FOREWORD The IEC (international Electrotechnical Commission) is a worldwide organization for standardization comprising all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote international cooperation on all questions concerning standardization in the electrical and electronic fields. To this end and in addition to other activities@ the IEC publishes International Standards. Their preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with may participate in this preparatory work. International@ governmental and non-governmental organizations liaising with the IEC also participate in this preparation. The IEC collaborates closely with the International Organization for Standardization (ISO) in accordance with conditions determined by agreement between the two organizations. The formal decisions or agreements of the IEC on technical matters@ prepared by technical committees on which all the National Committees having a special interest therein are represented@ express@ as nearly as possible@ an international consensus of opinion on the subjects dealt with. They have the form of recommendations for international use published in the form of standards@ technical reports or guides and they are accepted by the National Committees in that sense. In order to promote international unification@ IEC National Committees undertake to apply IEC International Standards transparently to the maximum extent possible in their national and regional standards. Any divergence between the IEC Standard and the corresponding national or regional standard shall be clearly indicated in the latter.