Understanding Maximum Available Fault Current: What It Is and Why It Matters
- Jeff Kershner
- Nov 25, 2024
- 4 min read
In the world of industrial electrical systems, safety and reliability hinge on understanding every element of an electrical setup—especially the Maximum Available Fault Current (MAFC). For electricians and engineers, understanding MAFC is vital not just to protect equipment but also to prevent severe electrical hazards.
What is Maximum Available Fault Current?
Maximum Available Fault Current (MAFC) is defined as the highest level of current that can flow through an electrical system during a short circuit or fault condition. This concept is essential for safe electrical system design, as it dictates the capabilities required for safety devices—like circuit breakers and fuses—to properly protect systems and personnel.
Think of electricity like water flowing through a pipe: the MAFC is akin to a sudden, powerful surge of water that a valve must handle without failing. If the valve (or breaker, in the electrical analogy) isn’t rated to manage this maximum force, the result could be catastrophic, including potential fires, equipment explosions, or severe injury.
Where does Maximum Available Fault Current come from?
Fault current originates at power sources, usually the utility or generators.
Source Size: Larger transformers yield higher fault currents.
System Impedance: Lower impedance (larger wires) results in higher fault currents.
Distance from Fault: Fault current decreases with distance from the source.
System Configuration: Parallel vs. series configurations impact fault current levels.
Calculating Maximum Available Fault Current
Unfortunately, you can’t just look at the size of a breaker or service and determine MAFC–It is completely independent of the breaker ampacity. A 400A breaker might have more MAFC than an 800A breaker. MAFC calculations are essential in arc flash studies, guiding equipment and protection choices. Rozel’s approach involves:
Source Characteristics: Obtain transformer data from the utility.
System Characteristics: Wire sizes, distances, and quantities.
Engineering Software: Software calculates MAFC at each system point, which can then be compared to the manufacturer’s rating.
Why Maximum Available Fault Current Matters for Safety
When an electrical fault occurs, the system’s fault current can spike to the MAFC. This current surge places an extreme load on protective devices. Circuit breakers and fuses are designed with an Interrupting Rating—the highest fault current they can safely interrupt without failing. If the MAFC exceeds this rating, the protective device may not function correctly, leading to equipment failure, fire risks, or arc flashes.
The 2024 version of NFPA 70E requires all components be rated for the MAFC or the equipment is not considered in normal operating condition, which would prevent any work or operation of the equipment unless it was first placed in an electrically safe condition.
The National Electrical Code (NEC) emphasizes the importance of installing equipment rated to the MAFC. Section 409.22 of the 2011 NEC states that an industrial control panel “shall not be installed where the available short-circuit current exceeds its short-circuit current rating,” and similar requirements apply to other equipment types. This is because a fault current can overpower an under-rated breaker, posing critical safety risks.
Real-World Application: An Example with a 100A Breaker
Consider a 100A breaker rated with a 14,000A interrupting capacity. This means it can handle up to 14,000A in a fault condition before risking failure. If the system’s MAFC is higher than 14,000A, that breaker may not interrupt the current, leading to equipment damage or personal injury.
In the example of a GE 100A breaker feeding equipment labeled "PANEL X," if PANEL X demands more than the breaker’s capacity, the breaker will trip—but this takes time. The amount of time is determined by the Time-Current Characteristic (TCC) curve provided by the manufacturer. For instance, if PANEL X pulls 400A, the breaker might trip in 15 seconds, but at fault currents over 3,000A, the breaker trips almost instantly, often in under 0.02 seconds. This quick response is critical for minimizing arc flash hazards.
Misconceptions and Safety Risks of Ignoring MAFC Ratings
Many people mistakenly believe that the ampere rating on a breaker, such as "100A," is the only rating that matters. However, the interrupting rating—often measured in Kilo Ampere Interrupting Capacity (kAIC)—is just as crucial. This rating shows the maximum fault current the breaker can manage. Under short circuit conditions, the actual current that flows can be far greater than the breaker’s ampere rating, making interrupting rating vital.
Without proper understanding, some electricians may unknowingly install breakers that are not rated for the MAFC in the system. If a breaker with a lower interrupting rating encounters a higher fault current, it may fail to trip—or worse, it could cause damage, sparking or melting wires, and exposing other components to excessive heat, ultimately increasing the risk of fires.
The Role of Arc Flash Evaluations and Equipment Assessments
During an arc flash evaluation, detailed data on the system’s fault current is gathered to ensure each breaker’s interrupting rating aligns with the calculated MAFC. The process involves:
Collecting data such as utility information, wire sizes, and transformer characteristics.
Creating a single-line diagram of the system, which helps engineers understand the flow of electrical current and identify potential fault points.
Calculating MAFC at each point in the system, comparing it against each breaker’s interrupting rating.
Recommending adjustments or replacements for underrated devices, known as over-dutied breakers, to align with MAFC requirements.
For companies concerned about workplace safety and operational reliability, this evaluation is a crucial step in preventing electrical hazards and ensuring the protective equipment performs effectively in the event of a fault.
Reinforcing the Definition
In summary, Maximum Available Fault Current is the maximum current that could flow in a short circuit scenario. Properly matching this value to equipment interrupting ratings is essential for safety. By understanding MAFC and ensuring equipment ratings align with system demands, companies can prevent equipment failure, reduce fire hazards, and safeguard personnel.