"Proving" safe voltage (detecting a very small % of a high voltage) - Page 1
Forum section: electronics / projects

author: tooki
author_rank: Super Contributor
timestamp: February 24, 2024, 10:55:37 am
content: Hey everyone,<background>I'm working on an experimental setup at work that involves a quite high voltage power supply. (The experiment requires the feedstock chemicals, in gaseous form, to flow through an electric field to catalyze a reaction somehow. We designed the electrodes as two cylinders separated by a gap. The lower electrode has the outflow in the center, so that the gas mixture cannot exit the reactor without passing between the electrodes.) This project was started before I was hired, and a high voltage power supply had already been ordered with a maximum voltage likely far higher than needed: 40kV DC. So even though they'll likely never run the experiment at that voltage, the entire setup has to be designed to protect human safety even if someone were to crank up the voltage.The power supply is current-limited to 1mA in terms of regulation, but the peak current as its caps discharge can reach up to 500 times that, according to the manual.Additionally, the 3 meter long HV cable, being the typical coaxial construction, has about 140pF of capacitance of its own, so on the advice of my old boss (who has experience with HV stuff) I'll be adding a discharge circuit (with analog moving-coil meter) and a series resistor to the electrode to limit current. Between my discharge circuit, and the discharge circuit built into the power supply, any voltage should be discharged within 1 second, which is faster than one could physically open the barriers between the user and the electrodes.There will also be interlocks, interfacing with the fume hood door, as well as the door to the metal cabinet that will house the reactor itself. (And inside that, everything HV is insulated.)</background>So the actual question:One of the things that the users would like is a light (like the typical industrial beacons) that indicates whether it's safe to touch or not. A theoretically simple one would be a light that simply signals when the HV output of the power supply is turned on (regardless of what voltage it's set to), but unfortunately the power supply does not have an output for that, only a 0-10V proportional analog output, so a safe-to-touch output voltage of 50V DC, for example, would mean an analog output of just 20mV -- and I can't be sure if that output would even operate if the power supply's mains input is turned off.* The even better option would be something that actually indicates a voltage of 50V or lessat the electrodes, but I am not sure how I would go about measuring that in any halfway reliable way.So how would one go about detecting that the voltage is 0-50V when the maximum is 40kV?Or is there some entirely different way of sensing this, especially something entirely passive?I'll be using a safety relay (probably something from Phoenix Contact) to monitor the interlocks and cut off power to the HV supply, and to control the beacon, if used.What wedon'twant to do is a light that doesn't actually indicate anything useful, or that could even be misleading. So I don't want the beacon to simply illuminate when the interlocks are closed (=ready to operate), but the HV supply isn't on, since that's signaling "danger" even when it's not, which could lead to complacency.I'm currently waiting on a reply from the power supply manufacturer to see if they can offer a solution. The power supply's own HV output on/off button illuminates exactly as I want, but I'm not opening and modifying an expensive, dangerous HV power supply without their say-so.Thanks for any ideas!*I did lash together a quick comparator circuit as a test, but all I had laying around was an LM339, whose input offset is too big for comparing 20mV to 0V. Even with hysteresis it flutters around.

author: Gyro
author_rank: Super Contributor
timestamp: February 24, 2024, 11:18:08 am
content: You probably want to go as passive as possible for safety, and maybe double up too. My first thought is a simple resistor chain and indicator, either a neon or very high efficiency LED. You could then rely on this to discharge the capacitances that you mentioned - for a neon you would need a high value shunt resistor across it to ensure that you get below 50V. The problem is that you have a relatively small amount of current to play with in terms of illumination. With the loading of the indicator, you could probably add a label saying something like 'Wait 10 seconds after the indicator is extinguished before opening'.As an alternative, you could use a MOSFET with gate clamp and high value shunt resistor in place of the LED / Neon to drive a larger indicator - as long as you can ensure that the supply to it is failsafe.The comparator method is reliant on an independent supply and other factors that might make it more prone to failure, not to mention the difficulty in handling such a wide input range without possible noise problems at the low end.

