A sag template is specifically design for transmission line according to their voltage levels. According the voltage levels and the conductor used there are specific design conditions are used.Sag Template is a very important feature for the surveyor by the help of which the position of tower can be decided on the Survey Chart so as to conform to the limitations of specified minimum ground clearance required to be maintained as per prevailing national rules, between the line conductor to ground telephone lines, buildings, streets, navigable canals, power lines, or any other object coming under or near the line and the limitation of vertical load coming on any particular tower.
The location of Structures on the ground profile with sag template is essential for both correct design and economy.
These Sag templates are consist of set of parabolic curve which design on transparent sheets (Commonly use acrylic clear sheets or celluloid sheets).
Sag Template consists of a set of parabolic curves drawn on a transparent paper, a celluloid or acrylic clear sheet duly cut In between the curves to allow surveyor to see through them on the Survey Charts placed underneath it.
Curves Under Sag Template
The typical set of curves under Sag templates consist following curves.
Cold sag curve or uplift curve: Corresponding to the vertical component of sag at worst load conditions of wind. This will shows the sag of power transmission line under minimum temperature.
Hot sag curve or maximum sag curve: Corresponding to the ‘maximum still air final sag’ at maximum temperature including sag tolerance to be accounted for, if any. This will shows the sag of power transmission line under maximum temperature.
Ground clearance Curve: Ground clearance curve are drawn parallel to Maximum Sag Curve. Drawn at a distance equal to minimum specified ground clearance, from and parallel to Hot sag Curve.
Support foot Curve: Tower Footing curve is drawn parallel to Ground Clearance Curve. Drawn at a distance equal to the bottom conductor height at tower, from and parallel to Hot sag Curve.
Hot sag curve is normally drawn through the origin ‘0’ at the center line of the template. The Cold and Hot sag curves are plotted and cut as a parabola on the minimum and maximum sag of the ruling span (the normal design span is the theoretical ruling span) which is extended by computing the as proportional to square of the span for spans both shorter and longer than the ruling span. Any particular span is considered spread-out by half of value on either side of origin ‘0’.
So its vital to get a good idea on sag template on transmission power line design stage.
Other Related Articles of Electrical Power Transmission Lines
Following are the article which related to power transmission line in electrical engineering industry.