Author Topic: Splicing 20KV CRT high voltage cable help  (Read 9173 times)

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Offline OmegaHyperionTopic starter

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Splicing 20KV CRT high voltage cable help
« on: August 20, 2017, 01:01:21 pm »
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.
 

Offline floobydust

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Re: Splicing 20KV CRT high voltage cable help
« Reply #1 on: August 21, 2017, 01:25:17 am »
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
 

Offline Ian.M

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Re: Splicing 20KV CRT high voltage cable help
« Reply #2 on: August 21, 2017, 01:57:42 am »
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.
« Last Edit: November 18, 2017, 12:01:02 pm by Ian.M »
 

Offline IanMacdonald

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Re: Splicing 20KV CRT high voltage cable help
« Reply #3 on: August 21, 2017, 11:46:22 am »
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.  |O
 

Offline oldway

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Offline ConKbot

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Re: Splicing 20KV CRT high voltage cable help
« Reply #5 on: August 23, 2017, 11:45:07 am »
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.
 

Offline OmegaHyperionTopic starter

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Re: Splicing 20KV CRT high voltage cable help
« Reply #6 on: August 23, 2017, 07:25:18 pm »
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?
 

Offline HighVoltage

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Re: Splicing 20KV CRT high voltage cable help
« Reply #7 on: August 23, 2017, 07:36:50 pm »
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.
 
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Offline floobydust

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Re: Splicing 20KV CRT high voltage cable help
« Reply #8 on: August 23, 2017, 08:29:50 pm »
... 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.

« Last Edit: August 23, 2017, 09:52:16 pm by floobydust »
 

Offline OmegaHyperionTopic starter

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Re: Splicing 20KV CRT high voltage cable help
« Reply #9 on: August 23, 2017, 09:31:03 pm »
... 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.
 

Offline OmegaHyperionTopic starter

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Re: Splicing 20KV CRT high voltage cable help
« Reply #10 on: November 18, 2017, 11:49:58 am »
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.
« Last Edit: November 18, 2017, 11:51:59 am by OmegaHyperion »
 

Offline HighVoltage

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Re: Splicing 20KV CRT high voltage cable help
« Reply #11 on: November 18, 2017, 12:07:56 pm »
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.


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Offline Ian.M

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Re: Splicing 20KV CRT high voltage cable help
« Reply #12 on: November 18, 2017, 12:09:48 pm »
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.
 

Offline OmegaHyperionTopic starter

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Re: Splicing 20KV CRT high voltage cable help
« Reply #13 on: November 18, 2017, 05:28:42 pm »
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...
 

Offline Ian.M

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Re: Splicing 20KV CRT high voltage cable help
« Reply #14 on: November 18, 2017, 05:51:50 pm »
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.
 

Offline OmegaHyperionTopic starter

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Re: Splicing 20KV CRT high voltage cable help
« Reply #15 on: November 18, 2017, 06:11:16 pm »
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
 

Offline Ian.M

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Re: Splicing 20KV CRT high voltage cable help
« Reply #16 on: November 18, 2017, 06:18:13 pm »
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.
 

Offline OmegaHyperionTopic starter

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Re: Splicing 20KV CRT high voltage cable help
« Reply #17 on: November 18, 2017, 06:26:37 pm »
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)
 

Offline Ian.M

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Re: Splicing 20KV CRT high voltage cable help
« Reply #18 on: November 18, 2017, 06:31:15 pm »
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.
« Last Edit: November 18, 2017, 06:34:28 pm by Ian.M »
 

Offline OmegaHyperionTopic starter

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Re: Splicing 20KV CRT high voltage cable help
« Reply #19 on: November 18, 2017, 07:23:54 pm »
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.
 

Offline OmegaHyperionTopic starter

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Re: Splicing 20KV CRT high voltage cable help
« Reply #20 on: November 18, 2017, 10:34:36 pm »
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.
 

Offline Ian.M

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Re: Splicing 20KV CRT high voltage cable help
« Reply #21 on: November 18, 2017, 10:52:50 pm »
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.
« Last Edit: November 18, 2017, 11:08:31 pm by Ian.M »
 

Offline OmegaHyperionTopic starter

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Re: Splicing 20KV CRT high voltage cable help
« Reply #22 on: November 18, 2017, 11:09:57 pm »
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)
 

Offline Ian.M

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Re: Splicing 20KV CRT high voltage cable help
« Reply #23 on: November 18, 2017, 11:18:11 pm »
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.
« Last Edit: November 18, 2017, 11:26:15 pm by Ian.M »
 

Offline OmegaHyperionTopic starter

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Re: Splicing 20KV CRT high voltage cable help
« Reply #24 on: November 28, 2017, 09:10:11 pm »
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.

 


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