Sheath Voltages

Hi,

The are several different approaches to this question and there are different values of permitted voltages depending on your country. IEEE and IEC have guides for these induced voltages. The use of VSL's is one approach, but there appears to be little problems encountered if cables are in Trefoil and are under 500 Mtrs long and the sheafs and armoring are earthed at both ends.

I had asked the question regarding circulating currents on MV single conductor cables run in parallel in ground with SVL's at both ends. Since the length of the cables exceeded 1.5 Km we had several joints in these cables The mid point joint had both the Screen and armor joined together at either side of the joint but separated from each other and brought to a link box and connected in a Wye and taken to earth.

We had several failures of two cables almost simultaneously and it was found that the joints had failed due to excessive heating at the Joint area. Taking readings at the Mid point it was found the circulating current was around 50-60% of load but onlyon the sleads coming from the connections to the load side of the cable .After discussion it was decided to isolate the armor at the cable gland on the transformer case which was the at load end, thus isolating the armor to earth and leaving the shields connected to the SVL at both ends.

The result was the circulating current was now negible, however we now have to inspect all the buried joints and it is apparent that we now have stressed joints which could fail alter on.

The answer is to test for circulating currents and induced voltages as soon as possible when applying load to these circuits, and also design engineers really need to review the problems encountered with long runs of cables.