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 24, 2024, 11:21:11 am
content: For that kind of voltage span, 50V to 40kV a passive method is not possible, because resistors will create a large voltage drop or the power dissipation will be too high.Maybe they can helphttps://saker-mv.com/voltage-detector-mvd30-mvd15/

author: Gyro
author_rank: Super Contributor
timestamp: February 24, 2024, 11:53:17 am
content: What's going to discharge the output capacitances down to <50V? The PSU? - one for tooki to check. I'm not saying a 40kV resistive divider is easy, but it is doable with proper insulation and layout. For a few 10s of uA dissipation isn't a problem, eg. 800mW @20uA. If it turns out that a discharge load is needed anyway to bring the voltage down in a sensible time, then you may as well make use of it.

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 24, 2024, 11:59:29 am
content: HV resistors are not that expensive. RS Pro 500Mohm rated for 20KV:https://uk.rs-online.com/web/p/through-hole-resistors/1754074?gb=sUse 4 of them to make 2Gohm 80KV + a final 200K sensing resistor. (1KV=100mV).2Gohm load (20uA @ 40KV, 800mW), giving a 10000:1 ratio or 4V ouput for 40KV input. Adjust the final ratio as required.Feed this to a instrumentation op-amp/comparator, these usually have GigaOhm input impedances.It doesn't need to come out for anything, just put it in a earthed metal enclosure. Use a small secondary psu for running the detection circuitry and turning the led on/off.Make the circuit redundand just in case, but the interlock should be the true safety mechanism, disconnecting the HV supply and applying a bleeding resistor unless the HV PSU automatically does this when off.As long as everything is referenced to ground (Protective earth), it shouldn't be dangerous.Dip everything in HV silicone, perform degassing if required, should perform great.Quote from: tooki on February 24, 2024, 10:55:37 amOr just a phototransistor reading this!But this probably isn't a real HV indicator, but simply turned on by the driving circuitry, assuming there will be HV output unless something is wrong.

author: tooki
author_rank: Super Contributor
timestamp: February 24, 2024, 12:15:22 pm
content: Quote from: Gyro on February 24, 2024, 11:18:08 amI am already planning to have a discharge circuit using a 100uA meter through 400Mohms, which will thus display the actual voltage, but also discharge. But of course once we're down to a few hundred volts, one won't have any useful resolution.The problem with lamps and whatnot is that this thing is never going to be run at 40kV -- I actually expect they'll be below 5kV -- so an indicator may not even light up with the series resistors that have to handle 40kV. :/

author: tooki
author_rank: Super Contributor
timestamp: February 24, 2024, 12:16:31 pm
content: Quote from: PartialDischarge on February 24, 2024, 11:21:11 amThanks, that's a great tip. Maybe they can do a higher-voltage model or can point me to one.

author: tooki
author_rank: Super Contributor
timestamp: February 24, 2024, 12:31:59 pm
content: Quote from: Gyro on February 24, 2024, 11:53:17 amThe power supply was ordered with the "quick discharge" option ("discharges to 10% of Vout within 2 seconds"), which does discharge any external capacitance as well, but of course in more time.The 400Mohm discharge and meter circuit described above should be able to discharge the cable alone to essentially zero in well under a second, and will help discharge the PSU's output cap as well, so between the two it should discharge fully within a second or so, plus I like having the two redundant discharge circuits.My current plan is to use four 100Mohm, 10W resistors in series (something from Ohmite, I don't have the model handy), mounted on solder eyelets screwed to nylon standoffs screwed into a thick plastic baseplate in threaded blind holes. Another branch will put one or two of those resistors in series with the output to the HV electrode to limit current in case of a discharge or arc. At around 8-10 cm between eyelets, this means the entire voltage is across a distance of around 35cm, far more than should be necessary for both clearance and creepage. There will be ample of both around the edges, too. I hadn't planned on potting or coating, since there's ample room in the setup to just keep things physically separated.The setup is a big aluminum box that will be grounded. The internal wire to the HV electrode isDaburn #2477/22-50, a 50kV wire that can handle an operating temperature of up to 250C (since the electrodes must be heated).

