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Short Circuit Current vs. Continuous Current: What You Need to Know

By Thomas Thrasher

To fully appreciate the significance of performing short circuit studies, it’s important to first understand short circuit current.

CONTINOUS VS. SHORT CIRCUIT CURRENT

In an electrical distribution system, we generally discuss current in terms of continuous current. For example, a 100-amp panelboard with 100 amp rated bus bars is designed to deliver up to 100 amps of continuous current. This current flows from the utility through the distribution system including feeders, bus bars, bus ducts, and other components. It follows a designed path and encounters a certain amount of impedance, which keeps the current at safe levels.

However, in a short circuit event, a conductor disrupts this designed path, creating a very low impedance pathway for the current to flow. The current in this scenario is much higher than the continuous current and is referred to as short circuit current.

HOW HIGH CAN SHORT CIRCUIT CURRENT REALLY BE?

In recent projects, our team has seen properties in New York City dealing with almost 200,000 amps of fault current. This extremely high fault current can be challenging to manage.

Electrical distribution equipment not only has a continuous current rating (like the 100 amps mentioned earlier) but also a short circuit rating, commonly referred to as the ‘AIC rating’ (Available Interrupting Current rating). This rating indicates how much short circuit current the equipment can handle until an overcurrent device stops the flow of power. If the equipment’s rating exceeds the available short circuit current, the equipment can be put back into service after a short circuit event.

While the lowest AIC rating on an electrical panelboard is 10,000 amps, we have seen roughly 200,000 amps short circuit current at many locations in New York City. As a result, we’ve replaced electrical distribution equipment with 200,000 amp rated gear, which can be very costly.

WHERE DO SHORT CURCUIT STUDIES COME IN?

A short circuit study analyzes the flow of fault current through distribution systems and confirms the AIC ratings required for the distribution equipment in a building. Once this is understood, the next step is to review arc flash energy levels and the protective gear required to work on energized equipment.

Have a question? Please feel free to reach out to me at tthrasher@wbengineering.com.