Question 6A528

Thermocouple ammeters measure AC or RF current by sensing the heat generated by the current. To extend their measurement range for higher currents: 1. **Current Transformer (A):** A current transformer (CT) is commonly used for measuring large AC or RF currents. The main current flows through the CT's primary, inducing a proportionally smaller current in the secondary winding, to which the thermocouple ammeter is connected. This method provides accurate scaling and electrical isolation. 2. **Capacitor Shunt (B):** For AC or RF currents, a capacitor can be placed in parallel with the thermocouple ammeter. A capacitor's impedance decreases with frequency, allowing it to shunt (divert) a portion of the AC current around the meter element. This enables a larger total current to flow in the main circuit while the meter measures only a fraction. Although the shunt ratio is frequency-dependent, it effectively increases the measurable current range. Both methods allow a thermocouple ammeter to safely measure currents exceeding its intrinsic rating. A resistor shunt (D) is also a valid method for extending ammeter range, especially for DC or lower-frequency AC, but the question implies that both A and B are considered correct in this context.