How to read trip curve of circuit breaker

A trip curve is a graphical representation of the relationship between the current and voltage of a circuit breaker, showing the point at which the breaker will trip (open) in response to an overload or fault current. Here's a step-by-step guide on how to read a trip curve:

Understanding the Trip Curve

A trip curve typically consists of three main sections:

  1. Normal Operating Region: This is the area where the circuit breaker operates normally, with the current and voltage within the rated values.
  2. Overcurrent Region: This is the area where the circuit breaker starts to trip in response to an overload or fault current.
  3. Fault Current Region: This is the area where the circuit breaker trips immediately, without any delay, in response to a high fault current.

Reading the Trip Curve

To read a trip curve, follow these steps:

  1. Identify the X-axis: The X-axis represents the current (in amperes) flowing through the circuit breaker.
  2. Identify the Y-axis: The Y-axis represents the voltage (in volts) across the circuit breaker.
  3. Find the Normal Operating Region: Look for the area where the curve is flat and horizontal, indicating the normal operating region of the circuit breaker.
  4. Identify the Trip Point: The trip point is the point where the curve starts to slope upward, indicating the start of the overcurrent region. This is the point at which the circuit breaker will trip in response to an overload or fault current.
  5. Determine the Trip Current: The trip current is the current value at which the circuit breaker will trip. This is typically indicated by a vertical line or a specific point on the curve.
  6. Determine the Trip Voltage: The trip voltage is the voltage value at which the circuit breaker will trip. This is typically indicated by a horizontal line or a specific point on the curve.
  7. Identify the Fault Current Region: The fault current region is the area where the curve is steep and vertical, indicating the immediate tripping of the circuit breaker in response to a high fault current.

Example

Suppose you have a trip curve with the following characteristics:

In this example, the circuit breaker will trip at 200 A, regardless of the voltage. If the current exceeds 200 A, the breaker will trip immediately. If the current is between 150 A and 200 A, the breaker will trip at 240 V. If the current is below 150 A, the breaker will operate normally.

Remember to always consult the manufacturer's documentation and follow the specific instructions for reading and interpreting the trip curve for your particular circuit breaker.