Stress Control On Paper Belted Cables

By on 18th September, 2013

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It is accepted that the use of paper belted cable has been virtually eliminated but there are huge amounts in the system and if it has to be re-terminated or jointed to polymeric cable the following sections will be relevant.

Initial Preparation Of Paper Belted Cable

When the outer metal sheath has been removed black carbon papers, which are wrapped around the three cores, are exposed.

These papers are removed to within 10-15 mm from the metal sheath - this dimension will be specified by the accessory manufacturer. This reveals the belt papers which are the same colour as the core insulation. The belt papers are then removed a further 5-10 mm beyond the carbon papers as shown in fig. 29.

Pictured: Figure 29 - Paper Belted Cable In Area Of Metal Sheath Removal

 

Many failures occurred in the early stages of developing dry teminations due to air being trapped in the area shown in fig. 29. Most of the time the failure was from phase to earth but another problem arose due to the first layer of heat shrink tubing not shrinking completely which in turn created voids.

The problem arose because the tubes were pushed as far down the cable as possible but then the flame used to carry out the shrinking did not penetrate between the cable cores and the tubing failed to fully shrink. To avoid necessary to initially shrink the tubes as shown in fig. 30, wait then twist in the direction of the cable papers and slide down into position. 

Pictured: Figure 30 - Correct Application Of Inner Tubing Heat Shrink Termination

Cable Breakout Area Of Unscreened Paper Cable 

It was mentioned in an earlier section that the cable breakout area of an unscreened paper cable contains areas where air could be trapped when using a dry joint or termination. Reference to fig. 12 in chapter 3 shows the potentially problematic sites.

Before the advent of dry terminations and joints this type of cable would be immersed in bitumen compound, which pushed all the air from this region.

The solution to this problem with modern cable accessories is to use a plug to fill the areas between the three cores at the break out and a self amalgamating tape wrapped around the three cores and onto the metal sheath. The tape will build up a profile onto which the cable breakout or other device can be installed without trapping air as shown in fig. 31.

The tape used may be stress relieving, but a good quality self amalgamating insulating tape such as butyl is equally as good. The important thing is to eliminate air from the critical regions of high stress.

Once the stress control tubes or preferably semi conducting tubes (see later) are fitted, the breakout is pulled down onto the cable and fits tightly to the void filling tape as shown in fig. 32.

Screening The Cable Cores

Pictured: Figure 31 - Treatment Of Paper Cable Prior To Installing Breakout

The need to screen part of the belted cable cores is essential when the cores of a termination have to be crossed. Many failures have resulted due to core crossing of paper belted terminations as discharging occurs in the stress control area between cores. The same is necessary when installing a joint.

Pictured: Figure 32 - The Breakout Area With Conductive Breakout Fitted

Paper belted cables can be converted into screened cables from the point of sheath removal quite easily but is not an option offered by all termination manufacturers.

This modification eliminates the problem often found particularly in terminations where core crossing occurs and unscreened areas of the termination are close or touching.

The practicalities of terminations in cable boxes will be considered in more detail in a later chapter. Figure 33 shows a typical arrangement for screening paper belted cable which is standard practice in a joint, but not on terminations.

Pictured: Figure 33 - Screening Paper Belted Cable

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