Which of the following cable insulation levels is NOT commonly used on an ungrounded system?

In ungrounded systems, the insulation level of cables is critical for ensuring safety and reliability under normal operational conditions. The insulation levels listed correspond to the ability of insulation to withstand voltage stress without failing, and they are usually selected based on system voltage levels and operational requirements.

The commonly used insulation levels for ungrounded systems include 100 percent, 133 percent, and 173 percent ratings, as these levels are designed to handle temporary overvoltages caused by factors such as switching surges or transient events. In contrast, 200 percent insulation is generally considered excessive for ungrounded systems. This level is more typically reserved for grounded systems where additional insulation is necessary to manage the higher voltages and provide extra protection.

Using 200 percent insulation in an ungrounded setting is uncommon because it does not align with the standard insulation requirements typically adhered to within ungrounded systems. Consequently, choosing a 200 percent level does not reflect the operational norms and practices typically observed for ungrounded systems, making it the option that does not fit with the expected insulation ratings.