Pulse Voltage Capability of cables - Page 1
Forum section: electronics / beginners

author: tastychickenwings69
author_rank: Newbie
timestamp: April 01, 2022, 12:06:48 am
content: Hi.I tried looking for information online but couldn't find anything.I think its known that cables can handle very high and short puls currents which are much higher than the cable can normally handle.But what about the pulse voltage capability of cables?Can a cable that's rated for 250V for example handle 10x the voltage, so 2500V if the single HV pulse is very short without permanently breaking or having anything arc over to another part of the circuithelp is greatly appreciated.

author: jonpaul
author_rank: Super Contributor
timestamp: April 01, 2022, 12:59:10 am
content: generally HV pulse and DC used thicker polyéthylène insulation coax like RG6/U, up-to 50 kV.See Belden Cable specs and app notes.Pulser required matched Zo eg 50 OhmAbove 50 kV special low E field silicone cable is used to control the E field. Vendors Rogan and Diélectrique SciencesBon chanceJon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 01, 2022, 01:27:08 am
content: Don't think you can draw general conclusions from so little information, but, there are some contexts in which that can be meaningful.Wire/cable rated for 250V AC, and probably including suitable approvals attesting to this fact (alas, I don't know what standards, or what's in them -- I don't make wire!), likely is also rated to handle mains transients of suitable category. Again, I don't know the exact standards, but from others I have seen, it's often an important factor to have e.g. reinforced type insulation and CAT II/III/... transient rating, between anything that carries hazardous mains voltage (dangerous voltage plus transients), and a user that might touch it. So, a power cable is an example, reinforcement being achieved by molding a jacket around individually insulated conductors. Whereas twin-lead (zip cord) sort of stuff, has just one layer, so wouldn't be rated as high. (But evidently is still readily available. Maybe it's adequate for the lower category; again, dunno!)So, given that some wire is rated for such voltages (transients), presumably it will also be functional (if not necessarily safe to handle) at higher voltages, perhaps up to and including continuous operation at that voltage.Realize that this is a chain of if's, and breaking any one of them breaks the whole chain. Better safe than sorry!As for physical limits, you can put, oh, 5kV+ easily on your average hookup wire, around which point it'll start emitting corona, and the free radicals, UV and ozone thus produced will very rapidly degrade the wire. Needless to say it's not safe to touch, for voltages anywhere near that high.As for something like coax, it's shielded, so as long as the shield is adequately grounded, it's safe to handle. The interior insulation is thick and can handle quite some voltage, more or less continuously, at whatever frequency you can get through it without also melting the stuff down, heh.Something like RG-58 is normally rated to a kV or so. Which is also about what the connectors (BNC) handle, so that's fine. It's possible to send much more down such cable, but the connectors arc over -- a higher voltage (e.g. SHV) connector is required in this case. Indeed I've done this before, a SHV to BNC adaptor on an EFT (electrical fast transient generator) machine, it arcs over, making a nasty noise at one of the BNC connectors, if you turn it up past 2500V or so, I think it was.For higher voltages, and especially for larger pulses, larger cable may be required both for peak current as well as voltage handling. 50kV in some nanoseconds -- if we're talking real man's-man pulses here -- is the stuff of physics labs, radio jammers, explosive initiators, and perhaps even nuclear weapons! Needless to say, these are quite high peak power levels, be careful...For less critical edge rates, yeah, "low E field" multi-core stuff works by widening the conductor, either with a layer of conductive plastic, or a spaced-out cage of wires. This shorts out E-field within the conductive area, raising the radius and thus reducing peak E-field around it, reducing the chance or magnitude of corona or partial discharge in voids within the insulating jacket, or in the air around it. For a similar reason, you see HV power lines arranged in groups: improves current and voltage handling without having to hang a truly massive conductor.Tim

author: PartialDischarge
author_rank: Super Contributor
timestamp: April 01, 2022, 06:39:38 am
content: Simply put the differential rating of a coax cable goes down with frequency, and it goes down a lot. And the same happens with mostly any insulation.At DC you may get 10kV to breakdown, which is I believe what you are referring to ,“breakdown” as tested with a Hipotronics. At 50Hz however this may well go down to 7kV. As the frequency grows your max voltage goes down.This is the reason why most cables are DC rated for breakdown, because it is a higher number. In general you can have DC or AC tests, 1sec, 5sec and 1min. And for long term use you need to measure the partial discharges of the cable.In short, depending on your frequency and how long this cable is going to be used continuosly you will need to derate the cable by a higher amount

