EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: SND on May 03, 2019, 10:58:06 am
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I need a good safe way to check harmonics, THD and wave form on 120 and 240V 1ph 60hz circuit, with possibly up to 125A/1ph breaker on them. I expect the main harmonic to be about 5khz but there could be other stuff in there, higher or lower.(IGBT/VFD's) Most of this will be done at the panel, and input of a few units.
I may like to be able to check some 3phase 60hz wave form as well and what harmonics might be going on there but its less important.
I can probably stick to just doing each leg to ground on their own but if its possible to go leg to leg then great. I'm still reading/learning about what these oscilloscopes can/can't do, but I do well know the dangers of the electricity I'm dealing with
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Would the Siglent SDS1202X-E or SDS1104X-E be a good choice for this application? Are the probes that come with it ok for this? I see they're just 10x probes. Should they be upgraded to something else and if so, which?
I also read a bit into the Hantek DSO-8060 handheld unit, looks interesting but I'm not sure if that's a better or worse choice.
I want something reliable and accurate enough that will last many years as I will need to check the THD levels periodically.
Are there any other units and accessories perhaps better suited for this that you'd recommend?
thanks
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Any time you want to scope mains (especially across phases) you don't want to use a typical grounded scope.
I would recommend battery powered scopemeter certified for the voltages and type of circuits you will be connecting to.
Exactly which one to recommend depends on your budget and needs. Do you need more one channel? Do you need advanced math FFT etc. Ability to upload waveforms to PC for analysis?
Fluke has some very nice offerings including a specialized "motor driver analyzer" series with harmonic measurements etc. -- warning - they're not cheap!
Dave
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Look for a Powerscope, I have an 881 Mk2.
These are made in Australia and are specifically intended for AC mains power work, 400V input on all channels.
They are now old and quite rare, download a copy of the manual to see what it does.
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Another way is to use a 100:1 probe or High Voltage Differential Probe.All depends or how high a voltage you intend on scoping.Running your scope from an isolation transformer might be a good idea .Maybe someone could comment on that idea.
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Is this a utility-fed mains circuit? Or some type of generator/solar system? Where is a 5KHz 'harmonic' coming from?
You need specialized equipment to do this safely. My choice would be some flavor of Fluke Scopemeter, especially since they often come with excellent leads and probes. There's no call for poking around in utility panels with cheap (or even expensive) regular oscilloscopes and their related probes. There's just too many things that can go way wrong way too fast.
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Another way is to use a 100:1 probe or High Voltage Differential Probe.All depends or how high a voltage you intend on scoping.Running your scope from an isolation transformer might be a good idea .Maybe someone could comment on that idea.
All those ideas are dangerous in one way/situation or another.
For a longer discussion, see the many other threads on the topic.
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Another way is to use a 100:1 probe or High Voltage Differential Probe.All depends or how high a voltage you intend on scoping.Running your scope from an isolation transformer might be a good idea .Maybe someone could comment on that idea.
All those ideas are dangerous in one way/situation or another.
For a longer discussion, see the many other threads on the topic.
I fully agree.
The only correct way of measuring mains on a 'scope is using high voltage differential probes. Everything else is a recipe for killing yourself.
Don't even think about battery 'scopes or isolation transformers.
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I read up a bit more and watched a few more videos about what these can and can't do. 120vac is all I really got to worry about for voltage, just leg to ground and it is power from the pole.
I can do each leg one at a time and I got room in there to work anyhow, I'm pretty used to working with live AC 1 and 3ph.
I mostly need it to check a wide enough spectrum of harmonics on the power line which it seems the FFT function should do. Find the worst frequencies, and have it give me a THD %. Since most of these oscilloscopes like the SDS1104X have a FFT math function, can I assume they also have a button that adds it all to a THD %?
I did think that the ground could be an issue as I'm pretty sure there's some noise on it from the units I want to test, but that might be fine as it's really to establish a baseline anyway.
I've no intention to try to run a plug in oscilloscope unit without a ground that's for sure... did notice some pretty sketchy setups while reading and checking youtube...
