Splicing 20KV CRT high voltage cable help

[OmegaHyperion]

So, again some cut cables, this time with the twist that i don't have the option of replacing this one. It needs to be spliced back together

The cable in question is a 20KV shielded high voltage cable powering a CRT. Since the cable i in a potted tank on one end, and an incline resistor on the other (with the anode cap for the CRT) i cant exactly replace it.

So, here is what i was thinking about doing:

Remove the outer insulation and shielding braid from about 3 cm of wire per side. Remove the inner insulation from about 5 mm on each side. Solder together.

Then using custom made PVC or Nylon Cylinders with a hollow cavity and hole, cover the solder joint, making sure shielding is not within this joint. Then fill this cylinder with Epoxy. Then once that is cured, place braided shielding over the joint, and connect to the original shielding (the cable is not RF, so i think i can probably get away with just a wire here). Then using 3:1 sweatshirt cover the joint.

So, before i go run off and make this, will this work?

I have the ability to test this up to 50KV AC if need by, but i would like your guys input on this before i set up my HV test system, and make a few test cables.

If anyone is wondering, this cable already has a splice on it, to accommodate the current limiting resistor installed by the OEM. So, i know its possible to do, i just haven't found an off the shelf product to do it.

Any input and or help would be much appreciated.

If anyone wants pictures of the cable in question, i would be happy to post some.

[floobydust]

High voltage splices are common in the electric utility industry. There are many kits, sleeves, epoxy, heatshrink etc. Tape is good for 1-5kV per wrap ages and gets loose so heatshrink is popular. You can use plastic tubing (i.e. Bic pen)  for a hard jacket.

I find most important is the connection must have no sharp points.
Otherwise you get corona or it pokes through your insulation. Much worse if the splice ends up near grounded metal verses free floating.
I solder the two bare wires side by side (parallel). I have put silicone caulk on top of the soldered wires and then heatshrink tubing over. This then squishes out all the excess silicone. Just to keep air out of the splice.

I do not understand the shielding you are talking about stripping back 30mm, I've never seen shielded HV cable in a CRT.

This fellow does it pretty simple: http://www.curtpalme.com/HV_Lead_Splicing.shtm

[Ian.M]

The key factors are no sharp points or edges in the joint itself and no voids when you rebuild the insulation.   Either can cause breakdown.

For small diameter joints, potting is questionable unless you can vacuum degas the compound before pouring.  Its not an issue when field jointing large HV cables as the mix in bag potting compounds have minimal gas entrainment to start with and any bubbles are much smaller as a proportion of the compound thickness due to the much larger joint size.

You can get clear silicone self-amalgamating tape with a dielectric strength of 7KV/20mil, but it would still be worth applying it over a thin smear of neutral cure electronics grade silicone sealant to be certain there are no voids.   You'll need to taper the ends of the inner insulation to get enough path length, and overlap the silicone tape outside it for about an inch.

If its got braid shielding, you should try to open it up and fold it back rather than cutting it away, and once you have made the inner joint and made good the inner insulation, you need to re-join the shielding, possibly by patching in a larger diameter length of braid.  That's going to be a real PITA, soldering enough braid strands without damaging the insulation under it.   With 1" overlap, you don't have to get all the strands, but you do need to solder enough for a reliable connection.  Then finally enough layers of adhesive lined heatshrink over the top with 1" minimum overlap on the undisturbed outer insulation for the final layer.

[IanMacdonald]

Above points are all good. Main issue is that the distance that can jump is considerably more than the insulation thickness, so you need to strip the braid back a fair distance and apply some form of tight-fitting cover to your join, such that an arc can't sneak along under it to the braid. 

For braid continuity I'd just link the ends with ordinary wire, insulated with heatshrink.

If using silicone you need RTV (electronics grade) type.

There are some videos on YouTube of how to do this on some rather larger cables, the principle if not the scale of the thing is the same.



Usual gotcha.. really neat job on the conductor, then you notice the outer sheath lying by the cable. 

[oldway]

I was using Scotch® Rubber Splicing Tape 23 , I recommend. 

http://solutions.3m.com/wps/portal/3M/en_US/EMDCI/Home/Products/ProductCatalog/~/Scotch-Rubber-Splicing-Tape-23?N=5433135+3294318588&rt=rud

[ConKbot]

Epoxy bonding to flexible silicone would be dubious with "good"stuff and even more so with whatever is at the hardware store. I'd try to use silicone self-fusing/self-amalgamating tape. Cut it into thin strips, so when you stretch it, you're left with appropriately small and skinny strands so you can get multiple wraps on, and hopefully exclude air by covering it slowly in multiple turns.

