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Protection riddle no.4 – Earthquake in nuclear power plant
After one sever earthquake happened in the hypothetical nuclear power plant area, following observations were reported by special investigation committee that were authorized to estimate earthquake demolition in power plant.The mentioned Power plant has two units. Each unit has one control room. The control rooms have many quite similar electrical circuits integrated in protection and control panels. The panels are fixed on the floor of the rooms. According to the committee technical reports, one of the generator protection panels that were installed in unit 1 control room, broke from bottom of its base plate and fell down due to the strong vibration. On the other hand protection panels located in other control rooms (unit 2) withstood against mechanical stress. After more verification they found that the protective relays installed in inversed panel had operated correctly and related generator circuit breaker had received correct trip commands. But the protective relays located in other protection panels (unit 2) couldn’t have suitable reflection against earthquake.

Consequently, one of the panels damaged due to falling but it could protect relevant generator, vice versa other panel that withstood against vibration didn't do its assignments (generator protection).
Regardless similar electrical circuits, the method of devices installation in dedicated panel were different from the other panel. In the selfless panel, heavy equipments (e.g. matching current and voltage transformers) were installed on the top of the panel and main generator protection relays were located in the bottom side but in the other panels, heavy equipments were installed down and protective relay were fixed on the top of the panels.
  
How can you explain different behaviors of protection panels against earthquake waves?
Author : Hamid
 
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