Working safely with high voltage - Page 1
Forum section: electronics / beginners

author: state_of_flux
author_rank: Contributor
timestamp: July 14, 2020, 01:41:50 pm
content: Hi All,I am going to designing a PCB for a high voltage, 6kV and 3kV @ 10mA/200mA (660W) power supply for my PhD. As you will know, the equipment needed to work at this level is very expensive and as a student the money available in my account is limited yearly.With this in mind, what are the quintessential safety precuations that will be needed for my lab work, and possibly within the DC/DC supply itself? For example, is short circuit or open circuit protection best, or should I have both? At the moment, all I have in my design is my isolated feedback circuit which will isolate the control board from the converter output. But I am worried that faults will destroy my equipment and they will be costly to replace.If anyone has any recommended techniques for open/short circuit protection at such high voltages, it would be recommended. I plan on having a pulse-to-pulse current limit of the primary transformer current, but I would like to add some more features to address my concerns of faults such as short circuit protection on the secondary current level. I am thinking of having a comparator circuit driving some LEDs as a voltage level indicator also, for example. This will show me how much voltage is on the secondary side and indicate when safe working voltage is reached.I would rather design in now for the most likely faults rather than worrying about it once I come to the bridge. The load is purely resistive for the purposes of my project.Any recommendations are appreciated.J

author: tkamiya
author_rank: Super Contributor
timestamp: July 15, 2020, 12:47:01 am
content: First thing I'd recommend is to fully realize you are dealing with lethal level of voltage/current.My method for working such voltage is, limit myself working with live circuit to bare minimum. Then have a method readily available to discharge the capacitor. They are often called Chicken stick, God stick, <whatever religion> stick. Legend goes, it is so called because if you don't use it, you will meet your maker. It's just a long insulated stick with hook at the end and tied to ground.Another thing is, be fully aware where my fingers are, especially the pinky on primary arm. Also, I consciously put my left hand to my back and tap once, reminding myself I know where my left hand is. This rule was born out of making a mistake and having a near fatal accident myself.Rest is being methodical, fully alert, and confirming every step I make.As far as circuit goes, you can do anything you'd want. This is one of the area you should have someone local to you who are well versed in high voltage coaching you. Short protection can be just a proper high voltage fuse. For metering, I won't use such a complicated circuit. A meter with high value resister will be just fine. I'd make it simple and straight forward so it can't fail and give a false reading.It kind of scares me that you have to ask these questions, and you are about to design and make such unit. I'm not trying to be condescending. I was lucky. My friend (who touched 2800v) was lucky. Next person may not be.Sincerely....

author: Ian.M
author_rank: Super Contributor
timestamp: July 15, 2020, 02:07:37 am
content: If your uni's still got in-house test equipment maintenance technicians, beg some time from the most senior bench technician (not a paper-pushing desk-driving superior) and ask what training they received to qualify to work on high current HV equipment. Also, your faculty adviser should be pushing you to agree a risk assessment with the departmental safety officer. If no-one's bought that subject up, its a big red flag!

author: TimNJ
author_rank: Super Contributor
timestamp: July 15, 2020, 02:52:07 am
content: As others have alluded...do not try to rearrange or alter your measurement setup once the UUT is powered up. If you're making a scope measurement, have your probes neatly set up before you turn the switch on. If you need to use a multimeter to troubleshoot, maybe do the same as for the scope, or at least limit yourself to a single probe, by tying the negative (black) multi meter lead to your circuit's ground/reference node via an alligator clip, mini-grabber, solder, etc. (Be careful with alligator clips though.) This way you limit yourself to one-handed probing, and you can put your other hand in your pocket, or under your butt. It's the classic electrician's rule. If you slip up, you won't end up with a high voltage across your two arms, and thus heart. In addition, it's less likely that you'll have a probe slip (and subsequent *CRACK* of a semiconductor) if you only have one probe to manage.Perhaps most importantly, don't get involved with a high voltage circuit if you are tired, or if your mind is on other things. You will mess up, almost guaranteed. No late nights in the lab.

author: Ian.M
author_rank: Super Contributor
timestamp: July 15, 2020, 03:11:57 am
content: Never work in the lab alone, make sure whoever else is present knows how to isolate and discharge your project safely*, and preferably that there is a qualified first aider present.* Probably the first safety feature you should design in is redundant high reliability auto-discharge if the mains supply is cut.