author: Wolfram
author_rank: Frequent Contributor
timestamp: February 24, 2024, 12:46:49 pm
content: If you only need to detect dangerous voltage, and not accurately measure it, you can use a lower divider ratio than planned, and clamp the measurement voltage under normal operating conditions. This means you can make your threshold voltage some hundreds of millivolts for example, instead of a few millivolts. This will greatly simplify the task of designing a threshold detector after the divider.

author: tooki
author_rank: Super Contributor
timestamp: February 24, 2024, 12:52:42 pm
content: Quote from: DavidAlfa on February 24, 2024, 11:59:29 amThis is sorta what I was envisioning/wondering about. But from what I gather, I would need to choose components fairly carefully to ensure it's "clean" given the tiny output voltage we'd be comparing, right?There will already be a 24V PSU to power the safety relay and the PID temperature controller that heats the electrodes. That could be used to power detection circuitry and the beacon lamp.(Cost isn't a concern, really. I'm certainly not gonna dick around with unknown-quality RS Pro crap when I can get Ohmite for a few bucks a piece... Heck, getting that 50kV Daburn wire ended up costing something like $200 after shipping and customs, for 12 feet of it. The electrode housing alone is made of hundreds of $ of PEEK plastic.)Quote from: DavidAlfa on February 24, 2024, 11:59:29 amAgreed. As mentioned above, the PSU has the quick discharge option (which does operate passively if mains voltage is lost), and since our actual load shouldn't haveanycurrent flow at all -- there should be no current flowing between the electrodes -- I can afford to leave my own discharge/meter circuit (also described above) connected at all times.I did float the idea of having a mechanically-operated crowbar circuit (some kind of mechanical linkage to the device door, such that when open, the HV output is shorted to ground via a bleeder resistor) but our safety official said that was overkill.Quote from: DavidAlfa on February 24, 2024, 11:59:29 amIt will be, for sure!The reactor will be housed in a heavy aluminum enclosure with a polycarbonate front door. All exposed metal will be connected to PE. This enclosure will be bolted to the back of the fume hood. The enclosure's door will have an end switch going to the safety relay. The fume hood's existing door slider sensors will also interface to the safety relay. The safety relay in turn will interface with the PSU's interlock circuit, as well as switching the PSU's 230V supply via a contactor with monitored contacts.The power supply's output cable's outer conductor is PE, but the PSU has an additional binding post for PE which I intend to use, plus my old boss (one building down in a different department, so he's come to look in person) suggested a solid PE connection within the fume hood. I can easily use the ground pin of an electrical socket within the fume hood (which is where the HV power supply will be plugged in), but he said that if possible, I should PE directly to a PE bus bar. I'm looking into whether I can gain access to one.Quote from: DavidAlfa on February 24, 2024, 11:59:29 amHow important is that, given the clearance/creepage distances described in the prior reply?Bear in mind that as it stands now, the electrodes are only about 5mm apart, so it's essentially a spark gap far, far smaller than any of the clearances.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 24, 2024, 12:54:35 pm
content: Use a diac and capacitor to flash the LED using the current from the sense resistor, that way it would remain visible even as the current drops to very low levels.

author: jpanhalt
author_rank: Super Contributor
timestamp: February 24, 2024, 12:57:18 pm
content: I am not into HV, but when you mention using a light to indicate safe, be sure to use two lights (one safe;other unsafe). If you have only one light, "on" should be safe; off unsafe. That's probably obvious, but maybe is worth repeating.

author: tooki
author_rank: Super Contributor
timestamp: February 24, 2024, 01:04:56 pm
content: Quote from: Wolfram on February 24, 2024, 12:46:49 pmIndeed, there are already two voltmeters planned: the digital readout of the PSU, and the analog meter I plan to put directly in the enclosure. The idea of the lamp is indeed an "idiot light", e.g. red lamp = dangerous voltage present, green lamp = voltage is safe to touch.I don't have enough experience with clamping circuits running on tiny currents.Do you mean something like this? Suppose a threshold of 0.5V. So for Vthr=0.5V when Vin = 50V, I'd need a 1:100 divider. That in turn means that at 40kV, Vthr=400V. So then you'd have a diode clamp or something to limit that to e.g. 5V for a comparator?The PSU is capable of 1mA, but of course I'd rather run it with as low a current limit as possible. Let's suppose 100uA. How would that affect the clamping?

