The question asks about the voltage withstand capability circuit breakers are designed for in systems operating below 400 KV, relative to the normal system voltage.

Circuit breakers are crucial components in power systems, responsible for interrupting fault currents and switching operations. They must be designed to withstand various voltage stresses, including normal operating voltage, temporary overvoltages, switching surges, and lightning surges. The design voltage level of a circuit breaker is determined by factors like the maximum system voltage and the required insulation levels.

In power system design, especially for high voltage systems, it is standard practice to design equipment, including circuit breakers, to withstand voltages higher than the nominal system voltage. This extra margin accounts for various transient overvoltages that can occur during switching operations, faults, or due to lightning strikes.

For systems operating below 400 KV (which includes common transmission and distribution voltages like 132 KV, 220 KV, etc., as well as the 400 KV level itself, as it's 'below or equal to'), the design standard often requires circuit breakers to withstand a certain percentage above the normal system voltage for a sustained period or during specific transient events.

Let's consider the options provided:

• 10% above normal system voltage
• 20% above normal system voltage
• 30% above normal system voltage
• 40% above normal system voltage

Standards for electrical equipment insulation and voltage withstand levels are set by organizations like IEC (International Electrotechnical Commission) and ANSI (American National Standards Institute)/IEEE (Institute of Electrical and Electronics Engineers). These standards specify the impulse withstand voltage (for lightning surges) and switching impulse withstand voltage (for switching surges) based on the nominal system voltage. Additionally, the continuous operating voltage rating is also higher than the nominal voltage.

A common design practice and standard requirement for system voltages up to and including 400 KV is that equipment, including circuit breakers, is designed to withstand a continuous operating voltage that is at least 10% above the nominal system voltage. This provides a safety margin for variations in system voltage and certain types of overvoltages.

For example, a circuit breaker designed for a 400 KV system would typically have a maximum continuous operating voltage rating of at least \(400 \, \text{KV} \times 1.10 = 440 \, \text{KV}\).

While switching impulse and lightning impulse withstand levels are significantly higher than this continuous voltage margin, the question specifically asks what the circuit breaker is designed to withstand above the normal system voltage without specifying the type of voltage (continuous, transient, etc.). Given the options are percentages above the normal system voltage, this refers to the design margin for continuous or temporary overvoltages slightly above nominal, which is typically around 10% for systems in this voltage range.

Therefore, circuit breakers for systems below 400 KV are commonly designed to withstand 10% above the normal system voltage as a continuous or near-continuous operating margin.