Lightning-Induced Overvoltages on Lines (Part One)

Section 1

Two Kinds of Lightning Overvoltage

This page explains how lightning that strikes near an overhead line — not the line itself — still induces dangerous voltages on the conductors, and how that induced-voltage flashover rate (IVFOR) is estimated.

Lightning overvoltages on overhead lines fall into two categories:

Direct-stroke overvoltages — the stroke terminates directly on a phase conductor, overhead ground wire, tower, pole or another part of the line structure.
Induced overvoltages — the stroke terminates on the ground or a nearby object, not on the conductor. The electromagnetic field of the return stroke couples to the line and induces voltage on the conductors.

This page focuses on induced overvoltages. They matter most for distribution lines, whose insulation level is relatively low: many have no overhead ground wire, so even a nearby stroke can drive enough voltage to flash an insulator over.

For higher-voltage transmission lines, induced overvoltages are normally less dominant because the insulation level is much higher — there, a direct stroke to the line or tower (and a possible backflashover) usually governs. Historically, before ~1930, line design was thought to be driven mainly by nearby strokes and induced voltages; the later direct-stroke theory shifted transmission-line performance toward direct strokes to ground wires and towers. Induced overvoltages nonetheless remain very important for lower-voltage systems.

What this page teaches

why a nearby ground stroke induces voltage without touching the line;
the Rusck equation for the maximum induced voltage \(V_c\);
the flashover condition \(V_c \ge \text{CFO}\) and the maximum distance \(X_m\);
the critical stroke current \(I_{sc}\) and the induced-voltage flashover rate IVFOR;
how a neutral or ground wire — and its grounding — changes the insulation stress;
why induced overvoltages dominate on distribution lines but rarely on transmission lines.

Section 2

Physical Meaning of an Induced Overvoltage

When lightning strikes the ground near a line, a very large current flows upward through the channel during the return stroke. This current produces fast-changing electric and magnetic fields that travel outward and interact with the conductors, inducing a voltage on the phase conductor, neutral or ground wire.

The induced voltage depends mainly on:

lightning current magnitude;
distance between the stroke and the line;
height of the conductor above ground;
return-stroke velocity;
presence or absence of a neutral or ground wire, and the coupling between conductors;
the insulation withstand level of the line.

Key idea

A stroke does not need to hit the line directly to cause flashover. If the induced voltage across the insulation exceeds the line insulation strength, the insulator can flash over.

Section 3

The Two Parts of the Calculation

Return-stroke model

The return-stroke model represents the lightning channel and the current flowing in it. In simplified studies the channel is assumed vertical, straight, and carrying a current waveform that travels upward from the ground. It is used to estimate the electric and magnetic fields produced by the stroke.

Coupling model

The coupling model describes how those fields interact with the conductors. Several models exist — two well-known approaches are the Agrawal model and the Chowdhuri–Gross model. For engineering estimates, the simpler method developed by Rusck is often used: it gives a practical estimate of the maximum induced voltage and is simple enough for hand calculation and performance estimation.

Section 4

The Rusck Equation for Maximum Induced Voltage

For a single overhead conductor with no neutral or ground wire, the maximum induced voltage is estimated by the Rusck equation:

\[ V_c = \frac{30\,I\,h_c\,K_v}{x} \qquad\text{with}\qquad K_v = \frac{1}{\sqrt{1 - v^2}} \]

\(V_c\): maximum induced voltage on the phase conductor (kV, with consistent units)
\(I\): lightning stroke current (kA)
\(h_c\): height of the phase conductor above ground (m)
\(x\): horizontal distance from the stroke point to the line (m)
\(v\): return-stroke velocity as a fraction of the speed of light
\(K_v\): correction factor for the return-stroke velocity

Section 5

Reading the Rusck Equation

The Rusck form is useful because it shows the main physical relationships directly. With the other parameters fixed:

Current: \(V_c \propto I\) — doubling the current roughly doubles the induced voltage.
Conductor height: \(V_c \propto h_c\) — a taller conductor is more exposed to the field, so induced voltage is greater on higher lines.
Distance: \(V_c \propto \dfrac{1}{x}\) — a nearby stroke produces a large voltage; a distant stroke a much smaller one.

The velocity term \(K_v = 1/\sqrt{1-v^2}\) shows the return-stroke velocity also matters. A common assumption is \(v = 0.3\) (about 30% of light speed):

\[ K_v = \frac{1}{\sqrt{1 - 0.3^2}} = \frac{1}{\sqrt{0.91}} \approx 1.048 \]

Note on the velocity correction

For the usual assumed value \(v=0.3\), \(K_v\) is close to 1. The velocity correction is important theoretically, but for typical assumptions it does not strongly change the final result.

