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Page 366

| Figure 17-30 Phase rotation or sequence indicators: (A) Using two lamps and a highly reactive coil; and (B) using one lamp, a noninductive resistance, and a reactive coil. |

| roundness, with a radius of from one ten-thousandth to three thousandths of an inch (0.0001 to 0.003 in.). Such dimensions, in turn, create tremendous pressures in terms of pounds per square inch. Polished jewels of extreme hardness and requiring little or no lubrication must therefore be used. |

| The remaining elements involve many separate coordinations but present few problems as a rule. Case design, including terminal arrangement, dial, glass, and cover depend on the application in question. Switchboard instruments differ entirely from portable ones. Size is, to a degree, associated with accuracy. More structural weight limitations are imposed on portable instruments than on a switchboard installation. Ventilation is important, especially for instruments carrying heavy currents, but it may also present problems of dust accumulation. In general, these concerns have been standardized satisfactorily. |

| Electronically activated instruments, more accurate and flexible, are supplanting electromechanical instruments; refer to Electronic Relays in Chapter 14. |

| Instrument Care: Operation and Maintenance |

| All instruments should be handled carefully to avoid injury to bearings and pivots. |

| If an instrument has more than one range, connection should be made to the highest range first in order to avoid damage should the load or voltage be underestimated. |

| Instruments should be used in their normal operating positionportable units horizontally and switchboard units vertically. |

| Check the instrument zero setting. If the pointer is slightly off, reset it by using the external zero shifter. Never try to compensate for a bent pointer by moving the zero shifter. |

| In connecting a voltmeter to a circuit, connect the leads to the instrument before connecting the leads to the voltage source. This method avoids loose leads that may be energized. Be sure that the voltmeter leads are properly insulated for the voltage to be measured. |

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