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Page 241

| Figure 13-4 (cont.) Overcurrent relay: (B) singlephase, and (C) three-phase. |

| used in place of the bellows. The rate of flow of oil interchange determines the time variations. The dashpot is a mechanical retarding device consisting of a piston and a cylinder that is usually filled with oil. The piston is attached to the plunger to introduce a time delay. This delay is governed by an opening in the piston that permits the oil to flow from one side of the piston to the other as it moves in the cylinder. |

| Although the accuracy of this type of relay is adequate, it is not as good as that available in the induction type. Many of them are presently installed, particularly in 2300- and 4000-V distribution circuits. Although apparently vanishing, this type of relay will continue to exist for many years to come, if only for economic reasons. |

| Other such relays employing magnetic attraction include balanced beam and hinged armature or clapper types, which are shown schematically in Fig. 13-5. Also shown are their operating characteristics, indicating typical orders of magnitude only. A diagram of the plunger type (or instantaneous type, defined as having no intentional time delay) is included for comparison. |

| Induction or Disc Type |

| These relays operate on the induction principle, being essentially small induction motors, the contacts of which are closed by the rotation of an aluminum or copper disc (Fig. 13-6A). When sufficient current flows through the electromagnet to produce the necessary torque (or tendency for rotation), the disc rotates until, at the end of its travel, it touches a stationary contact and |

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