HV Circuit-Breaker Switching Duties

Short-Line and Long-Line Faults

Faults on overhead lines impose some of the most demanding transient recovery voltage duties on high-voltage circuit-breakers — and the severity is not set by current magnitude alone. The fault distance, the line surge impedance, travelling-wave reflections and the pole-to-clear sequence all shape the recovery voltage. This note explains the short-line fault (SLF) and long-line fault (LLF) duties: why the line behaves as a distributed system, why the line-side TRV is triangular and very fast, the L90 / L75 and T10 duties, and how to model and check both.

Reading time ≈ 22 min · TRV behaviour of overhead-line fault interruption

Short-line faults and long-line faults are overhead-line fault duties where the transient recovery voltage (TRV) is strongly influenced by travelling waves between the circuit-breaker and the fault location. The interrupted current may be lower than the terminal-fault current, but the TRV can be more severe, because the line-side voltage can rise very quickly after current zero. Circuit-breaker suitability must therefore be checked against the TRV, the RRRV and type-test evidence — not only against short-circuit current magnitude. The severity is also shaped by the fault location, the line surge impedance, travelling-wave reflections, the pole-to-clear sequence, the local capacitances around the circuit-breaker, and the interruption technology.

Two important line-fault duties are the short-line fault (SLF) and the long-line fault (LLF). Both involve faults on overhead lines, and both involve travelling-wave behaviour — but they are not the same duty. A short-line fault is close to the circuit-breaker and is severe mainly because the line-side TRV rises very quickly immediately after current zero. A long-line fault is at a significant distance, and its waveform, peak value and pole-to-clear severity differ from the short-line-fault case. In both, the overhead line cannot be represented as a single lumped inductance and capacitance: at the high frequencies associated with TRV the line behaves as a distributed-parameter system.

Do not confuse a line fault with a terminal fault

A terminal fault normally gives the highest current, because the fault is at the circuit-breaker terminals. A short-line or long-line fault may give a lower current, but it can impose a more severe recovery-voltage duty because of the travelling waves on the overhead line.

Abbreviations and symbols used on this page

SLFShort-line fault

LLFLong-line fault

TRVTransient recovery voltage

RRRVRate of rise of recovery voltage

ITRVInitial transient recovery voltage

RVRecovery voltage (power-frequency, after interruption)

OHLOverhead line

CBCircuit-breaker

ACAlternating current

GISGas-insulated switchgear

SF₆Sulphur hexafluoride (interrupting gas)

\(Z_0\)Line surge (characteristic) impedance

\(L',\ C'\)Line inductance / capacitance per unit length

\(di/dt\)Current slope at current zero

\(du/dt\)Initial rate of rise of recovery voltage

\(k_\text{af}\)Amplitude (peak) factor

L90 / L75Short-line-fault duties (90% / 75% of \(I_\text{sc}\))

T10 / T30Terminal-fault duties (10% / 30% of \(I_\text{sc}\))

\(X_0/X_1\)Zero- to positive-sequence reactance ratio

\(v\)Travelling-wave velocity

\(I_\text{sc}\)Rated short-circuit breaking current

\(\Delta I\%\)Line-fault current as a percentage of \(I_\text{sc}\)

IECInternational Electrotechnical Commission

IEEEInstitute of Electrical and Electronics Engineers

Key idea

Travelling-wave duties, not lower-current terminal faults. SLF and LLF severity comes from line-side travelling waves, not from current magnitude — a lower-current line fault can be more onerous than a terminal fault.
SLF = very fast line-side RRRV. The line-side TRV rises at \(du/dt = Z_0\,di/dt\) into a triangular waveform; the first few microseconds after current zero are the challenge.
LLF = travel time and TRV peak. Lower current, longer travel time, the first pole to clear can be the most severe, the amplitude factor can reach about 2.4, and parallel circuits add components.
Check the TRV, model the line as distributed. Verify the RRRV and the first TRV peak against the circuit-breaker envelope, using a distributed-parameter line (overhead-line surge impedance \(\approx 450\ \Omega\)) — not a lumped \(L\)-\(C\).

Key terms used on this page

01Travelling wave

A voltage/current disturbance that propagates along the line at finite speed and reflects at discontinuities.

