An arc fault is the discharge of electricity through the air between two conductors creating large quantities of heat and light. It is widely accepted that there are many variables which affect the behavior of an arc in real switchboards, and thus, understanding the phenomena is difficult. This paper is based upon the results of several years of arc testing with circuits from 155 Vdc to three-phase 450 Vac and at power levels from tens of kilowatts to several megawatts. The goals of the testing were to understand the general behavior of arcing faults in low-voltage switchboards-well enough to predict the motion of arcs-and to design electronic monitoring systems that are capable of protecting switchboards against arcs. This paper will focus on the arc behavior, whereas our other papers will discuss its implications upon forensic investigations and upon the arc-fault protective systems.