Section 6

Return-Stroke Velocity as a Function of Current

The velocity may also be approximated as a function of stroke current — stronger currents tend to travel faster:

\[ v = \frac{1}{1 + \dfrac{710}{I}} \]

\(v\): return-stroke velocity (fraction of light speed)
\(I\): stroke current (kA)

In many engineering studies, using a constant \(v = 0.3\) instead of a current-dependent velocity does not significantly change the estimated induced voltage. A constant \(v = 0.3\) is therefore usually acceptable unless a more detailed lightning-performance model is required.

Section 7

Critical Flashover Voltage (CFO) and the Flashover Condition

The Critical Flashover Voltage (CFO) is the voltage at which there is a 50% probability of flashover under a specified impulse waveshape. In lightning-performance studies it is used as an approximate insulation withstand level. Below the CFO, flashover is unlikely; at or above it, flashover may occur. The basic condition is:

\[ V_c \ge \text{CFO} \]

\(V_c\): induced voltage on the conductor
\(\text{CFO}\): critical flashover voltage of the insulation path

Section 8

Maximum Distance for Induced Flashover, \(X_m\)

Setting the Rusck voltage equal to the CFO and solving for \(x\) gives the maximum distance at which a stroke can still produce flashover:

\[ \text{CFO} = \frac{30\,I\,h_c\,K_v}{x} \;\;\Rightarrow\;\; X_m = \frac{30\,I\,h_c\,K_v}{\text{CFO}} \]

\(X_m\): maximum horizontal distance at which the induced voltage still equals the CFO (m)

For a stroke closer than \(X_m\), the induced voltage may exceed the CFO; beyond \(X_m\) it is lower. So induced flashover is possible for \(x < X_m\) and unlikely for \(x > X_m\).

Section 9

Striking Distance and the Boundary \(D_g\)

Not every nearby stroke terminates on the ground — some terminate directly on the conductor or structure. The striking-distance concept decides whether a stroke attaches to the conductor, the tower/pole, the ground wire, or the ground near the line.

The distance \(D_g\) is the boundary: closer than \(D_g\) the stroke may terminate on the line; beyond \(D_g\) it terminates on the ground and may produce an induced overvoltage. For induced-voltage flashover, a stroke must satisfy two conditions — it must terminate on the ground, and it must be close enough to induce more than the CFO. This defines the contributing range:

\[ D_g < x < X_m \]

\(D_g\): limiting distance below which the stroke may hit the line instead of the ground (m)
\(X_m\): maximum distance at which the induced voltage still exceeds the CFO (m)

Only strokes within this range contribute to induced-voltage flashover.

Section 10

Critical Stroke Current for Induced Flashover, \(I_{sc}\)

For small currents the induced voltage may be too low, and the stroke may not satisfy \(X_m > D_g\). The minimum current for which induced flashover becomes possible is the critical stroke current \(I_{sc}\), defined by:

\[ X_m = D_g \quad\text{at}\quad I = I_{sc} \]

For \(I < I_{sc}\), \(X_m < D_g\) and there is effectively no distance range in which ground strokes produce flashover. For \(I > I_{sc}\), \(X_m > D_g\) and a region near the line exists where ground strokes can cause induced flashover. Induced flashover is therefore not caused by every nearby stroke — only when the current is high enough and the location lies within the effective range.

Section 11

Induced-Voltage Flashover Rate (IVFOR)

The IVFOR is the number of flashovers caused by induced overvoltages over a given line length and period, usually in flashovers / 100 km-years. The general form is:

\[ \text{IVFOR} = 2\,N_g\,L \int_{I_{sc}}^{\infty} (X_m - D_g)\,f(I)\,dI \]

\(N_g\): ground flash density (flashes/km²/year)
\(L\): line length
\(I_{sc}\): minimum stroke current for which induced flashover is possible
\(X_m - D_g\): effective horizontal exposure distance on one side of the line
\(f(I)\): probability density of lightning stroke current

The factor 2 accounts for strokes on either side of the line.

Section 12

What the IVFOR Integral Means

The equation looks complex but the idea is simple. For each possible current:

find how far from the line a stroke can be and still cause flashover (\(X_m\));
subtract the distance where strokes would hit the line directly (\(D_g\));
multiply by line length and ground flash density;
weight by the probability of that current;
integrate over all currents above \(I_{sc}\).

