Low-Voltage Switchgear
Low-voltage switchgear is a commonly used name for metal-enclosed or metal-clad low-voltage power circuit breaker switchgear rated for 600 V alternating current (AC) and below. The metal-enclosed switchgear assembly is completely enclosed on all sides and top with sheet metal (except for ven- tilating openings and inspection windows) and contains stationary primary power circuit switching or interrupting devices, or both, with buses and con- nections. The metal-clad low-voltage switchgear contains circuit breakers of removable (rack-out) types which are housed in individual grounded metal compartments. The construction features of metal-enclosed and metal-clad switchgear were described previously in Chapter 7. The service conditions for the design and performance of low-voltage switchgear are based upon ambient temperature from −30°C to +40°C, altitude not to exceed 6000 ft, and switchgear installed in nonexplosive atmosphere.
There are two basic types of low-voltage switchgear structures, they are indoor and outdoor types. Indoor switchgear consists of a front section con- taining circuit breakers, meters, relays and controls, bus section, and cable entrance section. The outdoor section is similar to the indoor switchgear except a structure that is provided around it for weatherproofing. Bus bars are available either in copper or aluminum. When aluminum bus is specified, bolted joints should be made with Belleville washers to minimize cold flow characteristics and maintain tight connections. Generally, bare bus bars are used. However, insulation can be specified on special orders. The normal clearance between phase to phase and phase to ground is 2 in. to minimize creepage for 600 V rated equipment. The standard high-voltage withstand is 2200 V AC for phase to phase and phase to ground for a period of 1 min.
Totally enclosed low-voltage switchgear, in its present form, began to gain acceptance through the 1950s. The manufacturers of the time marketed three- pole circuit breakers and their enclosures as equipment that was safer for the user’s personnel, more reliable, and as having advantages over fuses; namely, prevention of single phasing on three-phase AC systems. Today, low-voltage switchgear takes on many specialized forms and functions that combine meter- ing, monitoring, control, protection, and distribution. Major manufacturers, i.e., original equipment manufacturers (OEMs), and specialty panel shops now provide a wide variety of low-voltage switchgear designs, some of them very custom, to suit the user’s needs. These may or may not be Underwriters Laboratory (UL)-labeled. It is common to find installations where several different kinds of circuit breakers, automatic controls, and monitoring devices, and even automatic transfer switches, will be combined in the same line-up. This trend in integration has started to confuse the issue as to what low-voltage switchgear really is. It should be borne in mind that switchgear is still some principal combination of metal enclosures with multipole circuit breakers. There are many metal-enclosed, dead front, assemblies offered that are switch and fuse combinations. Although they look like and commonly are referred to as switchgear, they are really modern versions of equipment known as switchboards, and are called that by most manufacturers. Like their forerunners, these switchboards do not address the problems with single phasing on branch feeders due to a blown fuse; however, the incoming device may have phase loss or blown fuse detection included in it. Regarding current-carrying capacity, both fuses and switches have roughly kept pace with the developments in circuit breaker technology.
Low-voltage AC switchgear designs are still widely applied to low-voltage direct current (DC) distribution centers up to 250 V. Previously, manufacturers provided two-pole, draw-out circuit breakers for DC switchgear. Today, the same three-pole design, and three-phase bus arrangement, is provided for both DC and AC applications; with the extra pole either unused or placed in series with one of the others according to the particular manufacturer’s application preferences. As of this writing, direct-acting overcurrent trip devices are not offered for the new low voltage power circuit breakers. The directacting and electromechanical trip devices have been replaced by microprocessor based (electronic) trip devices for overcurrent protection. However, in the molded and insulated case low voltage circuit breakers both electronic and thermal-magnetic overcurrent trip devices are offered. The electromechanical and dicect-acting trip devices are still available in the secondary market as replacement for the older low voltage power circuit breakers.
Low-voltage generator paralleling switchgear continues to become more commonplace as utilities and consumers strike agreements for cogeneration or load curtailment contracts.Although similar in form to unit substation type switchgear, it is vastly more sophisticated in the areas of protection and control. It is common today to see low-voltage switchgear with protective relaying that used to be found only on medium-voltage switchgear in a utility’s generating station.