to determine a sheath voltage limiter (SVL) should have a continuous operating voltage of Uc higher than the calculated voltage at the cable end.

my point is why to choose higher than calculated voltage and not the same or less as to choose higher than the calculated value could be not safe for the cable system. 

It is important to realise that the intention of the SVL box is to ensure that the sheath/ armour is interrupted, being in effect broken into short sections so that the end to end induced voltage in any one section is non-hazardous (exactly how long that section is,  first requires the induced voltage to be calculated, and secondly a decision  on what non-hazarous means in this context).  The SVL is then in its high resistance state most of the time, and in normal operation no sheath/armour current flows.
However, during a fault, when we do need current to flow in the sheath/armour of the cable, we require the SVL, or SVLs to  breakdown into their low impedance condition, and carry the highest credible current, for long enough for whatever protective device is involved to operate and remove the supply, ideally with some margin.

The SVL is usually a large block of semiconductor, but if it helps you can imagine it behaving in some ways like a spark gap in that it has a definite 'flash over' threshold,  and the ability to carry high currents for a short time, and then a recovery time once the current has dropped, for it to go back to the high impedance state,

This is similar to a surge protector, but the way it is tested and specified is a bit different.

What we do not want is the SVL to 'flash over' during normal operations, like brief overloads, switching transients and so on, so the threshold voltage and the segment length need to be set so that the operating voltage is perhaps twice the maximum expected  armour voltage.

Mike

For a description of induced sheath voltage

https://electrical.theiet.org/media/1678/voltages-induced-in-the-non-magnetic-metallic-sheaths-and-or-armour-of-single-core-cables.pdf

For the maths that allow you to calculate it

https://mycableengineering.com/knowledge-base/sheath-induced-voltage-circulating-current

to determine a sheath voltage limiter (SVL) should have a continuous operating voltage of Uc higher than the calculated voltage at the cable end.

my point is why to choose higher than calculated voltage and not the same or less as to choose higher than the calculated value could be not safe for the cable system. 

It is important to realise that the intention of the SVL box is to ensure that the sheath/ armour is interrupted, being in effect broken into short sections so that the end to end induced voltage in any one section is non-hazardous (exactly how long that section is,  first requires the induced voltage to be calculated, and secondly a decision  on what non-hazarous means in this context).  The SVL is then in its high resistance state most of the time, and in normal operation no sheath/armour current flows.
However, during a fault, when we do need current to flow in the sheath/armour of the cable, we require the SVL, or SVLs to  breakdown into their low impedance condition, and carry the highest credible current, for long enough for whatever protective device is involved to operate and remove the supply, ideally with some margin.

The SVL is usually a large block of semiconductor, but if it helps you can imagine it behaving in some ways like a spark gap in that it has a definite 'flash over' threshold,  and the ability to carry high currents for a short time, and then a recovery time once the current has dropped, for it to go back to the high impedance state,

This is similar to a surge protector, but the way it is tested and specified is a bit different.

What we do not want is the SVL to 'flash over' during normal operations, like brief overloads, switching transients and so on, so the threshold voltage and the segment length need to be set so that the operating voltage is perhaps twice the maximum expected  armour voltage.

Mike

For a description of induced sheath voltage

https://electrical.theiet.org/media/1678/voltages-induced-in-the-non-magnetic-metallic-sheaths-and-or-armour-of-single-core-cables.pdf

For the maths that allow you to calculate it

https://mycableengineering.com/knowledge-base/sheath-induced-voltage-circulating-current