First, I suggest a search of this site. This topic repeats frequently.
The main points;
1> A power cable will induce a voltage in a sheath, braid, shield, and/or armour.
2> The induced voltage will be proportional to the length of the cable.
3> If the conductive covering is grounded at only one end, there will be a voltage present on the other end of the conductive covering.
4> If the conductive covering is grounded at both ends, a current will flow. The magnitude of the current will be dependent on the induced voltage and the impedance of the conductive covering and the ground path.
What to do;
1> If the conductive covering is grounded at both ends, some codes require the cable to be derated to allow for the extra losses and heating.
2> The conductive covering may be interupted at one or more places in the cable run.
For example, The conductive covering will be grounded at the supply end. At a point one third of the distance of the cable run the conductive covering will be interupted and insulated. The conductive covering imediately past the interuption will be grounded, and at the two thirds distance point the conductive covering will be interupted and re-grounded.
The conductive covering has now been divided into three sections and each section has been grounded at the supply end.
The cable may be divided into any number of sections, three sections was selected as an example.
3> The conductive coverings may be transposed. The conductive coverings will be interupted at intervals, but instead of being insulated and re-grounded, the incoming conductive covering on "A" phase will be connected to the outgoing conductive covering on "B" phase.
The incoming conductive covering on "B" phase will be connected to the outgoing conductive covering on "C" phase.
The incoming conductive covering on "C" phase will be connected to the outgoing conductive covering on "A" phase.
When transpositions are used, the cable is usually divided into multiples of three.
If your cable length is marginally over your guidlines, then your induceed voltages will be marginally over the guidelines. I would consider an extra layer of insulation on the ends of the sheaths. If your safety regime will not allow you to exceed the guidlines, I would suggest interupting, insulating and regrounding the sheaths at either the mid point or at some more convenient location that will result in both sections being within your guidelines.
respectfully