I.e. Your process would be 0) put heatshrink on the cable for layer 1) chamfer the ends 2) solder center conductor. 3) clean the flux and rest of the joint 4) wrap with silicone tape, tightly. 5) fold the shield back down and splice it however you want. 6) slide heatshrink over and shrink.

[OmegaHyperion]

I see.

Thanks for the replies so far.

One thing i find interesting about the tape method, is that the tape mentioned here, is rated at 800V/mm dielectric strength. So i would need 25mm thickness to get the insulation rating of the thin 20KV cable. (relatively speaking).  Seems a bit off to me, that the tape isn't rated higher.

Is there something i have missed about the tape method?

[HighVoltage]

Every time I did some similar splicing, I used a custom made tube out of PTFE that just slides over the cable. You can have new braiding cover the Teflon tube.
 

[floobydust]

... the tape mentioned here, is rated at 800V/mm dielectric strength. So i would need 25mm thickness to get the insulation rating of the thin 20KV cable. (relatively speaking).  Seems a bit off to me, that the tape isn't rated higher.

Is there something i have missed about the tape method?

Your units are off; 1 mil = 0.0254mm or 40mils/mm.

That 3M 23 HV splicing tape  is rated: Dielectric Strength (V/mil): 800 and is 30mil thick, so a single layer is 24kV but you might splurge and use more

3M 1350 transformer tape used in SMPS power transformers (UL approved yellow tape) is rated 5.5kV per 2.5mil layer.
It is much thinner than the electric utility-grade stuff.

Vinyl electrical tape like 3M 33+ is 1.15kV/mil and 7 mils thick, but rated for max. 600V use.

[OmegaHyperion]

... the tape mentioned here, is rated at 800V/mm dielectric strength. So i would need 25mm thickness to get the insulation rating of the thin 20KV cable. (relatively speaking).  Seems a bit off to me, that the tape isn't rated higher.

Is there something i have missed about the tape method?

Your units are off; 1 mil = 0.0254mm or 40mils/mm.

That 3M 23 HV splicing tape  is rated: Dielectric Strength (V/mil)?? 800 and is 30mil thick, so a single layer is 24kV but you might splurge and use more

3M 1350 transformer tape used in SMPS power transformers (UL approved yellow tape) is rated 5.5kV per 2.5mil layer.
It is much thinner than the electric utility-grade stuff.

Vinyl electrical tape like 3M 33+ is 1.15kV/mil and 7 mils thick, but rated for max. 600V use.

(facepalm)

Damn, i knew i would screw up with the non metric units at some point. Guess this is the time.  Ok, that explains the tapes ratings better. I will use it to splice the cable, and cover it with the 3M splicing tape. Then heat shrink, then the shielding, then more heat shrink. Alternatively some vinyl electrical tape from 3M instead of the inner heat shrink.

Thank you guys, you have bean a huge help! Ill post a pic of the finished joint once i get around to making it.

[OmegaHyperion]

I have finished the splice in question. Though i would call the result piss poor workmanship... Guess i should have practiced.

I warped 3 layers half lapped #23 tape onto the splice , then once the shield was connected i covered it with electrical tape. (i originally wanted heat shrink, but it turnd out to be to small to fit over this splice.

[HighVoltage]

honestly, that does not look good at all....

Plus you probably need a shield all the way around with a new braided tube, connecting both ends.

Depending on where you are, send it to me an I will make a proper repair for you.
Send me a PM.

[Ian.M]

Yes, you SHOULD have practised - if you could have found two 4" offcuts of similar cable.  However sometimes you don't have that luxury and have to make the best job of it you can first time.

That may hold, or it may not.    It looks more like self-amalgamating tape than ordinary PVC electrical tape so at least you are in with a chance.  Silicone self-amalgamating tape would have been a better choice.   Power it up in a dark room and watch it from a safe distance for any signs of breakdown.  If it holds for a long soak test you can fix the shielding with foil tape followed by more self-amalgamating tape to insulate it.

[OmegaHyperion]

The tape is 3M #23 high voltage splicing tape, its self bonding.