The battery scopes like the siglent 810 and hantek's don't see to get the best reviews and I have no use for their multi-meter side. A fluke 125b could be nice for at almost $4k, hard to justify for this.
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I read up a bit more and watched a few more videos about what these can and can't do. 120vac is all I really got to worry about for voltage, just leg to ground and it is power from the pole.
I can do each leg one at a time and I got room in there to work anyhow, I'm pretty used to working with live AC 1 and 3ph.
I mostly need it to check a wide enough spectrum of harmonics on the power line which it seems the FFT function should do. Find the worst frequencies, and have it give me a THD %. Since most of these oscilloscopes like the SDS1104X have a FFT math function, can I assume they also have a button that adds it all to a THD %?
I did think that the ground could be an issue as I'm pretty sure there's some noise on it from the units I want to test, but that might be fine as it's really to establish a baseline anyway.
I'd no intention to try to run a plug it unit without a ground that's for sure... did notice some pretty sketchy setups while reading and checking youtune.
The battery scopes like the siglent 810 and hantek's don't see to get the best reviews and I have no use for their multi-meter side. A fluke 125b could be nice for at almost $4k, hard to justify for this.
It's not wise to use stock 10x probes for mains use as a matter of course. 100x probes aren't expensive and offer higher voltage ratings and additional protection from surges.
Still, for regular use on mains it's better to use a HV differential probe and negate any risk of ground loops.
I don't see any provision for THD measurements in the Datasheet so interpretation of the FFT seems the only way with an SDS1104X-E.
https://www.siglentamerica.com/wp-content/uploads/dlm_uploads/2018/05/SDS1000X-E_DataSheet_DS0101E-E03B.pdf (https://www.siglentamerica.com/wp-content/uploads/dlm_uploads/2018/05/SDS1000X-E_DataSheet_DS0101E-E03B.pdf)
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I look a bit more into the HV differential probes, any brand/model in particular to consider?
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I look a bit more into the HV differential probes, any brand/model in particular to consider?
Pintek DP-25 is a good entry level diff probe
Others:
Micsig
EEVblog
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Not running a bench scope without a ground is the right attitude, but unless you have a differential probe, it means that all of your measurements will be referenced to ground. You won't be looking across 3 phases properly, and even single-phase properly between neutral and hot.
You can make use of differential readings to use two channels as a single differential input with normal probes, but this is usually considered a MATH function, and doing so may prevent you from performing other match function which may include FFT on some scopes. Differential probe is a much better way.
Scopes with integrated power THD measurements are not common. I believe some of the Fluke MDA series I mentioned earlier do perform a THD measurement, but for the higher end models you are getting up $10k+ (at least in Canuck bucks). Would it be an option to use a device that can store waveforms and upload to a PC later for detailed analysis later?
Someone mentioned using 100x probe - good idea. If you decide to get away with 10x which *should* be ok for 120v circuits, DON'T use a 1x/10x switchable probe! - they WILL get switched and bad things will happen!
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I look a bit more into the HV differential probes, any brand/model in particular to consider?
Our host makes one
https://www.eevblog.com/product/hvp70/ (https://www.eevblog.com/product/hvp70/)
It is not in stock but maybe that is negotiable. You can also use it as a frame of reference.
In industry, were we to make such measurements, we would need the full complement of Personal Protective Equipment (PPE) including flame retardant overalls, face shield, rated low voltage gloves and we would probably stand on an insulating mat.
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For many purposes, one can simply put a decent quality transformer on the mains and look at the secondary with ordinary precautions. Of course the transformer will distort the wave but it might not be serious, depending on what you are trying to do.
You could also try a couple of different transformers and see how the results compare. But most power line transformers won't have a lot of response into the kHz region.
Another type of transformer that might be better is an output transformer from an old tube type hi-fi amplifier. They usually can handle the voltage and are wound with more care.
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I fully agree.
The only correct way of measuring mains on a 'scope is using high voltage differential probes. Everything else is a recipe for killing yourself.
Don't even think about battery 'scopes or isolation transformers.
