Arc Interruption and Switching Phenomena

Introduction

The interruption of current is one of the most demanding functions performed by a circuit breaker. When contacts separate, the current does not immediately become zero. The voltage across the separating contacts rises, an electric arc forms, and the breaker must create conditions that cause the arc to extinguish and prevent its re-establishment. The interruption process is therefore an interaction between the electrical circuit, the arc, the contact geometry, the interrupting medium, and the speed of dielectric recovery.

Arc interruption is central to the protection of generators, transformers, transmission lines, industrial motors, and distribution networks. An inadequate interruption process can produce restriking, insulation failure, equipment damage, or prolonged faults. This chapter develops the physical principles of arc formation and extinction, transient recovery voltage, restriking voltage, current chopping, capacitive and inductive switching, switching surges, and the differences between alternating-current and direct-current interruption.

Learning Objectives

After studying this chapter, the reader should be able to:

1. Explain how an arc forms between separating circuit-breaker contacts.
2. Describe the mechanisms responsible for arc extinction and dielectric recovery.
3. Distinguish recovery voltage, transient recovery voltage, and restriking voltage.
4. Explain current chopping and its effects in inductive circuits.
5. Analyze capacitive-current interruption and inductive switching.
6. Describe switching surges and methods for controlling them.
7. Compare interruption under AC and DC conditions.
8. Apply basic circuit models to estimate transient voltages and interruption stresses.
9. Select practical mitigation measures for switching-related overvoltages.

1. Formation of the Circuit-Breaker Arc

When a circuit breaker opens, the contacts initially carry current through a small physical contact area. As the contacts separate, the area decreases and the current density increases. Local heating vaporizes contact material and produces metallic vapour. At the same time, the electric field between the contacts accelerates free electrons. These electrons collide with gas molecules and create additional electrons and ions by impact ionization.

The conducting path between the contacts is called an arc. It contains electrons and positive ions and is therefore a plasma. Its temperature may reach several thousand kelvin. The arc voltage is generally much smaller than the system voltage during the initial stage, so current continues to flow even though the contacts are physically separated.

Arc formation is promoted by:

• thermionic emission from hot contact surfaces;
• field emission caused by a strong electric field;
• vaporization of contact material;
• impact ionization in the interrupting medium; and
• residual ionization from the preceding current cycle.

The arc has a positive temperature coefficient in some operating regions: as current increases, the arc becomes hotter and its conductivity increases. A simplified arc relation is often written as

where v_a is the arc voltage, i is the arc current, and R_a is an effective arc resistance. This is not a fundamental constant-resistance law; R_a varies with arc length, current, pressure, temperature, and the interrupting medium. More advanced arc models, such as the Mayr and Cassie models, represent the dynamic conductance of the arc.

2. Arc Extinction

Arc extinction requires two conditions:

1. The current must pass through zero or be forced to zero.
2. The dielectric strength between the contacts must recover faster than the voltage imposed across them.

In an AC circuit, the first condition occurs naturally at each current zero. However, current zero alone does not guarantee interruption. The arc may reignite if the voltage across the contacts rises above the recovering dielectric strength.

2.1 Thermal and dielectric processes

Immediately before current zero, the arc channel is hot and highly ionized. Near current zero, the energy supplied to the arc decreases. Cooling, recombination of ions and electrons, diffusion, and removal of charged particles reduce conductivity. This is the thermal recovery process.

After current zero, the contact gap must withstand the applied voltage. The increase in withstand capability is called dielectric recovery. It depends on:

• contact separation and opening speed;
• pressure and composition of the interrupting medium;
• gas flow or oil movement;
• contact shape and field distribution;
• arc energy and duration; and
• contamination, humidity, and surface condition.

In air-break and vacuum breakers, the medium recovers differently. Air requires deionization and sufficient physical gap. Vacuum interrupters benefit from rapid removal of charged particles and vapour condensation, although metal-vapour arcs can remain important during the first microseconds after current zero. In SF₆ and other gas-insulated equipment, electron attachment and forced gas flow contribute to dielectric recovery. Modern equipment increasingly uses vacuum interruption and alternative gases because of environmental concerns associated with SF₆.

2.2 Restriking and reignition

If the arc is extinguished at current zero but the contact gap subsequently breaks down because of the applied transient voltage, the arc starts again. This is called restriking. In some texts, reignition refers especially to breakdown occurring very soon after current zero, while restriking may be used more generally. Both represent failure of the dielectric recovery process.

