Adding additional overvoltage protection to a DMM - Page 1
Forum section: electronics / general

author: jgvicke
author_rank: Newbie
timestamp: May 15, 2025, 04:23:32 pm
content: I have a test stand situation that will have a 7.5digit DMM from Keysight (rated at 1KV input) plugged into a switch matrix that if configured incorrectly could potentially connect the DMM to a 5KV @ 1A or 4KV @ 1.2A power supply. These supplies will have overvoltage and overcurrent protection that will shut them down if the outputs are shorted, but that will not be super fast (I think it was in the 50-100ms range) and has a fair bit of output capacitance that would have to be dissipated as well. While I know these DMMs have input protection for transients, they are not rated to take that many of them before you let out the magic smoke. I would like to put a more robust front end at my switch matrix to make sure that no matter how it is configured, the meter will be safe.There is also a high volt meter that the negative terminal has to be within 600V of chassis and I would also like to clamp this against switch matrix misconfiguration.The other devices in the switch matrix are all rated to handle the voltages, but I need to be able to make very accurate resistance measurements and low voltage measurements with the DMM when the supplies are not on.Are there any COTS items I can buy (like a DIN rail device) that would do this? Or can someone suggest a circuit I can build to achieve this?Thanks,John Vickers

author: paulca
author_rank: Super Contributor
timestamp: May 16, 2025, 09:14:45 am
content: I think no one is answering because if you are working with 5kV and don't already KNOW which equipment you can use and which you can't and are asking about modifying equipment you know you shouldn't use.... you are likely to kill yourself and they do not want any part of it.My advice. Seek professional assistance from someone with HV experience, tools and wit.

author: jgvicke
author_rank: Newbie
timestamp: May 16, 2025, 11:46:22 am
content: Thanks for the assumptions about my wit...To clarify: I do understand what can be connected, when, and to what. The setup involves a properly rated high-voltage switch matrix that manages the sequencing of multiple instruments. The DMM in question meets our requirements for low-voltage and resistance accuracy, but like most precision meters, it isn’t designed to handle sustained HV exposure. That's why we’ve implemented software-level interlocks to prevent it from ever being connected to active HV sources. This design has been reviewed and signed off by multiple engineers with HV experience.However, our safety department has requested that the system be made tolerant to possible future misconfiguration — as if the DMM were accidentally connected to an active HV source — and asked for a hardware-based mitigation, but without suggesting a method to do so. While I personally think planning around possible future software changes is a questionable path, I’m exploring how to add a hardware layer of protection regardless.What I’m looking for is a robust circuit — likely involving some combination of TVS diodes, MOVs, GDTs, and possibly PTCs — that can safely clamp or divert energy long enough for the HV supply’s OVP/OCP to kick in, without compromising the measurement accuracy under normal low-voltage operation. If you know of any existing solutions, or have experience designing such protection circuits, I’d genuinely appreciate your input — ideally without the personal commentary.Thanks,John Vickers

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2025, 02:45:54 pm
content: The K2001 uses FETs in the protection and thus could really want some extra protection. It may not like peaks over some 1.5 kV.One could add some protection with something like MOVs and for the voltage measurement some series resistance. It would usually be some series impedance (resistor, PTC maybe inductor) and some clamping device (MOV, spac gap, TVS diodes). One may get away with voltage clamps made for lightning protection at mains - possible only as a first step / part of the protection.Here it helps that the voltage source is somewhat power/energy limited. So the added protection could be OK for the HV supplies, but not safe for something like mains.On the downside the added protection can effect the measurement with the added resistance and leakage current. What can be used depends on the measurements and requirements for these. It would be tricky for high resistance like > 10 M or very low resistance like 10 ohm. It would at least want different types of protection.

author: PA4TIM
author_rank: Super Contributor
timestamp: May 16, 2025, 02:58:52 pm
content: What about active protection with a comparator? Those can react very fast. Or a crowbar circuit.There are commercial lightning protectors, a friend of mine had one mounted in the mains powergroup of his lab. I use simple ones in my lab (They are passive)

