Calculations show that the tank would have begun to overflow at about 05:20. [ 1 ] [ 10 ] There is evidence suggesting that a high-level switch, which should have detected that the tank was full and shut off the supply, failed to operate. [ 1 ] [ 10 ] The switch failure should have triggered an alarm, but it too appears to have failed. [ 1 ] [ 17 ] Forty-one minutes later, an estimated 300 tonnes of petrol would have spilled down the side of the tank through the roof vents onto the ground inside a bund wall—a semi-enclosed compound surrounding several tanks.