Most damage to a power cable during installation is invisible on the day. A pull that was too hard stretches the conductor, crushes the insulation at a bend or tears the armour bedding, and the cable passes its insulation test and goes into service. The fault appears years later, at the bend, and the cause is long forgotten. The way to avoid it is to work out three numbers before the pull starts and to measure the pull while it happens.
Number 1: tension along the route
On a straight, level section the tension needed to move the cable is the cable weight per metre, multiplied by the length of the section, multiplied by the coefficient of friction between cable and its support. On rollers spaced properly the coefficient is low; dragging a cable over the bare floor of a trench or through a dirty duct can double or triple it. On an incline, add the component of the cable’s weight acting down the slope when pulling uphill, and subtract it when pulling downhill.
At each bend the tension leaving the bend is the tension entering it multiplied by a factor that grows with the friction coefficient and the angle of the bend. Several bends in sequence multiply together, which is why tension at the end of a route with four 90 degree bends can be many times the tension after the first. Pull from the end that puts the bends nearest the feeding drum, where the tension is still low.
Number 2: sidewall pressure at bends
Sidewall pressure is the tension in the cable at a bend divided by the radius of that bend. It is the force pressing the cable against the inside of the bend, and it is what crushes insulation and displaces conductors. A cable can be within its tension limit and still exceed its sidewall pressure limit if the bend is tight. Large-radius bends, a bend sheave rather than a single roller, and reducing tension before the bend all bring it down. Whatever the calculated figure, the bend must also respect the cable’s minimum bending radius.
Number 3: the cable’s own limits
The allowable pulling tension depends on how the pull is attached. A pulling eye crimped to the conductors uses the conductor strength, and the limit is expressed per square millimetre of conductor area, higher for copper than for aluminium. A basket grip over the sheath uses the sheath, and the limit is much lower. The maximum sidewall pressure depends on the insulation and armour type. These values come from the cable manufacturer’s installation data for the specific construction; for MV and HV XLPE cables in particular the installation instructions should be requested with the cable, because a screened cable has more layers to protect than an LV one.
Site practice that keeps the numbers honest
Place rollers at close enough spacing that the cable never touches the ground between them, and use corner rollers or a sheave at every change of direction. Put a dynamometer or a winch with a tension readout in the line, set it to the calculated limit, and stop the pull if it is reached rather than adding another man on the rope. Keep the drum braked so the cable does not run off and loop. Use a swivel between the winch rope and the pulling eye so the cable cannot twist. Lubricate long duct pulls with a cable-pulling compound compatible with the sheath. And on any large LV XLPE cable, check the drum for transit damage before the pull starts; a cable that arrived with a flattened turn will not be improved by pulling it.
Where the calculated tension exceeds the limit, the options are a larger bend radius, an intermediate pulling point, a pull from the other end, or a joint. All four are cheaper than a cable fault under a road.