A test facility for a full-scale prototype of the Neutral Beam Injector (NBI) for ITER is planned to be built in Padova, Italy, in the framework of the European activities in support of ITER. Two possible sites were considered: the site hosting RFX (Reversed Field eXperiment), connected to the 400-kV transmission network, and the site hosting the National Institute for Nuclear Physics (INFN), connected to the 132-kV network. Analyses have been made to evaluate the impact on the two HV networks due to the additional NBI load. A particular aspect was studied in detail: during the NBI operation, very frequent accelerator grids breakdowns are expected, requiring fast de-energization of the main power supplies, followed by voltage re-application in about 50 ms; this can cause active and reactive power steps and consequent voltage fluctuations (flickers) on the HV networks. The analyses are described in the paper and the main results reported and discussed.