author: bdunham7
author_rank: Super Contributor
timestamp: May 16, 2025, 03:18:51 pm
content: Quote from: jgvicke on May 16, 2025, 11:46:22 amSometimes I see bench instruments that have obviously been exposed to very high voltages and I wonder how that happened. I guess a system like yours is one answer!You mention "transient" protection and state that the meters in question "can't take very many" of those hits. In fact, 5kVDC for more than a few hundred microseconds (at most) is going to cause internal fireworks. Your meter can't take ANY hits like that. I seriously doubt you can design better front end protection without affecting the accuracy and sensitivity of the meter. The only thing I can think of that would be transparent would be an interlock system that checks for high voltages before connecting the meter to the system and then disconnects it before each and every configuration change. That sort of system would have to be integrated into your setup.Is your safety department concerned about the meter or people? If, as I would assume, they're worried about people then you could just use 5kV fuses such as are found in microwave ovens on each and every input. This might not protect the meter but would prevent any fires or explosions. In addition, if all of your DMM measurements will be at very low voltages, then you could also simply add a lower voltage MOV network that would short out the inputs well below what the DMM is protected to. You would just have to make sure that the leakage currents were all low enough to not affect your readings. If you could add the details on exactly what range of resistances and type of voltages you need to test so precisely that would help us give you more specific advice. For example, if all of your resistance measurements are 4W and all of your voltage measurements are <12V so the meter can be in high-impedance mode, then you could also add series resistance to limit fault currents.Also, is this test stand isolated or is it operating with a ground reference?

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 16, 2025, 03:41:59 pm
content: Have you considered something like this: ?https://www.bourns.com/docs/technical-documents/technical-library/outside-plant-products/Bourns_SPD_Short_Form_Brochure.pdf

author: Gyro
author_rank: Super Contributor
timestamp: May 16, 2025, 04:44:40 pm
content: Yes, I was wondering about a European style mains consumer unit SPD. At least they're DIN rail mounted - I don't know about the US panel equivalents (if they exist). That will give you two chunky MOVs but the breakdown voltage will be (comfortably) lower than 1kV.

author: johansen
author_rank: Super Contributor
timestamp: May 16, 2025, 04:52:36 pm
content: there are several kinds of TVS diodes, metal oxide varisters..and there is also a third kind i just found out about.. thyristers that are configured to auto trigger above a certain voltage. i don't know if they self reset or not.so basically, for your 600 volt requirement, you just need some off the shelf 330vac rated movs' that begin to pass current at 600 volts and are fully clamped at 700 volts.and you need an appropriate resistor in series so the peak current is in the milliamps range instead of amps range when the 5kv hits it, if it does.such a resistor will not affect the voltage reading on a 10M ohm imput impedance volt meter (within reason, or just compensate for it)for current readings just use a separate meter and don't switch between volts and current on the same meter. that way if the 1 amp fuse blows.. you don't have 5KV across the fuse inside the meter. ...https://www.mouser.com/datasheet/2/54/actp250j1bj-777229.pdfQuote

author: Zero999
author_rank: Super Contributor
timestamp: May 16, 2025, 04:56:35 pm
content: What's the voltage relative to earth/ground? The meter's insulation is unlikely to protect the user, if it's floating at 5kV. At high voltages, a spark can easily jump through a gap in the case and potentially kill the user.This sounds like a very bad idea.

author: johansen
author_rank: Super Contributor
timestamp: May 16, 2025, 05:04:26 pm
content: Quote from: Zero999 on May 16, 2025, 04:56:35 pmthat's how a friend of mine killed this:https://www.eevblog.com/forum/repair/xantrex-56-4/msg5876382/#msg5876382we got it partially working. the pic survived, but there is esd damage to potentially more than one component. (all standard parts its just a matter of which one(s).

author: floobydust
author_rank: Super Contributor
timestamp: May 16, 2025, 05:08:46 pm
content: It looks like an unsafe design and then a focus to protect the multimeter, when it's already too late then.It's nothing for software i.e. Labview to encounter an error and halt the .vi, leaving some outputs/devices in the wrong or unsafe state.I've made/fixed test fixtures where hazardous voltages could make it to people removing boards/connectors on the pin bed, or to test equipment etc.I use a safety light tower stack, a H/W watchdog to remove any HV, and the switch matrix is split into a HV and LV section- they are not mixed.This test stand design could use some safety interlocks I think, as well as an engineer signing off on it.

author: Zero999
author_rank: Super Contributor
timestamp: May 16, 2025, 05:39:24 pm
content: Quote from: johansen on May 16, 2025, 05:04:26 pmYou appear to misunderstand. I'm more worried about the user being killed. 4KV @ 1.2A is a lethal combination.

