Surge Protection Calculator Formula

Understand the math behind the surge protection calculator. Each variable explained with a worked example.

Use the Surge Protection Calculator

Formulas Used

Clamping Voltage

clamping_voltage = v_clamp

Energy per Surge

energy_joules = surge_energy

System Peak Voltage

peak_system = v_peak

Peak Surge Power

peak_power_kw = v_clamp * surge_current_ka

Variables

VariableDescriptionDefault
system_voltageSystem Voltage (RMS)(V)230
surge_current_kaExpected Surge Current(kA)10
surge_duration_usSurge Duration(µs)20
clamping_ratioClamping Voltage Ratio2.5
v_peakDerived value= system_voltage * sqrt(2)calculated
v_clampDerived value= v_peak * clamping_ratiocalculated
surge_energyDerived value= v_clamp * surge_current_ka * 1000 * surge_duration_us * 1e-6calculated

How It Works

Surge Protection Device Sizing

SPDs must absorb surge energy while limiting voltage to a safe clamping level that protects downstream equipment.

Formulas

V_clamp = V_peak x Clamping Ratio

Energy = V_clamp x I_surge x Duration

The standard test waveform for SPDs is 8/20 microseconds (8 us rise, 20 us to half-value).

SPD Categories (IEC 61643)

  • Type 1: Lightning current (10/350 us), installed at service entrance
  • Type 2: Surge current (8/20 us), at distribution panel
  • Type 3: Combination wave, at point of use

    • Worked Example

    230 V system, 10 kA surge (8/20 us), MOV clamping ratio 2.5x.

    system_voltage = 230surge_current_ka = 10surge_duration_us = 20clamping_ratio = 2.5
    1. 01System peak: 230 x sqrt(2) = 325 V
    2. 02Clamping voltage: 325 x 2.5 = 813 V
    3. 03Energy: 813 x 10,000 x 20e-6 = 162.6 J
    4. 04Peak power: 813 x 10 = 8,130 kW

    Frequently Asked Questions

    What is an 8/20 microsecond waveform?

    A standard test surge that rises to peak in 8 microseconds and decays to half-value in 20 microseconds. It simulates induced lightning surges.

    How do I choose between MOV and TVS?

    MOVs handle high energy and are used in power systems. TVS diodes have faster response and lower clamping, ideal for sensitive electronics.

    How many surges can an SPD handle?

    MOV-based SPDs degrade with each surge. Rating should include expected surge count over the device lifetime, with replacement indicators.

    Ready to run the numbers?

    Open Surge Protection Calculator