When discussing circuit breakers, we like to have good images to show the difference between the main contacts and the arcing contacts. When a circuit breaker opens, the main contacts should open partially first before the arcing contacts start to open. There should be no arcing damage to the main contacts because the arc interruption takes place between the arcing contacts. This protects the current carrying surfaces of the main contacts so there is minimum contact resistance at the main contacts. Good contact surface means no overheating. For the same reason, when the circuit breaker closes, the arcing contacts close first, suffering any arcing damage. After the arcing contacts are closed, the main contacts close. All this keeps the main contacts in good condition. The arcing contacts are enclosed in something called an arc chute that extends and separates the arc until it is extinguished when the circuit breaker opens. When the arc chutes are contaminated with dirt or high humidity or interrupt very high loads or fault currents, the arc chutes take a beating. Whether you’re talking about Square D circuit breakers, Cutler Hammer circuit breakers, Siemens, Westinghouse or GE General Electric circuit breakers, molded case circuit breakers, power circuit breakers, or medium voltage air circuit breakers, the function of the arc chutes is the same, take a beating to protect the main breakers. The attached image shows the damage to the stationary and movable arcing contacts and to the arc runner that extends into the arc chutes to extend and divide the arc until it is extinguished. The main contacts, the 16 curved segments across the middle of the image, are in good shape. No arcing damage what so ever. This image is from a medium voltage air circuit breaker.