Fault-current breaking is one of the principal duties of a high-voltage circuit-breaker. The terminal fault — a short circuit at the circuit-breaker terminals — usually gives the highest current, but current magnitude alone does not define the duty. After interruption the circuit-breaker must withstand the transient recovery voltage, and in a three-phase system the first pole to clear often sees the highest voltage. This note explains current interruption, recovery voltage and TRV, and the first-pole-to-clear factor and its dependence on system earthing.
Reading time ≈ 20 min · Circuit-breaker duty, three-phase interruption & first-pole-to-clear factor
Fault-current breaking is one of the principal duties of a high-voltage circuit-breaker: the interruption of current under faulted network conditions. In a faulted network, current flows through an unintended path — usually from insulation failure, lightning, pollution, line galloping, falling trees, mechanical damage, fire, animals, equipment failure or operational error. The circuit-breaker is the device designed to interrupt that current; other switching devices may carry or make fault current, but the circuit-breaker is the one intended to interrupt it.
Fault-current making matters too. It occurs when a device closes onto an already-faulted network — for example when an earth connection is accidentally left installed after maintenance and the circuit-breaker is closed onto it. From the system point of view, closing onto an existing fault produces transients similar to a fault suddenly appearing on an energised system; the network cannot tell whether the fault came from an external flashover or from a device closing onto a faulted circuit. Both breaking and making must therefore be considered when specifying and applying circuit-breakers.
Making current versus breaking current
Breaking current is the current the circuit-breaker must interrupt after a fault has occurred. Making current is the current the circuit-breaker may experience when it closes onto an already-faulted circuit; the making duty matters because the first peak current can create high electrodynamic forces.
A terminal fault is a short-circuit located directly at, or very close to, the circuit-breaker terminals. It normally gives the highest short-circuit current at that circuit-breaker location, because there is almost no line, cable, transformer or reactor impedance between the circuit-breaker and the fault. However, the duty is not defined by current alone: after current interruption, the circuit-breaker must also withstand the transient recovery voltage (TRV) across its contacts.
Do not confuse the duties
A terminal fault should not be confused with a short-line fault (SLF), long-line fault (LLF), transformer-limited fault (TLF), reactor-limited fault (RLF) or out-of-phase switching (OOP). A terminal fault usually gives the highest current, while those other duties may create a more severe TRV or RRRV.
Abbreviations and symbols used on this page
TRVTransient recovery voltage
RRRVRate of rise of recovery voltage
RVRecovery voltage (power-frequency, after interruption)
\(k_\text{pp}\)First-pole-to-clear factor
\(k_\text{af}\)Amplitude (peak) factor
CBCircuit-breaker
ACAlternating current
TFTerminal fault
\(I_\text{sc}\)Rated short-circuit breaking current
\(X_0/X_1\)Zero- to positive-sequence reactance ratio
X/RReactance-to-resistance ratio
DCDirect-current (offset) component
puPer-unit
LLLine-to-line
HV / EHVHigh / extra-high voltage
UHVUltra-high voltage
\(Z_{0s}/Z_{1s}\)Zero- to positive-sequence surge-impedance ratio
SLFShort-line fault
LLFLong-line fault
TLFTransformer-limited fault
RLFReactor-limited fault
OOPOut-of-phase switching
IECInternational Electrotechnical Commission
IEEEInstitute of Electrical and Electronics Engineers
Key idea
The terminal fault sets the current rating. A short circuit at the circuit-breaker terminals gives the highest prospective current; the rated short-circuit breaking current must exceed it.
But current is only half the duty. After current zero the circuit-breaker must withstand the TRV — its peak and rate of rise — faster than the contact gap recovers.
The first pole to clear is usually the worst. With the other phases still conducting, it recovers against a raised voltage set by the first-pole-to-clear factor \(k_\text{pp}\).
\(k_\text{pp}\) depends on earthing. About 1.5 for non-effectively earthed systems and 1.3 for effectively earthed systems — driven by how far the neutral can shift.
Key terms used on this page
01Terminal fault
A short circuit at or very close to the circuit-breaker terminals — normally the highest-current duty.