First, make sure that your differential probe is specifically suited for the task (CAT III or IV rated in this case) and remember that they are not completely isolated and have non-trivial input loads in some cases.
Second, why do you dismiss battery operated scopes? A proper one is as safe as a proper DMM, and if you showed up as an electrician to work in a panel with anything else you'd be thrown out.
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As others have mentioned, you really can't go wrong with a CAT III or CAT IV rated scope such as:
https://www.fluke.com/en-us/product/electrical-testing/portable-oscilloscopes/190-series-ii/fluke-190-ii-190-102-s (https://www.fluke.com/en-us/product/electrical-testing/portable-oscilloscopes/190-series-ii/fluke-190-ii-190-102-s)
(edit) but perhaps something a bit cheaper but still reputable could be this Chauvin Arnoux:
https://www.aemc.com/products/oscilloscopes/oscilloscope-ox5022 (https://www.aemc.com/products/oscilloscopes/oscilloscope-ox5022)
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I fully agree.
The only correct way of measuring mains on a 'scope is using high voltage differential probes. Everything else is a recipe for killing yourself.
Don't even think about battery 'scopes or isolation transformers.
First, make sure that your differential probe is specifically suited for the task (CAT III or IV rated in this case) and remember that they are not completely isolated and have non-trivial input loads in some cases.
Second, why do you dismiss battery operated scopes? A proper one is as safe as a proper DMM, and if you showed up as an electrician to work in a panel with anything else you'd be thrown out.
Unless they offer isolation for all channels and normally they only offer proper isolation for one channel before getting into the old ground loop scenario. :rant:
Fully independent isolated channel scopes, handheld or bench top units are $ $ $.
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I fully agree.
The only correct way of measuring mains on a 'scope is using high voltage differential probes. Everything else is a recipe for killing yourself.
Don't even think about battery 'scopes or isolation transformers.
First, make sure that your differential probe is specifically suited for the task (CAT III or IV rated in this case) and remember that they are not completely isolated and have non-trivial input loads in some cases.
Second, why do you dismiss battery operated scopes? A proper one is as safe as a proper DMM, and if you showed up as an electrician to work in a panel with anything else you'd be thrown out.
Unless they offer isolation for all channels and normally they only offer proper isolation for one channel before getting into the old ground loop scenario. :rant:
Fully independent isolated channel scopes, handheld or bench top units are $ $ $.
Exactly. The OP's specs simply have way too much energy to fool around with cheaper alternatives.
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Unless they offer isolation for all channels and normally they only offer proper isolation for one channel before getting into the old ground loop scenario. :rant:
Fully independent isolated channel scopes, handheld or bench top units are $ $ $.
I would think that one channel at a time would be a given unless you have an instrument specifically designed for 3-phase power analysis--and I think that would be way beyond the OP's needs. Using only one ground lead is standard ScopeMeter technique. I suppose three HV differential probes would get the job done, but who has that?
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Unless they offer isolation for all channels and normally they only offer proper isolation for one channel before getting into the old ground loop scenario. :rant:
Fully independent isolated channel scopes, handheld or bench top units are $ $ $.
I would think that one channel at a time would be a given unless you have an instrument specifically designed for 3-phase power analysis--and I think that would be way beyond the OP's needs.
Not so much.
While not as good as an differential probe Siglent's ISFE works OK and offers 2 isolated channels:
https://www.siglentamerica.com/accessory/isolated-channel-optional-accessories%EF%BC%88isfe%EF%BC%89/ (https://www.siglentamerica.com/accessory/isolated-channel-optional-accessories%EF%BC%88isfe%EF%BC%89/)
Note, the BNC spacings and F-F adapters are such to fit directly onto any Siglent DSO and that may not allow other brands connection without additional BNC leads.
Using only one ground lead is standard ScopeMeter technique. I suppose three HV differential probes would get the job done, but who has that?
Those that want to view 3 phase mains with entry level equipment.
Cheap 4ch DSO + 3 differential probes is still cheaper than any isolated channel scope, HH or bench top.