author: jgvicke
author_rank: Newbie
timestamp: May 16, 2025, 06:08:27 pm
content: Thanks for all the responses — a lot came in while I was drafting this, so I’ll try to address the key points.First and foremost:the test stand is safe. The idea of adding protection to the DMM isn’t due to expected or frequent misuse — it’s to cover an extremely unlikely edge case where multiple layers of safety are deliberately bypassed. There’s already extensive engineering in place, both hardware and software, to prevent unsafe conditions.To clarify:The setup is an earth bonded, full-height 19" rack.All equipment is individually bonded to earth ground.The voltages mentioned are with respect to ground.The HV switch matrix handles all routing. The DMM is the only device on that side not rated for 5 kV.We do not connect the DMM to rails with over 1KV potential to earth ground — this is enforced by software.The power supplies have hardware interlocks tied to enclosures and UUT access points. If anything opens, they shut down.This inquiry came from a safety department request to explore adding a hardware-level backup, in case someone in the future modifies the software and creates an unsafe connection. I personally think that’s an unlikely scenario, but I was asked to evaluate options anyway.What I'm looking for:Clamp at <1 kV but allow accurate measurements up to 1 kV.Handle a fast transient from 5 kV sources (≤1.2 A) long enough for the PSU’s OVP or OCP to trip (~50–100 ms).Be always inline with the DMM so cannot be easily circumventedDMM usage:Precision resistance measurements (10–100 Ω) across HV current sense resistors when supplies are off.Precision voltage measurement across the sense resistors while supplies are on (<1KV potential to earth ground on all inputs)Leakage/bleed resistor checks (100 kΩ–100 MΩ) between HV rails for UUT safety confirmation.If the protection adds some resistance, that’s okay — the switch matrix allows me to short the DMM leads with the protection inline to measure that offset, then add in the sense resistor to determine the delta.So again, thanks to those who have offered practical suggestions. I’d appreciate it if we could keep the discussion focused on constructive input rather than assumptions about the safety of the setup.Thanks again,John Vickers

author: bdunham7
author_rank: Super Contributor
timestamp: May 16, 2025, 07:29:15 pm
content: Quote from: jgvicke on May 16, 2025, 06:08:27 pmThat makes it tough. Your meter doesn't have a lot of headroom for high voltages, as is shown by the ACV range being limited to less than 1kV. A meter that is rated for 1kVAC or more can be expected to withstand 1.5kV peaks on the input circuitry but I'm not sure yours would do well with that. So coming up with something that is transparent up to exactly 1kV and then is fully clamped by 1100V or so and does that in a few microseconds...well that's harder and would have to be an active circuit. The issue with MOVs is that there are big differences between operating, tripping and max clamping voltages.5kV at 1.2A is 6kW, 100ms of that is 600 joules. And that's assuming that the PSU actually limits the current during the fault as opposed to tripping off after some time. Have you characterized or do you know the actual fault current? A MOV network clamping at 100V or so could manage, at higher voltages that will take some big MOVs. They exist but will have a fair amount of capacitance--IDK if that matters.As far as precision resistance using 2W and always having the protection inline, the problem is that you'd have to measure the resistance of the protection each time and hope that it stays stable long enough to complete your measurement. That's not a trivial issue if you actually need 7.5-digit levels of precision.If the goal here is just to ensure personal safety in the unlikely event of a system malfunction, I think the simplest bulletproof method would be to install the 34470A behind a plexiglass shield and use it entirely remotely over the LAN connection. As long as it is grounded, even high voltages that nuke it wouldn't make it out on the LAN cable. Additionally, fuses wouldn't hurt. The Bussmann/Eaton HVR series might work, there is a 1/16A 5kV model available.QuoteI missed that detail--be aware that this is not acceptable. When I mentioned ground referenced, I was referring to the common/negative input being at ground potential not just the chassis of your stand. Your meter is not rated for 1kV common mode voltage, the negative terminals must not ever exceed 500V from ground. So if your setup doesn't have that terminal permanently grounded, you'll need to have protection from there to ground as well.

author: floobydust
author_rank: Super Contributor
timestamp: May 16, 2025, 08:06:56 pm
content: Why can't the test fixture have hardware interlocks that won't let it connect the DMM when the HV is on that bus?If the HV is on, then a permissive doesn't allow the DMM or other devices to get nailed. Instead of 100% control by the S/W.Hard-clamping that much energy is quite hard on the PSU and switching elements. I second just using an HV fuse and maybe a blast shield lol. Gas tubes are a bit slow and would basically be a use once and discard.There is the differential and common-mode voltage to think about, in that the DMM can become an unsafe device if either modes see HV beyond design.