Sadly sending the device this cable is in to anyone is a no go, seeing as it weighs about 1 metric tone (literaly). I can practice on some RG11 coax cable, i think i have some high voltage cable of similar dimensions around my lab somewhere. Been a while since i worked with high voltage (well, other then testing a RG11 cable on my 50KV AC test stand today (i stopped at 35KV since it held what it needed to do, its to replace another 20KV cable in the same machine))

I didn't take a picture of the finished tape btw, the above image is the high voltage insulation only, not the outer most layer.

The cable is thankfully long enough to remove this crap connection and redo it properly.

The thing i found to be hard, was to keep the tape stretched, and wrap it around this highly flexible cable. I think i will use a jig next time to hold the cable better. Or just have a friend with me.

Just testing and seeing if i can see corona discharge is a no go, the power supply must not be damaged, no matter what.

If anyone is wondering what the voltage applied to this cable is, its 10KV DC. The other cable that will be RG11, is also going to have 10KV on it.


As to the shielding, ill need to figure something out there...

[Ian.M]

If its that critical then you'll need to keep making practice splices with a jig to hold the cable, stretching the 3M #23 tape to its  recommended percentage elongation during application, and taking far far more care tapering the insulation till they can reliably pass a 25KV HiPot test, then do your absolute best work in situ.   

For re-building the braid, you should use some hollow copper braid of an appropriate size to slip a length over the cable and join to the existing braid at each end of the splice.  Any significant irregularities in the braid over the inner joint will stress the insulation, so don't joint them over the inner joint.  It sounds like the equipment is valuable enough to be worth ordering in braid for this, rather than trying to find large enough diameter coax to strip it from.  The same goes for a selection of larger diameter adhesive lined heatshrink tubes.

[OmegaHyperion]

I will definitely redo this splice, after some practice. Will some RG11 coax do as practice cable? I have some left over from the same machine, where it will replace another high voltage cable (same type as the one seen above actually) (also i chose this because based on the RG11 cable having 3mm of solid PE insulation, i think (and tested) it to withstand the 10KV this cable will be carrying. Though i am still thinking of maybe getting proper high voltage cable instead)

As for the joint, should i wrap the #23 tape with vinyl electrical tape, and then apply the copper braided sleeve?

Also, what is the best practice of splicing braiding together? Seeing as i never needed to do that before.

As for the jig, i am thinking of a U shaped one, with screw based clamp downs to hold the cable in place, and enough clearance to allow me to wrap the cable with #23 tape. I would join the inner conductor (not sure if i should use a lineman splice, or just do the splice i did above), then tension the cable with the jig. Then using a dremel with a drum sander attachment, i would shape the original insulation to be conical. Then clean with ethanol, or isopropal alcohol (probably the latter).

If you have any other ideas on how to do it better, please let me know

[Ian.M]

Sounds reasonable apart from the vinyl tape - I really wouldn't trust its adhesive long term.  If you want something tougher under the braid to protect the HV tape, use heatshrink, but it really shouldn't need it if you are splicing in a new length of braid and you stretch it so it contracts and lies evenly over the inner splice insulation.

[OmegaHyperion]

Then i shall go out and get the things i need for the jig. And built it tomorrow. (pics will be uploaded of it as well)

I will also order the braided sleeve (id say the 9.5mm from techflex should work, as it can contract to 4.8mm and expand to 15 mm, so should cover the #23 tape) I shall also order the adhesive heat shrink with 19mm diameter, though i would rather not buy it from techflex, as theirs is rather expensive. (however, if chineesium heat shrink would pose a problem, then i will buy the techflex stuff)

[Ian.M]

Chineesium heatshrink probably has a poorer shrink ratio, and is likely to be less resistant to chemicals and ozone, and may be less flexible, but it should still do fine to protect the braid.   Keep the cable straight while you shrink - you don't want the inner conductor shifting off center while the splice insulation is softened by the heat.

[OmegaHyperion]

If i can use normal 3:1 heatshrink (the non glue kind) i would go with techflex heatshrink in a heartbeat. Its just the high price of the glue type heatshrink.

A note to the environment this connection will be in, it will be more or less a home environment for now, but will eventually be in a lab environment.

[OmegaHyperion]

This is the splicing jig i came up with (the CAD drawing of it anyways, still need to build it)

The base is going to be made of some scrap wood i have left over, the pullers will be made of birch, and the holding plates of aluminum. Everything will be bolted together with machine screws. Ill post a pic of the finished jig once its done.