author: tooki
author_rank: Super Contributor
timestamp: February 24, 2024, 01:05:48 pm
content: Quote from: NiHaoMike on February 24, 2024, 12:54:35 pmHow would that work? I have absolutely zero experience with diacs.

author: tooki
author_rank: Super Contributor
timestamp: February 24, 2024, 01:10:35 pm
content: Quote from: DavidAlfa on February 24, 2024, 11:59:29 amIndeed, it's simply the "output enable" status. Of all the signals/indications currently available, without me actually building my own circuits, this is the closest to the users' desired function as possible.But of course then they could just glance over at the HV PSU itself and look at the button directly, since it'll be right next to the fume hood. Given changing ambient light (and the theoretical failure of that lamp itself!) I'm not sure that optically monitoring it is a good idea. (From everything I've seen, this is why real-world light curtains and free-space optocouplers always use a carrier frequency, just like IR remote controls.)

author: tooki
author_rank: Super Contributor
timestamp: February 24, 2024, 01:20:34 pm
content: Quote from: jpanhalt on February 24, 2024, 12:57:18 pmI did think that, but thank you -- it's definitely worth repeating when safety is involved.As I see it, the most essential component of the safety design is the discharge circuit that ensures that when the fume hood slider is opened, opening the interlock, the voltage will discharge to safe levels by the time you manage to open the device enclosure. That ensures it's safe even if something has gone wrong within the enclosure, allowing exposed wiring or electrodes. Normally, once the enclosure is opened, you'd still need to unscrew the entire reactor assembly within (which takes way more than a few seconds) until the electrodes are exposed. Within that amount of time, either one of the two discharge circuits (the one in the PSU, and my discharge/meter circuit) alone would have long since discharged it to zero.In a way I know that a lot of this is belt-and-suspenders design. But frankly, I don't want anyone to get injured or killed with something I built, even if someone else is legally responsible, and even if a user does something really, really dumb. So even though I've been told that training is part of the safety design, I want it to be as inherently safe as possible.So I'm a bit skeptical of relying on DIY monitoring circuits that could potentially fail, creating false safety.

author: tggzzz
author_rank: Super Contributor
timestamp: February 24, 2024, 03:59:06 pm
content: Quote from: tooki on February 24, 2024, 01:20:34 pmMaking sure it is very difficult to defeat the interlock is too importantThat's one advantage of moulded mains plugs over plugs that can be dismantled. (I have an example of that which I discovered on a scope. You can guess what a previous user had done.)Interesting thread

author: joeqsmith
author_rank: Super Contributor
timestamp: February 24, 2024, 04:14:58 pm
content: 1mA DC max, 40kV max, 150pF added capacitance. Well under 0.2J. You never supply a PN# for the supply. I assume you have checked with the manufacture about their internal filter and are working at much higher levels. I also assume you have followed some standard like the following to asses the risk.https://www.standards.doe.gov/standards-documents/1000/1092-BHdbk-2013There are products approved by safety agencies. If your design injures someone and you have chosen to make your own safety devices which have not been certified, you may be on the hook.

author: mag_therm
author_rank: Frequent Contributor
timestamp: February 24, 2024, 04:43:39 pm
content: Yes is likely that in the EU lab workplace, there will be regulations for this.There will surely be IEC or Swiss standards that Tooki can search for "Proving" safe voltage ...in R&D research labIn some jurisdictions, I read that the legal definition of high voltage equipment is now defined by bounds of [ kV, s/c A, stored mJ ]There is the USA OSHA "Lab Safety Guidance" which is not in itself regulatory.However, it points to the various ordinary workplace safety rules including electrical, which are mandated in labs, including employee training and qualification.In my experience (now dated) HV access permits reqired a time limited document with the procedure, signed on and off by all workers who were accessing.The procedure involved a group master padlock box, isolation lockout sub procedure, followed by use of approved and calibrated live line tester. (On a long pole)Then before worker access, a phase shorting/grounding bar was connected to the HV parts, with sufficient rating to clear the incoming breaker.And with DC:Capacitors and cables with high performance dielectrics that have been polarized for a long time can have a charge "rebound" if left open circuit after discharge.I did not find ref to this today. With the oil dielectric capacitors I was involved with, they had to be shorted with lock wire during storage and after removal from service.

