page_361

< previous page | page_361 | next page >

Page 361

| specific percentage of the total coil (Fig. 17-27A), and its reading multiplied by the ratio of the portion of the coil to the total. For example, if a 100,000-ohm resistance is connected across a 500-V circuit and a voltmeter is connected across 10,000 ohms of that coil, the instrument will read 50 V. Multiplying this value by 10 (the ratio between the total coil and the voltmeter portion) results in a value of 500 V. Although this scheme is usually limited to dc circuits, the connections of a potentiometer are the same as those of an autotransformer, and it is therefore applicable to ac circuits as well. |

| Potential transformers (Fig. 17-27B and 17-27C) are used with many of the instruments mentioned earlier; they are also used for pilot and signal lights. Standard potential transformers (PTs) are usually rated from 50 to 300 volt-amperes, with a secondary voltage of 150 or 300 V. One potential transformer can supply a number of instruments at the same time provided that the value of secondary volt-amperes, called the burden, does not exceed the PT capacity rating. They may be either air-filled (dry type) or oil-filled, depending on the magnitude of the voltage involved. |

| In order to protect against sustained short circuits, fuses (sometimes equipped with current-limiting resistors) are connected in series with the primary winding. The resistors limit the short circuit of the PT to values that a fuse can safely interrupt. |

| For additional information regarding potential transformers, refer to Chap. 6. |

| Resistance Measurement |

| Two instruments are used to measure the resistance of a circuit or circuit element and to check the continuity of a circuit: the ohmmeter and the Megger. The former is limited to measuring a few megohms (millions of ohms), whereas the range of the latter extends to over 1000 megohms. |

| Ohmmeter. |

| The ohmmeter is essentially a milliammeter in series with a dc voltage source of a known fixed value (Fig. 17-28A and 17-28B). It is placed in series with the circuit to be measured. Since, deriving from Ohm's Law, resistance is equal to the voltage divided by the current, the current reading represents the resistance, and the meter scale is so calibrated. |

| Megger.. |

| The Megger consists of two main elements, both of which are provided with individual magnetic fields from permanent magnets. One element, a hand-driven dc generator, supplies the necessary current for making the |

< previous page | page_361 | next page >