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| additional internal inspection. Connections to ground should receive special attention as a safety measure. Radiators should be inspected for dents, holes, rust, and discoloration. |

| 3. Deenergizing makes possible closer inspection of the transformer both inside and outside the unit. Gaskets may be inspected closely, samples of oil taken, discoloration of terminals, core, coils, terminal boards, connections, tap-changer contacts may be observed at close range; loose connections, core laminations and bracing may be checked by actual touch. Scale and rust on internal tank, radiator, and piping surfaces may also be observed closely. |

| 4. Ordinary mechanical maintenance includes: cleaning and repainting rusted, colored, or discolored areas; cleaning and tightening connections, bolts, lagging and internal supports; and replacing gaskets. |

| 5. Electrical maintenance includes the checking of oil and coil and bushing insulation. Oil may be checked by comparing the color of a sample to standard colors (Fig. 4-2), which give an indication of emulsification. In a breakdown test, a sample is placed between two electrodes and a voltage applied until flashover or breakdown takes place, which indicates the insulating adequacy of the oil (Fig. 4-3). Oil may be filtered and replaced, core, coils, terminal boards should be first washed down with clean hot oil under pressure. |

| 6. Coil and bushing insulation (as well as oil) can be checked by a Megger, an instrument which measures resistance (Fig. 4-4); the indicated values are compared to standard acceptable values. The Megger is a portable dc generator which produces a voltage of about 500 volts; the small currents flowing through the insulation (coil, bushing, or oil) are calibrated to read resistance in ohms or megohms (Fig. 4-5). |

| 7. When indications of moisture are encountered, the transformer may be dried out by heat applied externally through the circulation of hot clean air or oil, or both; or by internally generated heat obtained by short circuiting the windings and allowing current to flow through them; or a combination of both these methods (Fig. 4-7). |

| 8. Electrical grounds provide low resistance paths to unbalanced, or neutral currents, which may be particularly heavy during fault conditions; this is especially true of large installations, such as substations, where such grounds help insure positive operation of protective relays. They should be checked for continuity and connections tightened. Their resistance may be measured with a Megger (Fig. 4-8). |

| 9. Precautions on energized units include clearances, barriers, and signs. On deenergized units, leads should be disconnected with visible open breaks in the circuit, on both primary and secondary sides. If this cannot be done, visible grounds should be applied to both sides. Units should not be lifted or handled by bushings or attachments other than the lugs provided for that purpose. Care should be taken that nothing falls into the tank, or is left inside the tank, which may cause flashovers or unexplainable noises. |

| STUDY QUESTIONS |

| 1. What are the causes of overheating of transformers, and how may they be discovered? |

| 2. What is the limiting consideration in the safe operation of a transformer? What factors affect this limitation? |

| 3. When should transformers be inspected and what external and internal inspections should be made? |

| 4. Why should the condition of the oil in a transformer be checked? |

| 5. How should the condition of the oil be checked in the field? |

| 6. How may the condition of the oil or askarels in a transformer be restored? What special precautions should be taken? |

| 7. What is a Megger and where is it used in the maintenance of transformers? |

| 8. How may transformers be dried out and what precaution should be taken? |

| 9. Why are electrical grounds associated with transformers of great importance and how may they be checked? |

| 10. What precautions should be taken when working on deenergized transformers? |

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