author: David Hess
author_rank: Super Contributor
timestamp: February 24, 2024, 05:27:06 pm
content: Quote from: NiHaoMike on February 24, 2024, 12:54:35 pmOld high voltage stuff sometimes used the same concept with neon bulbs acting as the DIAC. Capacitive discharge through the bulb gives good intensity where microamps would not. This is probably what I would do although it does not indicate voltages below the neon bulb's strike voltage.

author: bdunham7
author_rank: Super Contributor
timestamp: February 24, 2024, 05:32:54 pm
content: Quote from: tooki on February 24, 2024, 01:04:56 pmEither learn or sub that part out.When I have issues like this my way of thinking starts with "how could I do that using regular lab (test) equipment" and then that helps me think of a way to design an actual specialized circuit. I can think of three ways of doing it that would all work quite reliably and I can demonstrate them all with regular stuff I have on my bench.1. Use a regular 1G 40kV voltage 1000X divider probe (e.g. Fluke 80K-40) and a 5.5-digit or better DMM set to the 100V range. You'll be able to clearly see the voltage with a resolution of 1V, so 50V is very easy to spot.2. Use a Picoammeter, set to (as an example) 100pA full range and a 500G resistor in series. 50V will show full-scale, but beyond that the meter will be pegged (I have an analog Keithley 414A) but that won't bother it due to the nature of picoammeter amplifier designs.3. Use a regular handheld DMM set to the 50mV or 100mV range and the same 1G 40kV 1000X divider, you'll get a 50mV indication at 50V and for higher voltages it will indicate overload.The last one utilizes simple clamping with a high-imedance input amplifier and a large resistance in series, which is probably the easiest reliable method to implement. However, I'd suggest at least setting up the first method for testing if for no other reason than to verify that your discharge circuits are working as rapidly as you expect them to. An exponential decay from 40kV can have a long tail if your lower threshold is 50V, especially if you pick up some extra capacitance unexpectedly.

author: Zero999
author_rank: Super Contributor
timestamp: February 24, 2024, 05:49:22 pm
content: Quote from: NiHaoMike on February 24, 2024, 12:54:35 pmI was going to suggest the same thing. The frequency will also increase with the voltage. The only thing is the sense resistor must pass more current than the leakage of the DIAC, but that shouldn't be an issue.I would also use more than one, so there's redundancy.

author: D Straney
author_rank: Frequent Contributor
timestamp: February 24, 2024, 05:54:07 pm
content: Another simple way to look at low voltages while blocking high voltages, stolen from common gate-driver-desaturation-detector circuits, is to have an HV-capable diode or diode string with its cathode at the output voltage, and its anode with a pull-up resistor to a lower DC supply. When high voltage is present on the output, the diodes block in reverse and the anode is at the lower DC supply. When the output voltage gets low, the diodes conduct and the anode voltage will be (Vout + Vf). Vf can be in the 10s of V in this case, but if you're not relying on a precise measurement and you find the right diodes it might do what you want. Maybe stick a large-value fusible HV resistor (value < pull-up resistor / 10) in series with the diode(s) to limit current just in case something arcs over.

