While testing automatic activation of 6 kV auxiliary power bus standby source of Unit 1 which was in maintenance and due to switching overvoltages, standby boiler's 6 kV cable insulation was damaged resulting in cable short-circuiting and burning. This resulted in the damage to Unit 2 cables laid nearby, deenergizing of two 6 kV auxiliary power buses, loss of power to two reactor coolant pumps causing the actuation of reactor scram. Breach of safe operation limits and conditions did not occur. Radiation situation remained normal. No injuries occurred while extinguishing the fire.