# Selecting of SVL Rating

Hi everyone,

in the HV Cable system, we have to install the link box for earthing purpose, however, the link box is having different types such as a) LB Direct earth b) LB with / without  SVL, ..etc, the  SVL (Sheath Voltage Limiter) is non linear resistance, designed to  work as Fues (just an example) to discharge the induced voltage on the HV Cables, during normal operation it will be switched off while in the short circuit it will be switched on for discharging to the main earth.

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.

Parents
• SVL is a new thing for me - so this is all guesswork (but might stimulate some useful discussion). The way I'm thinking - we know single core (non-ferrous) cable armours on a.c. circuits will develop a voltage between their ends (basically the armour forms a 1:1 transformer with the core, if a horribly inefficient one) - and generally we try to arrange things to keep this at safe levels (e.g. below 25V or 50V) and ideally try to avoid circulating currents in the armours. I think usual UK practice normally involves solid bonding of the armour at at least one point, with either gaps in the armour, or connections to armours from all phases 'rotated' at intermediate points so the induced voltage tends to cancel out.

Like I said, the SVL is new to me, but conceptually from what you describe it sounds a but like an SPD - shorting to earth if an overvoltage is present. I'm guessing it would be positioned where the armour would be normally gapped, intended to introduce additional earthing (or bonding?), but at the expense of allowing potentially large circulating currents. Perhaps when the cables are carrying substantially larger than the normal design current (e.g. during fault clearance times)?

So I'm guessing the thinking is that in normal circumstances you want to avoid circulating currents (as it's not only a waste of power, affects the voltage drop in the main circuit and generally overheats things - always bad for any conductor and especially its insulation) - so rate the SVL so it normally isn't called into action (i.e. > the voltage it would normally be exposed to) - but still lower enough that it can work on rare occasions and for short durations where the heating effect from circulating currents would be tolerable.

or I might be barking up completely the wrong tree....

- Andy.

• Andy and anyone else wondering what this is about - this intro to the topic may be helpful

https://www.powerandcables.com/svls-sheath-voltage-limiters-for-protection-of-mv-hv-cables/

Generally it is only needed when the currents within the armour of a cable do not cancel and induced sheath current would otherwise be a problem.

Cable with all three phases present inside the armour do not have this problem.

Mike