The plant was in mode 6 - refueling. Nuclear fuel was in the reactor vessel, reactor vessel head was removed and the reactor cavity was flooded. In accordance with the Technical Specifications one train of the Residual Heat Removal System (RHR) was in operation. At 10.00 a.m. testing of one train of batteries for DC power supply started. During the test the batteries were isolated from the safety bus 125 V DC and the bus was powered through the charger from 400 V AC power supply. Simultaneously welding was in the process on the metal support of the cabinet containing breakers controlling the line fro the 400 V AC power supply and the safety bus. One of the workers by accident triggered the breaker thus disconnecting power to the 400 V AC power supply resulting in all the equipment connected to this train to loose power. Power was lost tot he pressure transmitter which protects RHR pumps against overpressure when suction is from the reactor coolant system. The loss of power released the relay in the transmitter which caused the isolation valves on the suction side of both RHR pumps to close. The RHR pumps lost the suction therefore the operators tripped them manually. RHR system was not operable for 15 minutes when power was restored to the 400 V AC power supply, the isolation valve of the RHR pump was opened manually and RHR pump was put into operation. Explanation of rating: Manual III-3.1.1 replaced by C3.1.1, within TechSpecs requirements, time available before primary system starts to boil is relatively long - about 6 hours.