I need to solder together some wires which will eventually run at about five kV, and I'm trying to figure out the best way to insulate the exposed leads. Heat shrink (my usual go-to) is apparently not rated above about 600 V. Kapton seems to be rated much higher (100's of kV/mm), so I am thinking of wrapping the leads in several layers of this. Given the high voltage hazard, I'm looking for advice: Would this be a good solution, or is there a better way?
Kapton is not appropriate to wrap cables: it's relatively stiff, and thus won't be conformal to the cable, which leaves air gaps, which have lower insulation per mm.
So, even multiple layers of heat shrink would be better.
At 5 kV, I'd start by trying to keep cables mechanically separated far enough that the air distance ensures sufficient isolation, even in the absence of any dedicated isolating material.
Use high-voltage cables, which come with the necessary isolation. Don't connect them at the same distance – cut one conductor shorter than the other, so that you don't break isolation in close vicinity.
There's isolating potting that you can use to fixate a solder joint.
KAPTON (tm) is excellent (Polyamide) But must be sealed from humidity to prevent creepage.
Rubber insulated wire is best.
Exposed terminations can be somewhat sealed if dry when applied with RTV Silicone to 25kV/cm safely from high impedance sources.
Ignition wiring uses Rubber insulation but now is all carbon, but you can still get copper wire. Rated for spark plugs.
You MUST be aware of ARC FLASH protection gear if high energy source is being tested.
Kapton is a really good insulative medium and aircraft cable use to be wrapped in it. However... Kapton is susceptible to embrittlement over time
If it is to encapsulate the solder connections then yes you could rely on Kapton or go for some form of potting to encapsulate the joint