Is this safe enough way to probe 240VAC mains voltage? - Page 1
Forum section: electronics / beginners

author: BlackICE
author_rank: Regular Contributor
timestamp: September 26, 2020, 09:58:43 am
content: I want to monitor at 240V US mains voltage during a HVAC compressor startup. I have a Siglent SDS1104X-E and the scope probes are rated for 300V at 1x and 600V at 10x. At full scale at 50V per division should give me a 400V range, enough for at nominal 240VAC*1.41 I plan to use an inductive current probe on one channel and use 2 other channels in subtract mode to monitor the AC mains voltage, leaving the grounds on the probes disconnected. The ground is connected via the power cord, not idea but good enough for my purposes. I will power off the equipment to attach the probes and then never touching the probes or probe wires while the power is connected. The 1x to 10x switch is pretty easy to move accidentally so best to set and never get near the switch.

author: Ian.M
author_rank: Super Contributor
timestamp: September 26, 2020, 10:27:39 am
content: The fact you are asking here indicates you aren't real happy with the idea.It depends on the definition of 'safe enough'. Assuming you don't have a lapse of concentration and handle the test leads, and that the compressor is in an item of plug-in equipment on an outlet not near the breaker panel and with adequate upstream circuit protection (i.e. a CAT II zone), the risk to you is pretty minimal. OTOH you'd be taking a very large gamble with your scope as it doesn't sound like your probes are rated for the transients one would expect on the mains supply. 1KV transients are pretty much routine on the supply in most developed countries and significantly higher transients aren't uncommon.OTOH if its a hard-wired appliance, making it a CAT III zone, you couldn't pay me enough to get me to connect unrated probes to its mains supply terminals.Are you sure you cant get the readings you need from the output of a low secondary voltage transformer?

author: BlackICE
author_rank: Regular Contributor
timestamp: September 26, 2020, 10:51:12 am
content: I think you would classify it as a cat 3. It's installed appliance with a nominally 5 horsepower motor that draws 18 amps running and over 130 amps at startup. It's on a 50 amp breaker which is essentially 0 protection from getting electrocuted. You'll fry or kill anything that gets shorted before that breaker blows. I don't own a transformer for 240 volt mains but I guess I can get one.

author: tautech
author_rank: Super Contributor
timestamp: September 26, 2020, 12:04:31 pm
content: A 100:1 probe is what you need here for the additional voltage rating they have for your and the scopes safety.All mains work is inherently risky due to its low impedance current source.Be careful out there.

author: magic
author_rank: Super Contributor
timestamp: September 26, 2020, 02:30:48 pm
content: Strictly speaking, what you need is a CAT III rated probe, of any division ratio that suits your needs. AFAIK such things exist.I sometimes abused a 100:1 2kV CAT I probe on household mains behind circuit breakers, but technically it's not guaranteed for such jobs.The problem with high power installations is not as much electrocution (less than an amp suffices for that anyway), but the fact that when something breaks down and shorts the supply, it doesn't go out with a smoke but with a bang. CAT III gear is required to have more robust insulation for that reason.

author: WattsThat
author_rank: Frequent Contributor
timestamp: September 26, 2020, 03:33:02 pm
content: QuoteAny monies spent on a transformer would be far better spent on a proper differential probe.I don’t know if Dave has restocked his store but the EEVBlog probe is/was a great buy. Other similar probes are easy to find on Amazon. While I don’t use mine very often, I’m very happy to have it when I’m working with mains. As measurement safety goes, it was the best investment I’ve ever made.

author: David Hess
author_rank: Super Contributor
timestamp: September 26, 2020, 04:50:34 pm
content: Quote from: WattsThat on September 26, 2020, 03:33:02 pmI agree in general but for an application like this, I would prefer a transformer with 240 volt AC primary and low voltage secondary, or an oscilloscope with isolated inputs. High voltage differential probes are *not* galvanically isolated.The transformer can be wired directly into the circuit and access with probes only done on the safer low voltage side.

