Inverse Time Circuit Breaker Calculation

Inverse time circuit breakers are essential components in electrical systems that protect against overcurrents by tripping in a time-dependent manner. This calculator helps you determine the proper trip time for a given current and breaker settings.

What is Inverse Time Circuit Breaker?

An inverse time circuit breaker is a protective device that opens a circuit after a delay that varies inversely with the current. This type of breaker provides both instantaneous protection against very high currents and time-delayed protection against lower currents that might indicate a developing fault.

The inverse time characteristic is defined by the IEC 60255 standard, which specifies the relationship between current and tripping time. The most common standard curves are very inverse, extremely inverse, and long time inverse.

How to Calculate Inverse Time Trip

Calculating the trip time of an inverse time circuit breaker requires knowledge of the current multiplier (I), the time multiplier (T), and the standard curve type. The calculation follows the IEC 60255 standard formulas.

The basic formula for inverse time trip calculation is:

Trip Time (t) = T × (I / M)ⁿ

Where:

T = Time multiplier setting
I = Current multiplier setting
M = Current ratio (actual current / rated current)
n = Curve exponent (varies by curve type)

The curve exponent (n) varies depending on the standard curve type:

Very Inverse: n = 0.02
Extremely Inverse: n = 2
Long Time Inverse: n = 1

Formula and Assumptions

The complete formula for inverse time trip calculation is:

t = T × (I / M)ⁿ

Where:

t = Trip time in seconds
T = Time multiplier setting (typically 0.1 to 1)
I = Current multiplier setting (typically 0.5 to 1)
M = Current ratio (actual current / rated current)
n = Curve exponent (0.02 for very inverse, 2 for extremely inverse, 1 for long time inverse)

Assumptions:

Standard IEC 60255 curves are used
Rated current is the breaker's nominal current rating
Actual current is the measured current in the circuit
Multipliers are factory-set values for the breaker

Worked Example

Let's calculate the trip time for a very inverse curve breaker with the following parameters:

Rated current: 100A
Actual current: 150A (M = 1.5)
Time multiplier (T): 0.5
Current multiplier (I): 0.8
Curve type: Very Inverse (n = 0.02)

Using the formula:

t = 0.5 × (0.8 / 1.5)⁰·⁰²

t = 0.5 × (0.533)⁰·⁰²

t ≈ 0.5 × 0.97

t ≈ 0.485 seconds

This means the breaker would trip approximately 0.485 seconds after the fault current of 150A is detected.

Frequently Asked Questions

What is the difference between inverse time and inverse definite minimum time circuit breakers?: Inverse time breakers have a tripping time that varies with current, while inverse definite minimum time breakers have a fixed minimum trip time regardless of current.
How do I select the proper curve type for my application?: The curve type depends on the protection requirements. Very inverse curves are used for motor protection, extremely inverse for transformer protection, and long time inverse for feeder protection.
What happens if the current exceeds the breaker's rated current?: The breaker will trip according to its inverse time characteristic. The trip time will be shorter for higher currents, providing faster protection against faults.
Can I adjust the time and current multipliers?: Yes, most breakers allow adjustment of these multipliers to customize the trip characteristics for your specific application.
What safety precautions should I take when working with circuit breakers?: Always ensure the circuit is de-energized before working on breakers. Wear appropriate personal protective equipment and follow all safety guidelines in your electrical code.