After countless mishaps in controlling and monitoring the huge steel tanks that store waters with different degrees of contamination - from water with hardly any contamination (rainwater, water drawn from the wells for the underground water bypass plan) to water with extremely high beta nuclides (waste water after SARRY/Kurion and desalination) - and countless promptings from Nuclear Regulation Authority commissioners, TEPCO installed water gauges to all 1000 tanks.
The water gauges are monitored in a room dedicated to monitoring the tanks, using a PC.
On April 8, TEPCO quietly announced that alarms went off on two of the tanks, without the staff in charge of monitoring (TEPCO employees) noticing the alarms for two hours. They turned out to be false alarms, and there was nothing wrong with either tank.
Now, why didn't the workers hear the alarm?
Then on April 9, TEPCO further admitted that there were not just two but 433 tanks whose alarms on water gauges were turned off.
Why? Because someone had turned off the sound on the PC.
This has got to be one of the lowest level of (non-)performance, even for TEPCO. But hardly anyone pays any attention any more to anything happening or not happening at Fukushima I Nuclear Power Plant, and even the NRA commissioners seems resigned to the fact that nothing they say or suggest to TEPCO will be fully heeded by TEPCO anyway. Chairman Tanaka, in recent press conferences, reportedly said "TEPCO is just that, that kind of company..."
From Mainichi Shinbun (4/9/2014):
Alarms on water gauges on 433 tanks that store contaminated water were "off"
TEPCO revealed on April 9 that the water gauges on 433 tanks out of 1000 tanks that store contaminated water at Fukushima I Nuclear Power Plant had been set so that there would be no sound even if an alarm was sounded to indicate abnormal water levels. On April 8, some water gauges malfunctioned and alarms were issued, but since there was no sound [the workers] didn't notice for two hours.
According to TEPCO, an alarm goes off when the water level rises or drops rapidly. However, the sound of the PC that controls all water gauges was set "off". When the highly contaminated water leaked in February this year, the alarm did go off. TEPCO believes the sound setting was changed to "off" after the February incident, and says they are investigating whether the sound was turned off by mistake.
My guess is that there were so many false positives that the workers couldn't stand the alarm noise and turned off the sound on the PC.