In plain words, IVFOR counts how many nearby ground strokes are strong enough and close enough to flash the insulation over.

Why the current distribution is needed. Stroke current is not constant: \(f(I)\) describes how likely each current is. Low currents are common but may not induce enough voltage; high currents are rarer but can flash insulation over from farther away. IVFOR is therefore not based on one assumed current — it integrates over the full distribution.

Section 13

Effect of CFO and Conductor Height on IVFOR

From \(X_m = 30\,I\,h_c\,K_v/\text{CFO}\):

\(X_m \propto \dfrac{1}{\text{CFO}}\) — raising the CFO shrinks \(X_m\), so fewer nearby strokes can flash over. Low CFO → high IVFOR; high CFO → low IVFOR.
\(X_m \propto h_c\) — a taller conductor enlarges \(X_m\), so strokes farther away can still cause flashover. Conductor height increases exposure.

This is exactly why induced overvoltages matter more for distribution lines: their lower insulation level means a lower CFO, so even moderate induced voltages can exceed the withstand level. (In a complete study, height also affects direct-stroke attraction and shielding, so the relationship is not always simple.)

The driving relationship

\(X_m \propto h_c / \text{CFO}\). Insulation strength and conductor height set the size of the exposure band — and therefore the induced flashover rate.

Section 14

Effect of a Neutral or Ground Wire

A nearby neutral or overhead ground wire also picks up induced voltage. What stresses the insulation is normally the difference between the phase conductor and the grounded/neutral conductor, not the phase-to-remote-earth voltage. With an ungrounded neutral at a similar height, both conductors may rise together, and the difference — the insulation stress — can be small:

\[ V_i = V_c - V_g \]

\(V_i\): voltage across the insulation
\(V_c\): induced voltage on the phase conductor
\(V_g\): induced voltage on the neutral or ground wire

If the two are at different heights, their induced voltages differ — the higher conductor normally has the larger induced voltage. If the neutral or ground wire is grounded through a resistance, its voltage is controlled by the grounding path, which changes the stress across the insulation.

Section 15

Grounded Neutral or Ground Wire

When the neutral/ground wire is grounded through a footing resistance \(R\), part of its induced voltage discharges to earth, changing the difference between it and the phase conductor. The voltage across the insulation becomes:

\[ V_i = \frac{30\,I\,K_v}{x}\left[\, h_c - \frac{Z_m + 2R}{Z_g + 2R}\,h_g \,\right] \]

\(V_i\): voltage across the insulation
\(h_c,\ h_g\): heights of the phase conductor and the ground wire/neutral (m)
\(Z_m\): mutual surge impedance between phase and ground wire/neutral (Ω)
\(Z_g\): surge impedance of the ground wire/neutral (Ω)
\(R\): grounding or footing resistance (Ω)

The bracket acts as an effective height. For a single conductor the stress is proportional to \(h_c\); with a grounded neutral or ground wire it is proportional to \(\left[h_c - \frac{Z_m+2R}{Z_g+2R}h_g\right]\). The neutral or ground wire can therefore reduce the stress, because part of the induced voltage couples onto it.

\[ X_m = \frac{30\,I\,K_v}{\text{CFO}}\left[\, h_c - \frac{Z_m + 2R}{Z_g + 2R}\,h_g \,\right] \]

Setting \(V_i = \text{CFO}\) gives the maximum flashover distance with a grounded neutral or ground wire — the same form as before, but with the effective height replacing \(h_c\). A smaller bracket means a smaller \(X_m\) and fewer flashover-producing strokes.

Section 16

A Counterintuitive Point: Low Footing Resistance

For direct strokes to a tower or pole, reducing footing resistance is beneficial — it lowers tower potential rise and backflashover risk. For induced overvoltages the behaviour is different.

Watch out

For induced overvoltages, lower footing resistance can increase the voltage across the insulation. A well-grounded neutral or ground wire is held closer to earth potential, so the phase conductor still receives induced voltage while the grounded conductor's voltage is reduced — widening the difference across the insulation.

So: for direct strokes, low footing resistance is usually beneficial; for induced overvoltages it may increase the phase-to-grounded-conductor stress. This does not mean high footing resistance is preferred — only that induced overvoltages behave differently from backflashover.

It does not mean grounding should be made worse for line design. It only means the induced-voltage mechanism is different from the direct-stroke / backflashover mechanism. Overall grounding design must still consider safety, earth potential rise (EPR), backflashover, protection operation and utility practice — low footing resistance remains the correct default.

