AC Bridges Questions and Answers

Answer: A

It is the standard bridge for insulation testing, giving both the capacitance of a specimen and its loss tangent at a test voltage of many kilovolts. The loss tangent is the more valuable figure, since it rises long before insulation actually fails.

Answer: B

The bulk of the applied voltage falls across the specimen and the standard capacitor, which are the two high-voltage arms. The adjustable arms carry only a few volts, so their controls can be handled and the detector connected without hazard. Guarding and earthing then complete the safety.

Answer: C

The dissipation factor of the specimen is measured against that of the standard, so the standard must have a loss so small as to be negligible. Compressed gas capacitors achieve loss tangents of a few parts in a million and hold their value at high voltage.

Answer: D

Balance yields \( \tan\delta = \omega C_4 R_4 \), where the subscripted quantities are the capacitor and resistor forming the parallel adjustable arm. The dissipation factor therefore depends on frequency, so the test frequency is always quoted with the result.

Answer: A

Voids, moisture and contamination all increase the loss component of the current without changing the capacitance much, so the loss tangent climbs. Because it is a ratio it is independent of the size of the specimen, which makes it comparable from one test to the next.

Answer: B

Its balance depends on frequency, so once the arms are set the bridge is silent only at one frequency. That property makes it a frequency-measuring network and, with an amplifier round it, the frequency-determining element of the Wien bridge oscillator.

Answer: C

Balance at one frequency alone is exactly what a frequency measurement needs, and putting an amplifier around such a network makes it oscillate at that frequency. Most bridges are deliberately designed so that their balance is free of frequency, which is the opposite requirement.

Answer: D

Maxwell's inductance bridge and the simple De Sauty bridge are of this kind. Frequency drift then does no harm and the result may be quoted without stating the frequency. Harmonics still trouble the null, however, since the residual arms are not perfectly reactance free.

Answer: A

A pair of impedances in series across the supply is adjusted until their junction is at earth potential, and that junction is then earthed. The stray capacitances of the bridge corners to earth are thereby placed across a branch carrying no detector current, so they drop out.

Answer: B

The detector is first switched to the earth point and the Wagner branch adjusted for a null, then switched back to the bridge and the arms adjusted. Each disturbs the other, so the two are worked alternately until both nulls hold at once.