Free Tool · IEEE C62.22 / IEC 60099-5 · Insulation Coordination · Cross-Discipline

Insulation Coordination

MOV arrester selection per IEEE C62.22 (Metal-Oxide Surge Arrester Application Guide) and IEC 60099-4/5. Computes lightning/switching protective margins (PM_L/PM_S), TOV capability check, energy duty check, separation distance correction, and generates an arrester selection summary table. Universal HV/MV tool — solar context: arrester selection for PCS transformers, substation switchgear, and collection system.

System & Equipment
Equipment Withstand Levels
BSL = Basic Switching Level — switching impulse withstand of the protected equipment.
IEEE C62.22 §5: PM_L ≥ 20% for self-restoring insulation (transformers, cable terminations), PM_S ≥ 15% for switching impulse withstand.
Arrester Parameters
MCOV = Maximum Continuous Operating Voltage — must be ≥ system line-to-ground voltage × 1.05.
Physical distance between the surge arrester and the protected equipment. Larger separation increases travelling-wave magnification at the equipment terminals.
Arrester Protective Levels (datasheet)
LPL = Lightning Protective Level from manufacturer datasheet (discharge voltage at 10 kA for Distribution/Intermediate, at 20 kA for Station class).
SPL = Switching Protective Level from manufacturer datasheet (discharge voltage at 500 A switching current). Typically ≈ 1.6–2.0× LPL for ZnO arresters.
Temporary Overvoltage (TOV)
Expected temporary overvoltage in kV rms — import from a TOV Study or enter manually. Typically 1.3–1.6× MCOV for fault conditions.
Duration of the expected TOV event in seconds. Used with IEEE C62.22 TOV-withstand curve to determine allowable overvoltage.
Arrester rated voltage (Ur) from manufacturer datasheet — the voltage rating used to determine TOV withstanding capability.
Energy Duty
Surge energy duty in kJ per kV of MCOV — from TOV Study switching surge analysis. Default 5 kJ/kV is typical for switching surges.
Override arrester class auto-detection. "Auto" determines class from system voltage and LPL.
Coordination Results
Lightning protective margin PM_L
—
≥ 20% for self-restoring insulation
Switching protective margin PM_S
—
≥ 15% for switching impulse
Effective LPL at equipment terminals—
Effective SPL at equipment terminals—
Corrected PM_L at equipment—
Corrected PM_S at equipment—
One-way travel time (τ)—
TOV Capability
—
Energy Duty
—
Recommended arrester class—
Arrester Selection Table
Standard MCOV ratings sorted from minimum recommended upwards. Green rows pass all checks. The first green row is the recommended fit.
MCOV (kV) Rated V (kV) LPL (kVp) SPL (kVp) PM_L (%) PM_S (%) TOV Energy Rec.
IEEE C62.22 §5: PM_L ≥ 20% (self-restoring), PM_S ≥ 15%. TOV per IEEE C62.22 §6. Energy duty per IEC 60099-4.
Export coordination report
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Worked Example — 11 kV cable-fed substation
PASS Adequate insulation coordination — all criteria PASS. PM_L=265.4% ≥ 20%, PM_S=476.9% ≥ 15%, TOV within capability, energy within class rating.
Lightning protective margin PM_L
265.38%
≥ 20% for self-restoring insulation
Switching protective margin PM_S
476.92%
≥ 15% for switching impulse
TOV CapabilityPASS — 1.8× rated voltage
Energy DutyPASS — 42 kJ
Recommended arrester classDistribution class (IEEE C62.22 Class A)
Methodology & Formula Reference

1. Lightning Protective Margin (PM_L)
PM_L = (BIL / LPL − 1) × 100% — IEEE C62.22 §5
Pass if PM_L ≥ 20% for self-restoring insulation.

PM_L = (BIL / LPL - 1) × 100
pm_l_pass = PM_L >= 20

2. Switching Protective Margin (PM_S)
PM_S = (BSL / SPL − 1) × 100% — IEEE C62.22 §5
Pass if PM_S ≥ 15% for switching impulse.

PM_S = (BSL / SPL - 1) × 100
pm_s_pass = PM_S >= 15

3. Separation Distance Voltage Correction
ΔV due to travel time: ΔV = 2 × τ × (rate of rise)
Effective level at equipment = datasheet level + ΔV

τ = d / v   (one-way travel time in µs)
ΔV_L = 2 × τ × (LPL / 1 µs)   (lightning, 1 µs front)
ΔV_S = 2 × τ × (SPL / 50 µs)  (switching, 50 µs front)
effectiveLPL = LPL + ΔV_L
effectiveSPL = SPL + ΔV_S

4. Corrected Protective Margins at Equipment
Margins using effective protective levels at equipment terminals.

correctedPM_L = (BIL / effectiveLPL - 1) × 100
correctedPM_S = (BSL / effectiveSPL - 1) × 100

