How to install overhead cables

Installation of overhead cables to out houses and garages needs to be done correctly, this is achieved by means of a catenary wire with a cable fastened to it.

In other countries catenary wires are something you see every day, especially in the U.S.A, they use a lot of catenary cables for everything from getting the supply into their homes, telephone cables to cable TV. However we in the UK don't really use them at all apart from garages and out houses.

What is a catenary wire? well its basically a length of steel wire anchored at each end and tightened using a turnbuckle, a Steel wire armoured cable is then fixed to the steel wire and terminated at each end.

A typical kit is shown below, this kit is available off of the internet and can be sourced at http://www.attock.net/netpro/catenarywirekit.htm.

 

For the overhead suspension of cables between posts / buildings
Kit consists of:
1 x 50m coil of 3mm catenary wire
1 x turnbuckle
4 wire rope grips
2 x hookplates
2 x eyelets

Catenary Heights and distances between suspension points

a Suspension point heights must be chosen to allow for the sag between supports.

b Greater heights than given in the table will be necessary in certain cases, such as where cranes may be present.

c The 'inaccessible to traffic' column does not apply at agricultural premises.

d 'hofr' means heat and oil resisting, flame retardant.

e The conduit should be of not less than 20 mm in diameter and not jointed in its span.

f A minimum height of 5.2 m is permitted for an existing overhead line (but not where refurbished or rebuilt).

 A suspended cable or catenary must be allowed to sag, as shown in Fig 4, in order that the tension (or pulling force) in it should not be unacceptably great. The tension depends on

the weight per metre of the cable or catenary and the distance between suspension points, and is greater the smaller the maximum sag.

 


To allow for the sag, the heights of cable suspensions need to be greater than the minimum
height required for the cable above the ground. Where the suspension points are at the same height, the point at which the maximum sag occurs will be midway between suspensions. With suspension points at different heights, the point of maximum sag will be off centre towards the lower suspension.