author: jonpaul
author_rank: Super Contributor
timestamp: April 01, 2022, 08:58:27 am
content: For the REAL DEAL: Sandia National Laboratories Z Pulsed Power machine...1.5 mm plasma 26 M A 90 megabars 100 nShttps://en.wikipedia.org/wiki/Z_Pulsed_Power_Facilityhttps://en.wikipedia.org/wiki/Z_Pulsed_Power_Facility#/media/File:The_Z_Machine_(8056998596).jpghttps://upload.wikimedia.org/wikipedia/commons/9/92/Z_machine_cross_section.jpgThe "cables" are man sized diameter....ZAP!Jon

author: David Hess
author_rank: Super Contributor
timestamp: April 01, 2022, 06:30:27 pm
content: Usually breakdown will occur first at the cable interface to the connectors or the connectors themselves where the creepage distance is shortest. RF connectors are not designed to withstand high voltage.

author: TimFox
author_rank: Super Contributor
timestamp: April 01, 2022, 06:57:26 pm
content: We used RG-59/U cable with SHV and MHV connectors in grad school, up to 2 or 3 kV DC. Once, I accidentally used foam-PE cable instead of solid PE dielectric, which broke down at a much lower voltage.

author: coppercone2
author_rank: Super Contributor
timestamp: April 01, 2022, 09:50:29 pm
content: Well over current is a thermal effect but voltage is a voltage effect.So does a spark not occur at breakdown voltage if the duration of the voltage past the breakdown voltage is too short? Nothing like this comes to mind for me.But for the insulation, I don't know. That seems to do with dielectric capacitance material specific/intrinsic time constants ? Device held at 'maximum' polarization might have a 'damage curve' (% of damage before failure) per unit time associated with it. I imagine these effects would have a huge proportionality in regards to dielectric material type (compare plastics to ceramics).Plus this also might have to do with inclusions, it might be that certain inclusions into the dielectric can 'cap it out' (aka very unusual failure curve) at a much lower voltage then something made under 'high purity' conditions in a cleanroom environment with some kind of 'extended' feedstock quality/purity control or something similar.As this has to do with safety, I would just not screw with it and get overrated stuff and (which is often not allowed anyway) never work on the live circuit (but some people are 'comfortable' with working on HV because they trust the dielectics I am sure), or give it more caution then a standard HV circuit that you 'trust' since you don't have large scale statistics to back safety related choices of these kinds, i.e. consider the whole thing to be 'bare' high voltage energized materials when working on it).

author: MathWizard
author_rank: Super Contributor
timestamp: April 02, 2022, 03:41:30 am
content: How good or bad does typical enameled wire do ? Like say in a 120V to 12V transformer, or some little RF inductor ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 02, 2022, 05:11:22 am
content: Enamel is usually good for 100V or so, on up to a few kV. Depends on type, as always. TIW (triple insulated wire) counts as reinforced insulation for purposes of mains transients, so 2.5kV+.Enameled wire is... kinda dangerously generic, hard to find source and datasheets on it, it feels? If you can find a known source and distributor, you can check their datasheets. And potentially order some, but the downside is you might have to order 10kg+ spools at a time? Idunno.Most multi-winding transformers, don't recommend any more than nominal voltage between common windings. Like the iron-core dual winding kinds, 120+120:12+12 or what have you, they're usually bifilar wound, so obviously they aren't exactly worried about putting an extra 120V between them in 240V operation, but more than that, you're kinda on your own, certainly not good to use it as an isolation transformer.Other off the shelf examples:https://www.digikey.com/en/products/detail/pulse-electronics-network/PE-1812ACC510STS/5149116Plain enameled wire, 50V. Probably breaks down much higher than that, but why risk it. Also, connection is through the ferrite (metallized pads), you probably don't want to apply much anyway heh (MnZn ferrite is slightly conductive, NiZn much less so; so, depends on type).Though, interestingly there are single Ethernet transformers of similar design, so maybe the isolation isn't so bad after all (Ethernet requires 1.5kV).https://www.digikey.com/en/products/detail/pulse-electronics-power/PM2185-011NLT/13998582Looks like plain enamel wire on mine. Colorful. Though they hide the toroid inside the shell thing with a glob of silicone, what a shame.https://www.digikey.com/en/products/detail/pulse-electronics-power/PM2190-011NLT/13998599Have used these before, they're TIW, both windings. Low leakage too!Tim