents due to lower impedance.
Why is Source Strength Crucial?
If you have the same questions flashing through your brain about why source strength matters, here are the answers.
A stronger utility source has the following effects.
- Higher fault currents
- Requires circuit breakers with larger interrupting ratings
- Increases arc flash energy
- Places greater mechanical stress on equipment
A weaker source can cause the following issues.
- Produces lower fault currents
- May reduce equipment costs
- Can create relay sensitivity challenges
Typical Utility Fault Levels
| Supply Voltage | Typical Short-Circuit Current | Approximate Short-Circuit Capacity |
| 400 V | 20–65 kA | 14–45 MVA |
| 11 kV | 10–31.5 kA | 190–600 MVA |
| 33 kV | 20–40 kA | 1,140–2,290 MVA |
| 132 kV | 31.5–50 kA | 7,200–11,400 MVA |
Tip: Choose the maximum and minimum fault currents when examining the system for protection and safety purposes.
Factor #2: System Voltage
Fault power and voltages are directly proportional to each other. It means that as the voltage of the system increases, fault power also increases exponentially with the voltage.
For the same impedance, this is a direct relationship with the few options listed below.
| System Voltage | Fault Current | Short-Circuit Capacity |
| 415 V | 25 kA | 18 MVA |
| 11 kV | 25 kA | 476 MVA |
| 33 kV | 25 kA | 1,428 MVA |
Design Consideration
Higher voltage systems have a few unique challenges and options. For example, you need to install the following options for the higher voltage systems.
- Larger switchgear
- Better insulation
- Stronger protection coordination
- Higher interrupting ratings
Factor #3: Transformer Impedance
Impedance is the resistance to the passage of the fault currents. It is inversely related to the fault currents. In simple words, if impedance is less, there would be higher fault currents and easier passage of the current through the circuit systems.
Lower impedance = Higher Fault Current
If there is a lower impedance, there is less opposition to the fault currents and higher short-circuit values.
Here is how it defines the different values.
- Higher short-circuit capacity
- Larger breaker requirements
- Greater mechanical forces
Higher impedance = Lower Fault Current
In higher impedance systems, there is a lower fault current.
It improves the circuit protection but has the following issues.
- Larger voltage drops
- Reduced motor starting performance
| Transformer Impedance | Approximate Available Fault Current |
| 4% | 34.8 kA |
| 5.75% | 24.2 kA |
| 6% | 23.2 kA |
| 8% | 17.4 kA |
| 10% | 13.9 kA |
Factor #4: Distance From the Source
As you go farther away from the source, the fault currents decrease. It means fault currents are inversely related to the distance.
- Faults close to generators experience the highest currents.
- Faults at the end of long feeders experience much lower currents.
| Fault Location | Relative Fault Current |
| Generator terminals | Very High |
| Main switchboard | High |
| Distribution panel | Moderate |
| Motor control center | Lower |
| End of feeder | Lowest |
Factor #5: Cable Length
Cable Length also matters. It increases the resistance and reduces the fault currents.
With increasing cable lengths, especially at the feeder, here are a few features.
- Lower fault current
- Lower arc flash energy
- Higher voltage drop
- Possible relay coordination challenges
| Cable Length | Relative Fault Current |
| 10 m | 100% |
| 50 m | 92% |
| 100 m | 86% |
| 250 m | 72% |
| 500 m | 58% |
Factor #6: Conductor Size
Conducting cross-sectional areas is crucial for the fault current values. Usually, large conductors have a lower resistance and a higher fault current compared to the smaller conductors.
| Copper Cable Size | Relative Impedance | Fault Current |
| 35 mm² | High | Low |
| 70 mm² | Medium | Moderate |
| 120 mm² | Lower | Higher |
| 240 mm² | Very Low | Very High |
Factor #7: Generator Ratings
Generator Ratings also define the contribution to the fault currents. Remember, higher generator ratings mean higher fault currents, while lower generator ratings mean lower fault currents.
Here is how the fault currents and power values vary.
| Generator Size | Initial Fault Current |
| 250 kVA | 4–6 × rated current |
| 500 kVA | 5–7 × rated current |
| 1000 kVA | 6–8 × rated current |
| 2500 kVA | 7–9 × rated current |
Factor #8: Number of Parallel Sources
Are your conductors or generators connected in a parallel format?
When generators are connected in a parallel pattern, their fault currents add up and provide a combined value. It means two generators connected in parallel fashion will have a higher current compared to a single generator.
Here is how the values add up.
- One transformer has a fault current of 18 kA.
- Two transformers in parallel connection will produce a fault current of 35 kA.
- Three transformers will produce a current of 51 kA.
Factor #9: Motor Contribution
Large motors have higher induction currents. When a fault current occurs, they keep on rotating due to stored kinetic energy, adding to the fault current.
There are a few industries most commonly affected by motor contributions.
- Mining
- Oil and gas
- Cement plants
- Steel mills
- Manufacturing
- Water treatment plants
| Motor Type | Initial Contribution |
| Small motors | Negligible |
| Medium motors | Moderate |
| Large induction motors | High |
| Synchronous motors | Very High |
Factor #10: System Configuration
Network topology affects the fault currents.
Here are a few examples.
- Radial systems often produce a lower fault current.
- Ring main systems have a medium fault current.
- Meshed transmission systems have a higher fault current.
Meshed systems have multiple current paths and produce a higher short-circuit capacity than radial systems.
FAQs
- Why must short-circuit studies be updated?
Short-circuit studies help understand system modifications, such as adding transformers, generators, or large motors. They can increase the fault levels beyond the rated limits.
- How do generators affect fault levels?
Generators produce higher fault currents depending on the capacity and conditions. Their contribution is transient and ends after a short episode.
- Does cable length affect short-circuit current?
Yes. Long cables have a higher impedance and have lower short-circuit currents compared to shorter cables.
- Which factor has the greatest effect on fault current?
Utility source strength and transformer impedance are the two most crucial and essential factors with the greatest effect on fault currents.
- Why is short-circuit capacity important?
Short-circuit capacity is crucial as it helps select properly rated circuit breakers, switchgear, transformers, and protective devices.
- What is the difference between fault current and short-circuit capacity?
Fault current is the practical current flowing through the faulty system, while the short-circuit capacity is the related fault power. It is often expressed in MVA.
- How can excessive short-circuit current be reduced?
There are a few techniques to avoid excessive short-circuit currents. For example, higher-impedance transformers, current-limiting reactors, current-limiting fuses, bus sectionalizing, or higher-rated protective equipment.
- Can high fault current damage equipment?
Yes. Excessive current can cause severe thermal heating and mechanical forces. Therefore, it damages circuit breakers, busbars, cables, transformers, and switchgear.
Conclusion
Whenever choosing a short-circuit breaker, always focus on the fault currents and voltage. Avoid buying extremely high- or low-capacity circuit breakers. Also ensure you choose the right size and brand.
Do you want to buy high-quality generators that could easily escape circuit issues and perform? Contact Huaquan Power. We have different types, features, and capacities of circuit breakers. You’ll produce consistent energy and get high efficiency while cutting fuel costs. Contact Us right away!
