Ground of oscilloscope always connected to earth? - Page 1
Forum section: electronics / testgear

author: pascal_sweden
author_rank: Super Contributor
timestamp: January 01, 2014, 09:44:02 pm
content: I saw the video on potential risk of destroying your probe when connecting ground to wrong signal.Was surprised to read that the ground of the oscilloscope is always connected to earth in most scopes.Why is that not isolated in a scope? This would make it totally safe not?

author: notsob
author_rank: Frequent Contributor
timestamp: January 01, 2014, 10:15:29 pm
content: This is for your own safety, in the past people used to sometimes float the scope (ie NO EARTH connection) - so especially with the traditional metal case scopes, the case becomes LIVE - - you touch the metal or the shielded end of the probe you die ( or you get booted across the room with serious burns.Leave the earth connected and use an isolation transformer

author: pascal_sweden
author_rank: Super Contributor
timestamp: January 01, 2014, 10:17:29 pm
content: So you would not connect the scope directly to mains, but put isolation transformer in between?Which brand and type you would recommend? How expensive?

author: BravoV
author_rank: Super Contributor
timestamp: January 01, 2014, 10:26:54 pm
content: Or ... use a special designed portable "isolated" scope, both channels and both grounds are fully isolated, for example like this Tektronix portable scope design here.But remember, the devil is in the detail, NOT every "portable" scope have this kind of design, so just be careful.

author: Bored@Work
author_rank: Super Contributor
timestamp: January 01, 2014, 11:53:04 pm
content: Quote from: pascal_sweden on January 01, 2014, 10:17:29 pmNo, NEVER isolate or float the oscilloscope. If you want to use an isolation transformer isolate the device in which you want to measure. But be aware that an isolation transformer (or an isolated oscilloscope) reduces but not eliminates the risk.If you need an isolated oscilloscope, get isolated probes (note: differential probes not necessarily mean they are isolated, check if they are), or get one of those special isolated oscilloscopes.Also note that a battery operated oscilloscope is not necessarily properly insulated. E.g. blank BNC connectors are typically a sign that an oscilloscope is not. But non-blank connectors still don't mean it is properly isolated.

author: EEVblog
author_rank: Unknown
timestamp: January 02, 2014, 12:02:50 am
content: Quote from: pascal_sweden on January 01, 2014, 10:17:29 pmNo!You use the isolation transformer on the device under test.One like thishttp://amzn.to/1dUDopy

author: EEVblog
author_rank: Unknown
timestamp: January 02, 2014, 12:05:15 am
content: For proper floating measurements on an oscilloscope you need a proper high voltage differential probe like this:http://amzn.to/1iuKJSB

author: nctnico
author_rank: Super Contributor
timestamp: January 02, 2014, 12:10:47 am
content: An oscilloscope (like most equipment) is designed to be connected to ground. The safety rating is based on it. Don't forget a scope (and many other types of equipment) have exposed metal parts which could become live if there is a breakdown somewhere in the isolation.edit: and don't think that never will happen. A while ago I didn't wait long enough to let a scope warm up when it came from the cold outside. The moisture and dirt started to conduct which caused a breakdown between the mains and the chassis. Because the scope was grounded the fuse blew.

author: casinada
author_rank: Frequent Contributor
timestamp: January 02, 2014, 12:56:04 am
content: Unless they're not:http://www.tek.com/oscilloscope/tps2000http://en-us.fluke.com/products/portable-oscilloscopes/fluke-190-ii-portable-oscilloscope-190-204.html#fbid=Irx3Vyq4cGY?featureshttp://www.home.agilent.com/en/pc-1000004009%3Aepsg%3Apgr/u1600-series-handheld-oscilloscopes?nid=-536906711.0&cc=US&lc=eng

author: Nerull
author_rank: Frequent Contributor
timestamp: January 02, 2014, 01:01:34 am
content: Imagine what will happen if you float scope ground to 120/240V mains and then absent mindedly reach over and connect another probe on a second channel and it's live BNC connector.If you survive, you will know why you don't float scopes. A floating scope puts potentially lethal voltages on exposed conductors inches away from buttons you may be using while focused on the screen/DUT.

author: casinada
author_rank: Frequent Contributor
timestamp: January 02, 2014, 01:54:27 am
content: Isolated scopes have isolated BNC connectors. Without common sense anybody should stay away from this type of environments.

author: Nerull
author_rank: Frequent Contributor
timestamp: January 02, 2014, 03:01:21 am
content: BNC connectors, and depending on the scope the chassis, front panel, switches, etc.If you have common sense you use the right tool for the job.

author: rexxar
author_rank: Frequent Contributor
timestamp: January 02, 2014, 03:34:06 am
content: I don't understand why the chassis would ever be at mains potential just because the ground is not connected. I've used my oscilloscope without the ground connected (before I knew any better) and I never got zapped. I also used it in the new apartment before I discovered there isn't any earth connection to the outlets, still not dead.

author: Unknown
author_rank: Unknown
timestamp: January 02, 2014, 03:42:59 am
content: Quote from: rexxar on January 02, 2014, 03:34:06 amThen probably best you read up on why a scope is grounded, and how.Quote from: rexxar on January 02, 2014, 03:34:06 amI think that is more luck than anything as you appear to be unaware of how grounding and fault currents/paths in general work.