The clearance between the cable and the plate is 20cm btw, ample room to move the tape around i hope.

[Ian.M]

A made up jig that deep wont have much rigidity done in wood.  It crushes too easily unless you use dense hardwood and epoxy in metal inserts in all boltholes. You'd probably also need to set the uprights into tapered holes in a much thicker base.

I suggest a one-piece jig: cut a deep enough U out of plywood, and add shaped padded clamps for the cable.    You'll want a minimum of two reel diameters + a few inches depth in the U and a width of the splice length + two hands width so it doesn't obstruct you while you are gripping the reel and trying to maintain an even tension on the tape.

You'll also want to fasten the jig down so it doesn't move while you are stretching the tape.

On the glue lined issue - keeping air and humidity out of the braid is important - any corrosion of the braid could result in strand breaks and higher local fields, and possible damage to the core insulation, but you may be able to get away with an electrical grade flexible sealant applied and worked into the braid, then heatshink over it applied before its cured.

[OmegaHyperion]

I could just weld the U frame out of steel pipe. That should be rigged enough. Also, what exactly do you consider as padding?

As to the woods in the CAD drawing, are particle board, 2cm thick for the base, and 3cm round birch (hardwood) for the pilers, held together by steel machine threaded bolts, with threads cut into the pilers. (size M6)

[Ian.M]

Shaped wooden clamp - just drill through a block of wood at slightly over the cable diameter, lightly countersink both ends then split the block down the centerline of the hole.  Screw it back together to clamp the cable.  For padding you could use bicycle inner tube or similar or just wrap a turn of the amalgamating tape round the cable. It needs to be softer than the outer jacket.    Another possibility for really thin padding you can line the clamp jaws with is self-adhesive medical tape.

N.B. If its got foamed inner insulation, you'll need a longer clamp that's better fitted so you apply less crushing force and don't distort the cable. You'll also want to flare the hole ends slightly to avoid a hard spot applying a sheering nip to the insulation.

[OmegaHyperion]

Excuse the delay, i have been busy with work, so i only had time to draw this now.

This is the revised design of the splicing jig, based on welded steel.

Its inner section is 30 x 40 cm, which should give ample room to move the tape around, and keep the splice well free of the wood jaws.

The jaws will be made of some random wood (its softer then birch) i got. The gap will be that of my circular saw, which should be around 3mm, alternatively i could use a hand saw, with about 1mm width. Which ever you recommend.

Will this jig work? Btw, as to mounting, the rig will be clamped to the equipment under repair using some screw clamps. (the only way i could find to mount this on the machine, and not rip the cable from the CRT)

The metal used will be 20x20 mm steel tubing, with 1.5mm wall thickness. The jaw brackets are 4mm x 20mm steel.

[Ian.M]

Looks pretty good.   Start with the 1mm handsaw kerf - if it doesn't clamp the cable tightly enough you can always sand down the jaw faces.  Consider putting the bolts through tapped holes from below so you have studs protruding to locate the jaws on then using wingnuts for easy clamping.

[OmegaHyperion]

i should have added that the bolts are held in the bottom plates via a thread, and a tightened nut (basically clamping the threaded rod in place). The top nuts are knurled steel nuts, not wing nuts. I assume they will develop enough force to clamp the cable, though id have to test it. If need be i can just replace them with something that i can apply more force to.

[OmegaHyperion]

Here is a cutaway of how i intend the clamp to be constructed. This might be able to clear things up slightly i hope.

[OmegaHyperion]

Here is an update on the splicing jig, i built the clamps now, and tested them with RG11 cable. They work perfectly. It took a bit longer, since my handsaws motor decided it no longer wanted to live. (the stator coils burned out)

Next step is to build the U frame, and finish the parts.

I intend to oil the wood clamps using boiled linseed oil, it should not be problem, as the cables will clamp well free of the splice its self. As well as linseed oil drying.

[OmegaHyperion]

It took a while to complete (life got in the way, as per usual), but i finally got the jig done, well sort of, the only thing its missing now is for me to oil the wood with boiled linseed oil, and pain the metal.



Now to practice splicing. I have also come up with a useful way of soldering the shielding. I will fold the shielding braid back onto its self, with a fiberglass sleeve between the inner and outer part. (alternatively i could fold it onto the outer insulation, also having the fiberglass sleeve there to protect the insulation). Then slide the new braid over the connection. Then fold the original braid onto the new one, and solder in place. The fiberglass should prevent the insulation from being damaged, as well as allow a full solder joint of all strands to be made. This should achieve the best shielding-

Any suggestions on streamlining this process are welcome, as usual.