02Surge impedance

The characteristic impedance \(Z_0=\sqrt{L'/C'}\) seen by a fast travelling wave before the rest of the network responds.

03Distributed-parameter line

A line model in which \(L\), \(C\) and \(R\) are spread along the length, so voltage and current vary with position and time.

04Triangular TRV

The characteristic line-side TRV shape produced by wave reflection between the circuit-breaker and the fault.

05RRRV

Rate of rise of recovery voltage — the initial slope of the TRV, critical for thermal re-ignition.

06Amplitude factor

A factor \(k_\text{af}\) representing damping that scales the first TRV peak above the prospective value.

07Post-arc current

Current that flows through the still-conducting arc residue after current zero, driven by the rising TRV.

08Thermal re-ignition

Failure to interrupt shortly after current zero because the hot arc channel has not cooled enough.

09ITRV

Initial transient recovery voltage — very fast components from busbars and short connections near the circuit-breaker.

10First-pole-to-clear

The first of the three poles to interrupt; it often sees the most severe recovery voltage.

11L90 / L75

Short-line-fault duties at about 90% and 75% of the rated short-circuit breaking current.

12T10 duty

A terminal-fault duty at about 10% of rated short-circuit current, important for high-TRV-peak cases.

Section 1

Why line-fault TRV matters

A circuit-breaker interrupts current at current zero. Immediately afterwards, the dielectric strength between the contacts must recover faster than the transient recovery voltage rises across the open gap. If the TRV rises faster than the gap can recover, re-ignition or restrike can occur. For overhead-line faults the line-side voltage can change extremely quickly because of travelling waves, which is why line faults — particularly short-line faults — are among the most important duties for circuit-breaker design and testing.

The circuit-breaker must be capable of:

interrupting the fault current;
withstanding the initial TRV and its rate of rise;
withstanding the first TRV peak;
avoiding thermal re-ignition immediately after current zero;
avoiding dielectric failure once the contact gap has started to recover.

The hardest few microseconds

The most difficult period is often the first few microseconds after current zero. During this short time the arc channel still contains hot residual plasma, while the TRV may already be rising steeply.

The TRV is built from both sides

When a circuit-breaker interrupts a line fault, the voltage across the contacts is made up of components from both sides. The source side (the network) can be represented in simplified form by its inductance and capacitance and produces the source-side TRV component. The line side (the faulted overhead-line section between the circuit-breaker and the fault) produces the line-side TRV component, governed by travelling waves. The total voltage across the contacts \(u_\text{ab}(t)\) is the difference between the source-side component \(u_\text{an}(t)\) and the line-side component \(u_\text{bn}(t)\); for line faults the line-side component \(u_\text{bn}(t)\) is the one that makes the duty special, because it is governed by travelling-wave reflections.

Line-fault duties in context

Fault-current breaking duties include terminal faults, short-line faults, long-line faults, transformer-limited faults, reactor-limited faults and out-of-phase interruption. A terminal fault is at or very close to the circuit-breaker terminals and usually gives the highest current. Transformer-limited and reactor-limited faults are limited in current by a transformer or reactor, but their TRV can be severe because of the natural frequency and capacitance of the limiting equipment — transformer behaviour in EMT studies is a topic in its own right, covered in Transformer Modelling in EMTP® ↗. The remainder of this note focuses on the two overhead-line duties — short-line and long-line faults.

Section 2

Why a lumped L–C model is not enough

For low-frequency studies a line section may sometimes be represented by lumped resistance, inductance and capacitance. This is not sufficient for short-line-fault TRV studies, because the length of the line section between the circuit-breaker and the fault is comparable with the wavelength of the transient voltage. The voltage and current are not the same everywhere along the line at the same instant, so the line must be treated as a distributed-parameter system with inductance, capacitance and resistance per unit length (\(L'\), \(C'\), \(R'\)), a surge impedance \(Z_0\), and a travelling-wave velocity \(v\).

