It is the purpose of this standard to provide a procedure for determining certain operating characteristics of carbon, carbon-graphite, electrographite, graphite, and metal-graphite brushes under prescribed test conditions for all types of rotating ac and demotors and sim­ilar apparatus. The operating characteristics covered in this standard are: voltage drop, coefficient of friction, wear ratio as affected by low humidity, and commutating ability. The test procedures for properties of materi­als and properties of shunt connections are de­fined in ANSI C64.1-1970, Brushes for Elec­trical Machines (NEMA CB1-1970). Dimen­sions, tolerances, and descriptive terminology of brushes for test procedures should conform to ANSI C64.1-1970. Quantities expressed in parentheses are in the International System of Units (commonly called SI units).