author: bdunham7
author_rank: Super Contributor
timestamp: July 15, 2020, 04:30:14 am
content: I actually did this sort of thing in college, built some high voltage stuff for various science projects. The largest was a 6-8kV supply for an electron beam tungsten vapor deposition system and used a transformer of about 1 cubic foot. Nobody died, but we weren't all that safety conscious, TBH. All of this was done using spare parts from the high voltage dept at a synchrotron lab.I'd like to know a bit more about your project (the actual study or experiment--what field, what's the theory, etc) and the requirements. Is that 3kV and 6kV AC, DC, filtered DC? How does it need to be controlled--and how quickly? You say 'resistive loads', but what are they? Are the outputs isolated or ground referenced? And most concerning to me, you mentioned 'designing a PCB'. At first I thought you meant that this power supply was your PhD project and I thought "gee, PhDs are easy nowadays"but of course you need to put it together on the cheap. BTDT, so here's my first round of advice:1) Find labs in your university that use high voltage and have a talk with them. They may have spare parts and equipment. Anyone doing sputtering, particle accelerators, anything with discharge tubes, high vacuum with electrostatic pumps, etc etc.2) Consider the non-scientific commercial products that already operate at these levels. A microwave oven contains a transformer with an output that could be modified to work in your application. One transformer, some rectifiers and a voltage doubler or tripler and you'd have both requirements covered. Or, perhaps you would need two of them. Neon light transformers operate at voltages higher than what you have, look at how they are made to get an idea of what type of wiring and insulation you'll need. An older CRT oscilloscope commonly has voltages similar to and higher than your project, although not at those currents.3) Are you looking to design a professional-looking product or just get it done? You mentioned 'designing a PCB', but you'll be far better off using as many existing products you can find and putting them together to do what you want. Starting from scratch will cost you time and yield questionable results. Your battery gauge is an example--commercial products are readily available that do the same thing for a dollar or so.Do you already have a notion of the basic design of the supply? If you are going to use a custom HF DC-DC setup (you mentioned feedback and pulse control) then I really would recommend rethinking that unless you have a specific need or reason. My recommendation would be to have your short protection be an inherently short-proof design (it can just run shorted indefinitely--many supplies have this feature) and I'm not sure why you need open-circuit protection if you have a voltage feedback. The output should probably have a resistor across it so the current never goes to zero. Remember, unless I misunderstand you, this device does not have to be light, small or energy efficient. Many problems can be solved by a technique I call "Captain Overkill".

author: eblc1388
author_rank: Frequent Contributor
timestamp: July 15, 2020, 09:32:35 am
content: Show us the output on/off switch and terminals you would be using/consider to use.

author: Ian.M
author_rank: Super Contributor
timestamp: July 15, 2020, 12:05:46 pm
content: +1. I'd also like to see 'state_of_flux's answer to eblc1388's question.Quote from: eblc1388 on July 15, 2020, 09:32:35 am@eblc1388: That'sI'mHigh voltage and DIY on a limited budget is *NOT* a good combo for safety unless you've got access to, and permission to rob, the 'elephant's graveyard' of dead EHT PSUs and test equipment

author: state_of_flux
author_rank: Contributor
timestamp: July 15, 2020, 12:29:56 pm
content: Hi all,Sorry for the delayed reply, I am in the UK so have only just started work and seen these replies. The output of my converter is going to be pulsed by a variable on-time switch such as:http://www.behlke.com/separations/separation_c1.htm.This will be placed in the 3kV output so that the load current triggers from 200mA to around 10mA only. I Could have an additional switch in the 6kV but it doesn't seem worthwhile since an almost equivalent load change is seen with the single switch. The variable on-time is required since the output pulsing changes from high frequencies such as 100kHZ to low frequencies as low as 100Hz.The application is for a microwave power module, which included a "mini-" Travelling wave tube, TWT, for radar transmission applications. I am hoping to get the project done, and it is not expected that it will go into production at all - it is just a PhD project for now. I know that it would take much more work to get it to where it needs to be to ensure it is ready for actual producton. I have designed my own DC-DC supply, but I am creating that by using a demonstration board on the primary side. I have essentially designed my own transformer, but I need to run some tests on the design once I get into lab. I am using 10kV isolated transformer wire to compose the windings for that. For the resistors I am using some high power, high voltage resistors from a company in the UK. I believe that at my university we have some kind of high voltage chamber in which my converter will be placed during power on and test so that I am unable to touch the converter. I have also been given advice to place perspex boxes around the load resistances so that they cannot be touched. I will place bleeder resistors across the load capacitances and ensure I do not work on the converter until so many minutes have passed and there is some indication that there is a safe working voltage on the converter output. I doubt that this is enough and therefore am keen to hear more techniques.The output will be filtered DC-DC. I understand people may be worried about safety but that is what I am trying to do - get advice from people on techniques and things I can place in my converter to absolutely minimize the risk of injury or worse. I have a decent budget, I get around 1250£ per year from my university, with about 1000£ already in my account. So I will have around 2250£ by the time I can get back into lab and build the PCB. At current exchange this is just over 2840$ for my American friends here!I am asking this question because at the moment I am composing a detailed health and safety and risk assessment report for the lab technicians and my supervisors for when I return to work. Of course they are very focussed on this as well, so I want to make sure all ends are tied up and all measures are in place for safe working before I am allowed to go near the HV chamber! I'm not sure if I have missed anything. I am more than happy to answer any more questions and very, very keen to learn about these things. Thanks to all who have replied thus far.