The propagation velocity of a travelling wave depends on the distributed inductance and capacitance of the overhead line:

\[ v = \frac{1}{\sqrt{L'C'}} \]

\(v\): travelling-wave propagation velocity
\(L'\): line inductance per unit length
\(C'\): line capacitance per unit length

The line surge impedance represents the instantaneous impedance seen by a fast travelling wave on the overhead line:

\[ Z_0 = \sqrt{\frac{L'}{C'}} \]

\(Z_0\): line surge (characteristic) impedance
\(L'\): line inductance per unit length
\(C'\): line capacitance per unit length

What the surge impedance is

In plain language, the surge impedance is the “instantaneous” impedance seen by a very fast travelling wave before the rest of the network has had time to respond. For overhead-line short-line-fault duties, a surge impedance of 450 Ω is commonly used in IEC/IEEE-based standard test representations. This 450 Ω figure is the standardised overhead-line surge impedance used for the test representation, not the actual surge impedance of every overhead line — a real line’s surge impedance depends on conductor geometry, height, spacing and earth-return effects.

Section 3

The short-line fault

A short-line fault is a fault on an overhead line a short distance from the circuit-breaker terminals. The distance is short in power-system terms, but long enough for travelling-wave behaviour to dominate the line-side TRV. A typical SLF may be located from hundreds of metres to a few kilometres from the circuit-breaker, depending on system voltage, fault level and the current reduction specified for the test duty. The fault is usually treated as a line-to-earth or three-phase-to-earth fault; the circuit-breaker clears the current, and the line section between the circuit-breaker and the fault remains charged at the instant of interruption, then produces a travelling-wave response.

Key feature

The line-side TRV rises very fast immediately after current zero. This very fast initial rise is the main challenge for the circuit-breaker.

The short-line fault is severe because it combines high current with a very fast line-side RRRV. The fault is close enough to the circuit-breaker for the fault current to remain high, but far enough away for the overhead-line section to produce a travelling-wave TRV component.

Voltage distribution at current zero

At current zero in an inductive circuit, the source voltage is close to its peak. The fault location is effectively at zero voltage because it is short-circuited. The line section between the circuit-breaker and the fault therefore has a voltage distribution along its length — highest at the circuit-breaker terminal and decreasing to zero at the fault. This distribution cannot remain stationary; it launches travelling waves along the line.

The waves reflect at both ends: at the open circuit-breaker end the voltage wave reflects positively, and at the short-circuited fault location it reflects negatively. The result is a triangular line-side TRV waveform, one of the characteristic features of short-line-fault interruption.

Section 4

Line-side RRRV, time to peak and first peak

The initial rate of rise of the line-side TRV is proportional to the line surge impedance and the current slope immediately before current zero:

\[ \left.\frac{du}{dt}\right|_{t=0^{+}} = Z_0\,\left.\frac{di}{dt}\right|_{t=0^{-}} \]

\(\left.du/dt\right|_{t=0^{+}}\): initial rate of rise of the line-side recovery voltage, just after current zero
\(Z_0\): surge impedance of the overhead line
\(\left.di/dt\right|_{t=0^{-}}\): rate of decline of the current, just before current zero

The steeper the current crosses zero, and the higher the surge impedance, the faster the TRV rises. Because overhead-line surge impedance is high compared with cables, overhead-line faults are usually far more severe for RRRV than cable faults.

Why cables are less severe for this duty

Cables also support travelling waves, but their surge impedance is much lower — typically several tens of ohms, against several hundred ohms for overhead lines. Since the initial RRRV is proportional to surge impedance, the cable-side RRRV is normally much lower. This does not mean cable switching is simple — cables bring energisation overvoltages, trapped charge, resonances and sheath effects — but for the specific short-line-fault TRV duty, overhead lines are the main concern.

Time to first peak

The travelling wave moves from the circuit-breaker to the fault and back; the first line-side TRV peak is associated with this travel time:

\[ t_p = \frac{2l}{v} \qquad\qquad T_L = \frac{4l}{v} \]

\(t_p\): time to the first local peak of the line-side TRV
\(T_L\): period of the triangular line-side oscillation
\(l\): distance from the circuit-breaker to the fault
\(v\): travelling-wave velocity

The closer the fault, the shorter the travel time and the higher the frequency of the triangular TRV. A very close fault produces a very fast TRV, but the first peak may be smaller because the line section is shorter. A more distant fault produces a slower initial rise, but the first peak can be higher because the line section stores more energy. This balance is why standardised short-line-fault duties are defined at particular fault-current reductions.

