Eaton Miniature Circuit Breakers

Trip Curve Basics Part 1

There are two critical elements in miniature circuit beakers.

Bimetal strip

This strip has two dissimilar metals attached to one another. When a prolonged overcurrent occurs, these metals begin to bend. Because the metals are different, the rate at which they bend is different, therefore causing the strip to bend. If this bending occurs long enough, the bending strip will disrupt the electrical contacts inside the breaker, causing it to trip.

Coil or solenoid

The coil or solenoid is designed for larger overcurrent events like a short circuit or lightning strike. When a large overcurrent event occurs, the plunger in the solenoid is actuated, thereby tripping the breaker.

What is a trip curve?

It is both the prolonged overcurrent protection from the bimetal strip and the higher spikes in voltage and current protection from the solenoid that make up the circuit breakers trip curve. How fast or slow these events occur determines the shape of the curve. A trip curve is simply a graphical representation of the expected behavior of a circuit protection device, in this case, a circuit breaker.

This graphical representation looks at two separate pieces of data to provide the information needed to understand when a particular breaker will trip. The first is time, more specifically, the time the circuit breaker experiences a certain amount of overcurrent. The second is the amount of current. In this case, how much more current is passing through the breaker than the protection device is actually rated for.