author: jgvicke
author_rank: Newbie
timestamp: May 16, 2025, 09:34:29 pm
content: QuoteSo the supply itself can only output 1.2A max. The supply is supposed to handle hard shorts (for instance if a circuit fails) without damage. In the case of shorting the supply with a protection circuit, the delay to turn off is because of their internal control loop having enough time to see the current hit the limit level, then disengaging the input contactors on the transformer, and the circuit draining it's output capacitance. So there are a few cascading events that have to happen before there is no energy left to dissipate.QuoteWhile I would like to maintain full measurement capabilities of the DMM, I currently only have requirements for the resistances I mentioned above, and voltage up to around 100V. In other words if just putting a MOV in there that is somewhere between 100-1000V to be fully conducting, would that be fast enough to fully protect the DMM and withstand full current for less than 1 second and survive?The other input that needed protection (neg rail on the HV Volt meter rated for 600V from chassis) only ever needs to be connected to gnd, so it could have a MOV that turns on at very low voltage, even just single digit volts (but I would probably put a little more headroom than that) would be sufficient.QuoteI do need to connect the DMM in voltage mode to the gnd bus and a test point on the sense resistors while the supplies are on.I could put a relay energized by the interlock line on the supplies so if the interlock is enabled the DMM couldn't connect to the current switch matrix at all, add a couple secondary relays that allow the DMM to connect to just the sense resistor nodes. Those resistors are sized such that even if the max voltage and current of each supply is dumped through it, they can handle the power and the voltage drop would be limited to less than 100V.The only scenario that comes to mind that may not be protected from is this: Say a UUT was running normally, majority of the voltage drop is across the DMM with only a few volts on the sense resistor. Something happens in the UUT and shorts the supply. The supply will see a surge in current up to the max the supply can provide. At that current the voltage drop across the resistor will only be <100V, but I don't know what would happen to the node voltage as the UUT is shorting and that huge potential across the UUT collapses to 0V. Anyone know what the behavior in this type of event would be?Since the DMM will never have any large current draw, what about a MOV rated for the 100-1000V conducting range, then put a low current fast blow fuse between the supply and MOV. That way as soon as the MOV starts conducting the fuse will blow. Would that be fast enough to protect the DMM? I am fine having replaceable fuses in the mix.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2025, 10:00:33 pm
content: A combination of MOV and some kind of fuse is sensible. However fuses that can withstand 5 kV are large and expensive. The large size may need extra care with leakage. They also still need some time to respond.Depending on the output capacitance of the supply the MOV may not be enough to clamp the voltage enough. One may want something like 2 stages.For the resistance measurements it makes a difference in which range. Low resistors would need 4 wire ohms and thus more protection channels. The sense input may well have less protecion. High resistors like > 100 K may be effected by the leakage at MOVs / TVS, but could use 2 wires. One may still not want much resistance in series (e.g. from a fusible resistor chain).

author: bdunham7
author_rank: Super Contributor
timestamp: May 16, 2025, 10:14:31 pm
content: Quote from: jgvicke on May 16, 2025, 09:34:29 pmJust because the supply trips off if you exceed 1.2A that doesn't mean it won't momentarily supply more current in the short time before it does so. If it has output capacitors of any size, that peak current is probably much, much higher than 1.2A. It's just like the 15A breaker feeding a wall socket--if you short that you get a LOT more than 15A for a bit. I wouldn't be surprised if your peak fault currents were hundreds of amps. At 5kV you'd need a lot of resistance to cut that down and while your voltage readings would be fine with a 1k resistor in series (a .01% error for a 10M input meter) your resistance readings won't work. So you'll need something to absorb a lot of energy.Yes, an appropriate MOV plus fuse should be fast enough. The HVR fuses might be expensive but since this is intended only as a last stand when things that shouldn't go wrong do go wrong that's OK. I'd still consider isolating the DMM entirely and only accessing it remotely. Here's a possible MOV that would work both for common-to-ground and positive input-to-common, so you'd only need two assuming you only use the two input jacks so the current and 4W sense jacks are unused--and hopefully plugged. The only issue might be the fairly large 18nF capacitance, but if you aren't measuring AC that's likely OK. Note that unlike some manufacturers, this one doesn't provide typical leakage current graphs so you'd have to think about that.https://www.mouser.com/ProductDetail/YAGEO/181KD53?qs=PzGy0jfpSMt712afZuW8bA%3D%3D

