Three-phase indicator

When a three-phase mo-tor is being installed, it is essential to know the sequence of phases R, Y and B, since an error may have disastrous consequences. The letters R, Y and B are abbreviations of 'red', 'yellow' and 'blue', that is, the colours used to identify the three phases. Also, red-yellow-blue is the sequence universally adopted to denote that the e.m.f. in the yellow phase lags that in the red phase by 1200 (a third of a cycle), and the e.m.r. in the blue phase lags that in the yellow phase also by 120°.