author: Ian.M
author_rank: Super Contributor
timestamp: September 26, 2020, 05:06:53 pm
content: DoD surplus 480V 4:1 ratio instrument (potential) transformers can be found reasonably affordably on EBAY, On 240V that will give you 60V out.

author: bdunham7
author_rank: Super Contributor
timestamp: September 26, 2020, 05:36:58 pm
content: If I didn't have other equipment to do this--and I'm glad I do--I would set it up and do it the way you plan to. I'd certainly be nervous about it, but a bit of fear will keep you thinking straight. Presumably you'll set this up to trigger and record as needed. I certainly wouldn't want you to touch anything while the breaker was on and I'd prefer 100X probes, but I doubt anything bad will happen. OTOH, it does seem well outside of the intended safe use of the instrument so there's always a possibility of some sparkles and a melted scope or probes, especially if you make any mistake. You might get a pretty good spike if the breaker is shut off or trips with the motor running, so try to avoid that and make sure any shutoff occurs via the contactor in the condenser assembly.One question though--why? What is going on that you can't just use a proper multimeter (or two) and a current clamp? Is this a hard-start situation?I don't know if you really need 'subtract', you can just record each leg and see what is happening.

author: james_s
author_rank: Super Contributor
timestamp: September 26, 2020, 05:50:31 pm
content: I don't see any real clear answers here so I'll just say this. NO! This is absolutely not a safe thing to do, not for you nor your equipment. If you want to probe anything with the 240V mains a HV differential probe is a must.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 26, 2020, 06:33:14 pm
content: As for me, I never get out of bed.It's not safe.A long time ago however, I demolished a small mains timer for the box, and put int some shunt resistors, a double throw switch and a 1:10 voltage divider from a bunch of THT resistors in series for mains measurements.Then as a GND connection, I use the GND lead of the scope, which is in the same extension socket as this switch box, and don't use the GND connection of the scope probe.However I also add in an isolation transformer in the mix "just to be sure".An old trick for such a measurement box is to add a green and a red neon light.Green goes between Earth and phase, Red goes between Zero and Earth. You can also buy mains socket test plugs with such circuitry built in.If you want to go the transformer route:Put the primary windings of 2 (small) transformers in series for the mains voltage. This way each transformer only sees half the mains voltage, which makes them more linear. Normally mains transformers are operated near saturation, and they loose linearity.

author: BlackICE
author_rank: Regular Contributor
timestamp: September 26, 2020, 10:53:11 pm
content: Quote from: bdunham7 on September 26, 2020, 05:36:58 pmWhy is because of curiosity, using a multimeter the sample rate is too slow to catch voltage drops. Using subtract gives me one trace I want vs viewing 2. Unless something is wrong both should be mirror images and likely measuring only one side relative to ground would give the same information.I want to monitor current and voltage over time during compressor startup to see if my generator can safety startup my HVAC. I did a previous test without instrumentation that was costly. The current was excessive causing a voltage drop, opening the contactor, dropping the power, the voltage increased closing the contactor, over and over again many times a second! That fried the contactor. After the contactor was replaced the compressor soon failed from the startup abuse.I have a soft start I want to test to see the changes in current and voltage during startup. If the numbers improve enough, I will test with my generator.Since the ground ref of the scope is about the same as the neutral of the 240VAC the scope probes will see, not counting spikes, about a +- 120VAC at the inputs. Which is within the probe's 600v rating at 10x and even the 300v rating at 1x. The input to the scope I believe is 50v so at 10x should be able to measure +- 500v at the very limit. Almost a 3x "safety" margin. A some have pointing out, spikes over the line voltage should be expected, especially running a high current inductive motor and other unknown devices connected to the line as well.I'm not too worried about getting killed unless I make a huge mistake and break my rule of not touching any of the wiring while the power is connected. However as many have pointing out Murphy's law can take out the probes or scope.Using a 100x probe would give a much better safety margin from spikes to the scope's front end, but if the resistors inside the probe failed closed it would fry the scope. Would a cheap diff probe be much better? If the diff probes failed would the damage be more likely to be contained to diff probe box? None of these solutions are CATIII rated and they all have a galvanic connection to the scope's electronics.My choice are:1. Very carefully use my current equipment, small chance of failure2. Buy two 100x probes rated for a higher voltagehttps://www.amazon.com/dp/B083LM4P9Tsmaller chance of failure3. Buyhttps://www.amazon.com/Micsig-DP10013-Differential-Attenuation-Tektronix/dp/B074K4XPW3,even smaller chance of failure.4. Don't do it! No chance of failure, but no measurements either

