Abstract: A dielectric strength tester is disclosed for use with high-voltage, high-volt amperage electrical equipment. The step-up transformer has a high-voltage secondary winding which is connected through first and second primary windings of a sensing transformer to first and second test terminals. If an operator should contact the second test terminal, which is at high voltage, this would cause a fault-to-ground current, which is an imbalance on the two primary windings, and hence supply an output on the secondary winding of the sensing transformer. This voltage is rectified and passed through a set point resistor means to trigger a thyristor into conduction. This thyristor is connected to de-energize a triac in series with the primary of the step-up transformer. The sensing is rapid because it is carried out in the high voltage secondary, yet the control is in the lower voltage primary of the step-up transformer.