IEEE 1628:2009 pdf free download – IEEE Recommended Practicefor Maintenance of DC Overhead Contact Systems for Transit Systems
4.1 General
The OCS is the overhead electrical distribution system through which electrical power is transmitted fromthe traction power substations to the rail transit vehicles or ETB.There are several styles or configurationsof OCS which can be used for a variety of applications. One property or project may use one or more stylesthroughout the system or only on specific segments of the route.
The configuration of the OCs conductors is dependent upon two primary engineering factors; first, themethod of terminating and therefore，of tensioning the conductors，and;second，the type of supportarrangement for the contact, or trolley, wire.
4.2 Conductor tensioning
Two methods are in general usc. The first is fixed terminated (FT) or variable tensioning, in which the OCSconductors are anchored or terminated directly to a fixed structure at both ends of each wire run.Anyexpansion and contraction of the conductors, due to temperature change,will result in a change in thetension in those conductors, and also in the amount of sag of the conductors between their support points.
The second is automatic tensioning (AT), in which the OCS conductors have some form of tensioningdevice attached to one or both ends of the conductors. The most commonly used are balance weights(Figure 1), in which a set of weights operates through a pulley system to maintain relatively constanttension in the OCS conductors over a particular range of conductor temperatures，with the objective ofmaintaining the contact wire at a near-constant height and tension above rail/road level throughout thattemperature range.Other AT devices include spring or hydraulic/pneumatic tensioners,which are oftenused for short tension sections,such as crossovers, or for wire runs where the temperature variation issmall, such as in tunnels. These latter devices do not afford constant tension throughout the full temperaturerange.
4.3 Support method
Two basic types of support method are in common use. The first and simplest is direct suspension, singlecontact wire(scw) construction (Figure 2). With this support method,the contact or trolley wire isclamped directly to the primary support system, be it a cantilever bracket arm, a simple cross span wire, ora bridle suspension. This method usually affords the simplest Ocs wire arrangements, but needs supportsat frequent intervals， since span lengths are usually limited to about 30-38 m or 100-125 ft on tangent(straight) track by the amount of permissible wire sag. In addition, the contact or trolley wire often needsreinforcing electrically with parallel feeder cables, either aerial or underground.
The second support method is messenger suspension construction, the basic form of which is termed simplecalenary where the contact wire is supported by hangers from a messenger wire (Figure 3). By adjustingthe size or number of these conductors，the current-carrying capacity of a catenary ocs can usually besized to match the load demand and, generally, there is no need for parallel feeders.Messenger suspensioncatenary construction typically permits the use of longer span lengths than direct suspension contact ortrolley wire construction.Span lengths of about 6l-73 m or 200-240 ft on tangent (straight) track arecommon. In older OCS installations, catenary styles using more than two conductors are still in use.