PROTECTIVE DEVICES

Overcurrent protection and personnel safety requirements are provided for in a power distribution system by the use of air-break switches and fused cutouts. Pole-mounted oil switches may also be used. Lightning arresters protect the primary lines from overvoltage caused by lightning.

Switches and Fused Cutouts

The air-break switch has both blade and stationary contact equipped with arcing horns, as shown in figure 5-17. These horns are pieces of

^{Figure 5-17.\Three-phase ganged air switch.}

Table 5-9.\Holding Power for Commonly Used Anchors

*Screw or Helix anchors power installed. 5-25

metal between which an arc forms when a circuit carrying current is opened. As the switch opens, the arcing horns are spread farther and farther apart, lengthening the arc until it finally breaks. Air-break switches are usually mounted on substation structures or on poles and are used to isolate primary feeders.

The fused cutout provides fuse protection in addition to isolating the line. Fused cutouts must be installed on the primary side of all distribution transformers and capacitor banks.

The rating (size) of the fuse link used in a primary cutout is important. The chart in table 5-10 will help you select the correct fuse size. As an example, suppose you have installed a single-phase 75-kVA transformer that operates from a 2,400-volt primary. Using the chart, first find the transformer capacity (75) on the left side of the chart. Then proceed to the right until you intersect the primary voltage column (2,400 volts). The chart lists the full-load current as 31.3 amperes and the recommended fuse size as 50

Table 5-10.\Fuse Size for Transformer Installations

Note: To compute the full-load current for transformers not listed on this table, use the formulas in appendix II. 5-26

amperes. Therefore, for this particular installation you should use a 50-ampere fuse.

The protective requirements of a three-phase transformer installation are the same as those for a single-phase service. The size of the fuse, again, is determined by the total capacity of the transformer bank and the value of the primary voltage. The chart in table 5-10 will assist you in selecting the proper size fuse.