A successful interruption requires, at every instant after current zero,

where V _withstand(t) is the time-dependent dielectric withstand voltage and V _applied(t) is the voltage across the open contacts. The difference between these quantities is the safety margin.

Figure: Successful interruption occurs when dielectric recovery strength remains above the transient recovery voltage.

3. Recovery Voltage and Transient Recovery Voltage

The voltage that appears across the breaker contacts after current interruption is called the recovery voltage. It has two components:

• a transient component immediately following current zero; and
• a power-frequency component that remains after the transient has decayed.

The transient recovery voltage, or TRV, is the voltage component during the early period after current interruption. It is determined by the network inductance, capacitance, fault location, transformer characteristics, and breaker position.

3.1 Basic LC representation

A simple source-inductance and shunt-capacitance model gives an approximate natural frequency

and

where L is the equivalent inductance in henries, C is the equivalent capacitance in farads, ω₀ is angular frequency in radians per second, and f₀ is frequency in hertz. These expressions assume a lossless lumped LC circuit. Resistance and distributed-line effects reduce and distort the ideal oscillation.

For an initially charged capacitor and an inductive circuit, the characteristic impedance is

in ohms. A higher inductance or lower capacitance generally produces a higher characteristic impedance and a steeper voltage rise for a given transient current.

The rate of rise of recovery voltage, or RRRV, is an important breaker-duty parameter. In a simplified linear approximation,

where ∆V is the change in recovery voltage over the interval ∆t. Actual TRV waveforms may be oscillatory, exponential, or a combination of travelling-wave components, so a single RRRV value is an approximation used for specification and comparison.

3.2 Worked example: natural frequency

An equivalent network has L = 20 mH and C = 0.02 μF. Determine its natural frequency.

Convert capacitance:

C = 0.02 × 10^-6 F = 2 × 10^-8 F.

Therefore,

The product LC = 4 × 10^-10, and its square root is 2 × 10^-5. Hence,

f₀ ≈ 1/(2π × 2 × 10^-5) ≈ 7.96 kHz.

This is a natural transient frequency, not the 50 or 60 Hz power frequency. The high frequency explains why breaker insulation and measurement systems must withstand very rapid voltage changes.

4. Restriking Voltage

Restriking voltage is the transient voltage that appears across the breaker contacts immediately after current zero and may cause the arc to re-establish. It is sometimes used interchangeably with TRV, although technically the term emphasizes the voltage responsible for restriking.

The possibility of restriking depends on the relative slopes and magnitudes of two curves:

• the dielectric-strength curve of the opening gap; and
• the TRV curve imposed by the external circuit.

If the TRV crosses the dielectric-strength curve, breakdown occurs. The resulting arc may carry a high-frequency current. This current can pass through another current zero, causing repeated restrikes. Such repeated restriking is particularly severe in capacitive circuits and can generate cumulative overvoltages.

Breaker standards specify test duties using parameters such as peak TRV, time to peak, RRRV, and prospective short-circuit current. A breaker must be rated for the system voltage, frequency, short-circuit current, making current, and the specified TRV envelope.

5. Current Chopping

Current chopping is the interruption of current before its natural current zero. It is associated mainly with vacuum and air-blast breakers, especially when interrupting small inductive currents such as transformer magnetizing current, shunt-reactor current, or motor current.

An arc can become unstable at low current. If the breaker extinguishes the arc at a chopping current I_c rather than at natural zero, the energy stored in the inductance is transferred to circuit capacitance. For an ideal inductance-capacitance exchange, the approximate overvoltage is

where I_c is the chopped current in amperes, L is the inductance in henries, and C is the effective capacitance in farads. This formula assumes zero initial capacitor voltage, negligible resistance, and an ideal abrupt interruption.

The result follows from equating stored magnetic and electric energy:

Thus, current chopping is dangerous when L is large, C is small, or I_c is high. Repeated chopping and restriking can produce several times the normal system voltage.

Mitigation of current-chopping overvoltages

Common measures include:

• controlled switching at an appropriate point on the waveform;
• surge arresters at the transformer or reactor terminals;
• RC or R-C-D snubber circuits;
• opening resistors in selected breaker designs;
• breakers with suitable interrupting characteristics; and
• reduction of excessive cable-reactor resonance.

The mitigation device must be coordinated with insulation levels and the breaker’s actual current-chopping behavior.

