as some fuse link particles. There also may be a very loud report. Accordingly, when one is working in the vicinity of a fuse cutout, care should be taken to stay clear of the exhaust path. In addition, when closing a cutout, it is good practice to look down, away from the discharge path, since there is always the possibility of closing the cutout into a short circuit.

RECOMMENDED SIZE OF PRIMARY FUSE. - Table 4-3 gives the recommended size of primary fuses to use with different transformer voltage and kilovoltampere ratings. The table also gives the normal full-load primary current rating of the transformer.

It is general practice not to protect distribution transformers against small overloads. To do so would cause unnecessary blowing of the fuses and frequent interruption of the service, both of which are undesirable. It is customary, therefore, to provide fuses which have a higher current rating than the current rating of the transformer.

AIR SWITCHES. - As their names imply, air switches are switches whose contacts are opened in air. Air switches are further classified as air circuit breakers, air-break switches, and disconnects.

Circuit Breaker. - A device used to complete, maintain, and interrupt currents flowing in a circuit under normal or faulted conditions is called a circuit breaker. The circuit breaker has a mechanism that mechanically, hydraulically, or pneumatically operates the circuit breaker contacts. Insulating oil, air, compressed air, vacuum, or sulfur hexafluoride gas is used as an arc-interrupting medium and a dielectric to insulate the contacts after the arc is interrupted. If the circuit is to open automatically during overload or short circuit conditions, the circuit breaker is equipped with a tripping mechanism to accomplish this. Thus, circuit breakers normally are used where control of the circuit, as well as protection from overload, short circuit, and so forth is desired, such as at generating stations and substations.

Air-Break Switch. - The air-break switch can have both blade and stationary contacts equipped with arcing horns. These horns are pieces of metal between which the arc forms when a current-carrying circuit is opened. As the switch opens, these horns are spread

Table 4-3. - Fuse Size for Transformer Installations