5. TOV Capability Check
IEEE C62.22 §6 TOV-withstand curve: duration → allowable factor × rated voltage.
E.g. 1 s → 1.8×, 10 s → 1.3×, 60 s → 1.2×, 3600 s → 1.0×.

allowableTOV = factor(tovDurationS) × arresterRatedVoltageKV
tovCapabilityPass = tovExpectedKV <= allowableTOV

6. Energy Duty Check
Expected energy = energyDutyKJkV × MCOV (kJ)
Class thresholds: Distribution ≤ 1500 kJ, Intermediate ≤ 2500 kJ, Station ≤ 4000 kJ.

energyRequired_kJ = energyDutyKJkV × mcovKV
energyDutyPass = energyRequired_kJ <= classEnergyThreshold

7. Arrester Class Selection (IEEE C62.22)
Distribution (Class A): system ≤ 52 kV AND LPL ≤ 100 kVp
Intermediate (Class B): 52 kV < system ≤ 345 kV OR LPL > 100 kVp
Station (Class C): system > 345 kV OR LPL > 150 kVp

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Related Tools
TOV Study (IEEE C37.06) Arc Flash Study (IEEE 1584) Relay Coordination (IEC 60947-2) Busbar Rating (IEC 60871-1)

FAQ

What is the difference between PM_L and protective ratio (BIL/APL)?
PM_L (Protective Margin for Lightning) = (BIL/LPL − 1) × 100%, where LPL is the arrester Lightning Protective Level from the manufacturer datasheet. The protective ratio BIL/APL uses the arrester protective level (APL = max(MCOV, discharge voltage)), which may differ from LPL for station-class arresters. IEEE C62.22 §5 specifies PM_L ≥ 20% for self-restoring insulation (oil-filled transformers, cable terminations) and PM_S ≥ 15% for switching impulse.
Why does separation distance affect the protective level at equipment?
When a surge travels from the arrester to the protected equipment, the voltage at the equipment terminals can exceed the arrester limiting level due to travelling-wave reflection and propagation delay. IEEE C62.22 gives the voltage adder ΔV ≈ 2 × (separation/v) × (rate of rise). For lightning surges (1 µs front), even 50 m of cable at 300 m/µs adds ≈ 0.33 µs travel time × 2 × (LPL/1 µs) = significant correction. The corrected PM_L uses the effective LPL at equipment terminals.
How does the TOV capability check work in IEEE C62.22?
IEEE C62.22 §6 defines a TOV-withstand curve (similar to IEC 60099-4 thermal stability curve) mapping duration (0.001 s to 3600 s) to allowable overvoltage factor × arrester rated voltage. For example: 1 s → 1.8× rated voltage, 10 s → 1.3×, 60 s → 1.2×, 3600 s → 1.0×. If the expected TOV in kV rms exceeds the allowable level for the given duration, the arrester may lose its protective characteristic or overheat. Increase rated voltage or reduce exposure.
When should I choose Station vs Intermediate vs Distribution class arrester?
Distribution class (IEEE C62.22 Class A): systems ≤ 52 kV with LPL ≤ 100 kVp, energy duty < 1500 kJ — typical 11 kV, 22 kV, 33 kV cable-fed or overhead networks. Intermediate class (Class B): systems 52–345 kV or LPL > 100 kVp, energy duty < 2500 kJ — standard for 33 kV, 66 kV, 110 kV substations and primary substations. Station class (Class C): systems > 345 kV or LPL > 150 kVp, energy duty ≥ 4000 kJ — required for 132 kV, 220 kV, 400 kV substations and EHV/HV networks. Station-class arresters have the highest discharge current rating (20 kA) and lowest protective level.
What is the solar-specific context for this tool?
Solar PV collection systems at 33 kV use Distribution/Intermediate class arresters on PCS (Power Conversion System) transformer bushings, substation switchgear, and cable terminations. The 33 kV solar PCS transformer is typically protected with a 24 kV or 30 kV MCOV arrester (LPL ≈ 78 kVp). For large utility-scale BESS or hybrid plants at 132 kV or 220 kV, Station class arresters are used at the point of common coupling (PCC) and main substation bus. This tool applies to any HV/MV system — solar context is noted where relevant.
How is energy duty determined from the TOV study?
The energy duty (kJ/kV of MCOV) represents the surge energy the arrester must absorb during a switching transient. It is derived from the TOV study which models the system impedance, fault clearing time, and surge energy injection. Typical values: 4–6 kJ/kV for switching surges in 33 kV systems, 8–12 kJ/kV for EHV systems (220 kV+). The energy check compares expected energy (kJ/kV × MCOV) against the arrester class rating: Distribution ≤ 1500 kJ, Intermediate ≤ 2500 kJ, Station ≤ 4000 kJ. If exceeded, use a higher class arrester.