author: eblc1388
author_rank: Frequent Contributor
timestamp: July 15, 2020, 02:36:33 pm
content: If I understand correctly from your description, there will not be any manual operated switch and output terminals. That would ease the safety requirement a lot. The use of enclosure is the way to go.As high voltage capacitor will slowly charge up because of dielectric absorption, bleeder resistor can not be relied upon for safety. A standard feature in high voltage testing is the earthing stick(s). An insulated fiberglass pole with a metal hook at one end and a trailing cable connecting to earth. This is applied manually to the test circuit live parts and must stay connected at all times when work is carried out on the setup. For high energy circuits, a suitable voltage detector should be used prior to earthing operation to prevent accident.As the designer of the high voltage setup, it is your responsibility and duty to "touch" the circuit first before asking others to work on it.

author: David Hess
author_rank: Super Contributor
timestamp: July 15, 2020, 03:33:25 pm
content: Use the minimum amount of capacitance to keep energy levels low; I learned this the hard way once.Active series or shunt protection is difficult because high voltage series cascode switches are required, but it is not impossible.Passive protection with high voltage current limiting resistors is easy and should be used in addition to any other protection. Be sure to use high voltage rated resistors.Leave plenty of extra room in any chassis, which will also make neat wiring easier. Do not let anything including wires flop around.

author: state_of_flux
author_rank: Contributor
timestamp: July 15, 2020, 03:53:13 pm
content: QuoteThe capacitance I have at the moment on the high voltage outputs is 100nF for the 3kV and 25nF for the 6kV if I remember correctly. The supply is a high frequency SMPS type and the prupose of my PhD is to minimize the size of these caps so that is a good suggestion.By passive, do you mean that there is simply just a large resistance in the output which limits the current into the load? Or do you mean some kind of circuit set up which is used in the controller design? Do you happent to have a circuit you could show me, if so?Side note: Where can I buy one of these discharge poles that are mentioned in the thread? I can't seem to find any on RS components, Mouser etc. Is it something that I manually "touch" the circuit capacitors with or is it something that is included in the chamber connected to ground?

author: jmelson
author_rank: Super Contributor
timestamp: July 15, 2020, 05:27:17 pm
content: Quote from: state_of_flux on July 15, 2020, 03:53:13 pmYou make it to suit the equipment. If it is a cabinet big enough to walk into, then it has to have a long handle. If it is a small box, then a 200 mm handle is plenty. You get some Plexiglas/Lucite/acrylic rod, drill and tap a hole in the end and screw in a threaded rod. A nut secures the rod and a terminal with a ground wire.To access, you remove power, open a cover in the cabinet and reach in with the ground rod to discharge anything that could retain energy. Your hands always stay outside the cabinet, holding the plastic handle.Jon

author: PartialDischarge
author_rank: Super Contributor
timestamp: July 15, 2020, 05:32:00 pm
content: Voltage detectors are very useful. These are devices that get power from the line itself and will clearly show visibly with an led if there is voltage or not. Unfortunately their use is not very common even in power electronics where multi-kV DC rails are present in the busbars.In railway on-board inverters however their use is a norm

author: David Hess
author_rank: Super Contributor
timestamp: July 15, 2020, 11:41:05 pm
content: Quote from: state_of_flux on July 15, 2020, 03:53:13 pmBy passive I mean add series resistance to limit discharge current using a high voltage resistor which will not suffer breakdown. This may mean using multiple resistors in series.

