Title : Circuit Breaker with Coaxial Current Sensor Description Circuit-breakers for application at medium voltages usually need to employ current sensing devices or current transformers to detect primary current overloads and short circuits for monitoring and protection. In indoor applications these may be incorporated in the equipment of which the circuit breaker is a component. In outdoor applications, particularly in rural electrification schemes, where often the circuit breaker may be used in overhead line applications, the current sensor or transformer is preferably incorporated as an integral part of the circuit breaker. This is usually achieved by mounting a ring-type current sensor or transformer coaxial with an insulated conductor or bushing. Typical examples of conventional use are shown in Fig 1 and Fig 2. The interrupting device could by typically a vacuum or gas switch.

The design shown in Fig 1 usually requires some form of additional liquid or gaseous insulation such as oil or SF6 to keep the size of the circuit-breaker to acceptable levels and also to ensure the internal components are maintained free of moisture and contamination.

Later designs are typically as shown in Fig2. Here the need for a tank filled with oil or SF6 is removed. The current transformer or sensor is mounted at the side of the switch and electrically in series with it. This example utilises a vacuum switch and current transformer encapsulated in solid insulation.

In both cases, however, it is necessary for the insulation exposed to outside environmental conditions to have additional"creepage"length compared to insulation which is protected from the external environment. Thus, although the design shown in Fig 2 does not need liquid or gaseous insulation material to minimise the overall dimensions, it is still necessary to protect the internal surface (1) of the insulation below the switch from the effects of condensation. In exposed hostile environments this can only be done in a practical manner by filling the volume below the switch with a controlled environment such as dry nitrogen or SF6. This requires additional seals and monitoring and regular maintenance to ensure the internal surface does not become contaminated. It is vitally important to ensure the internal surface is kept clean and free from condensation and contamination otherwise there is risk of internal electrical discharge from the live conductor down the insulation to earth.

In Fig 1 the current flowing through the device is conducted via conductors item 1 encapsulated in suitable electrically insulating material. Item 2 such as epoxy resin or polymer concrete. Connection item 9 and flexible connection item 4 and switch item 7 provide the internal conducting path. Operation to open or close the switch item 7 is performed by actuator item 8 and lever item 5. The integrity of the internal insulation surfaces are maintained by using SF6 gas or oil.

In figure 2 the current flowing through the device is similarly conducted via conductors item 1 and switch item 2 through current transformer item 3. To maintain the integrity of internal surface 4 requires some form of controlled environment such as SF6 gas or dry nitrogen.

Thus, in both cases it is necessary to protect the internal insulation surfaces by using a