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 24, 2024, 06:08:03 pm
content: Paper about Non-Contact DC Electric Field Sensors:https://www.osti.gov/servlets/purl/971778

author: jonpaul
author_rank: Super Contributor
timestamp: February 24, 2024, 08:50:47 pm
content: neon bulb, connect 1 end to HV or divider with seriers R.Leave other end disconnected.E filed will lihgt it when HV is ON.Check power lineman's non-contacting HV detectors.j

author: Gyro
author_rank: Super Contributor
timestamp: February 24, 2024, 09:11:59 pm
content: That doesn't help with the <50V requirement though.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: February 24, 2024, 09:18:20 pm
content: Quote from: tooki on February 24, 2024, 10:55:37 amWhat voltage do they actually expect to use ?You will need a system that is 40kV safe, either roll your own with HV resistors (as above), but note 40kV multimeter probes are a somewhat standard item.The advantage is they can be plugged into their matching meter to confirm visually the voltage, and you have shifted all the HV work and calculations to them.Those probes seem to be designed for a 10Mohm meter load, with 1000M:1.1M internal, giving 1000M:1M when 10M meter is added.You only need to sniff that junction with a system that can sense 50mV, and tolerate 40V, with a somewhat high impedance, at least at 50mV.Modern opamps/comparators have sub-mV offsets so that should be relatively simple.You might choose to expand this simple 'safe' test, with other thresholds for levels of voltage, so the checker is seen to operate correctly in normal use.Addit:QuoteIf you already have voltmeters, you must already have dividers down to sane levels.You can sense directly across your analog meter driver, or add your own HV divider if you want fully isolated safety check.

author: Someone
author_rank: Super Contributor
timestamp: February 25, 2024, 12:12:14 am
content: Quote from: tooki on February 24, 2024, 10:55:37 amA super low current sense that is snappy with a 1:500 range.... well that solves half the problem. 50V - 25kV working range just needs a series stack of them. For the other side I'd like to see a shorting relay across the user accessible parts but that's probably too expensive.

author: max_torque
author_rank: Super Contributor
timestamp: February 26, 2024, 05:06:02 pm
content: The "Problem" you have here is nothing to do with sorting out a suitable circuit and EVERYTHING to do with ensuring your working practices meet all necessary HSE requirements (depending on where you are working, country wise)Having set up several test facilites that use hazardous voltages, the most difficult bit is to ensure your risk assessment and mitigation are fully documented and a suitable work instruction / procedure is created, ratified and signed off (by the CEO if necessary as the buck ultimately stops with them these days)What i can suggest is however:1) a mechanical disconnect or crow bar for the HV circuit. Ie opening the door of the test chamber will actually mechanically short the HV Supply to zero within a length of time deeamed to be sensible depending on the access level to live conductors within the chamber2) if your safety circuit reqiures external power, say a seperate 24v supply to operate, make the precence of this power MANDITORY for the HV to exist. For example a N/C relay that should the 24v power be lost, shorts the DC link to zero volts as per 1) above3) NEVER indicate a "safe" state unless you can 100% ensure this state. Normally i would use an orange and red beacon. When the test chamber is unpowered, neither will be lit. Applying power to the chamber which could therefore allow the HV supply to start (under fault conditions) lights the orange beacon to say "this chamber is potentially live).. When you enable the HV supply, just light the RED beacon, which should have a suitable delay period for turning off, ie it lights as soon as the HV supply goes live, but takes say 5min to go out when the HV supply is disabled.4) you may be able to validate the probability of the HV being present by simply measuring the input power to the HV supply itself, especially in the case of having passive discharge resistance that is NOT decoupled from the HV bus, ie the supply is expected to always drive this load5) the most dangerous case is stored charge, either during normal operation or from un-expected failure states. When working with HV, all capacitances (inc parasitic ones) should have robust methods to ensure that should then become charged they will self discharge. Knowing what the time period for this discharge could be allows you to proceduralise the way you terminate and remove DUTs from the test chamber.6) for all my test chambers, i put all instrumentation in the chamber, and use, where possible, a single cross boundary isolation point to move data out and power in. For example, use a suitably rated ethernet isolator (probably have to be optical fibre type @40kV) to 'remote desktop' in to a PC based DAQ system in the chamber. Here, in the case of gross fault or miswire, you may well damage equipment, but operators should be always protected. At these voltages, suitable fused and crow bar voltage limitation to PE is going to be required. ie your primary instrumentation should be specificed to deal with the maximum WORKING voltage of the DUT, but should that voltage get through your functional isolation / insulation, a second layer of voltage limitation is in place to either clamp the voltage to below a safe value or to blow a fuse and achieve the same effect on any external inetrface. Here, because the power you are working with is very low, this really shouldn't be very hard ie a some redundant 30v TVS diodes should easily be able to hard short the HV to ground without themselves going bang first.7) i highly recomend you use a suitable commercial PROING UNIT (ie martendale tester etc) to ensure that you have a final line of defense for all operatives that is commercially proven and established. Before ANY condutor is touched, this unit is required (by a written and witnessed procedure) to be used to "PROVE DEAD". There are legal ramifications of this step in most countries set by the relevant HSE.if your power supply can be set to 40kV then you need to design your safety case for this rating, unless you can absolutely and robustly limit the output voltage, note a software limit would not be considered suitable on it's own for this limitation

