As an ISO certified company, we have an obligation to investigate any of our product failures, but over the years we have been asked more and more to get involved in general failure analysis. We undertake to make an un-biased investigation into any failure involving cable joints and terminations regardless of who manufactured the product.
We were asked to make a report on an 11kV 3 core transition joint which was audibly discharging. The yellow stress relief mastic was applied correctly, to the ends of the extruded screen of the XLPE cables (Fig 1), and the applied screens of the PILC cable (Fig 2). These tapes should have been covered by the dual wall core insulation tube. Unfortunately this was not the case (Fig 3 and 4).
It can sometimes be difficult to be 100% accurate when attributing cause for any breakdown. However, in this case we can confidently state that the primary cause was the incorrect installation.
The cables being jointed were 185-240mm² and the installer had used the stripping dimensions for that range as per the installation instructions provided within the kit. Unfortunately the actual kit used was designed for cables ranged 35-95mm². The dimensional difference accounts for the dual wall tubing “floating” between the core screens.
Items within the kit such as constant force springs, under armour support rings, tubings and mastics, would have been sized to suit the smaller cables for which this joint was designed.
Figure 1
Figure 2
Figure 3
Figure 4
An 11kV 3 core transition joint which was audibly discharging.
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