[OmegaHyperion]

I have a new plan for the thermal insulation of the joint. I will use Fiberglass band to wrap the splicing tape, then fix it with kapton tape. After that i will put the braid over the joint, and place the braid of the original cables onto the new braid, and solder it into place. Then put glue heatshrink over that.

Any opinions on the joint idea?

I will test this on RG11 Coax before going to the CRT. I will also do a high pot test on the joint as well.

[OmegaHyperion]

I have made an experimental splice using some scrap RG11 cable

Here are the pictures of preparing the splice (the solder joint, and the splicing tape), as well as the post high pot test images, showing the spots it arced.


I only made the high voltage insulation part of the splice, and did not pay much attention to the shielding, it was connected and wrapped with wire to press it against the high voltage insulation to simulate the heat shrink.

The results of the high pot test where a arc voltage of 30KV (+-3 KV) Peak, or roughly 21KV (+- 2KV) AC 50Hz. This voltage was maintained for 1 min before a continuous arc was observed.

I will redesign my high voltage test bed to better do high pot. The above test was done as a sort of eyeballing test, to see if it even worked or not. The new test bed will have a more accurate high voltage volt meter permanently mounted, as well as a amp meter in the high voltage path (between ground and the transformer chassis). This should give me a more accurate reading. As well as a high voltage rectifier to test with both AC and DC (as needed), since i can only test with AC at the moment.

Also the scale bars are accurate, if anyone wants to do their own measurements.

[OmegaHyperion]

Conclusions from the test:

- The solder joint needs to be optimized, since it still has and allows spots to form where a corona discharge becomes more likely, as well as reduction of insulation thickness due to the soft splicing tape.
- Soldering with heatsinks is required to percent insulation damage due to heat from the center conduction (the center conductor was off center after soldering).
- 2 cm tapers should be increased to 3 cm tapers for more safety, as well as 1 cm of overlap of new and old insulation.


Possible solutions:

The use of crimp based joining of the high voltage conductor seems like a good idea at first, but would again create spots for corona to form. The best solution i can come up with at this moment is the use of heatsinks, and longer open conductor for soldering, as well as the use of a copper or brass tube filled with solder to mate the conductors, without forming sharp edges.  The new insulation should be wrapped to 1 1/2 the original diameter, half lapped.



Any input is welcome

[OmegaHyperion]

This is the redesigned solder joint. It eliminates all solder joint related problems found in the previous experiment.

It consists of a 2mm diameter 10mm long brass tube which was filled with solder. The conductors which will be mated where tinned and inserted under heat into the tube.

Due to a longer 15mm exposed cable, and this technique, no decentering of the central conductor was observed, due to the conductor temperature at the insulation interface staying below 100°C (melting point of PE)


This section of cable was a scrap from the previous test. It was not intended for a high pot test, it was only meant for a solder test.

[OmegaHyperion]

Im already using 3M high voltage splicing tape. Also in this particular CRT, there is no protection to trip, the high voltage power supply (not a flyback transformer) will just continue to pump power into the short, until i manually shut it down, or the fuse trips.

Thus i need to get this splice as close to perfect as i can get without spending thousands of euros in the process.

However, thank you for your comment none the less.

[OmegaHyperion]

Thus i need to get this splice as close to perfect as i can get without spending thousands of euros in the process.

For god sake. Just bodge it and wait it to fail. It will take considerably longer than you thought.
I won't be surprised if a street side repair shop will just bodge them in place and leave it to float in the air.

Well, good thing im not a street side repair shop then. I intend this splice to last at least as long as the piece of equipment the CRT is in, if not longer.

At the moment im looking into a better way to create the tapers on the high voltage insulation. So far i have used abrasion to grind the shape.

My new idea is to use a method similar to a pencil sharpener to taper it. Though i doubt a standard pencil sharpener will give me a long enough cone. I tested it on some RG11 scrap, and the cone on the 7.3mm insulation was roughly 10mm long, where as the ground tapers where 16mm long.

I will do some more testing and experimentations, until i have a way to make a splice which outperforms the cable its putting together.

[OmegaHyperion]

Well, good thing im not a street side repair shop then. I intend this splice to last at least as long as the piece of equipment the CRT is in, if not longer.