02Recovery voltage
The voltage across the contacts after current interruption.
03Transient recovery voltage
The high-frequency transient part of the recovery voltage just after current zero.
04First pole to clear
The first of the three poles to interrupt successfully; it often sees the highest recovery voltage.
05First-pole-to-clear factor
\(k_\text{pp}\) — how much the first-pole recovery voltage exceeds the post-clearing phase voltage.
06Amplitude factor
\(k_\text{af}\) — the overshoot of the oscillating TRV above the power-frequency recovery voltage.
07Effectively earthed
A neutral arrangement that limits neutral shift, giving a lower first-pole-to-clear factor.
08Symmetrical current
A fault current centred on the zero axis, with no DC offset.
09Asymmetrical current
A fault current with a decaying DC offset, set by fault inception and the network X/R.
10RRRV
Rate of rise of recovery voltage — the initial slope of the TRV.
11Making current
The peak current when a circuit-breaker closes onto a fault, set by asymmetry and X/R.
12Sequence reactance
Positive- and zero-sequence reactances \(X_1\), \(X_0\) describing balanced and earth-return behaviour.
Section 1
Categories of fault-current interruption
Fault-current interruption can be divided into two broad groups: short-circuit fault interruption and out-of-phase fault interruption. Short-circuit interruption occurs when current flows through an unintended short-circuit path; the location and nature of the fault strongly influence the duty. Out-of-phase interruption is different — two parts of the system are not in synchronism and active source voltages may remain on both sides of the circuit-breaker.
Typical short-circuit duties include:
terminal faults;
line faults — short-line and long-line faults;
transformer-limited faults;
reactor-limited faults.
This page focuses on terminal faults and three-phase fault-current interruption, because they explain the basic concepts of current interruption, recovery voltage, TRV and the first-pole-to-clear factor. The line-fault duties are covered in Short-Line and Long-Line Faults ↗, the limited-current duties in Transformer- and Reactor-Limited Faults ↗, and the two-source case in Out-of-Phase Switching ↗.
Section 2
What a terminal fault means
A terminal fault is a short-circuit fault located directly at, or very close to, the circuit-breaker terminals. True terminal faults are less frequent than faults some distance away on a line or cable, but they are very important for ratings and testing because they generally produce the highest short-circuit current the circuit-breaker may have to interrupt. With almost no impedance between the circuit-breaker and the fault, the current is limited mainly by the source-side network impedance, so the terminal-fault current is normally the maximum fault current at that location. The rated short-circuit breaking current should be higher than the prospective terminal-fault current at the point of installation.
Why terminal faults drive the current rating
Because the fault is at the terminals, no line, cable, transformer or reactor impedance reduces the current. Terminal faults therefore size the short-circuit current rating, the thermal and electrodynamic stresses, the arc energy and contact erosion, the arcing time and current asymmetry, the DC component, the peak making current and the mechanical withstand.
But not the worst TRV
The highest current is not always the most severe TRV case. Lower-current duties — transformer-limited, reactor-limited, short-line or long-line faults — can impose more severe TRV or RRRV. Terminal-fault current rating alone does not prove a circuit-breaker is suitable for every application.
Section 3
The terminal-fault TRV concept
For a terminal fault the circuit can be understood simply. The load side of the circuit-breaker is effectively short-circuited to earth, while the source side is represented by the equivalent source inductance and stray capacitance. After interruption at current zero, the voltage across the contacts is mainly supplied from the source side; the fault side remains at or near zero voltage because it is directly short-circuited. The voltage across the circuit-breaker contacts can be understood as the difference between the source-side and fault-side voltage components:
fault-side / load-side voltage component (\(\approx 0\) for a direct terminal fault)
The source-side recovery voltage dominates the TRV across the circuit-breaker. After current interruption, the voltage across the contacts is the recovery voltage; its high-frequency transient part, appearing immediately after current zero, is the transient recovery voltage (TRV). The TRV is critical because the contact gap is still recovering its dielectric strength after arc extinction — if the TRV rises faster than that strength recovers, the gap may fail and current may re-establish. A successful interruption requires the dielectric withstand to recover faster than the applied TRV.
