Multi Break Function of Circuit Breaker:
It has been pointed out earlier that a circuit breaker may comprise a single pair of contacts or a number of pairs in series operating simultaneously. Undoubtedly the single-break is the simplest arrangement, a single-break is generally desirable up to the echttps://www.digikey.inonomic limit of rating, which is usually about 66 KV and 1,000 MVA although these figures have been exceeded in a number of designs. However, the Multi Break Function of Circuit Breaker has the following advantages:
- The transient restriking voltage is divided over a number of breaks so that only a fraction appears across each.
- Each break can be separately tested at a voltage lower than that of the breaker as a whole (unit testing).
- The effective speed of arc lengthening is increased in proportion to the number of breaks.
The main difficulty with Multi Break Function of Circuit Breaker is to ensure that the restriking voltage distributes itself evenly across the breaks so that full advantage of item (i) above can be obtained. One of the main reasons of this uneven voltage distribution is the presence of capacitance between line and grounded parts of the circuit breaker. This phenomenon will be quantitatively dealt with later in this chapter.
Multi Break Function of Circuit Breaker have been developed for installations where rupturing capacities are high and very high speed operations are called for.
The construction of Multi Break Function of Circuit Breaker is based on the unitized design principle in which one unit consists of a vertical support insulator surmounted by a single or a pair of interrupting units in Y formation. By connecting the units in series, requirements for the network for different voltages and rupturing capacities can be met. This type has 2 to 3 cycles as total break time. The arcing time is 10 to 15 ms. Table (15.1) shows the typical design and ratings for minimum oil circuit breakers. It shows that 765 KV, 55,000 MVA breaking capacity minimum oil circuit breakers have been designed having 25 breaks per pole.