Low-Voltage Switchgear and Circuit Breakers:Selection and Application of Low-Voltage Equipment

Selection and Application of Low-Voltage Equipment

The modern distribution system has high short-circuit current available and therefore requires special consideration so that equipment may be applied within its rating. Furthermore, the switchgear should be protected against all types of faults, from low-level arcing faults to bolted faults. The protec- tion system should be selective; that is, the fault at a remote location in the system should be localized without unnecessary tripping of either the main breaker system or any intermediate breakers. The distribution system should be planned to provide continuity and reliability of service. This can be achieved by using two or more separate distribution systems instead of one large system. The continuous current rating of the main protective device should be adequate for the load to be served. Protective devices should not be paralleled to obtain a higher rating. As a general rule, the bus bars are

rated on the basis of not more than 800 A/in.2 of aluminum or 1000 A/in.2 of copper. The operation of protective devices is based upon an ambient temperature of 40 °C, and if these devices are to be applied at higher tempera- ture, the manufacturer should be consulted. The short-circuit rating of a bus is limited to the interrupting rating of the lowest rated protective device, and the available short-circuit current should not exceed this value.

The application of circuit breakers and fuses must be considered to determine which offers the most appropriate protection. Consideration should be given to anti-single-phase devices when three-pole interrupter switches with fuses are used, because fuses are single-pole interrupter switches. An arcing fault may not be stopped by a single-pole interruption. It can be backfed from the other energized phases. Because of this condition, severe equipment burn- downs may occur. Ferroresonance is the result of interaction between the reac- tance of a saturable magnetic device, such as a transformer, and system capacitance. Ferroresonance can also occur due to a single phasing condition. This phenomenon occurs mostly in high- and medium-voltage systems and results in a very high voltage on the order of three to five normal system volt- age which is imposed on the circuit involved, causing equipment failure.

Always keep in mind that fuses should be applied in systems where the system voltage is compatible with the fuse voltage rating. The reason for this is that the arc voltage generated by a fuse when interrupting is several times its voltage rating and, if misapplied, could subject the system to overvoltage conditions, causing equipment failure.

The current-limiting features of current-limiting fuses are definite strong pluses for many applications; however, use the fact wisely, for they do not limit current for all values of fault current. If the fault current magnitude is equal to or greater than the fuse threshold current, they will always be current limiting. However, if the fault current magnitude is less than the fuse threshold current, but greater than the current magnitude indicated at the intersection of the maximum peak current curve and fuse curve, the fuse may or may not be cur- rent limiting. For fault current magnitude indicated by the above curve inter- section, the fuse is never current limiting.

For this reason and arc-voltage considerations, when applying current- limiting fuses to increase the interrupting rating of other protectors, the fusing recommendations of the product manufacturers, not the fuse manu- facturer, should be followed. Fused equipment can be opened and closed manually or electrically to provide circuit protection. However, the fuse and equipment should be coordinated and tested as a combination. The fuse’s adequate performance as a circuit protector and switching device should be certified by one manufacturer.

When fused switches are electrically controlled, caution is required not to let the switch open due to fault conditions. The fault current, if not sufficient to cause interruption before the switch contacts or blades open, could be greater than the contacts’ or blades’ interrupting capability. This would result in a hazardous condition. Fused switches basically require the same application considerations as previously outlined in this section.

Fused motor starters applied in medium- and low-voltage systems avoid the ferroresonance problem owing to their location in the system. Auxiliary devices can be supplied with motor starters to provide a complete overload protection and anti-single-phasing protection. The selection and application of switchgear should be approached on an engineering basis. To provide reliability, ease of maintenance, and continuity of service, properly rated equipment and adequate circuit protection are necessary throughout the entire system, from the place where the power system enters the facility down to the smallest load.

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