author: Zero999
author_rank: Super Contributor
timestamp: May 17, 2025, 03:50:51 pm
content: Quote from: jgvicke on May 16, 2025, 06:08:27 pmHow much does the meter cost?Again, what about the safety of the user?Stick the meter in a perspex box, with an interlock on the lid. That will prevent the user if the meter goes pop. Meters are cheap, compared to the consequences of someone getting killed.

author: jgvicke
author_rank: Newbie
timestamp: May 17, 2025, 04:32:56 pm
content: QuoteSeveral people have mentioned something like this and it is a good safety layer in case the software interlocks somehow failed. Likely what I will do is mount this meter rear facing inside the rack. The rack door will be interlocked, so that adds a secondary large metal enclosure that is also ground bonded.On a fully automated test stand, the only reason to have any of the instruments exposed is for integration and debug, and for that type of activity it would be fine for the power supplies to be off and the door open.As mentioned in a previous post, this particular meter normally runs around $4K which is chump change in the long run for a large organization. It would be nice to prevent damage to it, but damage to people is absolutely worse!Thanks,John Vickers

author: johansen
author_rank: Super Contributor
timestamp: May 17, 2025, 05:41:40 pm
content: Quote from: jgvicke on May 16, 2025, 06:08:27 pmthink you should look into those thyristor surge components i linked to, when they turn on, they collapse to just a few volts, which is why they are used in series with an MOV.for the inline resistance to protect the 5KV power supply, you could use a 500 ohm resistor in series with the protection circuit, that will limit the current to 10 amps and the MOV's just have to be sized to handle the total joules of energy stored in the 5kv supply.500 ohms will not affect the voltage measurements enough to matter. however, it will get in the way of your precision resistance checks of the high voltage shunts. i don't see a way around that, but i also don't understand why you need to measure those shunts more than once.

author: Zero999
author_rank: Super Contributor
timestamp: May 17, 2025, 06:42:08 pm
content: Quote from: jgvicke on May 17, 2025, 04:32:56 pmWhy not just use a cheaper meter? Do you really need 7.5 digits?

author: floobydust
author_rank: Super Contributor
timestamp: May 17, 2025, 06:50:37 pm
content: Using MOV's, I think they won't give what you want - they have the problem of leaking a fair bit of current when below their hard-clamping voltage. Their clamping ratio is quite mushy, log10of current.So a part that hard clamps a couple amps at 1,100V will leak roughly around 1mA at 800V which can get in the way of readings.The 34470A manual mentions accuracy degradation due to internal heating of components during HV measurements.Maybe use something cheaper/disposable instead for the high risk tests. Or I would consider adding a HV differential probe, or at least 10:1 resistive divider that is comfy taking a few kV all day.Do you have coverage for the sense-resistor failing? I find they get beat up in a test fixture, overloaded due to product faults etc. and the shunt needs to be seriously over-rated.It would be bad if it went open, although it would just arc and burn up. You can diode-clamp the sense-resistor so it could never go with full HV across it.Test fixtures similar to this I have built using a light tower, a few mushroom emergency stop pushbuttons on the machine, yellow lanes markings on the floor or light curtain or a fence to keep people from strolling by or bumping into the operator, as well as PPE specs.Also no verbal instructions about operating this, it's all documented on paper. Never overestimate (IQ) how stupid people can be.

author: Echo88
author_rank: Frequent Contributor
timestamp: May 17, 2025, 11:51:26 pm
content: Maybe the HP 3458A Vin protection is applicable here.Theres no GDT with exact discharge voltage spec though and overload calculation based on your HV source is needed.Working principle of the +-Vin overvoltage protection:-GDT maintains low leakage during non-fault conditions-GDT and capacitor together work to strike the GDT and start passing any transient to the varistor-Varistor does its stuff and clamps the bottom of the GDT to some voltage,that is slightly higher than the nominal working voltage of the circuit so that,once the transient passes, working DC bias on the circuit can't hold the GDT in spark (or glow) discharge.-meanwhile the capacitor gets charged to the clamping voltage.Once the overvoltage transient has gone:-Varistor turns off, impedance to earth now too high to maintain GDT discharge-Resistor bleeds charge off the capacitor once the GDT has stopped being a low impedance pathNow reset, ready for next time.https://xdevs.com/doc/HP_Agilent_Keysight/3458A/doc/3458A%20CLIP.pdflast page

