What is Sag Template?
Definition: A sag template is a tool used to accurately determine the positioning and height of supports on a profile. It defines the limits for vertical and wind loads, and also establishes minimum clearance angles between the sag and the ground for safety. Typically crafted from transparent materials such as celluloid or plexiglass (and occasionally cardboard), the sag template features the following marked curves:
Detailed explanations of these curves are provided below.
Hot Curve: The hot curve is generated by plotting the sag values at the maximum temperature against the corresponding span lengths. It serves as a guide to determine the precise locations where supports must be positioned to ensure that the specified ground clearance requirements are met.
Ground Clearance Curve: Positioned below the hot curve, the ground clearance curve runs parallel to it. The vertical distance between these two curves is equivalent to the ground clearance value mandated by regulations for the particular line, thereby providing a clear margin for safety and compliance.
Support Foot Curve: This curve is specifically designed to aid in accurately locating the positions of supports for tower lines. It indicates the height measurement from the base of the standard support structure to the attachment point of the lower conductor. In the case of wood or concrete pole lines, however, there is no necessity to draw this curve, as these poles can be installed in any location that offers practical convenience.
Cold Curve or Uplift Curve: The uplift curve is derived by plotting the sag values at the minimum temperature under windless conditions against the span lengths. Its primary purpose is to assess the likelihood of conductor uplift on any support. Uplift of the conductor may occur at low temperatures, especially when one support is significantly lower than its adjacent supports, and this curve helps in identifying such potential scenarios.
Preparation of the Sag Template: Initially, the aforementioned curves are meticulously drawn on graph paper using the same scale as that of the line profile, with appropriate scales carefully selected for accuracy. Subsequently, with the assistance of a sharp - pointed probe, these curves are precisely transferred onto transparent celluloid or perspex sheets. Finally, the celluloid or perspex is cut along the line representing the maximum sag, which is the hot curve, completing the creation of the sag template.
