Rated Short Time Withstand Current Calculation

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Rated Short Time Withstand Current (Icw) Calculator

Calculate equivalent current withstand levels for different time durations based on thermal stress (I²t).

Thermal Energy (I²t) Constant:
Equivalent Withstand Current (I2):

Note: This calculation assumes the adiabatic condition where thermal energy remains constant.

What is Rated Short Time Withstand Current (Icw)?

The Rated Short Time Withstand Current (Icw) is a critical parameter for electrical switchgear, busbars, and circuit breakers. It defines the maximum amount of RMS current a component can safely carry for a specific duration (typically 1 or 3 seconds) without suffering mechanical deformation or thermal damage.

This rating is essential for ensuring that protective devices have enough time to clear a fault elsewhere in the system without the switchgear itself failing due to the massive electromagnetic forces and heat generated during a short circuit.

The Constant Thermal Stress Principle (I²t)

In electrical engineering, for short durations (usually less than 5 seconds), we assume an adiabatic process. This means that all heat generated by the fault current is absorbed by the conductor and none is dissipated into the environment. Under this assumption, the thermal stress is constant:

I₁² × t₁ = I₂² × t₂

Where:

  • I₁ = Rated Withstand Current (kA)
  • t₁ = Rated time duration (seconds)
  • I₂ = New equivalent current (kA)
  • t₂ = New time duration (seconds)

Example Calculation

If a circuit breaker is rated for 40 kA for 1 second, but your system requires a protection duration of 3 seconds, what is the reduced current it can withstand?

Using the formula:

I₂ = I₁ × √(t₁ / t₂)
I₂ = 40 × √(1 / 3)
I₂ = 40 × 0.577
I₂ = 23.09 kA

Therefore, a device rated for 40kA/1s is only thermally capable of handling 23.09kA if the fault persists for 3 seconds.

Importance in System Coordination

Properly calculating Icw ensures:

  • Protection Coordination: Ensuring the upstream breaker stays closed long enough for the downstream breaker to trip.
  • Equipment Safety: Preventing busbar melting or support insulator shattering due to magnetic stress.
  • Standards Compliance: Meeting IEC 61439 (for low voltage switchgear) or IEC 62271 (for high voltage) requirements.
function calculateIcw() { var i1 = parseFloat(document.getElementById("ratedCurrent").value); var t1 = parseFloat(document.getElementById("ratedTime").value); var t2 = parseFloat(document.getElementById("targetTime").value); var resultDiv = document.getElementById("resultsArea"); var thermalSpan = document.getElementById("thermalEnergy"); var currentSpan = document.getElementById("equivalentCurrent"); if (isNaN(i1) || isNaN(t1) || isNaN(t2) || i1 <= 0 || t1 <= 0 || t2 <= 0) { alert("Please enter valid positive numbers for all fields."); return; } // Calculation: I1^2 * t1 = I2^2 * t2 // I2 = sqrt((I1^2 * t1) / t2) var thermalEnergy = Math.pow(i1, 2) * t1; var i2 = Math.sqrt(thermalEnergy / t2); thermalSpan.innerHTML = thermalEnergy.toFixed(2) + " kA²s"; currentSpan.innerHTML = i2.toFixed(2) + " kA"; resultDiv.style.display = "block"; }

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