Why do high-grade relay test sets still use linear power amplifiers instead of lighter switching (class-D) amplifiers?

The amplifier is the heart of a relay test set: the relay decides on the waveform it receives, so the fidelity of that waveform determines whether test results are trustworthy. Linear amplifiers reproduce the reference sine wave with very low total harmonic distortion, because the output transistor operates in its linear region - there is no switching ripple to filter away.

Sine-wave purity - linear stages deliver a clean 50/60 Hz sine wave with minimal harmonic content, close to the waveform the relay sees in the real power system.

Precision at very low currents - the crucial differentiator. Pickup tests of sensitive protection (e.g. earth-fault elements set near 0.01 A) demand accurate, stable output at milliampere level. A switching amplifier's pulse-width modulation becomes noisy and coarse at such low amplitudes, while a linear amplifier stays clean and accurate.

Stability under load - linear designs tolerate the highly inductive loads of relay trip coils and current circuits without the oscillation or protection trip-outs switching designs can exhibit.

The trade-off - linear amplifiers are heavier and less efficient; switching amplifiers are compact and energy-efficient but compromise waveform purity and low-current precision. For protection testing, accuracy outranks weight.