Part 4 of the short-circuit series. Each 1.6 MVA transformer gives 37.4 kA on its own. Closing the bus coupler raises the fault level to 67.5 kA — beyond the 50 kA switchboard.
| Supply | 250 MVA, 11 kV grid feeding two identical 1.6 MVA, 6 %, Dyn transformers T1 and T2 |
| LV switchboard | Two sections A and B, each rated 50 kA, with a bus coupler between them |
Each section is fed by one transformer, exactly as in Part 1.
The two transformer impedances are now in parallel, but both are fed through the same grid impedance, so the current does not quite double.
Either keep the coupler open in normal service and interlock it so it can only close when one incomer is open (the common solution), or specify switchgear rated for the parallel fault level, or limit the current with a reactor or a fault-current limiter.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Coupler open | ≤ 50 kA | 37.41 kA | ✓ PASS |
| Coupler closed (paralleled) | ≤ 50 kA | 67.49 kA | ✗ FAIL |
Key insight: Paralleling sources is one of the most common ways a fault level grows beyond the switchgear rating after commissioning — for example when a second transformer or a generator is added later.
Every input in this example is editable in the live calculator — no signup needed.
Open IEC 60909 Short-Circuit Calculator calculator →Only if the switchgear is rated for the paralleled fault level, or the parallel period is so short and controlled that the user accepts the risk. Many specifications require the rating to cover it.
For Dyn transformers the zero-sequence impedance is close to the positive-sequence impedance and there is no grid zero-sequence path in series, so the single-phase fault can exceed the three-phase fault near the transformer.