author: coppercone2
author_rank: Super Contributor
timestamp: May 18, 2025, 06:56:58 am
content: don't you just need another switch that can't go to the meterTo me this is like asking how do I do this with one mux, the answer is you need another one? That is not connected to the HV node at all so no matter what state it jams or malfunctions into its not gonna connect those two things?IeMux 1 : dangerous stuffMux 2 : non dangerous stuffI don't see a block diagram and the description is very confusing. Why is something setup so it can connect 5000V where it should not be? To me its like saying nuclear bombs and 2,4,5 trioxin zombie gas will be shipped by UPS instead of requiring their own special delivery and logistics service service. And I think thats what legal would sayor perhaps something with combining the bicycle lane and the autobahn

author: floobydust
author_rank: Super Contributor
timestamp: May 18, 2025, 09:14:26 pm
content: The fault time is too long at 100msec for common protection components to handle that kind of energy. It takes you to the MV league which is very expensive and most are for motor protection, the huge cartridge fuses.Low cost 5kV fuses are 6.5x40mm (called 6x40) used in microwave ovens. They are not sand-filled but use a long spring to increase the trip gap. Rated 400mA at the lowest. They are cheap consumer components.Selittelbut I see they did not buy any time. They are slow over 100msec typ.Next is the MOV problem, a hard-clamp at 1,000V will be leaking a lot close to that at say 800V even a mA or more.There are large block MOV's that could take the fault energy.

author: johansen
author_rank: Super Contributor
timestamp: May 19, 2025, 12:43:18 am
content: Quote from: floobydust on May 18, 2025, 09:14:26 pmand you somehow know how much energy is stored in the power supply before it trips off due to over current?what model number is op's power supply?

author: floobydust
author_rank: Super Contributor
timestamp: May 19, 2025, 02:05:14 am
content: Quote from: johansen on May 19, 2025, 12:43:18 amYeah there's so many companies making those. Took the lid off and wrote down some capacitor valuesDo you really think it works that way?Magna-Power Electronics TS Seriesrise/fall time is 100msec. OP mentioned a contactor, mains might get yanked instead.

author: coppercone2
author_rank: Super Contributor
timestamp: May 19, 2025, 05:38:34 am
content: this gave me an idea. Let the meter blow up so they see that this is a bad ideaalso post pictures I am curious what will happen to the enclosure. The keysight has like copper block pipes for the inputs.

author: floobydust
author_rank: Super Contributor
timestamp: May 19, 2025, 06:49:24 pm
content: The DMM will just carbon track and burn up the main PCB. There's a few 344xx that got nailed and killed that way, it does happen. Lots of smoke, then a funeral.Could hit up Keysight and get pallet quantity pricing /s.My new product idea would be a precision bench DMM that can work in a tough environment, not the pristine pansy workbench but with high power gear. We see it with power analyzers, anything in 3-phase, they are toughened. Daniel B. should get on that

author: johansen
author_rank: Super Contributor
timestamp: May 19, 2025, 11:05:10 pm
content: Quote from: floobydust on May 19, 2025, 02:05:14 amthis forum would be better with less trolling.read the datasheetMaximum Slew RateModels with High Slew RateOption (+HS)4 ms, output voltage change from 0 to 63%8 ms, output current change from 0 to 63%If you put a 500 ohm resistor on the output in series with a mov of reasonable rating, the voltage will collapse far faster than 4 milliseconds (as a 1 amp rated power supply able to slew the output in 4 milliseconds... probably doesn't have pass transistors and shunt transistors capable of 10 times more than the rated output).004*5000*8amps is divide by 2 is 80 joules.well within the range of a single mov.

author: bdunham7
author_rank: Super Contributor
timestamp: May 20, 2025, 12:40:15 am
content: Quote from: johansen on May 19, 2025, 11:05:10 pmDid I miss something? I don't recall the OP stating the make/model of the HV PSUs at all let alone any high-slew options. What he did say is that it will trip off due to overcurrent within 100ms. No mention of maximum fault current.