author: max_torque
author_rank: Super Contributor
timestamp: July 16, 2020, 01:39:46 pm
content: Here are my simple 6 steps to staying safe (that i have used on numerous HV projects)1) PREVENT CONTACT WITH DUT WHERE EVER POSSIBLEIf you physically can't touch, or physically can't get near enough for any HV to jump out and touch you, then you level of practical safety jumps hugely. Define an "isolated area" and surround it with a suitable containment (usually insulated plastic or earthed metal enclosure). Define a SINGLE access point, where any instrumentation or controls interface must break through into the isolated area. At this point, install fully rated isolation to ensure ideally a complete galvanic barrier, but at the very least a resistive one of suitable value to limit fault currents to a survivabel level (suggest 50Kohm for human body model at 10mA max, preferebly lower). Access to the isolated area should be either interlocked and/or require tools to provide a fixed "time barrier" to entry. I suggest that if possible you do not include "Live working" (working where the system is >50V with uncovered / un-isolated conductors) in your normal procedures, but use a system of "specific deviation" to cover any occasion where that becomes a necessity (like fault finding etc). Each deviation would require an individual risk assessment and sign off from a second nomiated person. You department will need to consider it's responsibilities in this respect. I have used isolated areas where a physical key system allows the system to be powered with "the door open" but that key is held by a senior, and only released after the necessary checking, assesment (including the tools and instruments to be used, isolation impedance, signage, and skill / experience level of the user(s) and risk assessments have been completed and signed)2) WARN WHEN SYSTEM IS POTENTIALLY HAZARDOUS, PROVIDE A SIMPLE SYSTEM TO MAKE IT SAFEUse a warning beacon or similar mounted locally and visible at all times to show the state of the DUT. Never show a GREEN coloured beacon, nothing is 100% safe, use yellow if in doubt and red for known danger case (ie HV enabled etc). Drive that beacon ideally both via an indendant safety circuit (voltage sensing relay) and with a direct signal (the one you use to enable the HV supply) I typically use both a yellow and red beacon. The yellow beacon is on at all times the system is powered and the red beacon on when HVDC exists. I use a single high level simple ESTOP system that ideally has an interrupt capability at a high enough level to cut all power feed to the isolated area when triggered. When that ESTOP is set (must be seperate button to reset, so just clearing an individual estop button does not re-power the system, you may use a key for the reset to prevent un-authorised power-ons) the yellow beacon is lit. This also shows that the ESTOP itself system is working. If your system includes stored charge, include a time delay relay on the red beacon, meaning it stays on for a set period, warning of that potential (sic) for stored charge. You must test, meaure and record your discharge times and your chosen delay period for the DUT as part of the risk assement.3) DE-REFERENCE THE HVDC FROM EARTHUse transformer isolation on the LV supply, or ensure your HV supply has galavanic output isolation to make your DUT "one point" safe, ie you have to touch BOTH DC+ and DC- at the same time to recieve a shock. if you can afford it, use isometry to determine in near real time the level of isolation your DUT actually has from earth. This can warn of any loss of isolation, and shut the system down before you get to a low enough impedance for the system to lose its "One point safe" rating. (seehttps://www.bender-uk.com/for suitable devices)4) ISOLATE THE USER, KEEP THEM ISOLATEDBuy a suitably rated HVDC isolating mat, put it on the floor in front of your bench. Keep it clean, it could well safe you life by isolating YOU or others from an earth path. This is especially true for systems rendered "One point safe" as in step3. Use of PPE (HV gloves, face shield etc) is generally recommended, and MANDITORY for live working, and tools used must be rated and checked and recorded5) HAVE WRITTEN PROCEDURES AND CLEAR SIGNAGEPrint out and keep basic "check lists" for common activities, including live working requirements, basics such turn the system on and off, gaining access to the isolated area, what to do in the event of a fire, and recording any accidents, or incidents however minor. Keep these in a fixed (ideally bolted to something so it can't go walk about) folder next to the isolated area. You will probably be required to have a signed risk-assessment from a senior member for each and every activity you do. These should also be kept in that folder. Clear and obvious signage is critical, and depending on the level of access to the working area, will be a legal requirement (for example, can cleaning staff access the area, or the general public etc etc)6) DEADTESTING and LOCKING OFFYou are legally required to install, nominate and provide the necessary means to ISOLATE AND LOCK OFF any HV supply used. This is closely proscribed by the UK HSE. It requires physical disconnection or isolation of the supply(s) and a means to physically fix them into that state (normally padlocks). From a practical standpoint, you should develop a suitable DEADTEST using the most suitable test equipment (not a multimeter because that can be set wrongly, normally a simple LED stick / martindale tester etc). This MUST be used to check the potnential on any condutor before physically touching them in each and every case. Training for deadtesting is both manditory and must be specifically recorded for each user. This is your final life line, should also else fail.This can sound onerous, but although it takes a bit to set up, it's not very expensive, and is a proven system that works and keeps peope safe. If you are working as part of a department (rather than as an individual) they should already have the necessary HSE procedures and personel in place."Defence in depth" is also important, because making a mistake is human. Using the isolated area, the isolated supply, the PPE, the HV floor mat, and the isometry provides this, meaning single mistakes are mitigated against, breching a single measure does NOT lead to user exposure. This is also important for prototype / custom DUTs which may have un-expected failure modes and which are likely to be much less robust than any production device.The one thing to keep in the back of your mind is that HV exposes one to a risk profile that is highly non-linear. For the most part, it is really hard to injure oneself, until suddenly, it kills you stone dead. This is very different to the typical risk in the modern world, where you can expect a minor injury fairly often, but to actually kill oneself is prettty difficult, especially where "dangerous things" generally look and sound dangerous (HV looks the same at 0v as it does at 5kv)One thing to note, in the UK the HSE's "Low voltage directive" covers between 50 and 1Kv, and the "high voltage directive" from 1kV upwards. The HVdir is significantly more onerous because it necessarily recongnises that once above 1kV, HV can jump out and touch you from a distance, whereas below 1kV, direct contact with a conductor is required to recieve a shock. Your department will (and should have) familarised itself with it's responsibilites under those directive!