6. Capacitive-Current Switching

Capacitive currents occur when energizing or de-energizing unloaded transmission lines, cables, capacitor banks, and voltage-transformer circuits. Although the current magnitude may be modest, the voltage across the breaker can become severe because the load-side capacitance retains charge.

For an ideal capacitor,

where i_C is capacitive current in amperes, C is capacitance in farads, and dv/dt is the rate of voltage change in volts per second. For a sinusoidal voltage v = V_m sin(ωt), the current leads voltage by 90° and has peak value

where V_m is the voltage peak and ω = 2πf.

6.1 Opening an energized capacitive load

When a breaker interrupts a capacitive current, the source-side voltage continues to follow the source waveform, while the isolated load-side capacitor may remain charged near the voltage existing at interruption. The voltage difference across the contacts can therefore approach twice the phase-to-ground peak under ideal conditions. In a three-phase system, unequal interruption and coupling can produce even more complex stresses.

If the gap breaks down, the capacitor discharges through the breaker and may become charged with opposite polarity. This is called voltage escalation when repeated restriking progressively increases the capacitor voltage. Capacitor-bank breakers must also manage inrush current during energization and high-frequency currents during back-to-back bank switching.

6.2 Control measures

Capacitive switching duties are controlled by:

• restrike-free circuit breakers;
• pre-insertion resistors;
• controlled closing and opening;
• grading or damping reactors;
• surge arresters; and
• proper capacitor-bank configuration and grounding.

The term “restrike-free” means that the breaker has demonstrated acceptable performance for the relevant capacitive-switching test duty; it does not mean that breakdown is physically impossible under every abnormal condition.

7. Inductive-Current Switching

Inductive loads include transformers, motors, shunt reactors, and long unloaded lines viewed from certain switching locations. An inductor resists rapid changes in current, according to

where v_L is the inductor voltage, L is inductance, and di/dt is the rate of current change.

If current is interrupted abruptly, di/dt becomes large and the induced voltage may become excessive. In practical circuits, capacitances provide an alternative path, producing an oscillatory LC transient. Current chopping is therefore closely associated with inductive switching.

Transformer energization presents a different problem. The closing instant and residual flux determine the transient flux. If the core flux exceeds its normal range, a large magnetizing inrush current flows. Controlled switching can select a closing instant that reduces the flux offset. For a transformer, the voltage-current relationship is approximately

where v is winding voltage, N is the number of turns, and Φ is core flux in webers. Integrating voltage over time determines the resulting flux, so the point on the voltage wave at which energization occurs is important.

8. Switching Surges

A switching surge is a transient overvoltage or overcurrent caused by a change in network configuration. It may arise from fault clearing, line energization, transformer energization, capacitor switching, reactor switching, load rejection, or reclosing.

Switching surges differ from lightning surges in duration and energy. Lightning impulses are usually shorter and steeper, while switching surges may last from several hundred microseconds to several milliseconds. At high transmission voltages, switching surges can determine insulation coordination and tower or substation clearances.

8.1 Line energization and reclosing

When an unloaded line is energized, travelling waves propagate along the line. Reflections at the remote open end and at discontinuities can produce voltage doubling or other amplification. The actual maximum depends on source impedance, line length, shunt compensation, trapped charge, breaker pole scatter, and the closing angle.

Reclosing an unloaded line is especially demanding because the line may retain a trapped DC voltage after opening. If the source voltage has opposite polarity at reclosing, the initial voltage difference can be large. Controlled switching, pre-insertion resistors, and surge arresters reduce this duty.

8.2 Insulation coordination

Insulation coordination selects equipment insulation levels and protective devices so that expected overvoltages remain below withstand limits with an acceptable margin. The coordination process considers:

1. the source and load configuration;
2. possible switching and lightning surges;
3. arrester protective characteristics;
4. equipment impulse withstand level;
5. grounding and shielding; and
6. statistical variation in surge magnitude.

A surge arrester does not eliminate a surge; it limits the voltage by conducting current and dissipating energy. Its continuous operating voltage, temporary overvoltage capability, discharge current, and energy rating must match the system.

9. Interruption in AC Circuits

AC interruption benefits from natural current zeros. In a symmetrical short circuit, current can be represented approximately as

where I_m is the AC current peak, φ is the phase angle, I_dc is the initial DC offset, and τ = L/R is the circuit time constant for a series RL model. The decaying DC component produces an asymmetrical current and increases the first peak and making duty.

