Sag is a design parameter that answers safety rules according to a number of constraints:
- Voltage level, tower design, public health regulations,
- Conductor features, line load,
- Ground profile and obstacles as documented at construction stage.
At all times there is only one sag value for a given span. DLR is about operating a line while making sure sag remains within acceptable limits.
Events affecting clearance can be temporary (crane, vehicle, animal crossing the line…), at various locations beneath the line, in which case they may pose a safety issue, solved by access restrictions. These events can also be durable (vegetation growth, new road, new building, other overhead lines…), in which cases they are managed in terms of line maintenance (vegetationmanagement, tower upgrade, tensioning…). New sag limits may then be taken into account for DLR.