author: johansen
author_rank: Super Contributor
timestamp: May 20, 2025, 02:02:20 am
content: Quote from: bdunham7 on May 20, 2025, 12:40:15 amI dont either. which is why i asked fluuby dust what the model number was.OP does state its a 5kv 1 amp or 4kv 1.2 amp.I responded to flubbydust's claims of megawatt rated protect is nonsense for the power supplies he listed.Secondly what i did not point out, is an appropriately rated series resistor, is going to burn up 4/5ths of the energy in a hypothetical situation where a 1.1KV mov is used to measure up to 1kv, and gets hit with 5kvdc.The resistor need to be low enough to trip the overcurrent set point which will instantly (nearly instantly) turn off the power supply, and the only energy is whats in the output capacitor is what has to be absorbed.let me provide you an easily remembered axiom:The slew rate of an opamp/powersupply/etc. IS NOT the rate at which it can be slewed. Would slung be a better term?

author: voltsandjolts
author_rank: Unknown
timestamp: May 20, 2025, 03:17:41 am
content: The OP is likely aware but just to point out the obvious; the DMM in-guard/out-guard isolation could not be relied upon for such high input voltages. I'm guessing this ATE system uses DMM remote connection for readout.IMHO the safestprovablesolution would be a 5kV to (say) 1kV voltage divider, carefully designed (with dV/dT awareness), and built in a rack mount enclosure. Thenallsystem voltage measurements are made through this divider. Whether this is workable depends on the required accuracy of voltage measurements.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 20, 2025, 03:25:46 am
content: The stated problem is future unwise modification. Take steps to prevent that. Welded steel enclosure. No access to programming capability. Then let the safety department and those who want a usable test station battle it out.

author: EEVblog
author_rank: Unknown
timestamp: May 20, 2025, 04:09:18 am
content: Quote from: jgvicke on May 16, 2025, 11:46:22 amI doubt there is an existing plugin solution, so I think you'll have the roll your own.What that entails depends upon the possible exposure time to the HV before it disconnnects, that dictates the surge energy and fusing requirements.

author: EEVblog
author_rank: Unknown
timestamp: May 20, 2025, 04:11:57 am
content: Quote from: voltsandjolts on May 20, 2025, 03:17:41 amYes, I'd recommend that as a minimum. I don't understand the need for a 7.5 digit meter, why is that resolution needed?

author: johansen
author_rank: Super Contributor
timestamp: May 20, 2025, 05:03:24 am
content: Quote from: EEVblog on May 20, 2025, 04:11:57 ammy guess is OP doesn't know either.practically speaking its not possible given the original requirements of"Clamp at <1 kV but allow accurate measurements up to 1 kV."

author: dietert1
author_rank: Super Contributor
timestamp: May 20, 2025, 05:39:26 am
content: Safe buildings often have two entry doors one after the other, with a camera to look at the space in between and some method to interlock the two doors so only one of them can be open at the same time.If that MUX is one of them (A), one could use a second switch (B) in front of sensitive equipment, probably a relay with high voltage specs. Then use some hardware sequencing to keep B disconnected while operating the MUX. In case of malfunction, it can be detected before connecting B.Regards, Dieter

author: jgvicke
author_rank: Newbie
timestamp: May 21, 2025, 07:58:29 pm
content: Sorry for not replying sooner (for some reason I stopped getting emails about new posts). Lack of model numbers is from giving as little technical details as possible from a business perspective, but there really is no reason to shield the supply and DMM part numbers.QuoteSomeone knows their power supplies or is really good at google fu! We are using the Magna TS 5000 (1A) and 4000 (1.2A) models, and we are using the HS option. Somewhere in the manual there was a note that OCP takes about 60ms to engage, but that was unclear if that meant 60ms at full power before it starts regulating (and it doesn't say how long that will take), or is 60ms how long it takes to get to equilibrium after OCP kicks in. The DMM in question is the Keysight 34470A.Also, how do I do a quote that pulls the name of the poster?The power supplies don't have great accuracy on their current readouts (+/-7.5mA on one of the lower voltage supplies we are using), and the design engineers of the UUT want us to verify current draws at ATP preferably to sub mA resolution.Taking this discussion down another path for a minute. We were going the DMM and sense resistor route as that is a common solution that is easy to implement but does expose this safety concern. I should probably ask if anyone knows of any isolated current sense probes we could use that are rated to 6KV from chassis (to give a little head room) and can measure current to 1mA or better resolution. We could potentially use a clamp on current probe, but I have had very bad luck with the specs of those in the past and found them very unreliable (even name brand ones like Tektronix).As to the resolution, we don't NEED 7.5 digits, but that is what was readily available in a half rack mount for not much more cost than a lower resolution version. So to make current computations from resistance and voltage, I wanted as much resolution as was reasonable. So another possible path would be a DMM rated for 5KV that is lower resolution, but I didn't find any like that. If you know of one let me know! (the half rack size is no longer a requirement BTW).I do still have some straight resistance measurements I need to make on the UUT power inputs as part of a safe to turn on test, but I do have a way I can isolate the DMM from the switch matrix before the supplies are able to be turned on, which would be a hardware protection from the 5KV. That solution gives the DMM both a hardware and software isolation when the supplies are on, it just prevents me from making the high precision voltage measurement to get accurate current when the supply is on.Doing a 5:1 voltage divider may prove an interesting alternative. I should be able to set up the switch matrix to do a high precision measurement of the resistors that make up the divider at the start of the test while the supplies are off, then use that to back compute the real voltages through the switch matrix.Any other ideas on how I could make precise current measurements with >5KV isolation that would isolate the DMM or remove the need for the DMM altogether?Thanks,John Vickers

