<P> This construction can further be surrounded by a porcelain or silicone insulator for outdoor use, or by contraptions to enter the cable into a power transformer under oil, or switchgear under gas - pressure . </P> <P> Connecting two high - voltage cables with one another poses two main problems . First, the outer conducting layers in both cables shall be terminated without causing a field concentration, similar as with the making of a cable terminal . Secondly, a field free space shall be created where the cut - down cable insulation and the connector of the two conductors safely can be accommodated . These problems have been solved by NKF in Delft in 1965 by introducing a device called bi-manchet . </P> <P> Figure 4 shows a photograph of the cross-section of such a device . At one side of this photograph the contours of a high - voltage cable are drawn . Here red represents the conductor of that cable and blue the insulation of the cable . The black parts in this picture are semi-conducting rubber parts . The outer one is at earth potential and spreads the electric field in a similar way as in a cable terminal . The inner one is at high - voltage and shields the connector of the conductors from the electric field . The field itself is diverted as shown in figure 5, where the equipotential lines are smoothly directed from the inside of the cable to the outer part of the bi-manchet (and vice versa at the other side of the device). </P> <P> The crux of the matter is here, like in the cable terminal, that the inner bore of this bi-manchet is chosen smaller than the diameter over the cable - insulation . In this way a permanent pressure is created between the bi-manchet and the cable surface and cavities or electrical weak points are avoided . </P>

Voltage stress is maximum in a cable at the surface of the