author: state_of_flux
author_rank: Contributor
timestamp: July 16, 2020, 02:36:10 pm
content: Thank you all for your responses thus far. I like the variety of answers and I am geting a lot of good information that I will definitely use in the future.QuoteDo you happen to have any kind of reference which shows how I may set up such a circuit? Like I said in my original post this is exactly what I am looking for, some kind of indicator of safety (even if it cannot be "100%" considered safe. I take as many measures as possible, hopefully. I think I have my short and open circuit questions answered: I do not need to worry so much about open circuit due to the resistive load (but what if any of the resistors fail(?)). And that short circuit currents can be limited by limiting resistors. But maybe there is a way to indicate this and compeltely shut down the supply for safety. Any documents/references would be appreciated!

author: PartialDischarge
author_rank: Super Contributor
timestamp: July 16, 2020, 02:54:49 pm
content: The best safety assesment for any HV line is an independent device that visibly shows if voltage is present or not. No need to second guess or assume anything, this is why voltage detectors and indicators do exist

author: tkamiya
author_rank: Super Contributor
timestamp: July 16, 2020, 06:43:29 pm
content: A very simple SAFE-TO-ENTER indicator/protector is a combination of neon lamp and a heavy duty switch actuated by the front door.Say you have everything in an acrylic box with one door. As a gentleman said, when power is ON, red light comes on.If you open the door, a switch behind the door activates, shorts the output. If the power is still there, it will safely bleed to the resister. If excessive power is there, fuse will also blow. You should have a bleeder resister parallel with the output filtering capacitor. Without it, it will take forever to discharge. (by the way, this is where I almost killed myself - bleeder register was blown)As an "output present" indicator, I would recommend neon lamp - almost never fails and an ANALOG meter with proper dividers. You need to make it simple here with a goal of reducing possible safety/risk related failure.By the way, at this voltage, fuse, register, switches, and just about everything else (including wires) has to be considered very carefully. It could easily arc across an ordinary devices. You can not make this kind of device 100% safe. You do the best in design and construction and it will still remain lethal. The last safety device is your brain!If you want to see some practical examples, you could look at tube type linear amplifier power supplies, especially BIG ones. It will include all these element. There are some awful design, but some of them are great. I really suggest consulting a local expert on the subject, possibly tour/examine and existing device. You can learn a lot from what someone else did.

author: tkamiya
author_rank: Super Contributor
timestamp: July 16, 2020, 07:11:07 pm
content: Oh, one more thing.....If you have a high voltage capacitor, NEVER assume it's safe to touch, even if the power has been off overnight or for a year. Static from surrounding will charge the capacitor to enough amount to hurt you. That's why high voltage caps are always stored with terminals SHORTED. In your design, you'd walk around this by having a good bleeder cap over capacitors on output side. Especially oil filled capacitors, which you are likely be using, are bad about this.As you can see, there is A LOT to think about in making such a device.Have you considered looking at neon power supplies? They are available inexpensively and while 3KV ones are rarer, they do exist. 6KV and up are plentiful.