Section 17

Where the Neutral or Ground Wire Sits

Neutral below the phase conductor

On many distribution lines the neutral is below the phase, so \(h_c > h_g\). The phase is higher and receives the larger induced voltage. The neutral still reduces the insulation stress, but its lower position limits the benefit. A neutral below the phase can significantly cut the induced flashover rate compared with no neutral, but it is not as effective as an overhead ground wire above the phase.

Ground wire above the phase conductor

With the ground wire above the phase, \(h_g > h_c\). This is normally more effective: the higher ground wire receives a strong induced voltage and, through coupling, reduces the voltage across the phase insulation — and it can intercept direct strokes too. In the effective-height term, a larger \(h_g\) makes \(\frac{Z_m+2R}{Z_g+2R}h_g\) more significant, lowering the effective height and \(X_m\). Overhead ground wires above the phase are therefore generally more effective than lower neutrals at reducing induced-voltage flashover.

Section 18

Lines Without a Neutral, and the Direct-Stroke Threat

A line with no neutral or ground wire may see high conductor-to-ground induced voltage — but the actual flashover path matters. With no nearby grounded conductor, flashover may have to occur phase-to-earth, a path that can have a very high CFO (much higher than phase-to-neutral or phase-to-crossarm). So even a high conductor-to-earth voltage may not flash over unless a suitable insulation path exists — this is why a large calculated induced voltage does not automatically mean flashover. The more serious risk on such lines is a direct stroke to the phase conductor, which can become more severe than the induced-overvoltage case.

For a direct stroke to an ungrounded conductor, the launched voltage is approximately:

\[ V = \frac{I\,Z_g}{2} \]

\(V\): voltage launched on the conductor
\(I\): lightning current
\(Z_g\): surge impedance of the conductor/line path (Ω)

The factor of 2 arises because the surge travels away in two directions. For \(Z_g = 450\ \Omega\) and \(I = 10\ \text{kA}\):

\[ V = \frac{10 \times 450}{2} = 2250\ \text{kV} \]

This is far above the insulation level of many distribution systems — direct strokes to unshielded phase conductors are very severe.

Section 19

Protective Gaps and Surge Arresters

Surge arresters protect distribution equipment, but direct strokes can inject very large energy into them. A protective gap can be set to flash over at a controlled level, for example around 300–400 kV. The idea: most induced overvoltages stay below the gap level so the arrester handles normal surges, while a very severe direct-stroke surge flashes the gap over, limiting the voltage and easing arrester energy duty. It is a practical compromise between insulation coordination, arrester protection and line reliability.

Section 20

Waveshape of Induced Overvoltages

Induced overvoltages do not necessarily match the standard \(1.2/50\ \mu\text{s}\) lightning impulse. Measured induced waveforms may have a median 10–90% rise time around 1.6 µs (some below 1.1 µs) and a short time to half-value around 4–5 µs — rising quickly and decaying faster than the standard impulse.

Insulation strength depends on waveshape. For a non-standard short time-to-half waveshape, the effective CFO may be approximately:

\[ \text{CFO}_{\text{eff}} \approx 1.4 \times \text{CFO}_{\text{standard}} \]

Insulation may withstand a higher crest when the impulse is shorter — but this should be used carefully, only when the waveshape and insulation behaviour are properly understood.

Section 21

Induced Overvoltage versus Backflashover

It is important to separate the two mechanisms — they are driven by different things, and footing resistance acts oppositely.

Table 1 — Induced-overvoltage flashover compared with backflashover.

Item	Induced-Voltage Flashover	Backflashover
Cause	Stroke to ground / object near the line	Direct stroke to tower, pole or ground wire
Mechanism	EM field of the return stroke couples to conductors	Tower potential rise through footing resistance
Main drivers	Stroke current, distance, height, CFO, neutral/ground-wire layout, coupling	Footing resistance, current, tower surge impedance, span, shielding, insulation
Lower footing resistance	May increase phase-to-grounded-conductor stress	Reduces tower voltage and backflashover risk

The total lightning flashover performance combines both:

\[ \text{Total flashover rate} = \text{IVFOR} + \text{BFR} \]

For distribution lines, IVFOR can be a large part of the total — especially at low CFO. For transmission lines, BFR usually dominates where direct strokes to towers or shield wires govern. The balance depends on configuration, insulation, shielding, grounding and the local lightning environment.

Section 22

Typical Example Parameters and the Coupling Factor

The following example values illustrate the method — they are not universal design values.