At current zero, the arc is extinguished if its conductance falls sufficiently and if the gap withstands the TRV. The first few microseconds are critical because the thermal and dielectric recovery rates are finite. Breaker design therefore combines:

• adequate contact opening speed;
• controlled arc lengthening;
• cooling and deionization;
• suitable pressure and gas flow;
• field grading; and
• coordination with the network TRV.

The short-circuit interrupting rating is not simply the RMS fault current. It is associated with a specified voltage, frequency, X/R ratio, power factor, TRV, and making-current requirement.

10. Interruption in DC Circuits

DC interruption is more difficult because a steady DC current has no natural current zero. The breaker must create an artificial zero or force the current into another path. The inductive energy stored in the circuit must also be dissipated safely.

For a series RL circuit supplied by a DC source,

where V is source voltage, R is resistance, L is inductance, and i is current. When the circuit is interrupted, the inductive voltage can rise sharply unless a commutation or absorption path is provided.

DC breakers use several techniques:

• series arcs with magnetic blowout;
• commutation branches that transfer current to capacitors or resistors;
• resonant circuits that create an artificial current zero;
• power-electronic switches with forced commutation; and
• hybrid breakers combining mechanical and semiconductor paths.

In a forced-current-zero breaker, a capacitor is connected so that its discharge current opposes the fault current. The breaker interrupts when the resultant current reaches zero, while an arrester or resistor absorbs the remaining energy.

DC systems also exhibit polarity and voltage-dependent interruption behavior. A breaker rated for AC cannot automatically be assumed suitable for DC at the same voltage, because its arc extinction relies on AC current zero and its contact spacing may be insufficient for the DC recovery voltage.

11. Comparison of AC and DC Interruption

12. Worked Application: Switching a Shunt Reactor

Consider an illustrative single-phase shunt-reactor circuit with inductance L = 0.50 H, effective capacitance C = 0.005 μF, and a possible current-chopping value of I_c = 4 A. Estimate the idealized peak transient voltage.

First convert capacitance:

C = 0.005 × 10^-6 F = 5 × 10^-9 F.

Using the energy-equivalence expression,

The ratio is 1.0 × 10⁸, whose square root is 10,000. Therefore,

V_max ≈ 4 × 10,000 = 40,000 V = 40 kV.

This is an idealized value. System resistance, nonlinear arc behavior, arrester conduction, distributed capacitance, and transformer saturation would change the actual waveform. Nevertheless, the calculation shows why a seemingly small chopped current can generate a substantial overvoltage in a highly inductive, low-capacitance circuit.

13. Illustrative Case Study: Energization and Reclosing of an EHV Line

Background

A 400 kV transmission line connects a generating station to a remote receiving substation. The line is normally unloaded during a maintenance restoration. The line breaker uses conventional closing control, and the remote end is open.

Problem

During energization, the receiving-end voltage rises above the insulation-coordination study value. During a subsequent unsuccessful reclosing operation, a higher transient appears because trapped charge remains on the line.

Analysis

The open line behaves primarily as a distributed capacitance. Energization launches a travelling wave that reflects at the open remote end. During reclosing, the line voltage is not necessarily zero; it may retain a trapped charge determined by the opening instant and leakage. If the source voltage has opposite polarity, the breaker contacts experience a large initial voltage difference. Pole scatter can cause the three phases to close at different times, producing negative- and zero-sequence transients.

A digital transient study should model the source impedance, line frequency dependence, remote-end transformer, shunt reactors, breaker pole scatter, trapped charge, and arrester characteristics. A simple lumped-capacitance model is useful for understanding but may not predict the maximum voltage on a long line.

Approach and solution

The utility applies controlled closing based on measured source and line voltages. Shunt reactors reduce the steady-state charging current and help control the line voltage. Surge arresters are installed at both terminals. The reclosing sequence includes a dead-time sufficient for fault extinction and a check that residual line voltage is within the breaker’s controlled-switching window.

Outcome

The calculated maximum switching surge is reduced below the specified insulation-coordination level, and the breaker’s capacitive-switching duty remains within its tested rating. Monitoring confirms that arrester energy is acceptable during representative switching operations.

Learning points

1. An unloaded line can create severe voltage stress despite carrying little current.
2. Trapped charge makes reclosing more demanding than first energization.
3. Distributed travelling-wave models may be required for long lines.
4. Controlled switching must be coordinated with surge arresters and insulation levels.
5. Field measurements and validated electromagnetic-transient simulations complement breaker type tests.