First line-side peak

The first line-side TRV peak depends on the faulted-line inductance, the current slope at current zero and the damping represented by the amplitude factor:

\[ \hat{U}_\text{line} = k_\text{af}\,L_L\,\left.\frac{di}{dt}\right|_{t=0^{-}} \]

\(\hat{U}_\text{line}\): first local maximum of the line-side TRV
\(k_\text{af}\): amplitude factor representing damping
\(L_L\): inductance of the faulted line section
\(\left.di/dt\right|_{t=0^{-}}\): current derivative just before current zero

For overhead-line oscillations, an amplitude factor of 1.6 is commonly used in IEC-based short-line-fault representation. The total TRV across the circuit-breaker can be slightly higher than the line-side value alone, because the source-side TRV component is superimposed on the line-side component.

Section 5

Current reduction and the L90 / L75 duties

Short-line-fault test duties are often described by the fault current as a percentage of the rated short-circuit breaking current. The idea is that a fault on the line is not exactly at the circuit-breaker terminals: the line section between the circuit-breaker and the fault adds impedance, so the fault current is lower than the terminal-fault current. The two common SLF duties are L90 (line fault current about 90% of the rated short-circuit breaking current) and L75 (about 75%).

Check the applicable edition

The exact requirements, tolerances and applicability should always be checked against the project-specified edition of IEC 62271-100, IEEE C37.04, IEEE C37.09 or the applicable manufacturer type-test evidence. The key concept is that short-line-fault duties are not arbitrary — they represent onerous combinations of current magnitude, RRRV and line-side TRV peak.

Standards for line-fault duties

The relevant standards have distinct roles:

IEC 62271-100 — AC circuit-breaker ratings and line-fault (short-line-fault) test duties.
IEC 62271-101 — relevant where synthetic testing is used.
IEEE C37.04 — the rating basis for AC high-voltage circuit-breakers.
IEEE C37.09 — the test-procedure framework.
IEEE C37.011 — application guidance for transient recovery voltage of AC high-voltage circuit-breakers.

Do not mix IEC and IEEE criteria

Do not mix IEC and IEEE values, test duties or acceptance criteria without checking the project-specified standard edition and the manufacturer type-test report.

Why the L90 region is especially severe

For a very close fault, the current is high and its derivative at current zero is high, creating a very high RRRV — but the line section is short, so the line-side voltage peak is limited. For a more distant fault, the line section is longer, so the first line-side TRV peak can be higher — but the current and RRRV are lower. The most difficult duty occurs where these effects combine unfavourably. In practice the L90 region is especially important because it combines high current with very fast RRRV. For gas circuit-breakers this region can be very demanding, because the contact gap must recover thermally within a few microseconds after current zero.

Section 6

A practical numerical example

The following example is intended to show the order of magnitude of the SLF RRRV. It is not a substitute for a project-specific IEC/IEEE test-duty check.

Consider a 420 kV system with a rated short-circuit current of 63 kA at 50 Hz. For a 90% short-line fault, the interrupted current is approximately 0.9 times the rated short-circuit current. The current derivative at current zero, and the resulting initial line-side RRRV using a 450 Ω surge impedance, are:

\[ \left.\frac{di}{dt}\right|_{t=0^{-}} = 0.9 \times 2\pi \times 50 \times \sqrt{2} \times 63\,000\ \text{A/s} \approx 25.2\ \text{A}/\mu\text{s} \] \[ \left.\frac{du}{dt}\right|_{t=0^{+}} = Z_0\,\left.\frac{di}{dt}\right|_{t=0^{-}} = 450 \times 25.2 \approx 11.3\ \text{kV}/\mu\text{s} \]

\(0.9\): current reduction for the 90% (L90) short-line fault
\(2\pi\times 50\): angular frequency \(\omega\) at 50 Hz
\(\sqrt{2}\times 63\,000\): peak of the 63 kA RMS rated short-circuit current
\(Z_0 = 450\ \Omega\): overhead-line surge impedance

What the number means

An initial rise of about 11.3 kV/µs is very high: only a few microseconds after current zero, tens of kilovolts can already be applied across the contacts. The gap must change from a hot conducting arc path to an insulating gap in an extremely short time — this is the physical reason short-line-fault interruption is difficult.