author: tooki
author_rank: Super Contributor
timestamp: March 06, 2024, 08:23:15 pm
content: Quote from: joeqsmith on February 24, 2024, 04:14:58 pmIt's a Heinzinger PNC 40000-1 ump, with options 02 (interlock), 46 (voltage ramp), 52 (quick discharge curcuit), 56 (arc detection with auto shutdown), and 60 (output polarity switch).I haven't asked about the internal filter specifically. I assume you mean to find out its internal capacitance?As for standards: that one, like everything else I found, pertains to high-voltage, high-energy circuits, i.e. power distribution. I've really struggled to find any kind of documentation about multi-kVDCstuff like in this project. :/According to the department's safety official, because this is prototype/research hardware that isn't for sale or transfer, there aren't any hard standards I need to adhere to or certify, and it won't be my signature on the SOPs and other documentation. But of course, just because my name isn't on that line doesn't mean I will take this lightly, of course!

author: tooki
author_rank: Super Contributor
timestamp: March 06, 2024, 08:31:46 pm
content: Quote from: PCB.Wiz on February 24, 2024, 09:18:20 pmThat's actually a bit of a big question mark -- the reaction apparently needs a particular electricfield strength, not a particularvoltage. Early tests with a totally different reactor design used plates just a mm or two apart, and of course that arced over with just a few hundreds of volts. The current design has the electrodes around 5mm apart IIRC, with the ability to add a spacer of nearly arbitrary size to increase the spacing if needed. I expect a working voltage of 1 to a few kilovolts.Quote from: PCB.Wiz on February 24, 2024, 09:18:20 pmSorta. The PSU has its own digital voltmeter, using an internal divider. The analog one I am adding is using an ammeter (a 100μA one, specifically) with a 400Mohm resistor (4x 100Mohm in series) to limit the current. So I'm not actually actively creating a voltage divider as such.

author: tooki
author_rank: Super Contributor
timestamp: March 06, 2024, 08:35:44 pm
content: Quote from: Someone on February 25, 2024, 12:12:14 amI'll check this out when I have a chance! Thanks!

author: Nominal Animal
author_rank: Super Contributor
timestamp: March 06, 2024, 08:58:14 pm
content: There are electrostatic field detectors ('electrostatic sensor', see e.g. SMC IZD10-510), and high-voltage DC detectors, sold commercially; dunno if any are suitable here.I too want to emphasize using all available information for a safe/unsafe indicator. That is, I'd like the safe light to only be lit when both the supply says it is off, and the high-voltage dc detector says there is no high voltage present. Don't leave it to the humans to check them separately.

