The 99T666 series square instrument is an electromagnetic system and adopts a repulsive structure. The instrument consists of a measuring mechanism and an indicating device. The outer shell is made of flame-retardant ABS engineering plastic, with safe measuring terminals and efficient wiring methods. It also uses a printed dial and transparent glass cover, which is aesthetically pleasing and has a wide field of view.

The 99L666 series frequency meter, active power meter, reactive power meter, and power factor meter are rectifier square instruments composed of a magneto electric measuring mechanism and a measuring circuit. The shell is made of flame-retardant ABS engineering plastic, with safe measuring terminals and efficient wiring methods. It also uses a printed dial and transparent glass cover, which is aesthetically pleasing and has a wide field of view.