TRV severity is normally described by its peak value, rate of rise (RRRV), time to peak, waveform shape, source-side and line-side components, the system earthing condition, the first-pole-to-clear factor, and the damping / amplitude factor.
Two kinds of stress
Terminal faults are severe for current interruption because the fault current is high. However, other, lower-current duties can be more severe for TRV. A circuit-breaker must therefore be checked for both current-interruption capability and recovery-voltage withstand.
Section 4
Current zero, voltage maximum and asymmetry
In a predominantly inductive fault circuit the current lags the voltage by about 90 electrical degrees. Because circuit-breakers interrupt AC at current zero, for a symmetrical current in a highly inductive circuit, current zero occurs close to the instant when the system voltage is near its peak. As a result the recovery voltage can reach a high value immediately after interruption — which is why the maximum TRV condition is usually associated with a current that is about 90 degrees out of phase with the voltage.
For an asymmetrical fault current, the current zero may not occur at the same point on the voltage waveform, so the TRV peak after an asymmetrical interruption may be lower than after a symmetrical one, depending on the exact current-zero instant. Asymmetrical current can still be severe, however, because it increases arcing time, arc energy, mechanical stress and making-current duty.
Symmetrical and asymmetrical current
A symmetrical fault current is centred on the zero axis, with equal positive and negative peaks once the transient offset has decayed. An asymmetrical fault current includes a DC offset that depends on the instant the fault occurs relative to the voltage waveform and on the network X/R ratio. A high X/R ratio makes the DC component decay slowly, which can delay the first current zero and lengthen the arcing time, increasing energy deposition in the contacts and interrupter.
Two different stresses
Current asymmetry mainly affects peak current, making duty, contact forces, arcing time, arc energy and delayed current zero. TRV mainly affects dielectric recovery, re-ignition risk, post-arc recovery and contact-gap withstand. The highest TRV peak is linked to the voltage at current zero, so in inductive systems a symmetrical interruption can give a particularly severe TRV — both asymmetry and TRV must be considered, but they stress the circuit-breaker in different ways.
Section 5
Three-phase interruption and the first pole
In a three-phase system the three poles do not necessarily interrupt at the same instant — each pole interrupts at its own current zero. The first pole to interrupt successfully is the first pole to clear. After it clears, the network is no longer a symmetrical three-phase fault circuit: the remaining two phases continue to carry current for a short time until they reach their own current zeros. This change in network condition raises the voltage appearing across the first open pole, so in many cases the first pole experiences the highest recovery voltage. That is why the first-pole-to-clear factor is important.
Section 6
The first-pole-to-clear factor
In a three-phase circuit-breaker, the three poles do not interrupt at exactly the same instant. The first pole to clear sees a recovery voltage while the other two phases are still conducting; because the network condition changes after the first pole opens, the first pole can experience a higher recovery voltage than the later poles. This effect is represented by the first-pole-to-clear factor \(k_\text{pp}\).
The first-pole-to-clear factor \(k_\text{pp}\) describes how much the recovery voltage across the first clearing pole is increased relative to the corresponding single-phase recovery voltage:
\[ k_\text{pp} = \frac{\text{power-frequency recovery voltage across the first pole, before the others clear}}{\text{power-frequency voltage after all poles have cleared}} \]
The first pole can see a higher voltage because the other two phases are still connected to the faulted network when it has already opened. The value of \(k_\text{pp}\) depends strongly on the earthing of the power system:
Mathematical limit of the formula below — not a real three-phase system condition (even effectively earthed systems give \(k_\text{pp}>1\))
In practical three-phase HV systems, the values 1.3 and 1.5 are the most important for circuit-breaker application.
The 1.3 and 1.5 values are standardised, rounded values
\(k_\text{pp} = 1.3\) (and \(k_\text{pp} = 1.5\)) are standardised, rounded values used for practical circuit-breaker TRV representation. The actual value depends on the zero-sequence to positive-sequence reactance ratio \(X_0/X_1\) and on the network configuration.
