What is a short-circuit and coordination study?

The short-circuit half

The study calculates available fault current at every bus in the system, from the utility's declared fault level through each transformer's impedance and the conductors between equipment. Those values are checked against each device's interrupting and withstand ratings — the AIC numbers on breakers and gear. Equipment applied above its rating is not a paperwork issue; under fault conditions it can fail violently instead of clearing. The check matters most where systems have grown: a panel that was adequate on the original service may be underrated after a utility transformer change quietly raised the available fault current.

The coordination half

Coordination plots the time-current curves of series devices — main, feeder, branch — and adjusts settings so the device closest to a fault clears first and the rest stay closed. Done well, a faulted motor starter trips its own breaker and nothing else notices. Done badly, the feeder or main opens on a branch fault and takes healthy loads down with it. The code mandates selective coordination outright for defined life-safety and critical systems — emergency, legally required standby, critical operations power, healthcare essential systems — and good practice extends it as far as the device settings allow everywhere else.

What it is built from, and what it produces

Inputs mirror the arc flash study: a verified one-line, utility fault data, transformer and conductor data, and the actual devices and settings in the field. The study is modeled in software and delivered as a report with the calculated fault duties, any underrated equipment flagged, the coordination curves, and a settings table the electrician applies device by device. Its natural companions are the arc flash study — same model, and the coordination settings directly set the clearing times the arc flash energies depend on — and the equipment submittal process, where device selections can still be changed cheaply.

When to commission one

At design for any building with layered distribution, so device selections and settings are specified rather than improvised. In existing buildings: after service or transformer changes, after repeated whole-building trips on local faults, when an insurer or an authority having jurisdiction asks, and before adding large loads. Torch produces short-circuit and coordination studies as sealed engineering — modeled from field-verified data on existing systems, and from the design model on new work, with the settings table written for the electrician who will apply it. Send the one-line diagram and we will scope the study against your actual distribution.

The bottom line

Short-circuit analysis proves your equipment can survive a fault; coordination proves only the right device answers it. They share one model, and together with arc flash they are the complete electrical study package.

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