Introduction:
Vacuum circuit breakers are widely used in electrical power systems for interrupting the flow of current during fault conditions. One of the challenges in vacuum circuit breakers is the occurrence of current chopping, which refers to the interruption of current in the circuit before it reaches zero. This can lead to high transient voltages and cause damage to the equipment. To limit current chopping, certain properties of the contacts are necessary.

Explanation:
To understand why high vapour pressure and low conductivity properties are required to limit current chopping in vacuum circuit breakers, let's delve into the working principle of these breakers.

1. Vacuum Circuit Breaker Working Principle:
- A vacuum circuit breaker consists of contacts made of a conductive material, typically copper or silver.
- When the circuit breaker is closed, the contacts are in physical contact, allowing the flow of current through the circuit.
- During fault conditions, the circuit breaker needs to interrupt the flow of current. This is achieved by separating the contacts, creating a gap through which the current must be interrupted.
- In a vacuum circuit breaker, the contacts are housed within a vacuum chamber, which eliminates the need for any medium (such as oil or gas) to extinguish the arc formed during contact separation.

2. Current Chopping:
- When the contacts of a vacuum circuit breaker start separating, an arc is formed between them due to the ionization of the surrounding vacuum.
- The arc provides a conductive path for the current to continue flowing until the natural current zero-crossing point is reached.
- However, in certain cases, the arc can be extinguished before the natural current zero, resulting in current chopping.
- Current chopping can lead to high voltages across the contacts and cause damage to the insulation and other components.

3. Role of High Vapour Pressure and Low Conductivity:
- High vapour pressure refers to the tendency of the contact material to vaporize under the influence of the arc.
- Low conductivity indicates that the contact material has poor electrical conductivity.
- These properties are desired in vacuum circuit breaker contacts to limit current chopping.
- When the contacts start separating and an arc is formed, the high vapour pressure of the contact material promotes the vaporization of the contact material.
- The vaporized material fills the gap between the contacts and increases the resistance of the arc path, leading to arc extinction.
- The low conductivity of the contact material further aids in increasing the resistance of the arc path and facilitating arc extinction.
- Consequently, the interruption of current occurs close to the natural current zero, minimizing the occurrence of current chopping.

Conclusion:
To limit current chopping in vacuum circuit breakers, the contacts need to have high vapour pressure and low conductivity properties. These properties promote the vaporization of the contact material and increase the resistance of the arc path, leading to efficient arc extinction and minimizing current chopping.