Section 7
Why neutral earthing affects the TRV
The neutral earthing arrangement determines how much the neutral point can shift during an unsymmetrical interruption. In a non-effectively earthed system the neutral is not strongly fixed to earth, so after the first pole clears the neutral potential can shift; this neutral shift adds to the voltage across the first open pole, and the first-pole recovery voltage can reach 1.5 pu. In an effectively earthed system the neutral is more strongly referenced to earth, the shift is smaller, and the factor is lower — commonly 1.3. This is why MV systems, often non-effectively earthed, can have a higher first-pole-to-clear factor than many EHV systems, which are typically effectively earthed.
In a non-effectively earthed system the first open pole may recover against 1.5 pu, so both its TRV peak and RRRV can be higher than in a single-phase interruption. The remaining two poles interrupt later and, in a simplified purely inductive case, recover against a lower voltage of about 0.87 pu. This is why the first pole is often the critical pole, and why a circuit-breaker is not simply three independent single-phase switches — the interaction between phases during three-phase interruption must be considered.
Effective and non-effective earthing in IEC practice
For TRV purposes, systems are grouped by neutral earthing. Systems below 100 kV are often treated as non-effectively earthed unless otherwise specified; systems at 245 kV and above are generally treated as effectively earthed, because the neutral is normally earthed through a sufficiently low impedance. In the intermediate range — 100, 123, 145 and 170 kV — both cases may need to be considered depending on the actual network and the rating selected.
Do not guess the earthing
The earthing condition should be confirmed from the system design, the transformer neutral earthing arrangement, the zero-sequence network and the applicable circuit-breaker rating basis — not assumed.
Section 8
Relating \(k_\text{pp}\) to \(X_0/X_1\)
The first-pole-to-clear factor can be related to the ratio of zero-sequence to positive-sequence reactance. The following relationship shows how the neutral earthing reactance contributes to the zero-sequence reactance:
\[ X_0 = X_1 + 3X_n \]
\(X_0\)
zero-sequence reactance
\(X_1\)
positive-sequence reactance
\(X_n\)
neutral-earthing reactance
The ratio \(X_0/X_1\) is used to describe the earthing condition seen by the fault-current path:
\[ k = \frac{X_0}{X_1} \]
\(k\)
ratio of zero-sequence to positive-sequence reactance, \(X_0/X_1\)
\(X_0\)
zero-sequence reactance
\(X_1\)
positive-sequence reactance
The first-pole-to-clear factor can be related to the zero-sequence to positive-sequence reactance ratio:
\[ k_\text{pp} = \frac{3k}{2k+1} \]
\(k_\text{pp}\)
first-pole-to-clear factor
\(k\)
zero- to positive-sequence reactance ratio, \(X_0/X_1\)
This shows that \(k_\text{pp}\) increases as the zero-sequence impedance becomes higher relative to the positive-sequence impedance. For very low neutral impedance, \(k\to 1\) and \(k_\text{pp}\to 1.0\); for high neutral impedance or an isolated neutral, \(k\to\infty\) and \(k_\text{pp}\to 1.5\). This is why non-effectively earthed systems produce a more severe first-pole recovery voltage.
Surge-impedance ratio and RRRV
TRV severity is affected not only by reactance but also by surge impedance. By analogy with the reactances, the zero-sequence surge impedance includes the neutral-path contribution:
\[ Z_{0s} = Z_{1s} + 3Z_n \]
\(Z_{0s}\)
zero-sequence surge impedance
\(Z_{1s}\)
positive-sequence surge impedance
\(Z_n\)
neutral-path surge-impedance contribution
The ratio of zero- to positive-sequence surge impedance affects the RRRV seen by each pole:
\[ \xi = \frac{Z_{0s}}{Z_{1s}} \]
\(\xi\)
zero- to positive-sequence surge-impedance ratio, \(Z_{0s}/Z_{1s}\)
\(Z_{0s}\)
zero-sequence surge impedance
\(Z_{1s}\)
positive-sequence surge impedance
For overhead-line HV systems this ratio is often about 2 to 2.5, and a representative value may be adopted in standardised TRV representations. The key point is that RRRV depends on both the current derivative and the effective surge impedance seen by the circuit-breaker pole.
