"FOREWORD 1) The IEC (International Electrotcchnical Commission) is a worldwide organization for standardization comprising all national electrotechnical committees (IEC Nationa: Committees). The object of the IEC is to promote intcrnational 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 participatc in this preparatory work. International@ governmental and non-governmental organizations liaising with the IEC also participatc 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. 2) The formal decisions or agreements of the IEC on technical matters express@ as nearly as possible@ an international consensus of opinion on thc relevant subjects since each technical committee has representation from all interested National Committees. 3) The documents produced have the form of recommendations for international use and are published in the form of standards@ technical reports or guides and they are accepted by the National Committecs in that sense. 4) 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. 5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any equipment declared to be in conformity with one of its standards. 6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of patent rights. The IEC shall not be held responsible for idcntifying any or all such patent rights. This standard has been prepared by Working Group' 821: ""Signalling and sccurity apparatus for railways"" of IEC Technical Committee 1: Terminology. This Group is a joint IECKJIC (International Union of Railways) Working Group@ and comprises experts from both organizations."