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They certainly do make equipment just for this purpose. The one I use can measure all three phases, both current and voltage plus a spare channel. I would recommend you talk with some of the sales reps and have them bring in there systems for a demo.
Several companies offer them with various features depending what you need. For example:
https://www.testequity.com/products/33042/ (https://www.testequity.com/products/33042/)
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Second, why do you dismiss battery operated scopes? A proper one is as safe as a proper DMM, and if you showed up as an electrician to work in a panel with anything else you'd be thrown out.
No it's not.
Hint: look at the connectors:
DMM: fully shrouded with properly isolated test leads.
'Scope: exposed BNC and (thin) probe cables with insufficient insulation.
Generally: Do not let your test equipment carry mains voltage unless it's specifically designed for it.
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Thanks for all the replies.
I'm currently considering the AEMC 407 clamp meter, with their free bluetooth software it might get me close enough to what I'm trying to find, at least it has some THD functions. I'm not 100% sure it'll get to the higher frequencies that might be in there but for a bit over $500 could be worth a try.
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Second, why do you dismiss battery operated scopes? A proper one is as safe as a proper DMM, and if you showed up as an electrician to work in a panel with anything else you'd be thrown out.
No it's not.
Hint: look at the connectors:
Yes it is.
Hint: look at the connectors!
Discussing a "proper" battery powered scope (Scopemeter) ... Like a Fluke, not DSO-xxx hobby class rubbish.
My Fluke has the inputs (including the BNC's) insulated so that you cannot contact metal, even on an unconnected BNC socket. The probes are fully insulated, no metal of the BNC is exposed.
Bare input on the scope is rated max. 300v (4kv surge). With the standard 10x probe, it's rated 600v (6kv surge).
HV probes are available which are rated for many kv - Highest input range in scope mode, configured with a 1000x HV probe is 100kv!
And, yes .. I agree, if you showed up to my building planning to look at my 3 phase mains, or VFD outputs with a typical bench scope, I'd send you packing.
Dave
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Second, why do you dismiss battery operated scopes? A proper one is as safe as a proper DMM, and if you showed up as an electrician to work in a panel with anything else you'd be thrown out.
No it's not.
Hint: look at the connectors:
Yes it is.
Hint: look at the connectors!
Exactly: :-+
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The battery scopes like the siglent 810 and hantek's don't see to get the best reviews and I have no use for their multi-meter side. A fluke 125b could be nice for at almost $4k, hard to justify for this.
If you are on a budget, the siglent SHS1000 series would probably be more appropriate than the siglent 810 (SHS810 - SHS800 series). They seem to be very similar operationally, though the SHS1000 series has isolated channels with "Oscilloscope isolation level: CAT II 1000 V and CAT III 600 V" (this rating seems to be when using suitable 10X probes). SHS1000 costs more than SHS800 series, but a lot cheaper than the fluke equivalent, or most dedicated power analysers.
From the images here https://www.siglentamerica.com/handheld-oscilloscopes/shs1000-series-isolated-handheld-digital-oscilloscopes/ (https://www.siglentamerica.com/handheld-oscilloscopes/shs1000-series-isolated-handheld-digital-oscilloscopes/) it seems to have FFT but I don't see a THD calculation there. If you don't need accurate THD calculation perhaps you could roughly work it out relatively easily from the dB levels of the fundamental and biggest harmonic on the FFT display using an online calculator or a table:
http://www.sengpielaudio.com/calculator-thd.htm (http://www.sengpielaudio.com/calculator-thd.htm)
For example, if the fundamental is at -10dB and the biggest(loudest) harmonic is -50dB, the difference is 40dB. Then "Distortion attenuation a" = -40dB. Put -40 into the calculator and that gives THD% of 1%
Or just print out the "Table: Distortions and the dB and percent and the decibel" on that page and do a quick conversion. Probably easy enough to memorise too.
Depending on your business needs that might be close enough? Doubt you need THD% to 3 decimal places or anything but that is for you to determine. Perhaps you just need >5% is bad, <5% is good?
