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By studying pull-in behavior, we investigate the stability of single-crystal silicon, rectangular, electrostatic torsion actuators in a variety of ambients and configurations. These structures are designed for full travel range using partial-plate buried electrodes on either oxidized silicon or nitrided glass substrates. We find that the presence of charge in both systems, due at least in part to moisture on insulating surfaces surrounding the buried partial plate, has a large effect on pull-in performance, even in vacuum and flowing dry nitrogen. We are able to minimize this charge effect by using devices fabricated with ground shield electrodes. These improved structures exhibit stable actuation in air without pull-in at a travel range that exceeds 90%. This is among the highest measured stable values reported in the literature.