author: strawberry
author_rank: Super Contributor
timestamp: July 16, 2020, 08:53:27 pm
content: High voltage (microwave oven) fuse to prevent hot plasma arc .Use neon indicator bulb with ~10Mohm (with sufficient voltage rating) resistor/s to indicate high voltage is present at secondary winding/multiplier.Dry and clean surfaces especially for voltages above 5kV.Discharge anode to shield/gnd, some vacuum tubes can create HV capacitor .. and multiplier capacitors as well.EMI (square wave) could interfere with control circuits.high voltage is not dangerous ,ignorance is.

author: JohnG
author_rank: Frequent Contributor
timestamp: July 17, 2020, 12:17:37 am
content: Please pay attention to all the safety stuff. Remember you OR someone else can be killed by the supply. They can also be seriously injured if the shock causes them to slam into something.Make sure you have lots of voltage margin on your caps. If you have cables, transformer, etc., an arc can cause some quite impressive voltage increase, which can take out a lot of stuff. Use a second bleeder on your caps with an LED indicator, protected by a Zener. This will be a pretty good indicator of capacitor voltage. Use your imagination here, becuase you want the LED to be visible at pretty low voltage (Say 50V or even lower), but not blow out even at double the desired output (faults happen). Get some decent HV probes for your multimeter and check anyway.TWTs are prone to "spits," otherwise known as arcs. They are a momentary (you hope) short. Make sure your supply can handle them, and a current limiting resistor in the output is a real good idea if you don't want to fry the TWT. Best to make your power stage easy to replace as well.Make sure the supply can handle a open load condition without failure or the voltage going out of control.TWT output can be pretty sensitive to voltage, especially phase.It is possible to do this safely. The most safe way is to prepare for the worst you can think of. There are many jobs people do that are far more dangerous, but they take safety very seriously.Have fun!John

author: state_of_flux
author_rank: Contributor
timestamp: July 17, 2020, 10:38:16 am
content: Thank you all VERY much for all of this information, it is very useful and it is making me slightly more confident of being safe in my work. I will take all necessary precautions.QuoteCould you please help me with some of these safety features that you have given me. I know it is typical to have voltage ratings of 2xmaximum. Meaning I should have 6kV and 12kV caps. Is that about right or still not enough margin?With the level indicator you discuss in this message - do you mean something like the one I have given in my original post, or something else maybe more robust? By visible at 50V but not blow up at 12kV. Can you even get an LED rated at this voltage? Maybe these are the neon lamps people are talking about. With the bleeder resistor connected to a zener, would you not need isolation between this path and the LED? Since this would be a lower voltage region I assume you would need some form of isolation between the LED and the bleeder resistor with at least 12kV isolation.Could you help me with calculating the current limiting resistor value for the supply. Would you need one in series with both load resistors? 6kV@10mA and 3kV@200mA, are there any equations for calculating resistors which limit the current to be these at maximum during a fault?I am worried about the short circuit as you mention. Some of my equipment is expensive, such as the load resistors for my high power output, which are just over $100 each, and I need 10 of those to meet the power requirements. I would really appreciate some kind of circuit which protects these against open circuit and therefore voltage runaway during the fault. That is one of my main worries, obviously right behind mine and other peoples safety (the main purpose of this post).If anyone has any circuit recomemndations that I could look into/simulate to help with this process that would be very very much appreciated, as would any links to the safety equpment that is being discussed - I am not experienced in high voltage experiments therefore it is quite hard to know what equipment is being discussed.Thank you all again, this is extremely helpful and I am very grateful indeed.

author: state_of_flux
author_rank: Contributor
timestamp: July 17, 2020, 11:16:27 am
content: I have just came across (randomly) the existence of high voltage insulated "boots" which apparently insulate the user to avoid contact with "ground". Can anyone advise whether these would be worth buying? Any recommendations for a brand that is established and anyone has used before for such works? see:https://www.tester.co.uk/high-voltage-utility-and-rail/high-voltage-personal-protection/insulated-boots

