The apparatus used for switching, controlling and protecting the electrical circuits and equipment is known as switchgear. Switchgears are the switching devices that form the backbone of modern electrical distribution systems. Read more on What is Switchgear and its features.
The following are some of the important parts common to most of the circuit breakers:
- Circuit Breaker Contacts
- Instrument Transformers
- Bus-bars and conductors
Bushings of Switchgear
The use of bushing for a plain-break oil circuit breaker is shown in the figure. The high voltage conductor passes through the bushing made of some insulating material (e.g., porcelain, steatite). Although there are several types of bushing (e.g., condenser type, oil filled etc.), they perfom the same function of insulating the conductor from earthed tank.
The failure of the bushing can occur in two ways.
- Firstly, the breakdown may be caused by puncture i.e., dielectric failure of the insulating material of the bushing.
- Secondly, the breakdown may occur in the form of a flash-over between the exposed conductor at either end of the bushing and the earthed metal.
Fig (ii) illustrates these two possibilities. The bushings are so designed that flash-over takes place before they get punctured. It is because the puncture generally renders the bushing insulation unserviceable and incapable of withstanding the normal voltage.
On the other hand, a flash-over may result in comparatively harmless burning of the surface of the bushing which can then continue to give adequate service pending replacement.
Read full details of Bushings here : Electrical Bushings – Types, Purpose and Construction
Circuit Breaker Contacts
- Tulip type contacts
- Finger and wedge contacts
- Butt contacts
1) Tulip type contacts
It consists of moving contact which moves inside the fixed contacts. At contact separation, the arc is generally established between the tips of the fixed contacts and the tip of the moving contact. The advantage of this type of contact is that arcing is confined to the regions which are not in contact in the fully engaged position.
2) Finger and wedge contacts
3) Butt contacts
The butt type contact and is formed by the springs. It possesses two advantages.
- Firstly, spring pressure is available to assist contact separation. This is useful in single-break oil circuit breakers and air-blast circuit breakers where relatively small “loop” forces are available to assist in opening.
- Secondly, there is no grip force so that this type of contact is especially suitable for higher short circuit rating.
- Current transformer (C.T.)
- Potential transformer (P.T.)
- They isolate the measuring instruments and relays from high-voltage power circuits.
- The leads in the secondary circuits carry relatively small voltages and currents. This permits to use wires of smaller size with minimum insulation.
Bus-Bars and Conductors