A fault current is an unintended, uncontrolled, high current flow through an electrical system. Fault currents are caused by very low impedance short circuits. These may be shorts to ground or across phases. The resulting high current flow can result in overheating of equipment and conductors, excesses forces, and at times even serious arcs, blasts, and explosions. Electrical safety is critical in preventing and mitigating these hazards.

Fault currents can result from various factors, including lightning strikes, animals, dirt and debris, dropped tools, corrosion, and human error. The impact of a fault current can be severe, potentially damaging electrical infrastructures and jeopardizing critical power system safety. 

Fault current calculations rely on Ohm's Law in which the current (I) equals the voltage (V) divided by the resistance (R). The formula is I = V/R. When there is a short circuit the resistance becomes very small, and that means the current becomes very large.

If the resistance was zero, then the calculated fault current would go to infinity. However, even copper wire has some resistance; it is not a perfect conductor. Determining the fault current involves knowing the total resistance from the power source to fault location.

OSHA Electrical Panel Clearance and Compliance 

Knowing the available fault current is important when selecting protection devices, however it is also required by code. The National Electric Code (NEC) 110.24(A) states:

"Service equipment in other than dwelling units shall be legibly marked in the field with the maximum available fault current. The field marking(s) shall include the date the fault current calculation was performed and be of sufficient durability to withstand the environment involved."

This means there must be field installed labels on electrical equipment, such as service entrance equipment, that gives the available short-circuit fault current. This allows the Short Circuit Current Rating (SCCR) of the equipment to be easily compared with the maximum available fault current.

Any time there is a change in equipment the fault current calculation must be reevaluated. This is specified in NEC 110.24(B):

"When modifications to the electrical installation occur that affect the maximum available fault current at the service, the maximum available fault current shall be verified or recalculated as necessary to ensure the service equipment ratings are sufficient for the maximum available fault current at the line terminals of the equipment. The required field marking(s) in 110.24(A) shall be adjusted to reflect the new level of maximum available fault current."

Types of Electrical Faults

In an electrical system there are several types of possible faults that require a different safety approach:

• Short circuit fault:  Occurs when the current bypasses the normal load.
• Ground fault: Occurs when current flows into the ground instead of through the circuit.
• Three-phase short-circuit: Occurs between one or more phases, generating the highest fault typically currents.
• Open-circuit fault: Interrupts current flow and does not generate short-circuit currents, but still poses as an operational risk.

Protective systems must be in place to prevent equipment damage and maintain electrical load management across all types of faults. 

Bolted Faults vs. Arc Faults

Electrical faults are categorized as either bolted fault or arc faults:

In a bolted fault there is a solid connection between conductors, allowing current to flow through a defined path. These faults can might happen when an installer connects to a power source to ground instead of to the point where it should be connected. When the power is switched on there will be an immediate bolted fault that trips the protective device generate high fault currents, but often result in minimal damage if protective device operate correctly.  might happen when an installer connects to a power source to ground instead of to the point where it should be connected. When the power is switched on there will be an immediate bolted fault that trips the protective device. Because the current flow was contained, the damage is typically limited. However, a bolted fault creates the highest fault currents.

An arc fault results when conductors are close enough that electricity jumps across the gap, creating an arc. The initial arc ionizes the air creating a plasma that allows the current flow to rapidly increase and be sustained, resulting in an arc flash or arc blast. Arc  faults can escalate into arc flashes or blasts, requiring critical power system safety measures.  Calculating fault current helps establish safe protection boundaries and provide the information needed for arc flash labels, which must be installed in addition to the required NEC 110.24 fault current labels.

Fault Current in Three-Phase Systems

The IEC 60909 standard, "Short Circuit Currents in Three-Phase Systems" gives the accepted method for three-phase fault current calculations. Ina three-phase system, faults can be:

• Symmetrical (balanced): All three phases are equally affected.
• Unsymmetrical (unbalanced): One or more phases are impacted, making fault current calculations more difficult.

Before a fault current calculation can be performed, all of the possible sources of current must be identified. There are four possible sources of short-circuit fault current:

• On-site electric generators: These are close and the fault current is only limited by the impedance of the generator itself and the electric circuit.
• Synchronous motors: A synchronous motor is an AC motor in which the speed of the motor is proportional to the frequency of the electric power.  When the power fails, as will happen if there is a short-circuit, the inertia of the mechanical load on the motor will continue rotating the motor. The motor will then act as a generator supplying current, and this will contribute to the total current flowing to the fault.
• Induction Motors: This type of motor will also become a generator should there be a short-circuit fault somewhere else in the system. However, the fault current generated by an induction motor will only last for a few cycles.  The current will be approximately equal to the locked-rotor starting current of the motor.
• Electric utility system: Most of the fault current typically comes from the electric utility. The level of the short circuit current will depend on:
• the transformer secondary voltage rating and impedance
• the impedance of the generators
• the impedance of the circuit from the transformer to the short.

To simplify the fault current calculation it is assumed that all electrical generators in the system are in phase, and that they are operating at the nominal system voltage.

Bolted Three - Phase Condition

A short circuit study is conducted so that the fault current can be calculated. This normally involves looking at the worst case scenario, which is the bolted three-phase fault condition. Based on this situation other fault condition can be approximated.

The fault current contribution from motors in the system is important. In many cases motors can contribute four to six times there normal full load current. Even if the current is for a very short duration, it is critical that it be included in the fault current calculation.

When an arc flash study is being done, the fault current calculation should still be for the highest bolted three-phase short circuit current.

Fault Current Labeling and OSHA Requirements for Employers

Once the fault current has been calculated, electrical labels must be applied to equipment. NEC 110.24 requires labels that state the maximum fault current at each location. Additionally, OSHA requirements for employers mandate clear hazard communication, which includes accurate labeling for electrical safety. 

If an arc flash hazard exists, arc flash labels must be applied, with appropriate protective measures. Each label requires custom information that includes:

• Calculated fault current value

• Date of fault current assessment

• Warning symbols for high-voltage areas

• PPE requirements for personnel working near the equipment

DuraLabel Fault Current Labeling Resources 

Managing fault currents is a key aspect of maintaining electrical safety and compliance. Proper calculations help prevent system failures, ensure the correct selection of protective devices, and minimize downtime. Effective electrical load management plays a crucial role in mitigating risks associated with high fault currents, reducing the likelihood of equipment damage and hazardous incidents. Proper electrical labeling ensures that electrical systems remain compliant and workers are protected. Critical power system safety depends on accurate fault current calculations, which guide the selection of protective devices and safety protocols.

To help facilities implement a safer and more compliant electrical system, DuraLabel offers expert guidance and high-quality labeling solutions. Download our free Arc Flash Quick Action Guide today to learn more about effective fault current management and best practices for workplace electrical safety.

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