author: bdunham7
author_rank: Super Contributor
timestamp: September 26, 2020, 11:14:46 pm
content: I think that it's very unlikely any decent 100X probes (your link doesn't work) or the MicSig DP will break down. As for the 1x/10x, still unlikely but I'm not as sure. In any case, as you mentioned, the scope can take 400V-pk, or at least that's what is printed on it.However, you didn't answer as to why you are doing this or what you are looking for. If you just need to know what the voltage and current are doing, the instrument transformer(s) suggested by Ian.M above would be a safe bet. If you do this more than once, making a test box using a potential and a current-to-voltage transformer (or a current transformer with a load ballast) would make sense. If you are looking at high-frequency noise or some imbalance on the legs, they may not work for you.https://www.ebay.com/itm/INSTRUMENT-TRANSFORMERS-POTENTIAL-TRANSFORMER-460-480-PRI-480V-RATIO-4-1/362905931350?epid=2075227737&hash=item547ee10e56:g:T0kAAOSwrzBeO3yshttps://www.ebay.com/itm/Instrument-Transformer-Inc-ACV-Current-to-Voltage-Transformer-50-Amp/124271302781?hash=item1cef25987d:g:C9wAAOSw3dVfGj5Y

author: BlackICE
author_rank: Regular Contributor
timestamp: September 26, 2020, 11:29:43 pm
content: Thanks for the links to the transformers. I couldn't find any at a reasonable cost, that's why I left them out as options. I edited my previous post to answer your questions and fixed the link. The scope taking 400V PTP is assuming the two channels aren't coupled and that the 10x probe doesn't failed or get switched by accident to 1x. I have hit the switches by mistake many times. Very easy to do. If I used them, I would tape them in the 10x position.Now I'm down to 2 options.1. 1:4 transformer, lower cost safe.2. Diff probes, excuse to buy more tools that a may find useful in the future.

author: bdunham7
author_rank: Super Contributor
timestamp: September 26, 2020, 11:49:33 pm
content: Quote from: BlackICE on September 26, 2020, 11:29:43 pmNot that I'm recommending it, but channel coupling or probe failure wouldn't affect the scope's 400Vpk rating--and 400Vpk means a peak voltage 400 volts over ground, not 400Vp-p or peak-to-peak. Not that you should ever hook it up that way, but if you did it shouldn't instantly blow up.

author: BlackICE
author_rank: Regular Contributor
timestamp: September 27, 2020, 12:09:41 am
content: I found another option.https://siglentna.com/product/isfe-isolated-front-end/But I think diff probes would be a better choice for a little more money.DescriptionProvides two channels of optically isolated inputs for use with oscillscopes and other applications requiring an isolated ground.Input Voltage Range: -600 Vpk – +600 VpkBandwidth: <= 1 MHzAttenuation Range: 200:1Output Voltage Range: -3 Vpk – +3 VpkInput Power: 5 V +/- 5% (USB Power). <200 maFloat voltage between Channel and ground: 1000 VrmsFloat voltage between each channel: 2000 VrmsInput Resistance: 9 MOhm

author: tautech
author_rank: Super Contributor
timestamp: September 27, 2020, 12:45:49 am
content: They are a quite valid option and you can fit 2 of them to the 4ch SDS1104X-E to then have 4 isolated channels.While they have limited BW and are a little noisy ISFE do work OK.That you can buy 2 for the price of a single channel differential probe is very attractive IMO.Threads with additional info on them:https://www.eevblog.com/forum/testgear/siglent-isfe-scope-front-end-isolator-any-opinions/https://www.eevblog.com/forum/testgear/siglent-sds1204x-e-released-for-domestic-markets-in-china/msg2444670/#msg2444670https://www.eevblog.com/forum/testgear/siglent-sds1204x-e-released-for-domestic-markets-in-china/msg2556423/#msg2556423

