Fault types
The key idea
A fault is named by what it connects: all three phases, two phases, one phase to earth, or two phases to earth. The three-phase fault is usually the biggest. The single line-to-earth fault is by far the most common — and its size depends on the earthing, not just the network.
The idea
Not all short circuits are the same short circuit. A tree branch might bridge two conductors. A failed cable termination usually goes to earth first. A dropped busbar cover can connect everything at once. Each geometry is a different electrical problem:
- Three-phase. All three phases connected together. The system stays balanced — each phase sees the same thing — so the calculation is the simplest, and on most networks the current is the largest.
- Line-to-line. Two phases connected, earth not involved. Slightly smaller — about 87% of the three-phase value.
- Line-to-earth. One phase to earth. Around four out of five real faults start this way. The current can be anything from a few amperes to more than the three-phase value, because the earthing arrangement is in the loop.
- Line-to-line-to-earth. Two phases to earth — checked mainly where earth-fault and phase relays must coordinate.
The unbalanced types cannot be solved phase by phase — the phases no longer see the same thing. They are solved with symmetrical components, and each fault type uses a different combination of the three sequence networks.
Try it
Sequence networks needed
- Positive
- Negative
- Zero
Typical size
100% — the usual maximum
All three phases short together. The fault is balanced, so only the positive sequence network is needed. On most networks this is the largest current, and the one equipment ratings are checked against.
Why it matters
- Equipment ratings usually follow the three-phase fault — but not always. Close to a solidly earthed transformer, the earth fault can be the larger one. Both get calculated.
- Protection must see every type. Phase elements watch phase faults; earth-fault elements watch the small, common ones. A scheme graded only for phase faults leaves the most frequent fault type to chance.
- The earthing question is inescapable. For the two fault types involving earth, the transformer earthing dominates the answer — model it before trusting any earth-fault number.
See it in Phasor
Phasor calculates all four fault types from the sequence networks, at every bus you select — and reports the per-phase and sequence currents so you can see which type governs each location.