When working with high voltage, how do you properly splice a cable? - Page 1
Forum section: electronics / beginners

author: LoveLaika
author_rank: Frequent Contributor
timestamp: February 08, 2023, 11:42:25 pm
content: One of my lab colleagues has asked me to repair a piece of equipment. It's some kind of X-ray device, but the issue with it is at a high-voltage section. There was some arcing, so the wiring needs to be repaired. The voltage is 12.5 kV, and it comes in via a double-insulated coaxial cable (with insulation on the outside and inside between the shield and the conductor). At the end of the conductor is a flag terminal that's also insulated. However, in the original wiring, the coaxial shield was exposed and connected to a ground wire connected to the chassis. This exposed part was covered up with heat shrink, so after that, all that's left is the inner insulation and the high-voltage conductor. I was wondering if this is the proper way to run the cable, have the high-voltage part not be surrounded by the cable shield (even though there's insulation around the conductor). If not, what is the proper way to splice a cable in this case?

author: jonpaul
author_rank: Super Contributor
timestamp: February 09, 2023, 02:30:20 am
content: x-ray HV is lethal and in medical equipment chances of patients risk.Check your liability insuranceSee applicable safety compliancereplace entire cable with manufacturers partJon

author: alpher
author_rank: Frequent Contributor
timestamp: February 09, 2023, 03:04:17 am
content: 12.5KV isn't exactly a very high voltage, i'd look into replacing the whole cable but a couple of layers of decent heat shrink should do it.jonpaul is just paranoid

author: jpanhalt
author_rank: Super Contributor
timestamp: February 09, 2023, 03:21:56 am
content: You do not say whether the device is used for patient care. If it is, in the US, X-ray equipment and radiology departments are regulated by the FDA. Generally, a user would need to have all repairs done by the manufacturer or approved by the manufacturer. DIY repairs "in theory" could be done, but any such repair would require recertification by the FDA before the equipment could be used. Non-compliant equipment must not be used for patient care. The consequences are serious.If the device is not used for patient care but is used in or by any business entity, I suspect it is still regulated:https://www.fda.gov/radiation-emitting-products/home-business-and-entertainment-products/x-ray-particulate-products-other-medical-diagnostic-or-cabinet

author: David Hess
author_rank: Super Contributor
timestamp: February 09, 2023, 07:18:48 am
content: After soldering apply corona dope and then heat shrink tubing.https://www.amazon.com/MG-Chemicals-Corona-Liquid-Applicator/dp/B008OA7CAE/

author: EPAIII
author_rank: Super Contributor
timestamp: February 09, 2023, 08:53:50 am
content: Yes!Yes!andYes!I have worked with high DC voltages and what you describe sounds reasonable. You do not want to continue that grounded shield too close to where the inner insulation ends. That could cause an arc.And even the heat shrink tubing that you talk about should be an OEM part.DO NOT SUBSTITUTE ANYTHING. PERIOD! All OEM parts and materials if you like keeping your house and bank account. The potential for a real expensive law suit here is very great. Really!Also check with the OEM for any modifications/change orders that may be necessary. They may have seen this problem before and have engineered a change to fix it. They should be available for a phone conference. You are a baby in the woods here. TALK TO THEM BEFORE DOING ANYTHING!Quote from: jonpaul on February 09, 2023, 02:30:20 am

author: Gyro
author_rank: Super Contributor
timestamp: February 09, 2023, 10:14:17 am
content: Some general points in solder splicing low power high voltage cable...- Make sure that the connection is mechanically secure before soldering.- Ensure that the solder joint is completely smooth (no protruding strands), use extra solder if necessary. This will minimise corona discharge.- If possible, fill the joint area up to the level of the existing insulation with insulating material, corona dope, as mentioned previously, is ideal. Thin PTFE tape also works. This will prevent any air space where a corona discharge can start - the ozone from corona discharges eats insulation.- Clean and dry the cable insulation.- Use decent qualityadhesive linedheatshrink with a decent overlap at the ends of the joint. The adhesive helps prevent any tracking path along the cable. With proper size choice, it is possible to use two layers. Add the second layer after the first one has been shrunk and cut it longer than the first so that there are two overlapping adhesive seals to the cable.P.S. This thread shouldn't really be in the Beginners section.

author: HighVoltage
author_rank: Super Contributor
timestamp: February 09, 2023, 10:29:09 am
content: Can you share some pictures of the device ?

author: wizard69
author_rank: Super Contributor
timestamp: February 18, 2023, 04:04:51 am
content: jonpaul is right: replace entire cable with manufacturers part.Also read comments about FDA and instruments used for patient care.Even at lower voltages, such as with ionizers, at work we don't bother to try to fix cables. In this case it has been proven not to be worth the trouble. Generally new replacement cables just last longer.