author: ArthurDent
author_rank: Super Contributor
timestamp: September 27, 2020, 02:28:17 am
content: I've mentioned this before but I dismantled a piece of high voltage equipment that had several isolation amplifiers that were rated 2500 volt r.m.s. on a circuit board. They were powered by a single 15VDC supply and although limited to about 200Khz bandwidth work great for measuring mains voltage and frequency. I tested one with an A.C. supply I have set to 10.000VAC and with the 10:1 divider on the input the output read pretty darn close. I just have to build this into an appropriate enclosure with a selectable input divider and I'll be all set to measure mains voltage with an oscilloscope safely.This particular isolation amp cost a lot and isn't readily available but a search might turn up some other isolation amp that might be cheaper and do the job.

author: Ian.M
author_rank: Super Contributor
timestamp: September 27, 2020, 02:46:17 am
content: If you are using regular probes and are unhappy with the possibility of transients exceeding your scope's max input voltage rating, but it is 200V peak or higher, fit a NE2 neon bulb in a BNC connector and use it with a BNC T to protect your scope input by clamping excess voltage (the NE-2 breakover voltage is in the 100V region). You *MUST* use an attenuating probe in front of it - either a fixed x10 probe or a fixed x100 probe as the NE2 cant handle a lot of current for very long so a x1 probe on 120V line to ground would blow the s--t out of it. Make sure the probe you choose has enough compensation range to handle the extra capacitance of the BNC T and NE-2 bulb (which you can measure, assembled, on a good capmeter).The remaining problem becomes how to probe the circuit without the hazards of live working on a CAT-III installation. That's where you need to get smart and bring out the points you need to probe for voltage through 100mA fast acting ceramic fuses, inline with each wire except Ground, with as high a voltage rating and interrupt current rating as you can afford (e.g.https://mou.sr/2S3p0rG) in good quality screw-together inline fuseholders, close to the points you need to test. Use appropriately rated multi-core double insulated flex long enough to get you well away from the open terminal box, and securely cable tie it to the equipment chassis. At the end you want to probe, screw down a terminal block on a decent sized (so it stays put) wooden board, and make small bare wire loops on one side of the terminal block to clip the probes to. Secure the incoming cable to the board with cord grips, P clips or similar. Fix a perspex cover sheet over the terminal block supported by a cross-wise batten either end of it extending far enough to protect against accidental contact, but still allowing enough access to clip the probes on. Adding Docterandus's suggested neon indicators from each Line to Neutral, and Neutral to Ground would be worth doing.*FINALLY* lock out the breaker or isolator and hard-wire the D.U.T connections, don't clip them on!CAUTION: Due to the fuses, no voltage (or indicator lights) at the test board does *NOT* indicate the D.U.T is safely de-energized.I still wouldn't want to move the scope probes with the power on, but the risks of doing so on an appropriately fused and shrouded test circuit are far far lower than on bare CAT-III terminals.

author: bdunham7
author_rank: Super Contributor
timestamp: September 27, 2020, 03:23:03 am
content: Quote from: BlackICE on September 27, 2020, 12:09:41 amIf I didn't already have 3 other better options, I would consider the Siglent ISFE with two completely isolated channels for basic power measurements. I don't know the detailed specs, but don't let the 1MHz bandwidth put you off. The thing about high voltage and bandwidth is you generally can't have both anyway. You may see a spec for "Volt-Hz", which is another way of saying dV/dT. Or, you may see a frequency derating curve that shows the maximum voltage at a given frequency. At 1 MHz, that voltage is typically low enough that you don't need isolation for safety reasons any more.Disturbingly, I don't see a derating curve or spec for either the Siglent ISFE or the MicSig DP10013 models. Perhaps I missed it.

