- Circuit Breaker Definition: A circuit breaker is defined as a device that opens and closes electrical contacts to protect circuits from faults.
- Operating Time: Circuit breaker operating time includes the duration from the start of contact movement to the final closed position.
- Tripping Time: Circuit breaker tripping time is the period from the activation of the trip coil to the final open position.
- Reliability and Speed: Circuit breakers must operate reliably and quickly to prevent damage and ensure safety.
- Design Factors: The design of circuit breakers involves considerations like contact gap, travel distance, and velocity to ensure effective operation.
An electrical circuit breaker makes and breaks current-carrying contacts under load, overload or fault. Operating time on close is counted from the close-coil current to first contact touch. Tripping time on open is counted from the trip-coil current to arcing-contact part. A circuit breaker sits closed for most of its life and only rarely travels.
The mechanism still has to meet its rated times every time it is called. Springs, hydraulics or magnetics, plus latches and dampers, make that travel a designed machine rather than a simple switch.
Stroke, contact gap and contact speed set both interrupting duty and dielectric withstand for circuit breakers.
Gap, stroke and velocity follow the quench medium plus the current and voltage ratings.
The sketch below is a typical travel-versus-time curve.
The figure plots time in milliseconds on X and contact travel in millimetres on Y. The GIF itself has no numbered scale.
At T0 current enters the closing coil. Motion toward the fixed contact begins at T1. The contacts first touch at T2. The moving contact is fully home at T3. T3 – T2 is contact wipe and overtravel, not electrical overload. After T3 the moving contact can bounce, then settle by T4.
On trip, at T5 current enters the trip coil of the circuit breaker. The moving contact starts opening at T6. It parts the fixed contact at T7. IEC-style opening time runs from trip-coil current to contact part. (T7 – T6) is remaining wipe while the faces still touch.
The moving contact reaches its open stop at T8, then rings on the damper before it is still. Rest is at T9. The same travel applies to a local close/trip and to a remote control circuit breaker.
Circuit Breaker Opening Operation Requirement
Faster opening cuts arcing time and contact wear, so fault current is interrupted sooner. Stroke still has to leave enough open gap for power-frequency stress and lightning impulse while the breaker sits open.
Continuous current wants a low-resistance path. The arc in circuit breaker wants a refractory face. High-voltage breakers therefore put a copper main pair in parallel with arcing contacts of tungsten, copper-tungsten or molybdenum. The arcing pair has much higher resistance than the mains.
On opening circuit breaker operation, the main contacts part first and the arcing contacts last. Path electrical resistance and loop inductor of the two branches differ, so current takes a finite time to commutate fully onto the arcing pair.
As the contacts open, the gap grows. After current zero the gap must already be large enough that the arc does not reignite.
Travel beyond that point is dielectric margin plus deceleration into the damper.
Circuit Breaker Closing Operation Requirement
Closing needs the following,
- The moving contact must close fast enough to limit pre-strike. As the gap shrinks, the dielectric can fail before the metals touch.
- Closing displaces the quench medium, so this circuit breaker operation must supply enough mechanical work to compress gas or oil in the chamber.
- On impact the moving contact can bounce, and electromagnetic repulsion is worse on a fault close. Closing energy has to hold the contacts in against that force.
- On a spring-spring mechanism the opening spring is usually charged during close. Closing energy must also wind that trip spring.
