At approximately 10:01 am, May 27th, 2011, a fire was found at the 
distribution switchboard for lighting located in the power panel room for
the high pressure core spray system on the first basement level of the 
Reactor Building Annex Area of Unit 1 which was in the status of shutdown.
At 10:04 am, workers of a partner company extinguished the fire and the
operator of TEPCO confirmed it. Afterward, at 10:08 am, we reported it to
the fire station. Fire fighters also confirmed at site that fire was 
extinguished and determined that it was a small fire. There were no
injuries and no radiation leak to the outside environment from this
incident. (Previously announced on May 27)

Survey result, estimated cause, and countermeasure are as follows: 

<Survey result>
Survey revealed followings: 
•Most damaged part from the fire was the distribution circuit breaker
 located at the very bottom inside the distribution switchboard for
 lighting. 
•Insulation resistance of burnt distribution circuit breaker was measured
 on May 19 and the function was confirmed to be intact.
•There was seawater inflow (the floor was flooded to 5cm high) from 
 tsunami into the power panel room for the high pressure core spray system
•On the day of fire, while the room was not flooded, the humidity inside
 the room was high as the air conditioner was not working due to tsunami.
 Therefore, environment was not ideal for the power panel.
•As a result of inside analysis of the distribution circuit breaker which
 located near the burnt breaker, salt was found in the connecting parts
 inside the breaker. 
•During the inspection of the burnt power panel after tsunami, equipments
 including the distribution circuit breaker inside the panel were not
 inspected.

<Estimated cause>
As the inside of the power panel was not inspected, salt inside the
distribution circuit breaker was not found. Afterwards, dew condensation
water attached there. Therefore, insulation resistance decreased and the
electrification with this condition was considered to cause short circuit
and fire.

<Countermeasures>
Following countermeasures will be implemented. 
•Flooded electric equipments will be replaced or repaired.
•When using the electric equipments located in the flooded area, salt
 attachment will be inspected.
•When using the equipment for the first time after tsunami, insulation
 resistance and safety of the equipment as well as the environment will be
 confirmed.
•Above 3 items will be informed to TEPCO supervisors and workers from 
 other companies.
The burnt circuit breaker will be newly manufactured.

TEPCO will implement countermeasures in order to prevent the reoccurrence
of the fire. 

 Appendix: Fukushima Daini Nuclear Power Station Unit 1, Location of fire 
           occurance(PDF 42.6KB)