author: max_torque
author_rank: Super Contributor
timestamp: July 17, 2020, 11:19:25 am
content: Regarding the EStop, in the uk it would fall under the HSE's Machinery Directive, which mandates a suitable level of robustness for these devices. The situation is very complex, because it falls between lots of individual pieces of legislation and involves a lot of "best practice" You also need to be aware that as far as the machinery directive is concerned, even prototype / custom equipment becomes "point of use" when it is being used for the purpose for which it was designed, so in effect, by the letter of the law, if you build a bit of custom test equipment, as soon as you use that equipment, then it becomes "in use" and falls under the machinery directive!I've been designing, operating and training HV DC systems up to 1.3MW for the last 15 or so years in the UK and it's been quite the eye opener, and i've had a lot of discussions (perhaps arguments) with variou s HSE and auditing authorities over what is legal, what is best paractice, and what realistically, is "good enough". IME it is actually IMPOISSIBLE with the regulations as they stand today to meet every single obligation on any LV/HV project of any complexity, but that doesn't absolve you from trying! IME, HSE in the UK HATEs people who don't think about things, but they sensibly realise that thinking about something may uncover aspects that are non-ideal, potentially (sic) illegal as far as the letter of the law is concerned, but practically un-avoidable. It is exactly thise things that we rely on the risk assemssment and subsequent risk MITIGATION procedure to mop up!There are however some tricks you can use to make things easier to pigeon hole under various regulations:1) Make your DUT and test equipment socketted! ie, have a socket on the wall that supplys mains power (1ph/3ph), and your test kit has a flying lead to that socket. This achieves two important but somewhat arbitrary aimsa) anything upstream of that socket is only covered by normal UK building wiring regulations (18th Edition, which constantly changes), and your DUT/Test kit becomes "mobile equipment" under those regulations and hence is (broadly) excluded (connector earthing, GFD, and wire gauges for the trailing leads are still covered under 18th edition for these devices)b) That connector can become a "physical point of isolation" ie by disconnecting it physically, it can provide your legally required "LOCK OFF" point if you install suitable sockets that have the abilility to take a padlock and therefor be "LOCKED OFF" (most 1ph and 3ph connectors above 13A will have this capability)2) Put your MASTER ESTOP, the one you will legally be relying on, on the AC supply side, using as many off-the-shelf parts as possible. I tend to use schnider AC distribution boards that have a seperate 24v "hold on" solenoid, and use a basic din rail based relay system and a suitable number of physical Estop buttons to drive that solenoid. Here, the distributions contactor must be manually set ie simply providing 24v to the solenoid does NOT close the breaker, it must have 24v AND have a person pull the lever over! This makes it a suitable to meet the "must not automatically reset" requirements of the directives. Should your ESTOP power supply fail, then the breaker opens, so the system is fail safe (mostly). Importantly, i tend to avoid having too have a certififed SIL rating on my MASTER ESTOP because the schinder breakers can also be tripped off manually (by simply pulling the lever to off). This means your fail safe in the event that your MASTER ESTOP system fails to break the AC supply is to simple manually open the breaker. If you don't do this, you may be required to establish the actual SIL rating of your MASTER ESTOP, which is basically a very advanced, expensive and timeconsuming game of "lets guess some very big numbers that we may later rely on in a court of law" which tbh, is an even less fun game than it sounds? It also opens up tricky moral descisons on what failure rate IS acceptable for sucha system in your organisation (tyr asking your department head for example, if they think they should kill an employee once every 10 million hours or once every 100 million???)3) If the MASTER ESTOP is enabled, then show this with a yellow beacon. Because it is enable, it IS possible for the DUT/Test kit to be live, so a yellow beacon is that warning. It also shows that the MASTER ESTOP system is working correctly. You'd therefore normally expect the yellow beacon to be one pretty much all the time.4) DUT ESTOP - you may well have secondary Estops on your DUT or test gear. Make sure your master is clearly identifiable and labelled as such. Your documentation should have a check list for this system, with a clear picture of the MASTER ESTOP buttons on it. i tend to use the much larger (~80mm diameter) mushroom style estops for the master and keep the smaller (30mm dia) more typical estop buttons for secondary stops5) Drive a RED beacon on at any time the HV DC is, or could be, live. How you sense this is up to you, and it becomes tricky as votlages climb above 1kV or so. You may simply not be able to sense it directly, and in this case may have a beacon that turns on when your HV DC supply is enabled, and turns off a FIXED TIME after it is disabled. It is far better to show a red light when the system is in fact not at HV, than to fail to show one when it is. On my larger test cells (cells big enough to take an entire EV passenger car) the door to the cell is also interlocked and cannot be opened from the outside whilst the red light shows (there is a break glass override in case of evacuation / fire etc, and a proceduce covers the times that may be used (never been used yet in 12 years!). If the "isolated area" you have set up to do your HV testing is small, then you can avoid having to interlock it by simply using a mechanical fastner that need tool release, where it takes longer to undo those fatserners than your worst case discharge time. I dont' know what your HV DC supply architecture is, but i would normally be relying on that device to "safe" the DC link when it is disabled, and that system itself needs to be suitable rated and qualified.