author: BlackICE
author_rank: Regular Contributor
timestamp: September 27, 2020, 03:54:18 am
content: I like Ian.M suggestion. It got me thinking, if I'm going to construct a board to connect the scope probe to the DUT with fast acting fuses (excellent idea for piece of mind and safety) why not add some MOVs to clamp possible transients? Then I can safely use my current probes. I won't connect any of the probes ground wires up, then I can't make the mistake of connecting them to a hot lead. The scope and I are protected by the fuses and MOVs.Per NEC the compressor has a service disconnect less than 2 feet away. Always within sight while testing.Cheapest solution yet, with safety

author: AnasMalas
author_rank: Regular Contributor
timestamp: September 27, 2020, 11:13:23 am
content: Hello.I would like to preface this by saying that it is anabsolutely horrible ideato even tell you I did this. I am a beginner, and all the answers above are guaranteed to be much better (Ian.M is suggesting a way that maybe wouldn't cost a lot, well, unless shipping costs a fortune for you like it does for me). and here comes the "but" you've been waiting for:But... Well I too have an 1104X-E, and I had a problem with my monitor blacking out whenever my AC compressor started or stopped, and one day it drove me up a wall and so I did the stupid thing and decided to probe our 230V mains to see what is going on... I took a USB cable and connected that to the monitor internal USB hub, put that on channel 1 AC coupling, and let the ripple going away (monitor backlight going off) be the trigger condition. I disconnected my scope from mains, and added a non grounded 2 prong adapter. I got a C14 socket and a C13 cable, removed the ground lead from probes 3 and 4, and connected them to live and neutral with the probes going to either way (left and right). I set the probes at 10x, and taped the switch, and then covered the whole thing with a sheet of paper. I then plugged the scope in and powered it, set Math to display a waveform 3-4 and set all the needed settings, set up probe 1 and the trigger condition, stepped away and plugged in the C13 cable.I was able to probe mains with no apparent issue, of course that's just like saying driving without a seatbelt on a highway is ok because I did it and im still here (I never did, its just an example), and my scope could be ever so slightly damaged now for all I know... But at least there wasn't an instant bang, so there is that, I guess?Oh, and about my monitor problem, yeah no clue, scanned the entire captured waveform before the monitor went black (after I disconnected the mains of course), and there wasn't an obvious dip or anything, argh! Here is a goofing off photo I took before I dismantled everything and admit defeat

author: Ian.M
author_rank: Super Contributor
timestamp: September 27, 2020, 12:01:22 pm
content: Yes, as AnasMalas said, it is a horrible idea.Quote from: AnasMalas on September 27, 2020, 11:13:23 am*NEVER* float your scopeCompetent engineers who thought they knew what they were doing, and who had taken what they thought to be reasonable precautions against contacting a live part have been killed doing exactly that. e.g.https://groups.io/g/TekScopes/message/10754

author: tooki
author_rank: Super Contributor
timestamp: September 27, 2020, 12:38:24 pm
content: A scope isn’t really the right tool for this. There exist power analyzers/monitors for a reason.If you just need to measure the value of the dips, use a multimeter with a good peak capture mode.If you really wanna look at the waveform, invest in (or rent/borrow) a specialized isolated scope like the Fluke ScopeMeter series.

author: AnasMalas
author_rank: Regular Contributor
timestamp: September 27, 2020, 01:14:14 pm
content: Quote from: Ian.M on September 27, 2020, 12:01:22 pmI knew the risks, and I knew that I should never float my scope....Except that the scope is floating whether I like it or not. By doing this, I just isolated the ground from my power stip and nothing else. Houses here dont have a ground. We thought ours had ground in the kitchen only, turns out the ground wire is just connected to neutral at one of the plugs, and not behind the circuit breakers. We do have codes, but no inspections, and so wiring tends to be a complete disaster. Against all advice, my father (I am a college student, this is our summer house where I live alone for 9 months of the year) wont do anything about it, and I was 6 when we this house was built.If you advise me to wire the ground to neutral locally at this socket only, sure, ill do that, but something tells me this is worse than just leaving the scope floating, as it has been for a year now

author: BravoV
author_rank: Super Contributor
timestamp: September 27, 2020, 01:23:39 pm
content: One mistake .. is enough to destroy the scope .. or your limb ...... or your life.Question is, how often you made mistakes on probing say at low voltage ?

