One of the transformers in a shopping mall got unexpectedly switched off. It turned out that neither of its two cooling fans was active. A simultaneous failure of both fans is not impossible but hardly probable. Why hasn’t the failure of the first fan been noticed? Who is to be blamed and what actually happened?
“Having analyzed the situation it turned out that one of the fans was already out of operation,” says the Expert. “How come the maintenance worker overlooked it?” Well, he did not overlook, he checked and found out that both fans rotate. If he was more curious he would have noticed that only one of the fans is actually operating and that the rotor of the other one rotates in the wrong direction.
In order to observe it one must have looked very carefully and considered the facts. And we all know what the routine inspections look like – one checks only if the fan rotates. This one did. The rotating fan may be considered operational, and that was the case.
The transformers are located in the basement of the mall. To maintain the proper operating conditions, they had to be ventilated. Two axial fans on a transformer chamber were used. When the temperature exceeded the first threshold, one fan was to switch on, after exceeding the next threshold the other one switched on. During the inspection tour an employee checked the temperature of transformer windings and operation of fans. Since the rotors were rotating, he diagnosed that the operation is proper and ticked off the “inspection box”.
Nobody anticipated that when one of the fans fails it would still rotate during the operation of the other fan. The air pumped to the transformer chamber was escaping through the faulty fan and by flowing through, it was rotating the rotor blades. The failure of the fan was indicated only by rotor reverse rotation, which was not discovered during the inspection.
The failure of the first fan was caused by a short-circuit – the power cable burnt out. “There is no doubt that during an inspection tour an employee saw that the protection device tripped,” believes the Expert. “Instead of investigating the cause, he switched the protection device on and… it worked.” In which case he could continue with the tour.
One fan provided cooling for a significant period of time. When it failed the temperature rapidly increased above the safe value and the transformer switched off.
What about losses? They were significant. “The fans must have been supplied as soon as possible, which is not easy if you don’t have back-up ones,” explains the Expert. “There was a replacement fan, but only one. The second one was taken from other transformer chamber.” Rush, tension and complicated installation promptly made the costs increase.
If similar failures had occurred simultaneously in multiple locations, it might have turned out impossible to maintain fire protection system, in which case the whole facility must be closed. The losses would have increased immediately.
A similar scenario could have been true in case of loss of power in the ventilation system – if the facility was equipped only with mechanical ventilation, it would be equal to excluding it from the use.
The Expert concludes: “Human negligence was to be blamed with a little help from an unlucky coincident. Had the operation of transformers been monitored by an external system, the failure of the first fan would have been detected immediately after its occurrence.”
Fast stock level planning would have taken place to secure necessary equipment for the event of fan failure and transformer disconnection. It is almost sure that lessees would not have suffered from power supply interruption.