7 December 1975 - An electrician wanted to show his apprentice how to bridge electrical circuits. He decided to short-circuit the primary winding on one of the Unit 1 pumps by developing an arc following the edge of a wiring loom. The fire in the main trough destroyed the current supply and the control lines of five of the unit's six main coolant pumps. The fire was quickly brought under control by the fire-brigade and the pumps were temporarily repaired. After this near-disaster, fire protection within the power station was substantially strengthened and separate electrical lines for each pump were introduced. The incident was only made public in 1989. A few hours after the incident the
IAEA was informed by Soviet authorities, which classified the accident under
INES 4, later revised to INES 3.
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