Dealing with DC voltages >100V - Page 1
Forum section: electronics / general

author: T_guttata
author_rank: Regular Contributor
timestamp: June 03, 2021, 08:16:02 pm
content: Hi everyoneI have some questions related to DC voltages >100V.I'm a mechanical engineer and I also have some background related to electronics, but I just recently started doing eletronics in my spare time. So far I have a 30V 10A power supply and was working with low voltages. But obviously at some point you will need higher voltages for a certain project or encounter high voltage capacitors when trying to repair some circuitry.Now, I'm familiar with the limits which are considered non-lethal. I know how RCD work and in my home all sockets are protected. However, I don't have experience working in a lab. And obviously I don't want to test the limits of the human body at all. I think that working in the lab is more prone to errors than lets say doing AC (230V) stuff, for example installing a lamp. There are (more) faulty situations which are not protected by the RCD and there are many parts around which are conductive. Maybe someone can give me some input regarding the following questions:1) What are (the most important) things you must do and must not do related to safety in the lab?2) What is the maximum voltage when using breadboards? It's clear that high currents over a longer time will dissipate heat which might melt the breadboard.3) What equipment would you use to charge capacitors to higher voltages? I only found a few (affordable) DC power supplies >60VDC. I'm not really attired to use high voltage converter circuitry from aliexpress (example:https://de.aliexpress.com/item/1005001361171048.html?spm=a2g0o.search0302.0.0.3ebf573cfBp9uU&algo_pvid=38aeedf3-f500-4a4c-8450-1602e2c56aa3&algo_expid=38aeedf3-f500-4a4c-8450-1602e2c56aa3-5&btsid=0b0a556716227504365024617ee323&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_) because of messing around to set the voltage etc. Is there anything in between a $3 PCB and a DC power supply for several 100 bugs?4) Insulating gloves; is it recommended? ESD gloves; any safety benefits?5) Any recommendations regarding literature related to practical safety (prototyping, reparing stuff)?Thanks for your advice!

author: bob91343
author_rank: Super Contributor
timestamp: June 03, 2021, 08:45:23 pm
content: You ask good questions. However, as Louis Armstrong famously said, 'if you have to ask, you'll never know'.These things become obvious to those who work in the field, in short order. You will get some tingles and jolts. You should be especially careful around 240 V mains, although not so much 120 V. One hand in the pocket is good advice but severely limits your ability to work. Gloves are a pain and reduce sensitivity too much.Charging capacitors requires some current limiting resistance, although in most cases the charge period is so short that nothing has time to get hot enough to melt, other than fuses.You learn early on that when pushing a plug into a socket, you back away from the bench a bit and hold the other hand high if you can. It's generally accepted that you can't feel anything less than around 10 Volts unless you wet your fingers. Up to maybe 70 Volts won't distress you too much. This is for low frequency or dc. For high frequency, the rules change. A little rf can cause a burn on the skin. A lot of rf can do serious damage.I have been subjected to all of it, even thousands of volts, and I am still here with no permanent damage, at almost age 89. I still get jolts and they usually give me valuable information.

author: james_s
author_rank: Super Contributor
timestamp: June 03, 2021, 08:58:10 pm
content: It all depends. Generally speaking, anything under about 48V is considered "low voltage" and is relatively safe. Breadboards are for low voltage and I would never recommend going above 48V in most cases. I did once breadboard most of a ~1kW SMPS with a 340VDC bus on a breadboard but that is definitely a case of "don't try this at home, I'm a professional" and I would never recommend it to anyone.You can go a very long way in electronics without ever needing to mess with more than about 15V. When it comes to higher voltages standard electrical safety rules apply. Bottom line is if you are not completely sure what you are doing, don't do it. Don't forget that low voltage can be dangerous too if the current is high enough. A car battery is only 12V but it can produce enormous current. My dad had a permanent scar on his finger that happened when he had a wrench on the positive terminal of a car alternator and it short circuited to something else through a ring he was wearing which instantly got red hot. This is why every auto repair manual starts most procedures with "disconnect the negative battery cable"! It is also advisable remove any metal jewelry before working with electricity, even small batteries of certain types can produce a surprising amount of current, enough to cause a serious burn.

author: David Hess
author_rank: Super Contributor
timestamp: June 05, 2021, 02:28:18 am
content: For high DC voltages, it is important to use as little capacitance as possible to limit the energy of any discharge through you. I learned this the hard and painful way.High voltage circuits also often have large value high voltage current limiting resistors.

author: Zero999
author_rank: Super Contributor
timestamp: June 05, 2021, 05:50:17 pm
content: The general rule is that DC, carries a lower risk of shock and death, than mains frequency AC. The only additional hazard is, in many cases DC is from a battery, which can't always be isolated.To be pedantic, high voltage is above 1kV. Mains, is low voltage. The reason for the official definition for high voltage being so high is, below that voltage, you need to actually touch a live conductor to get a shock. High voltages can arc, so you can be shocked, just by going too close.Extra low voltage is anything below 120VDC, or 50VAC. The reason for this is voltages below those levels are unlikely to deliver a lethal shock, in dry conditions. Of course they can still shock and still carry a small risk of death, but it's unlikely. Appliances using separated (meaning it's separated, i.e. isolated from the mains and earth) extra low voltage, doesn't require an earth connection.Voltages below 60VDC and 25VAC, don't require all parts to be insulated from the user and are safe to touch, in dry conditions. They carry a very low risk of shock and you probably won't even feel it, unless you have damp skin, or the contact area is large.In other areas, where there's an increased risk of shock, such as next to a pool, even lower voltages need to be used and in some cases, it's not permissible to have any uninsulated conductors.

