What Actually Breaks: Fault Types and How Often

7 min · difficulty 3/10

A feeder one-line shows four fault scenarios: one conductor to ground, two conductors together, all three, and two-to-ground. Before any theory, rank which you would expect to see MOST often on a real overhead feeder. Pick first.

The most common by far is single-line-to-ground (SLG): one phase faulting to ground. It is roughly 70 to 80 percent of overhead faults. The reason is physical. A tree limb, a squirrel, or a flashover across a dirty insulator almost always bridges one energized phase to a grounded object first, not all three at once.

Fault types and how often they happen on an overhead distribution feeder A three-phase overhead feeder one-line shows four fault types by code: SLG single-line-to-ground, LL line-to-line, DLG double-line-to-ground, and 3PH three-phase. A horizontal bar chart of relative incidence on overhead feeders gives SLG about 75 percent, LL about 15 percent, DLG about 8 percent, and 3PH about 2 percent. The most common fault, SLG, is not the most severe; 3PH is rarest but usually carries the highest fault current. Fault types and how often they happen Overhead distribution feeder, three-phase conductors A / B / C A B C source earth / ground return SLG single-line-to-ground LL line-to-line 3PH three-phase DLG double-line-to-ground Most overhead faults are temporary: about 70-80% self-clear on a reclose. Underground cable faults are almost always permanent. red marks = fault Relative incidence share of overhead faults SLG 75% LL 15% DLG 8% 3PH 2% 0 50% 100% Most common is NOT most severe. 3PH is the rarest fault but usually carries the highest fault current, so it sizes interrupting ratings.
On overhead feeders single-line-to-ground is by far the most common fault (~75%) while the three-phase fault is the rarest (~2%) but usually the most severe, so common does not mean worst-case.

The four types and their abbreviations: SLG (single-line-to-ground, one phase to ground), LL (line-to-line, two phases touching with no ground), 3PH (three-phase, all three conductors together), and DLG (double-line-to-ground, two phases to ground). SLG leads the count; the others trail well behind.

Here is the distinction that trips up new engineers. Most common is not the same as most severe. SLG dominates by frequency, but a three-phase bolted fault is the rarest fault and usually the highest fault current. That highest current is the number that sizes a device’s interrupting rating, so you design the gear around the rare event while you operate the system around the common one. Plan your interrupting ratings for the worst-case three-phase fault, but expect single-line-to-ground every day.

The other half of this lesson is whether a fault stays. About 70 to 80 percent of overhead faults are temporary. The limb falls clear, the animal drops, or the arc across an insulator de-ionizes once the current is interrupted. Cut the current for a moment and the cause is gone. That single fact is the entire reason reclosers exist: trip, wait, and re-energize, and most of the time the line comes right back.

Underground is the opposite. A buried cable has no limbs, no animals, and no wind. When a cable faults, the failure is insulation breakdown inside the cable itself, and that is almost always permanent. The cable does not heal. Reclosing onto a cable fault just drives current through the damaged spot again, so reclosing onto known cable is discouraged. This is why an overhead lateral and an underground lateral get protected with different logic, even when they sit on the same feeder.

Hold these two ideas together. The fault you will chase most is single-line-to-ground, and on overhead line it has good odds of clearing on its own. The fault you design the gear to survive is the three-phase bolted fault you will rarely see.

A coordination study is only as good as the device data behind it. DistroForge Insider covers protection equipment sourcing and the OEMs actually shipping relays, reclosers, and fuses on current lead times.

Question 1 of 3

On a typical overhead distribution feeder, which fault type do you see most often?

Educational material only. This is not engineering, safety, or procurement advice. Confirm any value against manufacturer documentation and a licensed professional before specifying equipment.