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| installation and other factors. Values will vary with temperatures, and excessive readings should be compensated for any difference in temperatures at times of test. |

| Transformer Troubles |

| Troubles which may be indicated by transformers acting abnormally may be caused by electrical or mechanical failures within the units themselves and the parts associated with them or may be caused by line troubles which may affect the transformer's installation, or combinations of these. It is impractical to attempt to list all possible troubles, situations, and sources of trouble. A few of the more prevalent ones are listed in Table 7-2. These are meant to be representative; other troubles not listed in the table may be found, their causes determined and proper remedies applied by extending the logic and the methods outlined in the tabulation. Those which have been selected for inclusion have been derived from handbooks, manufacturers' maintenance manuals, technical papers, and the author's experiences over many years. To this list, the worker may add those of particular individual importance based on the particular circumstances. |

| Review. |

| 1. Tests are made on transformers to ascertain that prescribed standards are met; to determine operating and maintenance requirements, as part of maintenance; to determine kinds and sources of trouble; and to ascertain that repairs have been effective. Tests can be made with the application of low voltage or high voltage. |

| 2. Low-voltage tests include: resistance tests (Fig. 7-1) to obtain copper losses and check internal connections; polarity tests (Fig. 7-2) to check for relative direction of windings on single-phase transformers (for three-phase transformers see Table 7-1); ratio tests (Figs. 7-3 and 7-4) to check transformation ratio; impedance tests (Fig. 7-5) to determine regulation of the transformer; transformer loss tests (Fig. 7-6) to determine core losses and copper losses; continuity test for TCUL transformers to ascertain that the circuit remains closed while taps are changing; and temperature test (Fig. 7-7) or heat run to determine temperature rise of the unit under load (for three single-phase or three-phase transformer, with winding connected in delta) (Fig. 7-8). |

| 3. High-voltage tests include: high-voltage test (Fig. 7-9) to check adequacy of insulation; impulse test (Figs. 7-10 and 7-11) to prove that insulation will withstand voltage surges caused by lightning or switching; induced |

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