A line fault is read in four steps: what type of fault it was, where it happened, what the protections and breakers did at both ends, and whether reclosing cleared it. The location given by the relay is an estimate that depends on the line parameters and the fault resistance; it must be checked against a field inspection.
Short answer
A line fault is analyzed in four steps: what type of fault it was, where it happened, what the protections and breakers did at each end and whether reclosing cleared it. This separates a transient fault, which the line overcame on its own, from a permanent one, which requires repair before re-energizing. The general reconstruction method is in electrical fault analysis and the reading of waveforms in how to read an oscillography record. Here we explain what is specific to lines.
What makes a line different
A line is long, exposed to the weather and has two or more ends with different protections and sources of infeed. That has three consequences for the analysis:
- The fault may be far from the relay; it has to be located to send the crew to the right place.
- Each end sees the fault differently, so the record from both ends is needed along with the sequence of each breaker.
- A good share of faults are transient, and the reclosing logic is designed for them.
Step 1: the fault type
The type is read from the currents and voltages in the record: one phase to ground, two phases, two phases to ground or all three. On overhead lines, a phase-to-ground fault appears when the phase-to-ground insulation gives way to lightning, a branch or contamination. The type matters for two reasons: it defines which functions should have operated (phase or ground distance, ground directional, differential) and it points to the cause (a phase-to-phase fault on an overhead line suggests contact between conductors or an object bridging them).
The general method for reading the fault records is in Fault analysis from oscillography records.
Step 2: the location
Distance relays (ANSI 21) and recorders calculate a distance to fault from the impedance seen up to it. The estimate is affected by:
| Factor | Effect on the estimated distance |
|---|---|
| Set line parameters different from the real ones | Error proportional to the difference |
| Fault resistance | Can shift the distance, especially if the other source feeds current into the fault |
| Parallel line with mutual coupling | Error on ground faults if the relay does not compensate for it |
| Taps or sections of a different conductor | The line is no longer homogeneous and a calculation with a single value fails |
| CT saturation or capacitive voltage transformer transients | Distorted measurements at fault inception |
That is why the distances from the two ends are always compared: their results should add up to approximately the line length. The estimate guides the patrol; the cause is confirmed by inspection. Once the place is known, the protection analysis is done with the zone in which the fault fell, as explained in distance protection 21.
Step 3: the protection sequence
At each end the chain of events is reconstructed from the event log and the oscillography record:
- Pickup of the protection functions and which one tripped first (zone 1 distance, line differential, ground directional).
- Sending and receiving of the teleprotection signals, if the scheme uses them. If the fault was on the line and the signal did not arrive, there is a channel failure, described in teleprotection channel failure.
- Tripping of the breakers at each end and their times. If one end opened later, that fact is the key to finding the cause: a scheme without teleprotection, a failed channel, a zone 2 with delay or infeed from that side.
- If there was a switch-onto-fault, that is, a manual close or a reclose with the fault still present, the SOTF logic must have tripped without delay.
A line with two main protections requires comparing both; see line protection main 1 and main 2. If a protection did not operate or operated when it should not have, the classification of the operation is in the fault analysis article already cited.
The voltage and current phasors of the fault are interpreted with Phasor diagram.
Step 4: reclosing
Automatic reclosing 79 exists because most faults on overhead lines are transient. The record tells which case it was:
- Successful reclose: the current returns to normal and the line stays energized. The fault was transient.
- Failed reclose: the fault current reappears and the relay trips again. The fault is permanent, or the dead time was not enough for the arc to extinguish.
- No reclose: the logic did not allow it. The usual causes are that the trip came from a scheme that blocks reclosing, that the fault was three-phase, or that the synchronism check did not give permission; see synchronism check 25.
- Was the line energized after the trip?
- Yes, and it stayed energized: Transient fault; review the cause and patrol the line if it repeats
- Yes, but it tripped again: Permanent fault or insufficient dead time
- There was no reclose: Why was it blocked?
- Trip that blocks reclosing, three-phase fault or no synchronism permission: review the 79 logic
How a distribution feeder is protected and reclosed is explained in Distribution feeder protection.
Common field causes
The most usual causes on overhead lines are lightning, contact with vegetation and contact by birds or animals. Next come insulation contamination, damage by third parties (a vehicle, a crane, a fire under the line) and the breaking of a conductor or a fitting. On underground lines the fault is almost always permanent and is located with the techniques of cable fault location. The record guides but does not identify the cause; that is what the patrol and the comparison with earlier events on the same section are for.
What not to do
- Do not re-energize a line that tripped twice in a row without inspecting it: the fault is probably permanent.
- Do not assume the relay distance is exact. Send the crew to a section, not to a pole.
- Do not change the distance protection settings on the basis of a single event without reviewing the record from both ends.
- Do not dismiss a failed reclose as bad luck: it is information about the fault.
When to call us
If your line has repeated trips, a location that does not match what was found in the field, or one end that opens later than the other, send us the records from both ends. We quote the trip analysis and deliver the sequence, the cross-checked location and the cause. If the problem is in the settings, we correct them with the remote relay configuration and review selectivity with the protection coordination study.
Related guides
- Distance protection 21
- Line differential protection 87L
- Fault analysis from oscillography records
- Distribution feeder protection
- Phasor diagram
References
- IEEE C37.113-2025, Guide for Protective Relay Applications to Transmission Lines.
- IEC/IEEE 60255-24:2013 (IEEE C37.111-2013), COMTRADE.
Frequently asked questions
How accurate is the distance to fault shown by the relay?
It is an estimate, not a measurement. It is calculated from the impedance the relay sees up to the fault and the line impedance per kilometer. If the set parameters are not the real ones, or there is a parallel line with mutual coupling, a tap or a high fault resistance, the error grows. It helps guide the line patrol; it does not replace the inspection.
Why do the two ends of the line give different distances?
Each relay calculates from its own measurement, and the fault is seen with different infeed from each side. The sum of the distance seen from one end and the one seen from the other should come close to the line length. If not, there is a parameter error, a mutual coupling or a high fault resistance distorting one of the two measurements.
Does a successful reclose mean the cause has disappeared?
It means the fault was transient: the arc extinguished and the insulation recovered. The cause that originated it may still be there, for example a nearby branch, a contaminated insulator or a lightning strike that damaged it. If the same line has several successful recloses within a few weeks, it is worth patrolling it.