14. Practical Design and Diagnostic Considerations

Protection engineers should distinguish the fault-current duty from the switching duty. A breaker may successfully interrupt a high short-circuit current but still require special evaluation for transformer, reactor, capacitor-bank, or line switching.

Important engineering practices include:

• obtain accurate network equivalent inductance, capacitance, resistance, and X/R ratio;
• evaluate the worst credible TRV, not only the nominal system voltage;
• include breaker pole scatter and prestrike or restrike behavior in studies;
• coordinate arresters with equipment insulation and breaker characteristics;
• inspect contact wear, gas pressure, vacuum integrity, operating mechanism timing, and travel curves;
• use calibrated high-voltage probes and suitable bandwidth for transient measurements; and
• validate electromagnetic-transient models against factory tests or field records.

Digital protection and monitoring systems can identify abnormal switching times, repeated restrikes, or excessive arrester energy. Artificial intelligence may assist in condition monitoring by detecting patterns in coil current, travel data, acoustic emissions, and high-frequency transient records. However, AI predictions should support—not replace—type-test evidence, engineering judgment, safety rules, and applicable standards.

15. Challenges, Limitations, and Safety

Simplified equations are valuable for first estimates but cannot represent every breaker phenomenon. Actual interruption involves distributed parameters, nonlinear arc conductance, moving contacts, frequency-dependent losses, magnetic saturation, statistical pole scatter, and complex gas or vacuum behaviour.

Safety is fundamental. Switching-transient testing involves lethal voltage, high fault energy, explosive mechanical forces, and electromagnetic interference. Tests must be performed under approved procedures with remote operation, interlocking, grounding, safe discharge of capacitors, and verified isolation. Relevant national and international standards should be consulted for breaker ratings, type tests, TRV envelopes, insulation levels, and capacitive-switching duties.

Environmental governance is also relevant. The selection of interrupting media must consider global-warming impact, leakage control, end-of-life recovery, worker exposure, and lifecycle emissions. Vacuum interruption and alternative gas technologies are important developments, but they must be evaluated for interruption performance, manufacturing complexity, reliability, and serviceability rather than by environmental claims alone.

Chapter Summary

Arc interruption is governed by the competition between arc extinction and the voltage recovery imposed by the network. Contact separation produces an ionized conducting path, and successful interruption requires current zero together with sufficiently rapid dielectric recovery. In AC systems, natural current zeros assist interruption, but the breaker must withstand the transient recovery voltage and its rate of rise. Restriking occurs when the applied transient voltage exceeds the recovering dielectric strength.

Current chopping is early interruption of an inductive current and can convert stored magnetic energy into a large capacitive overvoltage. Capacitive switching produces stresses from trapped charge, restriking, voltage escalation, and inrush or high-frequency currents. Inductive switching produces high voltages because an inductor opposes rapid current change. Switching surges arise during fault clearing, line energization, reclosing, transformer energization, capacitor switching, and load rejection.

DC interruption is more difficult than AC interruption because there is no natural current zero. DC breakers therefore use forced commutation, magnetic blowout, semiconductor assistance, or energy-absorption devices. Practical breaker selection requires consideration of short-circuit current, TRV, RRRV, making current, capacitive and inductive switching duties, insulation coordination, arrester performance, and environmental and safety requirements.

Key Terms

Arc: Ionized conducting path between separated electrical contacts.

Arc extinction: Loss of arc conductivity so that current cannot continue.

Current zero: Instant at which instantaneous AC current becomes zero.

Dielectric recovery: Increase in voltage withstand capability of the contact gap after current interruption.

Recovery voltage: Voltage appearing across breaker contacts after current interruption.

Transient recovery voltage (TRV): Transient component of the recovery voltage immediately after current zero.

Restriking voltage: Transient voltage that may cause a previously extinguished arc to re-establish.

Reignition: Reappearance of an arc shortly after current zero.

RRRV: Rate of rise of recovery voltage.

Current chopping: Interruption of current before its natural current zero.

Capacitive switching: Switching of capacitor banks, cables, or unloaded lines.

Inductive switching: Switching of reactors, transformers, motors, or other inductive loads.

Switching surge: Transient overvoltage or overcurrent caused by a network switching operation.

Trapped charge: Residual voltage retained by an isolated capacitive circuit.

Voltage escalation: Progressive increase in capacitor voltage caused by repeated restriking.

