Where the voltages are symmetrical and equal, one light will always be out. When testing unsymmetrical voltages (such as two phase wires and the neutral of a Y system) one light will be bright and the other dim; the phase sequence will then be that indicated by the brighter light.

The TESCO Phase Sequence Indicator (Catalog No. 500) can be used on polyphase voltages from 115V to 600V, at 60 cycles. The instrument is completely self-contained, and does not require a range-changing switch. There are no moving parts, and the two leads are permanently attached to the body. The three electrodes are conspicuously numbered, to facilitate phase identification.

The instrument is extremely easy to use. The lead with the clip (#2) is attached to one phase terminal. The insulated prod (#1) is then touched to the second phase terminal and the prod (#3) on the end of the instrument body is touched to the third phase wire: the positions of the prods can be quickly interchanged to reverse the indication.

The body of the instrument is made of insulating material and measures 1 inch (2.54 cm) in diameter and 6 3/8 inches (16.17 cm) long overall. The total weight including leads is four ounces (.114 kilos), making it extremely convenient to pack for every job.