At what voltage level are secondary class arresters typically applied?

Secondary class arresters are typically applied at voltage levels of 999 volts or less. This classification reflects their intended use in protecting the low-voltage distribution systems commonly found in residential, commercial, and industrial settings. These arresters safeguard equipment from voltage spikes due to lightning or other surges, ensuring that sensitive devices are protected from damage.

In contrast, the other options pertain to different categories of electrical equipment. For instance, medium voltage class often refers to voltage ranges above 1 kV and below 35 kV, which are beyond the typical operating range for secondary class arresters. High-voltage transmission systems operate at much higher voltages, generally above 69 kV, and would require different protective devices designed to handle those substantial levels of surge energy. The reference to 1000 kVA is a measure of apparent power rather than voltage, making it irrelevant to the question regarding voltage levels for secondary class arresters. Thus, option A accurately captures the standard application range for these devices.