What Is the Infinite Bus Method for Transformer Fault Current Calculation?
- Jeff Kershner
- Dec 9, 2024
- 2 min read
What Is the Infinite Bus Method (for Transformer Fault Current)?
Arc flash calculations always start at the source of power. If a facility is powered from a generator then generator characteristics are the starting point. If a facility is powered from a utility, then fault current provided by the utility (typically by a transformer) is necessary to begin an arc flash evaluation.
Fault current is the amount of current which rushes into a system in the event of a fault. During an arc flash evaluation, Rozel requests the available fault current value from the utility provider. The utility provider typically responds with calculated fault current values (this is our preference) but sometimes the response will be a maximum value which was provided by using the infinite bus method.
The Infinite Bus Method is a widely used electrical engineering technique to estimate the maximum current a transformer can supply during a fault. It assumes the transformer is connected to an "infinite bus," an idealized voltage source with zero impedance that can provide infinite current, and calculates the fault current under this worst-case scenario. While effective for equipment sizing and initial fault current assessments, understanding the method's strengths and limitations is essential for accurate electrical safety and arc flash evaluations, which directly impacts employee safety and productivity. Used in isolation, the method can dramatically overestimate or underestimate the incident energy and arc flash risk.
Why Is the Infinite Bus Method Important?
Utilities often use the Infinite Bus Method because it provides a conservative estimate of fault current, simplifying their process for delivering data to customers. This value is critical for:
Sizing Equipment: Ensures breakers, fuses, and other devices are rated for the maximum fault current they may encounter.
Validating Utility Data: Provides a baseline for checking whether utility-provided data is realistic and aligns with system capabilities.
However, the Infinite Bus Method has drawbacks, particularly for arc flash evaluations. Overly conservative estimates can cause unnecessarily high incident energy calculations which, in turn, result in high PPE requirements, increasing worker discomfort and reducing productivity. In other situations, the Infinite Bus Method will underestimate available incident energy by lowering clearing times of protective devices.
How Does the Infinite Bus Method Work?
At its core, the infinite bus method determines the maximum fault current a transformer can deliver, factoring in its internal limitations. A transformer contains an iron core surrounded by windings. The physical properties of the iron core limit the amount of magnetic flux it can conduct from the primary to the secondary side. When enough current flows through the primary side, the iron core saturates, creating a limit on how much current can pass to the secondary side.
How to calculate transformer fault current using a basic Infinite Bus Method (3-phase)?
Conclusion
The Infinite Bus Method is a tool for transformer fault current calculations. It offers simplicity and is conservative for purposes of calculating maximum fault current for sizing equipment. However, relying on it alone for arc flash calculations can dangerously over or underestimate hazards, dramatically impacting employee safety and productivity. Limitations with the method necessitate expertise to interpret results accurately and implement effective safety measures. Partnering with Guidant ensures your arc flash studies incorporate the best practices and nuanced calculations needed to protect your team and operations.
