IEC 61857-21:2009 describes a general-purpose model (GPM) and a tall channel alternative model (GPM-TC) which can be used for the evaluation of wire-wound electrical insulation systems (EIS) where specific electrotechnical products are not available or required. The third edition cancels and replaces the second edition published in 2004, and constitutes editorial revisions to make this standard compatible with Parts 1 and 22.