A short circuit study calculates the available fault current at every bus in your distribution system and compares it against the interrupting and withstand ratings of the equipment installed there. Its job is to find underrated or overstressed equipment before a fault finds it for you.
Equipment applied above its rating does not fail gracefully. A breaker asked to interrupt more current than it is rated for can fail to clear, and the resulting damage is rarely confined to the device.
Why available fault current changes
Most facilities were correctly rated on the day they were built. What changes afterward:
- The utility upgrades or reconductors the service, raising available fault current at your service entrance
- A transformer is replaced with a larger unit or one of lower impedance
- On-site generation — solar, storage, or standby generators — adds fault contribution the original study never accounted for
- Motors are added, contributing to the fault for the first few cycles
- Feeders are shortened or upsized during a remodel, lowering impedance
Equipment intended to interrupt current at fault levels must have an interrupting rating at nominal circuit voltage at least equal to the current available at the line terminals. Circuit impedance and other characteristics must be selected so that the circuit protective devices clear a fault without extensive damage to the electrical equipment.
Service equipment at other than dwelling units must be field marked with the maximum available fault current, including the date the calculation was performed. The marking must be verified or recalculated when modifications affect the available fault current.
What the study produces
- Three-phase and line-to-ground fault current at every bus
- Momentary and interrupting duty comparisons against nameplate ratings
- A pass/fail equipment evaluation identifying anything applied above its rating
- Series rating evaluation where applicable
- Available fault current field marking data for NEC 110.24 compliance
- Recommendations where equipment needs replacement, current limiting, or reconfiguration
How it fits with the other studies
Short circuit, coordination, and arc flash are usually performed together, because each depends on the one before it. The short circuit study establishes fault current. The coordination study sets protective device behavior against that fault current. The arc flash study uses both to calculate how much energy is released and how long it lasts.
Buying them as one package is almost always cheaper than buying them separately, because the system model — the expensive part — is built once.
Talk to the engineer who does the work
No sales team. You speak directly with a California licensed Professional Engineer about your equipment, your schedule, and what the study will actually cost.