Natural frequency: Oscillation frequency determined principally by equivalent inductance and capacitance.

Artificial current zero: A forced zero created in a DC or other non-natural interruption circuit.

Insulation coordination: Selection and coordination of insulation and protective devices for expected overvoltages.

Pre-insertion resistor: Resistor temporarily inserted during closing to limit transient current or voltage.

Surge arrester: Nonlinear protective device that limits overvoltage by conducting surge current.

Review Questions

Short Answer Questions

1. Why does an arc form when circuit-breaker contacts separate?
2. What is meant by dielectric recovery?
3. Define transient recovery voltage and RRRV.
4. Why is current chopping hazardous when switching a reactor?
5. Why is DC interruption more difficult than AC interruption?

Long Answer Questions

1. Explain the complete sequence of arc formation, current zero, thermal recovery, and dielectric recovery.
2. Discuss the origin of TRV and the factors that determine its magnitude and rate of rise.
3. Explain capacitive-current switching, trapped charge, restriking, and voltage escalation.
4. Compare the interruption mechanisms and breaker requirements for AC and DC systems.
5. Describe switching surges on an unloaded transmission line and methods used to control them.

Analytical / Application Questions

1. An inductive circuit has L = 0.20 H, effective capacitance C = 2 μF, and chopped current I_c = 3 A. Using the ideal energy method, estimate the peak transient voltage and state the assumptions.
2. A breaker interrupts a capacitor-bank current at current zero, but the load side remains charged. Explain how a restrike can reverse the capacitor voltage and why repeated restrikes may escalate the voltage.
3. A DC feeder contains substantial inductance. Propose a breaker interruption strategy, identify the energy-absorption path, and explain why an ordinary AC breaker rating is insufficient.

Multiple Choice Questions with Answers

Question 1

Which condition is essential for successful AC arc interruption?

A. The contact gap must remain at zero voltage

B. The dielectric recovery must exceed the applied recovery voltage

C. The load current must increase after contact separation

D. The circuit inductance must be eliminated

Correct Answer: B. The dielectric recovery must exceed the applied recovery voltage

Question 2

The natural frequency of an ideal LC circuit is given by:

A. f₀ = 2π√(LC)

B. f₀ = L/C

C. f₀ = 1/(2π√(LC))

D. f₀ = C/L

Correct Answer: C. f₀ = 1/(2π√(LC))

Question 3

Current chopping is most directly associated with interruption of:

A. A purely resistive load

B. A small inductive current before natural current zero

C. A balanced capacitor current at voltage peak only

D. A zero-current circuit

Correct Answer: B. A small inductive current before natural current zero

Question 4

For an ideal inductive circuit undergoing current chopping, the approximate peak overvoltage is:

A. V_max = I_c√(L/C)

B. V_max = I_c√(C/L)

C. V_max = L/(I_cC)

D. V_max = I_cLC

Correct Answer: A. V_max = I_c√(L/C)

Question 5

For a sinusoidal capacitor voltage, the capacitive current leads the voltage by:

A. 0°

B. 45°

C. 90°

D. 180°

Correct Answer: C. 90°

Question 6

What is the principal reason DC interruption requires special techniques?

A. DC always has two natural current zeros per cycle

B. DC has no natural current zero in steady operation

C. DC contains no inductive energy

D. DC breakers never experience recovery voltage

Correct Answer: B. DC has no natural current zero in steady operation

Question 7

A restrike occurs when:

A. The breaker closes before fault initiation

B. The contact gap breaks down after an initial arc extinction

C. The current remains exactly zero permanently

D. The system frequency becomes zero

Correct Answer: B. The contact gap breaks down after an initial arc extinction

Question 8

Which device limits switching overvoltage by conducting surge current?

A. Surge arrester

B. Current transformer only

C. Synchronizing relay only

D. Isolator without an arc path

Correct Answer: A. Surge arrester

Question 9

An unloaded transmission line is especially vulnerable to switching surges because it behaves substantially as:

A. A short circuit with zero capacitance

B. A capacitive distributed network with travelling-wave reflections

C. A purely resistive load

D. A DC generator

Correct Answer: B. A capacitive distributed network with travelling-wave reflections

Question 10

If L = 20 mH and C = 0.02 μF, the ideal LC natural frequency is approximately:

A. 80 Hz

B. 796 Hz

C. 7.96 kHz

D. 79.6 MHz

Correct Answer: C. 7.96 kHz