The tank was also fitted with an independent high-level switch (IHLS) set at
a higher level than the ATG alarms. This was intended to stop the filling process
by automatically closing valves on any pipelines importing product, as well as
sounding an audible alarm should the petrol in the tank reach an unintended high
level. The IHLS also failed to register the rising level of petrol, so the ‘final alarm’
did not sound and the automatic shutdown was not activated. By 0537 hrs on
11 December, the level within the tank exceeded its ultimate capacity and petrol
started to spill out of vents in the tank roof.