Most aircraft (both fixed wing and helicopters) have over voltage protection relays specifically for protecting their aircraft systems from being damaged by high voltages from GPUs. Unfortunately, there is no industry standard for the setting of these relays. However, most are 30 to 31 volts. Some GPUs can range from 28 to 30 volts depending on the brand and their age. A 28 volt battery pack, such as the START PAC 6028QC, will actually read 30.5 volts when fully charged. One example of an aircraft where the voltage protection relay is set at an unusually low value, is the Eclipse 500 at 29 volts. At many airport facilities, the GPUs exceed this value.
The VRU 600 and VRU 1200 START PAC voltage reduction units, therefore, have been developed so that when connected between the GPU and the aircraft, the voltage is lowered by 0.6V or 1.2V respectively. In many cases, this slight voltage drop will allow the aircraft to accept the GPU.