author: Someone
author_rank: Super Contributor
timestamp: May 22, 2025, 12:15:19 am
content: Quote from: jgvicke on May 21, 2025, 07:58:29 pmYou'll probably need to share much more detail of the system and include some schematics to get further help. A big part of HV instrumentation is moving measurements to ground referenced wherever possible (measure current only when its near ground and easier to protect), or floating with optical coupling if it has to be differential (rather than trying to do everything with a single DMM).Quote from: jgvicke on May 21, 2025, 07:58:29 pmVarious solutions exist but I'm not playing the game of blind guessing of your constraints.

author: EEVblog
author_rank: Unknown
timestamp: May 22, 2025, 12:22:31 am
content: Quote from: Kleinstein on May 16, 2025, 10:00:33 pmYes, you are into specialised territory at 5kV fusing.

author: dietert1
author_rank: Super Contributor
timestamp: May 22, 2025, 06:24:35 am
content: No, the 5:1 divider should be separate from the MUX and look at the line from MUX output to DVM safety relay, like the camera watching the space between two entrance doors. The voltage divider should not be connected to the DVM input but a hardware interlock. I mean some software independent detector/comparator that inhibits the safety relay in case of overvoltage. Another hardware interlock inhibits operation of the MUX while the DVM safety relay is closed. Checking the interlocks requires some care, yet it can be automated, too.Regards, Dieter

author: bdunham7
author_rank: Super Contributor
timestamp: May 25, 2025, 06:06:23 am
content: Quote from: jgvicke on May 21, 2025, 07:58:29 pmIDK why a testing stand would be a trade secret but not explaining precisely what it is that you need to measure makes it difficult to give precise answers to what very likely is actually an extremely simple question. Thus we end up providing useless answers to this complicated X-Y question. Other than all the self-testing of the stand, etc, is the ultimate goal to simply measure the current draw of your DUT when supplied a specific voltage by your PSUs? And are you measuring just the current? I'm guessing at that because you while you've discussed a voltage divider you hadn't already specified the need for one and obviously your meter can't measure 5kV.If you can't tell us what the DUT is, can you say whether there is some reason that the current measurement cannot be done on the low side with a ground-referenced system? And what actual current levels do you expect and what would be the acceptable tolerances for your DUT?

author: jgvicke
author_rank: Newbie
timestamp: May 30, 2025, 01:01:32 pm
content: It has been a bit since I replied to this thread and wanted to give a final status. Lots of helpful input and ideas.I was able to go back to the drawing board and rework my switch matrix with hardware interlocks in such a way that I have the DMM connected only when the power supplies are not (so there is no way to get 5KV into the 1KV DMM directly). Then when the supplies are connected the DMM is only connected through a 10:1 voltage divider. In the DMM configuration I can measure both sides of the voltage divider to get very precise measurement of the ratio, then use that to accurately compute the voltage at the top of the divider when in power supply mode.All of this configuration is done through hardware interlocks so it is impossible for the configuration to be wrong from a software command.Through this redesign I was also able to add some additional layers of hardware interlock protection around some other portions of the circuit so overall a much more robust design.So, thanks again to all the people offering help and suggestions!John Vickers