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Page 103

| consideration in the choice of transformer size and design. Starters are usually designed so that they cannot rest in the starting position, or in any position which will make inoperative the protective devices in the circuit. |

| X-Ray Equipment |

| X-ray equipment usually requires dc voltages from 10,000 volts to 500,000 volts or higher for operation. The voltage wave may be a sharp peaked one and the duration of the current flow only a fraction of a second. Current values may range from milliampere (1/1000 ampere) to several amperes. The high voltage is obtained by means of a transformer, usually operating at 240 volts primary, and is converted from alternating current to direct current by means of vacuum tubes or rectifiers. The transformer may be oil filled or air cooled, and is usually situated as near the x-ray tube as possible to reduce the length of high-voltage wiring required. Should the transformer fail, the secondary may come in contact with the primary voltage of only 240 volts; hazards from the primary voltage are low and hence elaborate construction or precaution is not usually required. In addition to medical uses, x-ray equipment is also employed in the examination of fabrics, metals, plastics, fruits, and other purposes. |

| Welding Transformers |

| Welding transformers are usually single-phase and operate with primary voltages of 240 or 480 volts. Secondary voltages are low as may be expected from the high current requirements. The load created by the arc is highly inductive and, if the transformer should be disconnected on the primary side (by safety devices or for other reasons), the arc across the switch or breaker contacts would tend to persist. Switches used to control equipment of this type should have ample interrupting capacity. |

| Small Intermittent-Use Equipment.. |

| Small transformers for such uses as bell ringing, annunciator systems, indicating lights, and other intermittent uses are usually of the dry type and operate from a 120 volt source for their primary and up to 15 volts for their secondary voltage. Their input is usually not over 50 watts. They are intentionally designed to have poor regulation, that is, the secondary voltage falls very rapidly as the load is increased, the current flowing increases only slightly, thereby decreasing their probability of failure. They may be protected with a small fuse (5 ampere) on the 120 volt side. |

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