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Page 132

| (1 volt-ampere, or 1 volt multiplied by 1 ampere) is a very small quantity, the short-circuit rating, sometimes called the circuit breaker duty, is given in kVA (kilovolt-amperes, or thousands of volt-amperes) or often in MVA (megavolt-amperes, or millions of volt-amperes). |

| Circuit breakers must conform to the particular characteristics on the system to which they are applied. The specifications for a circuit breaker, therefore, must provide for the following: |

| 1. Operating voltage of the circuit |

| 2. Normal operating or maximum load current |

| 3. Maximum abnormal or fault current that must be interrupted |

| The first characteristic determines the insulating requirements; the second, the requirements of the normal or load-carrying parts; and the third, the mechanical requirements of the breaker itself and of its supporting structure. |

| Principle of Operation.. |

| In interrupting a circuit, the circuit breaker actually makes a physical separation in the current-carrying or conducting element by inserting an insulating medium sufficient to prevent current from continuing to flow. In so doing, the persistence of an arc across the gap is prevented. |

| The circuit is usually opened by drawing out an arc between contacts until the arc can no longer support itself. The arc formed when the contacts of a circuit breaker move apart to interrupt a circuit is a conductor made up of ionized particles of the insulating material. With the advent of higher voltage power systems, the length of arc that would be necessary before the arc would extinguish itself in air would require breakers of such physical size as to be impractical. Hence, whenever voltages and currents are going to be large, other forms of insulation are used in place of air to extinguish the arc as quickly as possible. The magnetic fields set up and the heat generated will persist as long as current continues to flow through the contacts and the arc. Should this current be fault current, the enormous magnetic fields and the great amount of heat will tend to cause damage in terms of their own duration. |

| The most commonly used insulation is oil. For higher voltages and larger capacities, the insulating medium may be a vacuum or an inert gas, such as sulphur hexafluoride (SF6). |

| Other means are also employed to limit and extinguish the arc as quickly as possible as well as to reduce the magnitude and duration of the fault current, both of which are factors in the size, reliability, and cost of circuit breakers. |

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