author: tooki
author_rank: Super Contributor
timestamp: March 06, 2024, 09:16:52 pm
content: Quote from: max_torque on February 26, 2024, 05:06:02 pmThanks for this, quite helpful.Regarding specific points (numbers <1 added in quoted text above):-1) As a research/development device at a research institution (public research university), we don't have to meet quite the same requirements as a device that would be offered for sale, according to the safety officer.0) Luckily, I am not responsible for that document -- that will ultimately be the responsibility of the research group lead. But I will certainly contribute, and I'll have my old boss (electronics tech at another department, who has done HV stuff and HV/high-ish energy stuff, including for external transfer that required documentation/self-certification) look at it too, since he's been advising us on this project.1) I proposed a mechanical crowbar, but the consensus was that this is overkill, given the other safety provisions. Between the PSU's internal discharge circuit and the secondary one I'll be installing myself, by the time one has opened the fume hood door (which will open the interlock) and then opened the device cabinet door (which also opens the interlock), the charge should be discharged to ~0V. Actually exposing the HV electrode is a matter of minutes of disassembly. So it should be inherently safe insofar as reaching the HV electrode breaches the interlock twice, and the discharge circuits have ample time to do their job before one actually exposes the electrode.2) I'll be using a safety relay (probably Phoenix Contact) with monitored contacts, and indeed, without power it won't close.3) This is exactly why I was reluctant to provide a "dummy light". After discussing this issue, and the technical difficulty of actually providing anything approaching 100% certainty, we are not going to provide one.The power supply manufacturer could add a "HV out ON" output as a custom modification, but at exorbitant cost, and it would only mirror the state of the output button on the PSU itself, which is within sight of the device itself. So not especially useful. Anything else would require actively monitoring the voltage inside the device, which seems fraught with opportunities to fail to actually "prove dead".4) Eh, not likely. Since it will likely end up running at far below the maximum voltage, and because the steady-state output current should never exceed the 100uA of the discharge/analog meter circuit, the change in input power is likely small.5) The parasitic capacitance of the 3m cable is around 150pF/m , so according to my math, at 40kV through 400Mohm, this should discharge to under 300mV in 2 seconds.40kV×(e^−(2s/(400Mohm×(3×141pF)))) = 294mVThe PSU's built-in discharge circuit is specified to discharge to <1% Vout within 1 second. The manufacturer has verified that this circuit will also discharge external loads. Since 1% of 40kV is still 400V, 1 second isn't enough, but since reaching the electrode is a matter of minutes, not seconds, this should be ample to discharge the internal capacitance.So my thinking (and my old boss, who reviewed this, agrees) is that between these two circuits, a) there's always a discharge circuit present, even if the cable should be disconnected or severed, and b) in normal operation the two discharge circuits provide generous ability to discharge.6) In this case, the output of the system is a gas, which flows through plastic tubing to some type of analyzer (probably a gas chromatograph, but don't hold me to this). (The inputs are also gases, created by a setup adjacent to the device, within the same fume hood). There is no DUT as such, but rather just two reagents in vapor form, with the hope that under the influence of the electric field, they react into the desired reaction product. (I wish I could tell you more but that's literally all I know about the chemistry.)In essence, we are using the fume hood as the test chamber, and within that, the device itself is a Faraday cage (all aluminum exterior, except for the polycarbonate door), and within the Faraday cage, the HV electrode is housed within the reactor, which is made of PEEK plastic with generous clearance and creepage distances. Opening the reactor (which shouldn't be needed in normal operation) requires unscrewing the lower half, which is basically a screw press (the pressure being needed to maintain tight dimensional tolerances).7) I will look into this.8) That's exactly why I have said from the beginning of this project that even though it's exceedingly unlikely that it will ever operate at or near 40kV, the entire safety design has to be designed around it because there's no way to actually prevent someone from setting the voltage that high. The PSU lets you select between the front panel knob or an analog control input, but the switch to select this is... on the front panel. Restricting access to the front panel is not an option because the only way to turn on the HV output is via the front panel button, there is no remote input for that. The PSU was not ordered with the serial port option, so I haven't investigated what possibilities it would have provided, but I would never rely on software configuration alone anyway, just as you say.

author: tooki
author_rank: Super Contributor
timestamp: March 06, 2024, 09:20:25 pm
content: Quote from: Nominal Animal on March 06, 2024, 08:58:14 pmI'll check that out!Quote from: Nominal Animal on March 06, 2024, 08:58:14 pmAs mentioned above, after discussion with the safety officer and project lead, we will not implement the indicator after all -- a half-assed "safe" indicator is worse than none at all.But if we do revisit the idea, that isdefinitelyhow I'd want it to behave.So, folks, for the moment, the issue is resolved. I really appreciate the replies, however. Thank you!!