Which insulation level is NOT typically used on an ungrounded system?

In an ungrounded system, the insulation levels are classified based on their capability to withstand potential overvoltages without the benefit of a ground reference. The terms "100 percent," "133 percent," "173 percent," and "200 percent" refer to the voltage ratings of insulation in relation to the system voltage.

The insulation level of 100 percent corresponds to the nominal system voltage, which does not provide any additional margin against transient overvoltages that can occur due to switching or faults. In an ungrounded system, this minimal level of insulation is inadequate since it does not account for the overvoltage conditions that might occur during operation.

On the other hand, insulation levels of 133 percent, 173 percent, and 200 percent offer increased margins that protect the system from transient overvoltages. These higher percentages allow for greater safety against potential insulation breakdown and result in more reliable operation in ungrounded systems where transient events can significantly affect performance.

Therefore, the insulation level that is not typically used on an ungrounded system is the 100 percent level, as it lacks the necessary protection against the higher transient voltages associated with an ungrounded configuration.