Section 7

Initial TRV and the time-delay effect

Initial transient recovery voltage (ITRV) is associated with very fast voltage components immediately after current zero. Although short-line faults focus on the overhead-line section on the load side of the circuit-breaker, other conductors also produce travelling waves. Busbars, short conductors and connections near the circuit-breaker can create very high-frequency TRV components. Busbars can have surge impedances of several hundred ohms, and because busbar lengths are much shorter than line lengths, the associated oscillation frequencies can be very high — often above 1 MHz.

This is why ITRV can be relevant for some circuit-breaker arrangements, especially where the interrupter is directly connected to an overhead busbar. In dead-tank and GIS circuit-breakers, the earthed enclosure and bushing capacitances can reduce the ITRV to less significant levels; in live-tank circuit-breakers, ITRV may need more attention. The practical message is that local geometry and capacitance near the circuit-breaker can shape the first microseconds of TRV.

Time-delay effect

Local capacitance to earth between the circuit-breaker and the fast TRV-producing element can slow the initial voltage rise — this is called time delay. Sources of local capacitance include current and voltage transformers, bushings, support insulators, nearby equipment, and intentional grading or shunt capacitance.

\[ t_d = C_\text{dl}\,Z_0 \]

\(t_d\): delay time
\(C_\text{dl}\): local capacitance at the line entrance
\(Z_0\): line surge impedance

Capacitance is not a free improvement

The time delay is useful for understanding test circuits and prospective TRV, but it should not be misunderstood. In a real interruption the TRV does not simply wait while a capacitor charges from zero — the arc voltage before current zero already interacts with the capacitance. Local capacitance can reduce the initial RRRV, but it may also increase the TRV peak because the oscillation is modified. Adding capacitance is therefore not always a simple improvement: it can reduce the early steepness but worsen another part of the duty.

Section 8

Why the gap is challenged: thermal recovery

The main challenge in short-line-fault interruption is the speed of dielectric recovery required in the interrupter. Immediately before current zero the circuit-breaker contains an arc — a conducting plasma channel between the contacts. The gas or vacuum gap is not yet a healthy insulating gap. Immediately after current zero the TRV begins to appear, and for a short-line fault it can rise very rapidly. The circuit-breaker must remove or neutralise the hot arc residue quickly enough to stop the TRV re-igniting the gap.

If the arc residue remains too conductive, a post-arc current flows, driven by the TRV through the remaining plasma. If the energy input into the post-arc plasma exceeds the cooling capability of the interrupter, thermal re-ignition can occur. Conceptually:

\[ u_\text{ab}(t)\,i_\text{pa}(t) > P_\text{cool}(t) \]

\(u_\text{ab}(t)\): transient voltage across the circuit-breaker contacts
\(i_\text{pa}(t)\): post-arc current
\(P_\text{cool}(t)\): cooling power available to remove energy from the arc residue

This is not a design calculation for the system engineer, but it explains the mechanism: if the TRV supplies more energy to the hot gap than the interrupter can remove, the interruption fails thermally.

Thermal re-ignition and arc conductivity

Thermal re-ignition is a failure of interruption shortly after current zero, because the arc channel has not cooled sufficiently and the post-arc current develops into a new conducting path. For SF₆ circuit-breakers it is closely related to the first few microseconds after current zero, where the steep short-line-fault TRV can feed energy into the residual plasma before the gap has recovered. The risk depends on current magnitude, current derivative at current zero, arc duration, contact gap, gas pressure, nozzle condition, cooling flow, arc voltage before current zero, post-arc current, TRV rate of rise and local capacitance.

The remaining conductivity of the arc near current zero is a useful indicator of interruption performance:

\[ g(t) = \frac{i_a(t)}{u_\text{ab}(t)} \]

\(g(t)\): arc conductivity
\(i_a(t)\): arc current before current zero
\(u_\text{ab}(t)\): arc voltage across the contacts

A high arc conductivity shortly before current zero means the gap still contains a strong conductive path, increasing the risk of thermal re-ignition; a low conductivity means the arc has been cooled and de-ionised more effectively, giving a better interruption margin. For the system engineer the lesson is simpler: a circuit-breaker may fail a short-line-fault duty even when the current is below its rated short-circuit current, because the TRV rises too quickly for the arc gap to recover.

