The common three-phase electrical system isn't difficult to understand: three identical ac voltages that pass through their respective peaks and valleys at evenly spaced times. In any real circuit, each voltage will be associated with the flow of a "phase current." The three phase currents are represented by a second set of rotating vectors, also spaced apart by 120 degrees, each one related to the corresponding voltage by a power factor angle . The disadvantages of delta connections lie in the grounding arrangements needed for safety, and the necessity to insulate the transformer windings for full line voltage rather than the lower line-to-ground value. The delta advantage is that failure of any one transformer allows three-phase service to continue with the remaining two. Also, cost and space savings can sometimes result from supplying the total load from only two transformers. Future load growth can be accommodated by adding he third transformer at a later date.