I am not an electrical engineer or anything so no idea if that is an effective or correct way of determining THD%. It is quick and easy though.
One other option though I don't think it fits your requirements, the Keysight U1242C multimeter (not an oscilloscope) measures/calculates something it calls "Harmonic Ratio" based on measuring RMS AC, and average responding AC. Details here: http://literature.cdn.keysight.com/litweb/pdf/5989-7687EN.pdf (http://literature.cdn.keysight.com/litweb/pdf/5989-7687EN.pdf)
Not sure if the AC bandwidth of the U1242C is sufficient? Doubt it goes up to 5kHz.
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If you are on a budget, you may want to look at a used one. I've used one of these and they are pretty nice. The problem is that they are no longer supported by HIOKI. You can still get batteries but if anything were to get damaged, they will no longer service them.
https://www.ebay.com/itm/HIOKI-3196-POWER-QUALITY-ANALYZER/183792935792?epid=1501714264&hash=item2acae9a370:g:JwgAAOSwCtJcx4Zp (https://www.ebay.com/itm/HIOKI-3196-POWER-QUALITY-ANALYZER/183792935792?epid=1501714264&hash=item2acae9a370:g:JwgAAOSwCtJcx4Zp)
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found this - UNI-T UT283A Single Phase Power Quality Analyzer Energy meter
relatively cheap. Seems to have THD%. Don't know if it is any good or safe.
edit:
Harmonic analysis: Up to 51th
Does that mean eg 60Hz fundamental * 51 = 3060Hz ? So might not show the 5kHz+ that you need.
(http://www.uni-trend.com/uploadfile/2018/1213/20181213094757733.jpg)
http://www.uni-trend.com/html/product/General_Meters/Power_Quality_Analyzer/UT283_Series/UT283A.html (http://www.uni-trend.com/html/product/General_Meters/Power_Quality_Analyzer/UT283_Series/UT283A.html)
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Normally if I am doing this sort of work, an typical oscilloscope I would use for electronics would be useless. The first problem is I not only need the equipment to survive in the field, I need it to capture some potentially large events (transients). I normally need both current and voltage. Complex triggers are a must. I also want long term recording of the events and the ability to post process them.
When I was first looking for one of these, HIOKI (and others) had brought in demo units. I ran transient tests on their demo units (with the approval of the sales men). Not the little toy generators I use to benchmark the handheld meters, but the real deal.
I like the new HIOKI units. Cost is about $10K USD but well worth it if you need this sort of tool. I've used them to solve some pretty odd problems.
Then again, if the goal is just to look at the AC line voltage at one of your home's outputs for the sake of looking at it, this would be a total waste.
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Thanks for all the replies.
I'm currently considering the AEMC 407 clamp meter, with their free bluetooth software it might get me close enough to what I'm trying to find, at least it has some THD functions. I'm not 100% sure it'll get to the higher frequencies that might be in there but for a bit over $500 could be worth a try.
AEMC 407 clamp meter does "Individual Harmonics (to 25th)", so I would think that means 60Hz x 25 = 1500Hz only? So probably won't show 5kHz.
https://www.aemc.com/userfiles/files/resources/datasheets/Clamp-On-Meter/2139-51.pdf (https://www.aemc.com/userfiles/files/resources/datasheets/Clamp-On-Meter/2139-51.pdf)
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If it were to just for a home outlet and a few small devices, a standard scope may be fine. Here is mine making some decent power measurements.
https://youtu.be/04I7nHA_HxM
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Another budget alternative is the uni-t 81b portable scope which is CATIII 600 but keep in mind it hasn't got isolation on the inputs jacks and some come with glass fuses :P
http://www.uni-trend.com/html/product/General_Meters/Digital_Multimeters/UT81_Series/UT81B.html (http://www.uni-trend.com/html/product/General_Meters/Digital_Multimeters/UT81_Series/UT81B.html)
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There is no danger measuring high voltage PROVIDED your at the SAME POTENTIAL as the line your measuring.This means you do not complete a circuit to ground or any of the other lines.This is why birds can sit on high voltage service lines .If you equipment is at the same potential as the line your measuring theirs no risk of damage.This is what isolation is for.Provided you don't exceed the maximum input voltage of the probe or the scope.
