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Page 356

| Figure 17-23 Typical recording meters: (A) single-pen, single-range voltmeter, and (B) strip-chart ammeter. |

| Measurements Exceeding Instrument Ranges.. |

| Shunts and Current Transformers |

| If the values of current involved in any of the measuring devices might possibly exceed the range of the instruments, two schemes are used to measure only a smaller but definite proportion of the total current. One makes use of a shunt whose resistance is a known proportion of the internal resistance of the ammeter (Fig. 17-26A). For example, if the internal resistance of the ammeter is 9 ohms, and the resistance of the shunt is 1 ohm, then 9/10 or 90 percent of the current will flow through the shunt and 1/10 or 10 percent through the ammeter. The ammeter reading is then multiplied by 10 to obtain the current in the circuit. |

| If the current is associated with a high-voltage circuit, a current transformer (Fig. 17-26B and 17-26C) is usually used instead of a shunt. The standard current transformer (CT) secondary is usually rated at 5 amperes, |

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