Post-arc current and arc conductivity

Post-arc current is the small current that flows through the residual plasma after current zero, driven by the TRV. Arc conductivity describes how conductive the arc path remains close to current zero: a higher arc conductivity near current zero means the gap is less recovered, so the risk of thermal re-ignition is higher.

Section 9

Arc–circuit interaction: SF₆ versus vacuum

In gas circuit-breakers, the arc voltage near current zero strongly interacts with the external circuit. The arc voltage is not constant — it depends on arc length, gas type, arcing-contact material, cooling intensity, current magnitude, nozzle condition, and pressure and flow pattern. In SF₆ circuit-breakers the arc voltage can rise sharply just before current zero; this is the arc-voltage extinction peak, and it can charge capacitance connected across or near the circuit-breaker.

\[ i_C = C_p\,\frac{du_a}{dt} \qquad\qquad i_\text{circuit} = i_a + i_C \]

\(i_C\): current through the parallel capacitance
\(C_p\): parallel capacitance
\(u_a\): arc voltage
\(i_a\): arc current
\(i_\text{circuit}\): total current supplied by the external circuit

Some current is therefore diverted from the arc into the capacitance. Near current zero, even a small capacitive current can significantly change the arc current, reducing the arc-current derivative at current zero and giving the arc more time to cool. So capacitance near the circuit-breaker can sometimes improve interruption probability by helping the current transfer away from the arc before current zero — though it also changes the TRV after current zero, possibly reducing RRRV while affecting the TRV peak and the completeness of isolation.

Vacuum circuit-breakers behave differently

In vacuum, the arc voltage is much lower — usually only a few tens of volts near current zero — and the vacuum arc does not have the strong extinction peak of a gas arc. The pre-current-zero arc–circuit interaction that matters in SF₆ circuit-breakers is therefore much weaker in vacuum circuit-breakers. Vacuum interrupters also have only a small post-arc current (typically a few amperes); their interruption is governed mainly by post-arc dielectric and charge recovery in the first instants after current zero, rather than by a large post-arc current.

Two different recovery mechanisms

SF₆ circuit-breakers are strongly influenced by arc–circuit interaction before current zero; vacuum circuit-breakers are more influenced by post-arc behaviour after current zero. Experience shows vacuum interrupters can often withstand very steep TRV stresses effectively, but the physical recovery process differs from SF₆ — which matters when interpreting test results or applying mitigation.

Section 10

The long-line fault

A long-line fault occurs when the fault is at a significant distance from the circuit-breaker. Like a short-line fault it involves travelling waves, and the line-side TRV component can still be triangular from reflections between the circuit-breaker and the fault. But it differs in several important ways. The fault current is mainly determined by the reactance of the overhead line between the circuit-breaker and the fault: as the distance increases, the line reactance increases and the current decreases. The line-side travel time is longer, so the triangular waveform is slower than in a short-line fault. And the first-pole-to-clear behaviour becomes very important — in some LLF cases the first pole to clear experiences the most severe TRV, which differs from some short-line-fault representations based on last-pole phenomena or single-phase-to-earth clearing.

Why the first pole can be severe in long-line faults

In a three-phase fault on a long overhead line, the pole-to-clear sequence strongly affects the recovery voltage. For long-line faults the first pole to clear can experience a high RRRV and a high TRV peak, because the line current, line distance, induced voltage from other phase currents and travelling-wave effects all contribute to the line-side voltage. The overhead-line zero-sequence to positive-sequence reactance ratio \(X_0/X_1\) is typically about 2 to 3.5 (commonly around 3), which affects the relative magnitude of three-phase and single-phase-to-earth fault currents; for overhead lines the three-phase short-circuit current is normally considerably larger than the single-phase-to-earth fault current when the line reactance dominates.

