Fuses breaker coordination

To achieve the above the characteristic of the fuses should lie well above the characteristic of the overcurrent and short-circuit releases of the breaker for the lower region of currents, such that only the breaker operates. However, it should lie well below the eharacteristic of the breaker in the higher region of currents to ensure that the fuses operate sufficiently quickly and long before the in-built releases. The breaker is thus prevented from operating at currents that are in excess of its breaking capacity. Figure 12.22 illustrates such a coordination. 

On faults exceeding ol breaker. I Rl (fuses) < I Rt (breaker) Figure 12.22 Coordination of fuses with a breaker... 

Fuses should not be used to protect secondary voltage feeders. The time current characteristics of fuses above 100 A will not coordinate with the groimd fault pickup currents and time delays of the main overcurrent protection (circuit breaker or fused disconnect switch) ground fault protection. A main load break disconnect switch can be equipped with current-limiting fuses to reduce the available short-circuit current from the utility and should have a three-phase voltage relay for single-phase protection. 

When fuses or moulded case circuit breakers are applied to a circuit it is necessary to ensure that their /-squared-t characteristics coordinate properly with the thermal capabilities of the downstream equipment, especially the cables. In order to determine the /-squared-t characteristics of a protective device it is assumed that the current in the device suddenly changes from a normal load value to the fault value in a very short period of time, i.e. similar to a step change in a control system. Hence for each value of current along the x-axis of the device s time-current characteristic the value of the current squared multiplied by the corresponding time can be plotted. For cables and busbars the /-squared-t function equals a constant (k) for each cross-sectional area of conductor, as explained... 

Where moulded case circuit breakers are chosen for a plant in favour of fuses the coordination of cascaded units becomes a little more difficult than with fuses. This difficulty arises from the fact that these circuit breakers have a definite or near definite minimum time limit to their time-current characteristic. This causes the lower part of the circuit breaker protection curve to be almost horizontal at a low value of time, typically in the range of 0.003 and 0.01 second. 

The necessity for an instantaneous tripping function is the same as for a high voltage motor. This function can be provided by a magnetic repulsion device within the moulded case circuit breaker, by a (50) relay or by upstream fuses. If fuses are used then the contactor must be capable of carrying the duty until the fuse completes its function. To minimise the stressing of the contactor it should be coordinated with the fuses as recommended in IEC60947 Part 2, as a Type 2 requirement. 

In a combination starter, the magnetic or sohd-state contactor or starter is coordinated with a circuit breaker or set of fuses so that the equipment will successfully complete a fault interruption up to its pubhshed rating. The same coordination is built into the self-protected combination controller. 

The common enclosure design (Fig. 5.141) and the use of combination starters offer both economy and ease of installation in multiple motor control installations. In addition, motor control centers (MCCs) provide proper coordination between short-circuit protective devices and the controller. Since MCCs are engineered systems, the components are closely coordinated to work together, and the unit is rated for a particular value of short-circuit interrupting duty at the point of its installation. MCCs may contain a molded-case circuit breaker and starter, or a fused switch and a starter. 

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