author: AnasMalas
author_rank: Regular Contributor
timestamp: September 27, 2020, 01:40:18 pm
content: Quote from: BravoV on September 27, 2020, 01:23:39 pmI know, it is a risk you shouldnt take, I did it once, and wont do it again. Ill be honest and say that I did want to do this for the longest time, which played into it.Ive made one major mistake probing low voltage with my DMM, I was trying to troubleshoot something for someone, went for a toilet break, and when I returned they had were playing with the DMM... They placed the probe in the 10A socket, and I didnt check. But I wasnt trying to be careful, I was working "normally".Now Ian.M's Tek story was spooky honestly, I kinda wish we knew what happened exactly, but I only now saw that you shouldnt touch the controls, well, uh, why?If I had known that therubberbuttons and dials are even slightly risky I wouldn't have done this without connecting the scope to my PC wirelessly and leaving it alone

author: Ian.M
author_rank: Super Contributor
timestamp: September 27, 2020, 02:26:20 pm
content: Except in the rare case of scopes designed and intended for fully floating operation (e.g. Fluke ScopeMeters), your scope was designed under the assumption that no hazardous voltages would be present on its chassis (or front panel PCB ground plane). You don't know how good the insulation is between the scope chassis or ground plane and your finger at the supposedly insulated control you are touching. e.g. conductive dirt may have built up on the sides of a button from fingerprint oils and ambient dust. If your scope isn't properly grounded, all it takes is one bad mains spike at the wrong moment i.e. while you are pushing a button, and there's a significant risk of one dead engineer.There was absolutely no need to float your scope. If you were probing within its (and its probes) ratings, with the ground clips removed and the scope set to display the difference between two channels (with equal Y attenuator settings), your scope would have been safer properly grounded.The matter of not having a proper ground at your workbench is another issue:Quote from: AnasMalas on September 27, 2020, 01:14:14 pmPersonally if I was in your situation, I'd get a ground to my bench no matter what. As long as its a single occupancy dwelling, driving a ground rod, and running a 4mm2ground wire to your bench should be possible. ATT earthing systemis recognized as safe and is commonly used in rural areas in many countries. You can set up just the socket strips on your bench that way without having to change any wall outlet or breaker panel wiring. A RCD (GFCI) is *ESSENTIAL* especially in hot dry climates as you are likely to have difficulty getting a low enough ground resistance to reliably trip ordinary circuit breakers or fuses. *DO* *NOT* use water (or any other) pipes for the TT ground connection - if their grounding isn't as good as you thought it was, one unnoticed fault on your bench could make every water tap in the whole building lethal. All socket strips on your bench should have their grounds tied together, and you shouldn't be physically able to come into contact with any metal pipes or building metalwork while at your bench. If the floor is dirt or concrete, you'll also need a good quality rubber or plastic insulating mat for your entire work area. Don't get a black one as it may be somewhat conductive due to being carbon filled.If its actually a multi-occupancy building (e.g. apartments etc.) and/or you cant drive a ground rod and run a ground wire, ask about reasonably safe options that don't involve having to find an honest, competent electrician to come in to rewire it properly.

author: BlackICE
author_rank: Regular Contributor
timestamp: September 28, 2020, 12:49:28 am
content: Different countries have different standard of "safety." Japan for example has both 50 and 60Hz system depending on where in the country. I don't know about other areas, but around Tokyo they use 100VAC at 50Hz non-grounded, non-polarized outlets in homes. My understanding is that the power isn't grounded at the service entrance box at is left floating. Are plug in devices all class 0?Decades ago I remember my home having a plug in light fixture that gave a buzz if you ran you moist fingers over the metal shade. I've been to a friends home that was built in the 1950s when you touched the refrigerator's metal handle you also get a buzz. The refrig was really dangerous as you could touch it and a water facet at the same time! Both the devices had two prong non-polarized plugs connected in reverse or a reversed outlet wiring! If you ever looked at old homes with knob and tube wiring it you may be shocked to see some wires with crumbling cloth insulation. So you have bare copper running inside the walls. I will never buy a home with groundless knob and tube wiring unless I was prepared to rewire the whole home.I have had a few experiences of dead short across as 120VAC 20amp wall outlet, not fun. A large loud spark is emitted before the breaker trips. I wouldn't ever want to find out what a 240VAC with a 50amp breaker would do!https://en.wikipedia.org/wiki/Appliance_classes