Table 2 — Typical parameter set used in example calculations.

Parameter	Symbol	Value
Ground wire / neutral surge impedance	\(Z_g\)	450 Ω
Mutual surge impedance	\(Z_m\)	130.5 Ω
Coupling factor	\(C\)	0.29
Phase conductor height	\(h_c\)	10 m or 8 m
Ground wire / neutral height	\(h_g\)	10 m or 8 m
Footing resistance	\(R\)	20 Ω
Ground flash density	\(N_g\)	1 flash/km²/year
Return-stroke velocity	\(v\)	0.3 (fraction of \(c\))

The coupling factor is approximately the ratio of mutual to self surge impedance:

\[ C = \frac{Z_m}{Z_g} = \frac{130.5}{450} \approx 0.29 \]

A higher coupling factor means a stronger link between the two conductor voltages. Strong coupling can reduce the insulation stress because the conductors tend to move together electrically during the surge.

Section 23

The Exposure Width \(X_m - D_g\), and Typical Voltage Levels

The term \(X_m - D_g\) is central to IVFOR: it is the effective exposure width on one side of the line where a ground stroke can cause flashover. If \(X_m < D_g\) the difference is negative — there is no effective region for that current. If \(X_m > D_g\), the band between \(D_g\) and \(X_m\) contributes; the wider it is, the higher the IVFOR.

Why induced voltages are often a few hundred kV. Induced voltages are often thought to be around 300 kV or less — consistent with the inverse-distance behaviour of the Rusck equation. Although currents can be large, the voltage falls with distance and only part of the field couples into the line. So induced overvoltages are usually less severe than direct strokes, but still large enough to be critical for distribution insulation: 250–300 kV may be unimportant for an EHV transmission line yet very significant for an 11 kV distribution line.

Section 24

A Step-by-Step Method for Estimating IVFOR

Estimating IVFOR

Define line parameters — \(h_c\), \(h_g\), \(Z_g\), \(Z_m\), \(R\), CFO, \(L\) and \(N_g\).
Select the return-stroke velocity — commonly \(v = 0.3\), then \(K_v = 1/\sqrt{1-v^2}\).
Calculate \(X_m\) — without a ground wire use \(X_m = 30\,I\,h_c\,K_v/\text{CFO}\); with a grounded ground wire use the effective-height form.
Determine \(D_g\) from the chosen striking-distance model.
Find the critical current \(I_{sc}\) where \(X_m = D_g\).
Integrate over the current distribution: \(\text{IVFOR} = 2\,N_g\,L \int_{I_{sc}}^{\infty}(X_m - D_g)\,f(I)\,dI\).

The striking-distance model matters. Different equations (Brown–Whitehead, Love, Young, IEEE-92) give different \(D_g\). Since \(D_g\) appears directly in \(X_m - D_g\), the IVFOR can change significantly: a larger \(D_g\) shrinks the exposure band, a smaller \(D_g\) widens it. When comparing studies, always check which striking-distance model was used.

Section 25

Engineering Interpretation: Distribution vs Transmission

Distribution lines

Induced overvoltages are important here because insulation levels are low, neutrals may sit below the phases, many lines have no ground wire, arresters may face high energy duty, and nearby ground strokes are more frequent than direct strokes to any single small conductor. On wood-pole lines, raising insulation strength can substantially reduce IVFOR, though very high levels are not always economical. Protection usually combines a suitable insulation level, surge arresters, neutral grounding, shielding where justified, protective gaps in selected cases, good earthing and appropriate arrester spacing.

Transmission lines

Induced overvoltages are usually less dominant because the CFO is much higher, ground wires are normally installed, phase-to-tower withstand is high, and direct-stroke shielding and backflashover govern. They should not be ignored entirely, though — they can matter for lower-voltage sub-transmission, unshielded lines, low-CFO designs, special insulation configurations, lines near tall objects or forests, and areas of high ground flash density.

Section 26

Key Practical Lessons

The key lessons

Induced overvoltages come from nearby ground strokes, not direct strokes to the line.
The Rusck equation gives a simple estimate of the maximum induced voltage.
Induced voltage rises with current and with conductor height, and falls with distance from the stroke.
CFO strongly controls the flashover rate — low CFO gives high IVFOR.
A neutral or ground wire reduces the insulation stress; a ground wire above the phase is more effective than a neutral below it.
Low footing resistance helps backflashover but can increase the induced-voltage stress between phase and grounded conductor.
The striking-distance model strongly affects the calculated IVFOR.
Distribution lines are far more sensitive to induced overvoltages than transmission lines.