The best and safest solution to measuring mains voltage is through a step down transformer and measure at low voltage.Any anomalies in mains will show up on the low side.then just do the math.
Or better yet don't measure mains at all.Why? If you have to ask then you are not qualified to do so and need to get a service person that is qualified to do so.They have the training ,knowledge and equipment for the task.
I simply don't understand why people ,when they get their first scope .need to run to the first outlet they see and stick the probes in.It's an AC sine wave .It's always going to be an AC sine wave no matter which country your in.
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Either use high voltage differential probes with the oscilloscope of your choice or an oscilloscope with galvanically isolated inputs which is commonly designed specifically for power line measurements anyway. But do not combine these two options.
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Another budget alternative is the uni-t 81b portable scope which is CATIII 600 but keep in mind it hasn't got isolation on the inputs jacks and some come with glass fuses :P
I have one of these, it's "OK" - UNI-T has been around for a while, and their stuff seems somewhat decent, but not up to the standards of the likes of Fluke or other top players.
It's basically a DMM with graphical display ability - quite useful at times, but not what the OP is looking for. It has no math or real measurement capability. It doesn't have storage other than the trace on the screen, no zoom etc. In "scope" mode it functions like a fairly basic analog scope, except that the trace hangs around till the next trigger.
The DMM leads are isolated/shrouded like most other DMM's. It's rated CAT II/1000 and CAT III/600 .. not sure I'd trust it like the Fluke, but I have no trouble poking around 120v NA mains with it. It is well built and insulated and there is no exposed metal. It has a BNC adapter which is insulated like the Fluke (but lower quality) not clear what if any ratings apply when the BNC adapter is used, unless you have a properly isolated probe it wouldn't apply anyway, I don't use the BNC around mains.
It does come with an optical USB interface, and it's easy to make a optical RS-232 serial interface. I reverse engineered the protocol (which I documented somewhere) and wrote code to interface to it. It does let you save/recall the displayed waveform, so it would be possible to do some post-analysis on it, but it's just the display.
There a review on it:
https://www.eevblog.com/forum/reviews/review-uni-t-ut81b-scopemeter/ (https://www.eevblog.com/forum/reviews/review-uni-t-ut81b-scopemeter/)
And a teardown of the UT-81C which is just a faster version of it:
https://www.eevblog.com/forum/testgear/uni-t-ut81c-scopemeter-teardownreview/ (https://www.eevblog.com/forum/testgear/uni-t-ut81c-scopemeter-teardownreview/)
Dave
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Thanks for all the replies.
I'm currently considering the AEMC 407 clamp meter, with their free bluetooth software it might get me close enough to what I'm trying to find, at least it has some THD functions. I'm not 100% sure it'll get to the higher frequencies that might be in there but for a bit over $500 could be worth a try.
AEMC 407 clamp meter does "Individual Harmonics (to 25th)", so I would think that means 60Hz x 25 = 1500Hz only? So probably won't show 5kHz.
https://www.aemc.com/userfiles/files/resources/datasheets/Clamp-On-Meter/2139-51.pdf (https://www.aemc.com/userfiles/files/resources/datasheets/Clamp-On-Meter/2139-51.pdf)
I'm not sure, Specs say it displays to 25th, but the android app for the 407 and 607 models has a picture showing higher harmonics on a graph so maybe it senses them but just can't display higher than 25th? It does say the hz setting goes to 20khz but I'm not 100% clear on if it'll show me data up to that in the app, can't seem to be able to open the app I downloaded without having the unit in hand. I like that it appears capable to check THD on current and voltage.
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I'm not sure, Specs say it displays to 25th, but the android app for the 407 and 607 models has a picture showing higher harmonics on a graph so maybe it senses them but just can't display higher than 25th? It does say the hz setting goes to 20khz but I'm not 100% clear on if it'll show me data up to that in the app, can't seem to be able to open the app I downloaded without having the unit in hand. I like that it appears capable to check THD on current and voltage.