As a result, the first pole to clear in a long-line fault can see a more severe TRV than the last pole, depending on the location and fault type. The line-side amplitude factor in long-line faults can be higher than the standard short-line-fault value of 1.6; the literature quotes severe-case values up to around 2.4. Unlike the short-line fault, however, “long-line fault” is a descriptive term rather than a defined IEC test duty (such as L90 or L75), and this elevated amplitude factor is not a standardised test value — so long-line-fault TRV generally needs a network-specific calculation rather than a fixed factor.

Section 11

Short-line versus long-line fault

Short-line and long-line faults are related, but their severity mechanisms are not identical.

Table 1 — Short-line fault versus long-line fault.

Aspect	Short-Line Fault (SLF)	Long-Line Fault (LLF)
Fault location	Close to the circuit-breaker	Far from the circuit-breaker
Fault current	High	Lower (limited by line reactance)
Current slope at zero	High	Lower
Travelling-wave time	Short — very high-frequency TRV	Longer — slower triangular TRV
Dominant stress	Very high initial RRRV	TRV peak & pole-to-clear severity
Most severe pole	Often last-pole / single-phase basis	First-pole-to-clear can be worst
Amplitude factor	~1.6 (standard)	Can reach ~2.4
Main challenge	Very fast thermal gap recovery	Travelling waves, peak, parallel circuits

Effect of parallel circuits

Long-line faults can be complicated by parallel overhead lines. In a double-circuit line, or several single-circuit lines in parallel, a switching surge generated by fault clearing on one circuit can travel through the parallel circuit and affect the TRV seen by another circuit-breaker. The total TRV can then consist of three parts: the triangular waveform from travelling waves on the faulted line; the response of the system at the busbar side of the circuit-breaker; and the switching surge associated with clearing at the other end or on a parallel circuit. Long-line-fault TRV therefore cannot always be evaluated with a simple single-line equivalent — for important transmission circuits, especially long double-circuit lines, the actual network configuration should be represented.

The T10 duty and long-line faults

Long-line faults may produce relatively low currents compared with terminal faults, but the TRV peak can be severe. In some applications LLF behaviour is associated with lower-current terminal-fault duties such as T10, where the interrupted current is around 10% of the rated short-circuit breaking current. (T10 and T30 are formally terminal-fault duties representing faults remote from the source, which produce the highest TRV amplitude factors.) Low current does not mean low TRV severity: a lower-current duty can have a more severe TRV peak or waveform than a higher-current one. The literature notes that high TRV peak values during long-line-fault clearing were one reason for adapting the T10 parameters in IEC 62271-100. The practical message is to never assess circuit-breaker suitability from short-circuit current magnitude alone — the TRV must be checked separately.

Section 12

How SLF and LLF differ from out-of-phase switching

Short-line and long-line faults are line-fault duties: a fault exists on the overhead line and the circuit-breaker interrupts fault current flowing into it. Out-of-phase switching ↗ is different — the circuit-breaker separates two live systems that are not synchronised, and both sides remain energised by active sources after interruption.

The difference can be summarised:

SLF is severe mainly because of very fast line-side RRRV.
LLF is severe mainly because of travelling-wave behaviour, fault distance, pole-to-clear effects and a possibly high TRV peak.
Out-of-phase switching is severe mainly because the circuit-breaker opens between two active source voltages with a phase-angle difference.

Consequence

All three duties can produce severe TRV, but for different physical reasons. A circuit-breaker that is satisfactory for one duty is not automatically satisfactory for another.

Section 13

Modelling requirements and what to check

For short-line-fault studies, the model should include:

a distributed-parameter overhead-line representation;
an accurate line surge impedance;
the fault distance;
the source impedance, rated voltage and fault level;
the L90 or L75 duty where applicable;
source-side capacitance and inductance;
local equipment capacitance near the circuit-breaker;
the circuit-breaker terminal arrangement;
TRV measurement across the circuit-breaker contacts.

For long-line-fault studies, the model should also include:

the full line length to the fault;
positive-sequence and zero-sequence line data;
phase coupling;
the pole-to-clear sequence;
parallel circuits;
remote-end reflections;
the busbar configuration;
connected lines or cables;
source strength at both ends;
the actual network configuration at the time of fault clearing.