Section 27

Summary of Key Equations

These equations should not be read as isolated formulas — they form a chain. The Rusck equation gives the induced voltage \(V_c\); the CFO condition \(V_c \ge \text{CFO}\) decides whether flashover is possible; \(X_m\) sets the maximum dangerous stroke distance; \(I_{sc}\) sets the minimum stroke current that creates a valid exposure region; and IVFOR integrates that exposure over the lightning-current distribution. Read top to bottom, they are the full workflow from a single nearby stroke to a flashover rate.

Equation Summary

Rusck induced voltage

\( V_c = \dfrac{30\,I\,h_c\,K_v}{x} \)

Velocity correction

\( K_v = \dfrac{1}{\sqrt{1 - v^2}} \)

Current-dependent velocity

\( v = \dfrac{1}{1 + \frac{710}{I}} \)

Flashover condition

\( V_c \ge \text{CFO} \)

Max distance (no ground wire)

\( X_m = \dfrac{30\,I\,h_c\,K_v}{\text{CFO}} \)

Insulation stress (grounded GW)

\( V_i = \dfrac{30\,I\,K_v}{x}\!\left[h_c - \dfrac{Z_m+2R}{Z_g+2R}h_g\right] \)

Max distance (grounded GW)

\( X_m = \dfrac{30\,I\,K_v}{\text{CFO}}\!\left[h_c - \dfrac{Z_m+2R}{Z_g+2R}h_g\right] \)

Induced flashover rate

\( \text{IVFOR} = 2 N_g L\!\displaystyle\int_{I_{sc}}^{\infty}\!(X_m - D_g) f(I)\,dI \)

Direct-stroke voltage

\( V = \dfrac{I\,Z_g}{2} \)

Section 28

Final Engineering Understanding

Lightning-induced overvoltage is a coupling problem. The stroke does not need to hit the line; a nearby return stroke creates electromagnetic fields that induce voltage on the conductors. For a single conductor the whole idea is captured by \(V_c = 30\,I\,h_c\,K_v/x\): stronger current, taller conductor and closer stroke all raise the voltage, while the velocity term modifies it slightly.

Flashover occurs only when \(V_c \ge \text{CFO}\); the exposure region is \(D_g < x < X_m\); and the rate follows from integrating over the currents that satisfy it. A neutral or ground wire changes the problem because the stress becomes the difference between conductor voltages, modified by coupling and grounding — which is why the location and grounding of the neutral or ground wire matter.

For distribution systems, induced overvoltages can be a major source of flashover because the insulation level is low. For transmission, direct strokes and backflashover usually dominate, though induced voltages can still matter in special cases. The central conclusion: induced-overvoltage performance depends on both electromagnetic coupling and insulation coordination. Knowing the lightning current alone is not enough — line geometry, conductor height, neutral/ground-wire arrangement, CFO, grounding resistance and the current distribution must all be considered together.

Reader should remember

Induced voltage is mainly controlled by lightning current, conductor height, stroke distance and CFO. IVFOR is the result of combining the dangerous exposure distance \(X_m - D_g\) with the probability of lightning currents \(f(I)\) — not a single assumed stroke.

This is Part One of a two-part self-study series on lightning-induced overvoltages. Part Two extends the treatment to nearby objects — trees, forests and structures — with striking-distance models, worked flashover-rate examples, surge-arrester spacing and a field-data comparison.

Section 29

Glossary

Table 3 — Key terms used on this page.

Term	Meaning
Induced overvoltage	A voltage produced on an overhead conductor by the EM field of a nearby ground stroke.
Direct stroke	A stroke terminating directly on the line, tower, pole, ground wire or phase conductor.
Return stroke	The high-current upward wave in the channel after ground connection is established.
CFO	Critical Flashover Voltage — 50% flashover probability under a defined impulse.
IVFOR	Induced-Voltage Flashover Rate, normally per 100 km-years.
BFR	Backflashover Rate — flashovers from tower/pole potential rise after a direct stroke.
Ground flash density	Lightning flashes to ground per km² per year (\(N_g\)).
Surge impedance	Characteristic impedance seen by a travelling surge on a conductor.
Mutual surge impedance	Surge coupling impedance between two conductors (\(Z_m\)).
Coupling factor	A measure of how strongly two conductors are electromagnetically coupled (\(C\)).
Striking distance	The distance over which a leader can attach to a conductor, structure, ground wire or ground.

Lightning-Induced Overvoltages on Overhead Lines