It’s true that switchgear use to perform the most arduous duty when it is in operation. For better understanding we will learn the about the circuit breakers operation in switchgear operating principles. It will be easy to understand switchgear operation principle if we know circuit breaker operation principle. So let’s go.
Circuit Breaker Operation Principle:
Construction: Once I dismounted a 1000A MCCB. I found out the heart of a traditional circuit breaker is the contact system. Circuit breaker usually consist with a set of moving contacts, a set of fixed contacts also available, their current carrying conductors or you can say leads with an opening mechanism which is often spring-loaded.
Most of the time, the fixed and moving contacts are made of pure copper with sufficient size and cross section. But it will carry the rated current continuously when it is connected between incoming and outgoing load. Contact tips or faces are attached or plated onto the copper. It usually made of metal like silver or an alloy.
This contact tip is the main point at which the fixed and moving contacts both are touch and it enable the current to flow. It is an important requirement that a low contact junction resistance is maintained all time. Also the contacts are not welded, unduly eroded by the high thermal and dynamic stresses of a short circuit situation.
Related: Introduction of Switchgear
Closing Operation: When device closes that time the contact faces are forced together by spring. Some others kinds of pressure can be used which is generated from the mechanism. This type of spring or external pressure is required to reduce the contact junction resistance. It is the ohmic heating at the contacts; it also assists in the destruction of foreign material such as oxides that can contaminate onto the contact faces during the operation time.
Generally the closing process inside of this device is further assisted by arranging the geometry to give a wiping action as the contacts come together. This wiping action helps to ensure that points of purely metallic contact are formed during the closing operation.
Breaking Operation: Contact breaking action is made with particularly difficult by the formation of an arc inside the device when the contacts separate. The arc inside the device is normally formed as the current reaches a natural zero in the AC cycle. This mechanism usually assisted by drawing the arc out to maximum length. Therefore increasing its resistance and limiting the arc current during breaking operation.
Usually the interruption of a resistive load current is not a problem in operation time. In this case the power factor is close to unity. Normally when the circuit breaker contacts open it draw an arc and interrupt the current then the voltage rises slowly from zero to its peak value following its natural 50 Hz or 60 Hz shape. The buildup of voltage across the opening contacts is therefore relatively small. It can be sustained as the contact gap when increases to its fully open point. In many circuits the inductive component of current is much higher than the resistive component.
Short Circuit Operation: We are using switchgear or circuit breaker for protecting our electrical equipments. Now if a short circuit occurs near to the circuit breaker, not only is the fault impedance is very low and the fault current at its highest value, but also the power factor can be very low and it can be below 0.1 and the current and voltage are nearly 90 degree out of phase. When the contacts open and the current are extinguished at its zero point, the voltage tends instantaneously to rise to its peak value.
This results in a high rate of voltage rise across the contacts that aiming for a peak transient recovery voltage which is considerably higher than the normal peak system voltage.