Section 9
Why \(X_0/X_1\) is not constant across the network
The ratio \(X_0/X_1\) is not a single fixed value for an entire system — it changes with location, configuration and which equipment is in service. Overhead lines often have relatively high ratios, typically about 2 to 3. Transformers with earthed neutrals and delta windings can have much lower ratios, sometimes below 1. The effective ratio at a circuit-breaker location depends on how much fault current arrives through lines, transformers, generators or other elements.
If the fault current is dominated by overhead-line reactance, the ratio may be high and the single-phase-to-earth fault current may be lower than the three-phase fault current. If it is dominated by transformer paths with low zero-sequence impedance, the ratio may be lower and the single-phase-to-earth current may become comparable to, or even greater than, the three-phase current. This is why fault-current and TRV studies should use the actual network configuration rather than generic assumptions alone.
Outages change the duty
\(X_0/X_1\) changes when network elements are out of service — a transformer or line out, a generator disconnected, a busbar split, a parallel path unavailable, a different transformer neutral arrangement, a maintenance outage, a restoration condition or a temporary arrangement. Because \(X_0/X_1\) changes, so can the first-pole-to-clear factor and the recovery-voltage stress. For critical substations and important circuit-breakers, consider credible operating configurations, not only the normal intact network.
Section 10
Pole-by-pole duty
During three-phase fault clearing, the first, second and third poles do not necessarily experience the same duty. The first pole may see the highest recovery voltage because the other phases are still connected when it clears. The second and third poles may have different current-zero timing, recovery voltage and RRRV depending on the earthing system and network parameters. In a non-effectively earthed system the first pole can recover against 1.5 pu while the last poles recover against lower values in the simplified purely inductive case; in an effectively earthed system the first-pole factor is lower, but the second pole may still see significant recovery voltage. TRV assessment should therefore not assume that all three poles see the same waveform.
Section 11
Amplitude factor and the TRV peak
The TRV waveform is not only a power-frequency recovery voltage — it also includes an oscillatory transient component. The peak of this oscillation depends on damping, captured by the amplitude factor \(k_\text{af}\). In an undamped theoretical circuit \(k_\text{af}\) could approach 2; in practical circuits damping reduces the peak, and standardised values are used for testing and rating. The terminal-fault TRV peak can be estimated conceptually as:
This is a simplified explanation; the exact TRV envelope should be taken from the applicable standard and circuit-breaker rating.
Why terminal-fault current is not enough
A circuit-breaker can be correctly rated for terminal-fault current yet still need further checking: a short-line fault can give very steep RRRV; a long-line fault high TRV peaks from travelling waves; a transformer-limited fault a high-frequency TRV; a reactor-limited fault very high RRRV from small reactor capacitance; and out-of-phase switching a very high recovery voltage. A complete application study considers both the current rating and the TRV rating.
Section 12
IEC and IEEE standard references
For IEC-based applications the main standard for general high-voltage AC circuit-breakers is IEC 62271-100 — High-voltage switchgear and controlgear — Part 100: Alternating-current circuit-breakers. It applies to three-phase AC circuit-breakers for indoor or outdoor installation, operating at 50 Hz and/or 60 Hz, on systems above 1 kV; it includes direct testing methods for making and breaking tests and refers to IEC 62271-101 for synthetic testing methods.
For IEEE-based applications the main rating standard is IEEE C37.04 — Ratings and Requirements for AC High-Voltage Circuit Breakers with Rated Maximum Voltage Above 1000 V, with the related test-procedure standard IEEE C37.09. In practice: IEC 62271-100 for IEC ratings and test duties; IEC 62271-101 where synthetic testing applies; IEEE C37.04 for IEEE rating requirements; IEEE C37.09 for IEEE test procedures; and manufacturer type-test reports checked against the edition specified for the project.
In short, each document has a distinct role:
IEC 62271-100 — AC circuit-breaker ratings and 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.