Okay, so it's not a cheap TV, it's actually a test gear. That makes sense.
If I were you, I would consider finding a plastic tube that's seamless and looks fine for 20kV (hint: gel ink pen refill), put the splice junction inside, and fill the thing with epoxy or silicone.

You are correct, its part of a piece of lab equipment im restoring.

I was considering the method you described, however previous members in this forum have discouraged me from perusing this rout. You can read the previous posts to see what this forum recommended and or came up with.

[andy2000]

It might be helpful to know exactly what this HV cable is used in (brand and model).  Are you certain the complete HV cable assembly is unavailable? 

[andy2000]

He mentioned that it's potted at one end (is the the entire HV power supply, just a HV multiplier, or something else?).  He said the other end is the anode cap, which is presumably removable from the CRT.  Is the shield grounded, or is it also carrying a high voltage?  More information might generate some alternatives to making a very difficult splice.  For all we know, it's just the coaxial HV cable required by older Sony Trinitron CRTs.  If it's something ultra exotic, it would be interesting to find out what it is.

It's hard to believe that no parts are available for something that appears to be very valuable. 

[OmegaHyperion]

Well, i might as well say what it is im working on. Its the photo CRT of a Scanning Electron Microscope (id say it dose not get more exotic then that). The shield is grounded inside the potted secondary high voltage tank (sadly no oil or SF6), thus i cant remove and replace the entire cable.

The other end of the cable is connected to a potted high voltage resistor, and then the anode cap of the photo CRT. A cap id rather not remove from the CRT.

As to available replacement parts, those have not been made since the early 90's, and finding one in unknown condition is hard enough, finding one in good condition is even harder. Thus id rather fix a known working component that was lightly damaged by an incompetent decommission team.

(they removed the photo CRT assembly to get the control unit threw a door where, even with the correctly installed CRT, a clearance of 30 cm on each side would have been present. (and instead of calling me they just decided to cut the cable...))

So, this is not your average TV im working on. Nor can i just simply move the unit to some repair shop. Thus im stuck repairing it in its installed location.

My previous splice test is capable of handling the voltage present in the high voltage cable, though id rather perfect the splice before i implement it in the microscope.  Just because something is hard to do, is no deterrent for me, i see it as a challenge.

edit:
I should probably add that the high voltage power supply that powers this CRT, also powers the other CRT's and other parts of the microscope. So having a short is a no go.

I did not say what it is im working on, since i thought that no one would reply if i stated i was working on an exotic CRT in an electron microscope. This way i got constructive replies, and am on the right track i think to fixing this cable.

[andy2000]

There's no substitute for unbroken insulation.  What about stripping back (and removing) several inches of shield on each side of the splice (2-3")?  Then insulate the splice with heat shrink tubing rated for at least 20kV plus a comfortable margin.  Finally, replace the missing shield with foil tape and one final layer of heat shrink over the entire splice. 

[Gyro]

Agreed, it's a simple matter of creepage distance and dielectric strength. The OP does seem to be making a bit of a meal of it (sorry).

There is no reason not to use continuous high dielectric heatshrink, the cable may have big things at each end but it is parted in the middle, so there is no difficulty in sliding on decent shrink ratio heatshrink before joining, (EDIT: probably 4 layers - two from each end).

From the creepage perspective, you can't assume that anything shrunk over the cable inner insulation is going to form a dielectric 'seal' (the same applies to the overlap between turns if insulation tape is used). The outer braid needs to be stripped back far enough to be able to resist 30-40kV on its own in open space.

The inner conductor joint area should be as small as possible (not long bare wire and splice) to minimize the airspace in which ionization can take place and degrade the insulation. coating the inner conductor joint with some silicone or dielectric grease would also help to fill the airspace. The heatshrink needs to overlap the inner conductor insulation at each end for a creepage distance capable of resisting the same 30-40kV.

Using copper foil tape over the splice area rather than braid is also a good idea. As the center conductor is +ve, the corona discharge will mostly come from the -ve charged screen near the inner core splice. That is probably what caused the insulation breakdown in the OP's wrapped tape insulation test - from the sharp ends of the braid overlaying it. Use of smooth copper tape will minimize any corona formation. It doesn't really matter how the braid is arranged outside that, the outer heatshrink will keep everything compressed and tidy.

With a little pre-planning it ought to be a reasonably easy joint to make.