It certainly does look like a useful device for THD and as a general multimeter/clampmeter. I like it too from what little I know about it and I would not mind owning one.
From the manual it says "The frequency of the fundamental is determined by digital filtering and FFT for the network frequencies of 50, 60, 400, and 800Hz".
800Hz * 25 = 20kHz. So maybe you need the fundamental to be 800Hz to see that bandwidth? Or maybe you can see it for everything including 60Hz fundamental? I don't know. You could ask their customer support.
Also from the manual it says:
"Frequency Analysis Band - 0 to 25 times the fundamental frequency, from among the network frequencies 50, 60, and 400Hz
- 0 to 12 times the fundamental frequency of an 800Hz network". So that might be a limit of up to 10kHz?
I think maybe the 5kHz you are talking about is not really "harmonic" distortion related to the fundamental frequency of 60Hz. So something that just analyses harmonic distortion might not take it into account and probably should not be expected to. It could be considered some other sort of distortion (there are different types), or noise, or a seperate fundamental frequency. You have not really said why you expect a 5kHz noise to be present and a significant contributor (unless I missed it).
So if it is just "harmonic" distortion you are interested in, something like this device sounds great and is relatively affordable. If you really need to measure/view other sorts of distortion and noise, it may or may not do it - not sure. Hope it works out for you.
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I expect most of the noise to be from big IGBT's that run at 5khz. Its in the audible range anyway. There probably is some other regular harmonics since its tied to other vfd's and 3ph motors.
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My 2 cents, think of your probing mistakes in the past at low voltage circuits, and count as you can memorize, how many ? 1 ? 10 ? or even more ? :P
Now, when it comes to high lethal voltage, all you need is just "one" mistake, is enough to zap your expensive measuring equipment, your circuits or even worst ... your limb or your life.
Again, think about it, is it worth the saving ?
As many had pointed out, the easiest & possible the most bang for buck is using HV differential probe. As probing mains related devices will be very comfortable, an example scope that was probing the mains line that powered the scope it self.
(https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=408726;image)
Also many pointed out, battery powered scope is not always a suitable one for probing lethal voltage.
Here, a design illustration, how a "proper" battery powered handheld scope that was designed for probing mains. Watch carefully how they separate & isolate each functions in the scope.
(https://www.eevblog.com/forum/testgear/tektronix-ths710a-at-$190-grab-it/?action=dlattach;attach=61659;image)
Also a proper scope and its accesories have NO exposed metal, including the BNC and also the whole probe's body.
An example of Tektronix THS7xx handheld scope for mains probing, watch the BNC connectors at the scope, no exposed metal.
(https://www.eevblog.com/forum/testgear/tektronix-ths710a-at-$190-grab-it/?action=dlattach;attach=61787;image)
Also the suitable probe, watch again carefully, all covered by non conductive material and its not easy for human body parts can touch the metal parts.
(https://www.eevblog.com/forum/testgear/tektronix-ths710a-at-$190-grab-it/?action=dlattach;attach=61791;image)
The bad news is, handheld scope that is qualified for probing mains line, even used, especially top brand ones, still very expensive, and most of the times still more expensive than a brand new "decent" HV diff. probe.
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As many had pointed out, the easiest & possible the most bang for buck is using HV differential probe. As probing mains related devices will be very comfortable, an example scope that was probing the mains line that powered the scope it self.
Also many pointed out, battery powered scope is not always a suitable one for probing lethal voltage.
An HV rated portable scope is as suited to probing high voltages as an HV rated differential probe. In most industrial cases the battery scope is the better solution.
In addition to being great at demonstrating probing mains sockets, a bench scope with an HV-diff probe is ideal when looking at non-ground referenced HV signals on an appliance on your repair bench.
But things like distribution panels, VFDs, large main connected inverters etc. aren't usually conveniently on your bench when operating under a load. These things are also not (usually) in the realm of things a typical do-it-yourself'er works with, but if you have particularly exotic hobby needs, you might "make due" with a bench scope and HV diff. probe.