Manufacturer type-test reports must be checked against the exact standard edition specified for the project.
Do not mix IEC and IEEE criteria
Do not mix IEC and IEEE values or acceptance criteria without checking the project specification, the standard edition and the manufacturer type-test report.
Section 13
Assessment, modelling and common mistakes
Engineering assessment checklist
Rated maximum voltage.
Rated short-circuit breaking current.
Rated short-circuit making current.
Prospective terminal-fault current at the installation point.
X/R ratio and DC component, and the resulting current asymmetry.
System earthing (effectively or non-effectively earthed).
First-pole-to-clear factor \(k_\text{pp}\).
TRV peak and RRRV against the applicable envelope.
Applicable standard and edition.
Manufacturer type-test evidence — including whether other duties (SLF, LLF, TLF, RLF, OOP) apply.
Typical modelling approach
For a terminal-fault TRV study, represent the source-side equivalent network accurately; the fault side is usually a direct short circuit at the circuit-breaker terminals. Include the source equivalent voltage and positive- (and where relevant zero-) sequence impedance, source-side and local substation capacitance, transformer and line contributions, the earthing arrangement, the circuit-breaker pole representation, current-zero timing, and TRV measurement across each pole. Allow each pole to be observed separately. Standard TRV envelopes suit screening; for critical installations, unusual configurations, or where the calculated TRV may exceed the standard envelope, carry out an electromagnetic transient study.
Common mistakes
Common terminal-fault TRV mistakes
Do not check only the RMS fault current — adequacy depends on both current interruption and recovery-voltage withstand.
Do not assume the highest current is always the worst TRV — lower-current duties can give a more severe TRV.
Do not assume all three poles experience the same recovery voltage — the first pole to clear can see more.
Do not use \(k_\text{pp} = 1.3\) or \(1.5\) without confirming the earthing condition.
Do not assume \(X_0/X_1\) is fixed for the whole network — it changes with configuration, fault location and outages.
Do not assume terminal-fault capability automatically covers SLF, LLF, TLF, RLF or OOP duties.
Section 14
Report wording and key message
Suggested report wording
“Fault-current interruption by the circuit-breaker was assessed considering the terminal-fault duty and the associated transient recovery voltage. A terminal fault represents a short circuit directly at or very close to the circuit-breaker terminals and normally gives the highest prospective short-circuit current at the circuit-breaker location. For three-phase fault interruption, the first pole to clear may experience a higher recovery voltage than the other poles because the remaining phases continue to conduct until their respective current zeros; the first-pole-to-clear factor depends on the system earthing arrangement, and non-effectively earthed systems can impose a higher first-pole recovery voltage than effectively earthed systems. The circuit-breaker rating should therefore be checked not only against the prospective short-circuit breaking current but also against the applicable TRV envelope, RRRV, first-pole-to-clear factor, system earthing condition and manufacturer type-test evidence. Additional duties such as short-line faults, long-line faults, transformer-limited faults, reactor-limited faults and out-of-phase switching should be assessed separately where relevant.”
Key message
Terminal faults matter because they normally produce the highest short-circuit current at the circuit-breaker location — but current magnitude alone does not define the duty. The circuit-breaker must also withstand the TRV after interruption, and in three-phase systems the first pole to clear can see an increased recovery voltage that depends strongly on the earthing arrangement. Always check the short-circuit current, making current, current asymmetry, first-pole-to-clear factor, TRV peak, RRRV, neutral earthing, standard rating basis and type-test evidence. A circuit-breaker is suitable only when both the current-interruption duty and the recovery-voltage duty are within its assigned capability.
A practical five-part series on the fault-interruption and switching duties of high-voltage circuit-breakers — from the terminal fault and overhead-line faults, through transformer- and reactor-limited faults, to out-of-phase switching and post-interruption breakdown events.
01
01Part OneReading now
Fault-Current Breaking & Terminal-Fault TRV
How a terminal fault sets the maximum short-circuit current at the breaker location, and why suitability also depends on the TRV, RRRV, first-pole-to-clear factor and earthing.