author: PwrElectronics
author_rank: Regular Contributor
timestamp: June 06, 2021, 06:07:33 pm
content: We regularly deal with "class B" voltages in the lab at work. Defined as >60VDC. We've got power electronics projects going on up to about 1000VDC. A lot of the lab DC power supplies are on 480V 3ph due to the power levels.The lab rules are you cannot be hand probing on a live circuit. Instruments must be connected with the power off, and a cover or shield placed over the unit before power applied.I've certainly bent these rules on occasion but keep in mind where your hands are and what you are doing!!!! Use only one hand and do not have your body grounded someplace while doing it.

author: tkamiya
author_rank: Super Contributor
timestamp: June 06, 2021, 06:28:53 pm
content: Anything higher than say, around 48V, I start taking precautions. Once, I was hit by about 1000V and the result was quite ugly. I lost use of right arm for a whole day. That was in vacumme tube era. These days, use of high voltages are rare in usual lab environment.As to precautions, I always assume voltage is always present until I measure and make sure there isn't. I don't merely switch it off. I unplug and short out the high voltage line to ground just to be 100% sure. If I have to measure something, I connect my instruments then turn on the power. I also use what's called a chicken stick. It's just an insulated stick with an electrode at the tip which is grounded. Touch potential danger and discharge.PCB can be used for quite high voltage. For example, rectifier board for around 3000V are often made with regular FR4 board. Beyond that, teflon or ceramic stand off are often used. Soldering takes special care, too. Don't leave sharp edges. Try to make a round mound. Especially be wary of capacitors. They retain charge. Make sure bleeder registers are working. (that was my reason for getting shocked)Around live voltage, non dominant hand stay in pocket or otherwise known location. I have a habit of making a fist with left, put it behind my back and knock once. On right hand, always be aware where your pinky is. If you wear a glove, be sure it's rated. Thin medical stuff doesn't do much.If you don't think you are ready for all this, don't try. Have someone supervise you. All it takes is once.

author: coppercone2
author_rank: Super Contributor
timestamp: June 06, 2021, 07:14:48 pm
content: there is something everyone missed, that is, to pay the correct amount of money from a proper supplier for all cables, interconnects, crimps, crimp tools, wires, etc.so1) proper components, cat ratings, insulation, etc.2) get the good fluke probe kit, think before measuring, ideally don't interact with circuit while its powered on3) put indicator lights (neon) to make sure you know when something is energized4) clean the work area, use a different outlet for the circuit under test then your equipment (don't plug it all into the same power strip), the main close calls I had were because of this, i.e. unplugging the wrong thing. This is when I suggest having two work areas, one for testing, the other for any disassembly, repairs, changes etc. So basically, ban the screw driver from being on the test bench ever, so you do your changes, move it to the test bench, test it, then take it back to the other bench to do anything (including stuff like changing breadboard components)5) use fuses6) clean tight grounds7) good way to get zapped is to lean your hands on a chassis for stability with probes etc, the problem is usually that there is a fuse holder, power inlet, etc, with exposed wires there, then you can easily bridge the circuit (especially on large equipment). If you can, design so there is nothing on the side panels that is non insulatedfollow peoples advice on isolation/gfic/etc, I am not going to get into that, it should be secondary protection from good practice and design9) when destroying old equipment with cutting (i.e. you found a bad cable), drag it away from where you are working so you see both ends in plain sight and then cut it, don't do this while sitting at the bench after you were working for a long time10) get tools with insulation, even the crappy rubber comfort grips can protect you a little bit (i.e. klein wire strippers), ditch the stuff with compromised handles for the pure mechanical work area (protects from #9 fuck ups). if you short it, throw it away after arcs occur where they are not supposed to, you have no idea whats going on there exactly11) take time to get rid of fiddly stuff that you think is bad (i.e. if you clip to something and it keeps falling off, just stop until you figure out a reliable way, buy the right connector, some kind of helping hand, etc)... unless you are trying to get something done in the middle of a goauld attack.. its not battlefield repair12) inspection, check for insulation damage, alligator clip alignment, equipment stability, etc#7 and #9 are the ones that are most likely to get you IMO, with work fatigue, because we are often workahaulics

author: RJSV
author_rank: Super Contributor
timestamp: June 28, 2021, 05:54:53 am
content: One question folks maybe missed asking:Certainly, you don't want CONDUCTIVE FIRE / FLAME SUPPRESSANTS !That LAB safety consideration is likely in place, in any local fire codes, or special safety codes in a LAB.I wonder how varied that compulsory / regulation is,place to place?

author: G7PSK
author_rank: Super Contributor
timestamp: June 28, 2021, 01:05:08 pm
content: Decen insulated tools are a must for HV work. The real HV fun starts a few years down the road when home mechanics start "fixing" EV's in the driveway.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 29, 2021, 02:32:01 am
content: Quote from: G7PSK on June 28, 2021, 01:05:08 pmEven a girl can disassemble an EV battery, how hard can it be really?