hi
I would like to know, why the airplane must be electrical systems off while the fuel tanks remains open?
thanks.

Hi,
You are not allowed to have power on when you have the fuel tanks open due to the safety reasons and because there are Flammability Zones around an Airplane with the open Fuel Tanks. Mainly, the reason why you are not allowed to have power is due to the electrical faults and wiring problems , these two possible situations can cause sparks which can ignite the fuel vapours.
For more info. you can read AMM: 28-11-00 / PB. 201 (B737-NG) and also find useful information in the following article: http://www.boeing.com/commercial/aeroma ... 2011_q1/3/

I hope to be useful to you.
Regards, Octavian

But if the tanks are open for a long time, and forced ventilated for several days, and the vapor is all gone, and the fuel remains are cleaned from the tank, and you did a test with the LEL meter and concluded there are no vapors anymore anywere, you can apply power again to the a/c.
This happens during long D-checks for example. There is no danger involved if you have the above situation.
But then again it may be company policy or local regulations.

We did this in the past too, and there is not any reason you can not do this since all danger of explosion is eliminated.