Cable tension calculations

Calculation formula for cable tension load

Where
Weight (in kg/m): Apparent weight of one cable meter
Tension (in daN): Calculated tension load applied to the cable and clamp
Span (in m): Distance between two poles
Sag (in m): Vertical distance at the center of the span, usually equal to 1% of the span

For example, for a 96 fibre count distribution cable, whose weight is 0.1kg/m, deployed on a 50 meters span with a sag of 1% (0.5meter), the calculated tension is :

Tension = 0.1 x 502₂ / 8 x 0.5 = 62.5daN

Bad weather conditions induce additional load on overhead infrastructures. The ice load increases the cable weight as well as the total surface subject to the wind.

The above formula takes into consideration weather conditions and their impact. Thus, wind and ice loads are integrated in the apparent cable weight calculation :

Cable data is provided by the cable manufacturers, while climatic data can be usually found in national standards for building/ infrastructure. As for example, in the USA, the National Electric Safety Code (NESC) Rule 250B defines 3 regions with typical values for ice thickness, temperature, and wind pressure :

A good knowledge of the topographic parameters (span, gradient of the ground) and of the climatic conditions allows to anticipate the loads and the overloads impacting the overhead infrastructure. This is also the best starting point for choosing the right material, cables and clamps, adapted to the considered area for a qualitative and future-proof network.

Pole banding or drilling: which functional differences ?

As a general rule, two attachment methods are possible to install pole hardware: by performing pole banding applications or by driling the support. The choice between these two solutions is first of all made by considering the pole’s characteristics. For non pre-drilled concrete, metal or composite poles, the pole line hardware can only be mounted by using pole band. However, on wooden poles, cross-arms or brackets can be installed at convenience either by using pole band or by performing drilling applications.

Due to its peripheral tightening, pole band does not alter the original structure of the support and allows the maintain of its exact mechanical properties. The different pole band dimensions and grades meet the various needs of mechanical and corrosion resistance.

When a wooden pole is subject to harsh climatic conditions, its diameter can reduce to a smaller or higher extent. In this particular case, it is advisible to use the drilling method for mounting pole hardware. It is also important to respect a minimum spacing of 5cm between each hole, regardless of the pole’s side and the length of the used material (pass through bolt or lag bolt) Poorly performed drilled holes can reduce the mechanical resistance of a pole, endangering thus all the existant infrastructure as well as the direct neighbouring areas (pedestrians, houses, road crossings, ...).

Cable-clamp compatibility for a future-proof network

Considering the sustainability and reliability of the network, it is essential that pole hardware and anchors to be adapted and qualified for the network on which they will be installed. The mechanical connection between the anchor and the cable constitutes therefore an important issue for overhead networks. The compatibility between anchors and cables is checked by carrying out the following qualification tests:

• Tensile tests at the short-term tensile load of the cable (Maximum Allowable Tension) according to EN 60794-1-2 standard - modified Method E1 involving a couple of anchoring devices on a cable length greater than 1 meter. There should be no slippage of the cable inside the anchoring clamps, no deterioration of the cable, nor deterioration of the signal (attenuation less than 0.1dB).
• Galloping test for anchor clamps according to EN 60794-1-2 standard - Method E1. This consists into applying 10 undulations to cables with a smaller or equal diameter to 6mm (drops), 3 undulations to cables with a greater diameter than 6mm (distribution and feeder cables) and a measurement of optical losses for 300 hours. A test is considered conclusive when registered optical losses are less than 0.1 dB throughout the test.

Droptic® LM4 – the drop cable solution enabling ZERO signal loss

In FTTH roll-outs, transitions between outside plant deployments and inside premises installations are nerve centers of the network architecture. Indeed, due to improperly performed splicing applications between outdoor and indoor drops, often optical budgets are overstretched. To avoid this and ensure the fastest speeds to end-users, Droptic® LM4 double sheathed optical drop cable is an effective solution. Designed as a multi-purpose drop cable, the LM4 drop enables simple, fast and reliable outdoor-indoor transitions.

Offering a rugged and watertight construction, the LM4 drop suits for both aerial and duct deployments. In overhead FTTH configurations, LM4 can be installed for spans up to 70m. Moreover, the dimensions of this drop cable have been reduced for better wind behaviour performances and a compatibility with duct installation on several hundred of meters.

Built with a double sheath construction, Droptic® LM4 presents two ripcords that can be used to strip off its HDPE outer sheath in a couple of seconds. Thanks to this operation, a Low Smoke Zero Halogen Flame Retardant LM1L drop cable is reached. This can be used for performing effective indoor applications. No splice is thus required to make the transition between the LM4 drop and the LM1L indoor drop. This functional convenience makes possible a constant transmission of the optical signal from an outdoor distribution point up to the telecommunications outlet without weighing down the optical budget.

• One drop for aerial, facade, duct and indoor FTTH roll-outs
• High mechanical performances