Use a travelling-wave line model

For both SLF and LLF, use a travelling-wave-capable (distributed-parameter) line representation. A lumped-impedance model is not sufficient where the line section is electrically long at TRV frequencies.

Engineer’s checklist

Is the fault a terminal fault, short-line fault or long-line fault?
Is the circuit-breaker connected directly to an overhead line? Is there a busbar/short conductor producing ITRV?
Is the circuit-breaker live-tank, dead-tank or GIS, and is local capacitance significant?
What is the line surge impedance and the fault distance?
What is the current reduction from terminal-fault current (L90, L75, T10, T30 or project-specific)?
What are the initial RRRV, the first TRV peak and the source-side TRV contribution?
Does the total TRV exceed the circuit-breaker envelope?
Are there parallel circuits that add travelling-wave components?
Is the first-pole-to-clear or the last-pole-to-clear more severe?
Is the system effectively earthed or non-effectively earthed?
Is the circuit-breaker type-test evidence applicable to this duty?

Common mistakes

Common line-fault TRV mistakes

Do not assume the highest current always gives the most severe TRV.
Do not model SLF or LLF using only lumped impedance when travelling-wave behaviour matters.
Do not ignore local capacitance and time-delay effects.
Do not assume cable faults and overhead-line faults have the same surge-impedance behaviour.
Do not confuse a short-line fault with a long-line fault.
Do not assume terminal-fault capability automatically covers line-fault TRV duties.
Do not use 450 Ω as the actual surge impedance of every overhead line.
Do not describe T10 as a long-line fault unless the standard or project basis supports that interpretation.

Section 14

Report wording and key message

Suggested report wording

“Faults on overhead lines can impose severe transient recovery voltage duties on high-voltage circuit-breakers. In short-line faults the fault is a short but electrically significant distance from the circuit-breaker; the line-side TRV is governed by travelling waves between the circuit-breaker and the fault, producing a steep triangular component immediately after current zero, with an initial RRRV approximately proportional to the line surge impedance and the current derivative at current zero. The line should be represented as a distributed-parameter system rather than a lumped impedance, and the commonly used standard representation adopts an overhead-line surge impedance of 450 Ω. Short-line-fault duties such as L90 and L75 represent high-current line-fault cases and should be checked against the applicable IEC or IEEE standard edition and the circuit-breaker type-test evidence. Long-line faults also involve travelling-wave behaviour, but the longer fault distance changes the fault current, travel time, TRV peak and pole-to-clear severity; in some cases the first pole to clear experiences the most severe TRV, and parallel circuits can introduce additional travelling-wave components. The assessment should therefore consider both the interrupted current and the TRV duty: the circuit-breaker rating should not be checked only against the maximum terminal-fault current, but also against the SLF and LLF TRV envelopes, the line-side RRRV, the first TRV peak, local capacitances, busbar ITRV and the applicable type-test evidence.”

Key message

Short-line and long-line faults are not simply lower-current versions of terminal faults — they are travelling-wave TRV duties. Short-line faults are severe because the TRV rises extremely quickly in the first microseconds after current zero. Long-line faults can be severe because fault distance, line reflections, induced line-side voltage, the pole-to-clear sequence and parallel circuits raise the TRV peak and complicate the waveform. For both, the circuit-breaker must be checked against the actual TRV stress, not only against the short-circuit current magnitude.

Book cover of Switching in Electrical Transmission and Distribution Systems by Smeets, van der Sluis, Kapetanovic, Peelo and Janssen (Wiley)

Reference & further reading Switching in Electrical Transmission and Distribution Systems René Smeets, Lou van der Sluis, Mirsad Kapetanovic, David F. Peelo, Anton Janssen — Wiley

Part Two Reading now

Short-Line and Long-Line Faults

Travelling-wave TRV duties on overhead lines — the triangular line-side TRV, its rate of rise, surge impedance, the L90 / L75 and T10 duties, ITRV and thermal re-ignition.

Series progress 2 of 5

Part Three

Transformer- and Reactor-Limited Faults

Low-current but high-TRV duties: why a transformer or reactor that limits the current can still create a fast, high-frequency recovery voltage, and how to mitigate it.