author: max_torque
author_rank: Super Contributor
timestamp: July 17, 2020, 11:22:31 am
content: If your test area is relatively contained (ie bench top) then it's better to use a suitable HV flooor mat (not very expensive) that is kept in front of the bench, because ime, it's very easy to "forget" to put on PPE, whereas the mat just sits there all the time protecting EVERYONE who stands on it!

author: David Hess
author_rank: Super Contributor
timestamp: July 17, 2020, 11:29:31 am
content: Quote from: state_of_flux on July 16, 2020, 02:36:10 pmMy favorite is a neon bulb in a relaxation oscillator circuit but this is less suitable above 100s of volts.

author: JohnG
author_rank: Frequent Contributor
timestamp: July 17, 2020, 02:49:09 pm
content: I can't really provide much more info on detailed circuit design for a number of reasons:-Your specs aren't very detailed.-It's not clear whether your load is resistors, a TWT, or both. They behave much differently. But, what kind of protection you need depends on these things.-I don't know the context of the rest of your system, but it could make a big difference in how you approach the entire problem.The LED circuit is just a couple resistors and a Zener that you hang in parallel with one of the segments of the main bleeder resistors. A few mA will make a visible light. The Zener is there for when the voltage gets higher. This all depends on how much current you allow in the bleeders under nominal conditions, etc. It's for your own piece of mind - it's a nice visible reminder that a cap might be charged up.-Short and open circuit protection depend a lot on your power supply topology, controls, size requirements, etc.Don't take this harshly, but this kind of design is far from trivial, and it seems from the questions you are asking that you are a long ways from having this together. It's not that hard to get 6 kV, but it is hard to build a robust and safe 6 kV supply that meets any kind of semi-modern performance capability. That's why these supplies are expensive. If you need something beyond a line frequency transformer with a lot of leakage inductance followed by a multiplier, I would really recommend looking for some used HV supplies where this has been done for you, or you can sink a LOT of time into this, and it does not appear that your Ph.D. is about HV power supplies.But, if you want to go down this route anyways, find someone local with experience in the topic, and pay them for their expertise. If they are any good, you will save money and time.Cheers,John

author: PartialDischarge
author_rank: Super Contributor
timestamp: July 17, 2020, 05:02:32 pm
content: The problem with resistors combined with zeners and leds is that the light intensity will not be constant, power dissipation in the resistor grows with the square of voltage and light does not blink as it should for proper discrimination.A well designed indicator should offer 1) constant light intensity over the detected voltage span 2) blinking lights and 3) more than one led/neon to account for failure of a single element

author: JohnG
author_rank: Frequent Contributor
timestamp: July 17, 2020, 07:36:37 pm
content: You need bleeder resistors anyway. They would generally be a bunch of resistors in series. You put your indicator in series with one of them. If you use a Zener in the circuit you can get fairly constant LED current with a couple extra resistors. I didn't draw it because it is (or should be) trivial, and if one can't figure it out, they have absolutely no business designing a HV power supply. Note that I generally have put this so that the LED is on or near the cap bank. It's for my own benefit when I work on a supply. Tthe fancier it is, the more likely something goes wrong with it.You can make it as fancy as you want, etc, and yes, you should have a separate front panel output indicator, a relay plus load to discharge the caps on power off or disable, etc. There should be many other features as well.Blinking indicator lights are bad for a few percent of the population (I am a member of that percentage). For that percentage, they are extremely distracting and an impediment to safety, especially if the frequency is higher than one Hz or so. If the blinking is on/off with an LED or other fast-responding light source, it's worse (due to the harmonics).John

author: state_of_flux
author_rank: Contributor
timestamp: July 20, 2020, 12:44:09 pm
content: The load is purely resistive for the purposes of my PhD I will not be using an actual TWT load.Okay, so something like this - which uses a Zener diode and LED as mentioned:http://www.learningaboutelectronics.com/Articles/Voltage-level-indicator-with-a-zener-diode.php.This placed in the 6kV bleeder resistor chain, which I had calculated as 400Mohm or something along those lines for a 1 minute discharge. Does this seem like a good idea.I was not sure whether this would be a good idea due to the high voltage, and whether a level indicator might be better placed on the low voltage control side following the feedback chain and opto-isolator? Would it not give the same function?Since my load is purely resistive - is there a need for some kind of overload condition since the load resistance is defined?