author: nightfire
author_rank: Frequent Contributor
timestamp: October 10, 2020, 11:51:01 pm
content: At my last job we had a Fluke 435 available for such things. The current clamps would provide Volts back to the Scopemeter and they were fast enough to cope with inrush current from capacitors of server PSUs- I measured those to get some feeling how a 19" Rack of servers would behave after a complete blackout and coldstart, where you will have a severe capacitor inrush current after providing voltage again...Seriously, this sounds as a CAT III zone, and even some errors in wiring those leads can short something out. Here you should only use power clamps rated for that environment and a metering device that is capable of dealing with that kind of transients usually present in the supply infrastructure.As we were back then replacing a distribution cabinet for the HVAC systems in our main data room I asked for implementation of a 4mm panel to get some test points for that scopemeter, so that I could measure not only currents, but power whilst running those systems- the power leads from that cabinet have been bent at different distances so I could fit those current clamps without hassle...

author: BlackICE
author_rank: Regular Contributor
timestamp: October 12, 2020, 04:07:24 am
content: Thanks to all that contributed with warnings and suggestions. I made a small PCB with 50ma fuses and 240V MOVs from HV to GND to short out the transients. It worked well for less than 10 test cycles. Then I wasn't getting any voltage readings. I checked the fuses and all 3 were blown! If I didn't make that PCB maybe my scope's front end would have been fried! I had a curiosity about the voltage reading but it wasn't really needed. So I didn't want to buy a real diff probe.I knew that the transients spikes and voltage drops from the large motor starting and stopping would likely cause problems with the scope's operation. So I ran it off a battery powered inverter, grounding the scope to earth. However my inverter soon failed, so I tried using a wall outlet, no dice! I kept getting premature false triggers when only the small fan motor started. I had to grab a UPS from my PC to power the scope to get back to testing.Anyway I got some great information and think because of the improvements I can run my 5 ton AC compressor using my generator. Testing that Monday.Summary unmodified the startup lasted 0.22 seconds with a RMS current of 163A.With soft start the startup lasted 0.45 seconds with a RMS current of 44A.The 1st run using the soft start the startup lasted 0.2 seconds with a RMS current of 65A.After 5 learning cycles, the current draw was further reduced by extending the startup time and reducing the duty cycle of the power given to the motor.Net result a 73% reduction in startup current while extending the startup time by 100%.Without any modsWith soft startCurrent waveform using soft soft.

author: wizard69
author_rank: Super Contributor
timestamp: October 16, 2020, 05:26:09 am
content: Quote from: BlackICE on September 26, 2020, 09:58:43 amIf you are worried it probably isn't good enough. You didn't mention if this is single phase or three phase, and what exactly you are looking for. However the first thing that come to mind is do you really need to monitor the voltage with a scope? Generally a decent DVM or even a good old analog meter will be good enough to monitor line conditions at start up. Voltage generally isn't as interesting as current draw where a good current probe on a scope can be useful. If you think you are suffering from excessive voltage drop for an extended time, you should be able to see that on a meter.To put it another way the best way to avoid the hazards of a dangerous measurement is not to make the measurement in the first place. Instead consider if you really need to know the values in the first place. If you do need to know, consider equipment that makes getting the data safer. Being able to hook us a meter before start up is also a huge advantage as you don't need to touch anything to get your data.As others have indicated short circuit currents can be excessive before a breaker can activate. In a home it might be more limited but even 10,000 amps for a cycle or two can generate a huge bang. This also assumes a breaker will actually actuate fast enough or at all. While currents in industry can be significantly higher I've seen what can happen when motor controls can't handle the currents they need to interrupt - they literally explode. So safety should always be a concern when working with mains power.