What is the first step when dealing with an electrical arc from the OBS window?

Turning off the window heat switch is the appropriate first step when dealing with an electrical arc from the OBS window. This action helps to eliminate or reduce the electrical current that may be causing the arc, thereby minimizing the risk of fire or further electrical hazards. It is critical to prioritize safety by addressing the source of the problem directly and stopping the potential for further electrical discharge.

Once the power to the window heating element is cut, additional actions may be taken to ensure safety, such as evacuating the area or investigating the source of the arc. However, those steps become relevant only after the immediate danger posed by the electrical current has been mitigated by switching off the heat.