This setup doesn't work on a job site.
Below is a small collage of images (shamelessly snarfed from web) which shows places where you might see someone comfortably taking mains related scope measurements on a job site.
I suppose you *could* use bungee cords to hang your bench scope on the panel. Or perhaps drag in a table and extra long leads, making do with the scope where you can't quite see it (also fun when you need to move between several pieces of equipment). But... where would you plug it in? Better bring some extension cords too!
I suppose you *could* juggle a 1054Z while standing in front of a panel/equipment wearing heavy protective gear... but IMO much more likely to make a mistake, and again where to plug in.
I suppose you *could* balance your bench scope on the motor ... Not as stable as the flat rubberized scopemeter, but hopefully it won't fall off... But what if it does. How badly will it be damaged?
Making measurements of this kind, you avoid extra cables (the bench/diff-probe adds 2), you avoid things that need to be plugged in, you avoid things that are hard to hold, you avoid things that are breakable ... bench scope fails on all of these counts. The scopemeter designed for this purpose is the right tool for the job.
And yes, sometimes the right tool for the job is expensive. If you're doing it once (or very infrequently) you might make due like MacGyver. If it's part of your job, you get the right tool, it will pay for itself (otherwise you may encounter a different meaning of "bang for the buck").
Dave
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An HV rated portable scope is as suited to probing high voltages as an HV rated differential probe. In most industrial cases the battery scope is the better solution.
...<snip>... This setup doesn't work on a job site.
I suppose you *could* ... <snip> ...
I suppose you *could* ... <snip> ...
I suppose you *could* ... <snip> ...
Dave, nothing against your post there, infact I do own a Tektronix THS710A, apart from that clumsy DS1054Z bench scope + HV diff. probe. :P
Its just my response to OP which I see he just started on this matter, and there is nothing wrong on a bit cautious at spending big bucks, especially at early stage in venturing in this HV probing domain.
Looking at current HV diff. probe's price, imho, is an ideal starting point, rather than have to jump directly buying an expensive "decent" handheld HV isolated scope.
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An HV rated portable scope is as suited to probing high voltages as an HV rated differential probe. In most industrial cases the battery scope is the better solution.
...<snip>... This setup doesn't work on a job site.
I suppose you *could* ... <snip> ...
...x3
Dave, nothing against your post there, infact I do own a Tektronix THS710A, apart from that clumsy DS1054Z bench scope + HV diff. probe. :P
Its just my response to OP which I see he just started on this matter, and there is nothing wrong on a bit cautious at spending big bucks, especially at early stage in venturing in this HV probing domain.
Looking at current HV diff. probe's price, imho, is an ideal starting point, rather than have to jump directly buying an expensive "decent" handheld HV isolated scope.
Fair enough, and I'm not saying it can't be done that way, or even that you would never do it that way "If you're doing it once (or very infrequently) you might make due like MacGyver."
But what I am trying to do is counter the arguments that battery-powered scopes should not be considered, and that benchtop devices are better. In most industrial/on-site situations the opposite is more true.
I don't know the details of the OP's setup, however the original post mentions, VFDs (plural), 125A feeds, and 3-phase. This suggests to me that it is an industrial application and that they do not bring in someone properly equipped to monitor/maintain it. I could be wrong. Perhaps it is a lab environment (*1). But I think it inadvisable to make general statements that benchtop scopes with HV-diff probes are better for mains/power work than the tools professionals in the field use every day.
(*1) some years ago I was developing control systems for wind turbines. Mostly smallish, IIRC the biggest was 50kw. We did have a lab with among other things captive turbine heads driven by honkin big motors on VFDs. These were primarily used for developing/testing the regulation sub-systems. The heads produce 600v 3-phase. In the lab, I did in fact use benchtop scopes and HV-diff probes. That was because the high-power systems were actually on (or at least beside) my bench. I never took (or saw taken) a benchtop scope to the test rigs, or on site visits - there we used battery powered scopemeters.
Dave