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Page 280

| the ability of this protection scheme to discriminate between load current and fault current. |

| Relay Maintenance |

| Protective relays have two characteristics that determine their performance: |

| 1. Current magnitude in the relay coils |

| 2. Time to close the contacts. |

| Relay Setting.. |

| Electromechanical types. In these types, the current characteristic of the relay coil may be regulated by taps on the current coil or by the distance the plunger, clapper, or armature must move. In general, the greater the distance these must move, the greater the current required to pull them into closing position. |

| The time feature is controlled not only by the distances the moving parts of this type relay must travel but also by the restraining devices associated with them. These may include bellows or dashpots for the plunger type of relays (Fig. 14-13) and springs or counterweights for the clapper and armature types. Adjustments may be made by adjusting screws or nuts. |

| Induction types. Current characteristics of this type of relay may also be regulated by taps on the current coil or coils. The taps are brought to a terminal board where the proper tap may be selected by inserting a plug into a slot or jack associated with the tap (Fig. 14-14). |

| The time feature here depends on the rotation of the disc or cylinder restrained by damping magnets and a spring. The damping magnets are moved along the surface of the disc and serve to keep the disc from moving or creeping when no current flows in the relay coils (usually as a result of incidental magnetic fields in its vicinity). The speed of rotation of the disc depends on the strength of the magnetic fields set up by the relay coil or coils. These magnetic fields set up currents in the disc, which in turn produce their own magnetic fields. The latter fields react with the former and cause the disc to rotate under the restraint of a spring. By varying the distance the movable contact has to travel, the time characteristic of the relay can be varied. |

| Electronic types. Refer to Electronic Relays on page 254. |

| Inspection. Protective relays and their associated auxiliaries must be periodically checked for proper operation, chiefly for the following: |

| 1. The assembly must be free of dirt, dust, and other foreign matter. |

| 2. The contacts must not be burned, pitted, or corroded. |

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