author: ConKbot
author_rank: Super Contributor
timestamp: January 02, 2014, 02:05:14 pm
content: Quote from: rexxar on January 02, 2014, 03:34:06 amA little thought exercise. Lets say you want to measure the gate waveform (Vgs) on a top mosfet in a full-bridge power supply. You diligently get an isolation transformer with a ground lift to isolate your scope, run it off of that, clip the groundclip of the scope probe to your source pin of the mosfet, and clip the probe onto the gate. Huzzah, you're getting a nice Vgs waveform on the scope now! Lets add a second probe to measure Vds on the mosfet also. You grab the metal BNC of the probe to plug it in, and as soon as the 2 BNC connectors touch... you get zapped by 400V@200KHz because the ground clip of the first probe is connected to the mid-point of the full bridge in the power supply, which goes between 0V, and 400V as the mosfets are switched. Ouch!

author: grumpydoc
author_rank: Super Contributor
timestamp: January 02, 2014, 02:29:35 pm
content: QuoteIndeed!It's important to realise that once you connect the 'scope ground lead to some point in the (isolated) DUT you've just ground referenced it again. If the point you chose is at 400V referenced to the DUT ground then you can create a situation where the DUT case is now at -400V to earth ground.Isolation transformers allow certain measurements which would otherwise blow you and/or your 'scope up to be made without letting the magic smoke out - but they are not a substitute for careful appraisal of what voltage goes where, nor a panacea for safety.

author: BravoV
author_rank: Super Contributor
timestamp: January 02, 2014, 06:12:14 pm
content: Quote from: ConKbot on January 02, 2014, 02:05:14 pm+1 , and that "Ouch!" sometimes means .... you're dead.

author: Kjelt
author_rank: Super Contributor
timestamp: January 02, 2014, 07:35:02 pm
content: I think you all forget that if the scope and the DUT are behind their own isolation transformers there is absolutely no reference to earth ground.So there is no current flowing to earth ground period. Beware of the rule never to use both hands simultaneously, if you touch with both hands a high voltage then you have a (lethal) problem.At our company we work for 20+ years with whole measurement benches behind a big isolation transformer just for safety. Just be aware there should nowhere a normal earth ground mains socket be used, everything should be behind the isolation transformer and the DUT should be behind a different isolation transformer so they are also seperated from eachother.Still if measuring on dangerous high voltages you still should always use differential probes. For very high voltages there are high voltage probes.Always be alert, think twice before measuring on dangerous voltages and with very high voltages build a dead mans switch and be sure there is someone else in the room with you that can toggle that switch (because with high AC voltages it twinges the muscles so you can not let go).

author: HooRide
author_rank: Regular Contributor
timestamp: January 02, 2014, 09:24:09 pm
content: Quote from: EEVblog on January 02, 2014, 12:05:15 amThat looks remarkably similar to the Agilent one that is twice the price (same internals?):edit:https://www.eevblog.com/forum/testgear/lecroy-25mhz-hv-differential-probe-teardown-repair/I'll be buying the BK for measuring things that can not easily be isolated.

author: csar
author_rank: Newbie
timestamp: November 12, 2018, 12:46:44 pm
content: The bench multimeters also have metal chassi, but the probes are floating. Why is not the same thing done with oscilloscopes?Why is the multimeter safe, but the scope is not?

author: Fungus
author_rank: Super Contributor
timestamp: November 12, 2018, 01:24:39 pm
content: Quote from: csar on November 12, 2018, 12:46:44 pmDoes your multimeter have a tempting row of BNC connectors along the front that could all be connected to a live wire via. the ground clip of a probe?

author: Scratch.HTF
author_rank: Regular Contributor
timestamp: November 15, 2018, 12:51:53 am
content: A Pace satellite receiver service manual had an instruction to float the ground of the oscilloscope (and not to touch it) when taking waveforms on the primary side of the power supply; high voltage differential probes (or alternatively in some cases, an isolation transformer) should be used instead.In a Philips test equipment manual, the notice regarding grounding had the line "intentional interruption is prohibited".

author: Fungus
author_rank: Super Contributor
timestamp: November 15, 2018, 07:31:42 am
content: Quote from: Scratch.HTF on November 15, 2018, 12:51:53 amI believe it was fashionable for a while in the 1970s because it makes life easier.Then two channel 'scopes became more widely available, the bodies started piling up, and people decided that "easier" isn't always the best way.

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: November 15, 2018, 11:31:39 am
content: I have a large HP isolation transformer that my scope runs through. As in any test, you have t think about what you're doing when you work with high voltages. I stuck a note on the front of my scope saying "GROUND IS ISOLATED!"Take care my friends.

author: tggzzz
author_rank: Super Contributor
timestamp: November 15, 2018, 11:38:15 am
content: Quote from: rexxar on January 02, 2014, 03:34:06 amIt is good that you know you don't understand. Now, what are you going to do about that?QuoteAh, the "I've walked across busy roads without looking and I'm still not dead" contention.

author: tggzzz
author_rank: Super Contributor
timestamp: November 15, 2018, 11:46:38 am
content: Example fatality fromhttps://groups.io/g/TekScopes/topic/7632418#10795with myemphasisQuoteExcerpt from Tektronix' "Fundamentals of Floating Measurements and Isolated Input Oscilloscopes Application Note"https://info.tek.com/www-fundamentals-of-floating-measurements-and-isolated-input-oscilloscopes_ty.html,with myemphasisQuote

author: cdev
author_rank: Super Contributor
timestamp: November 16, 2018, 02:02:31 am
content: Its true, I have an old Tek 2200 series scope and it had a battery add on and they give instructions in the manual how to float it using this external source of power. Nowadays they would never tell people to do or how to do that.Quote from: casinada on January 02, 2014, 12:56:04 am

author: vk6zgo
author_rank: Super Contributor
timestamp: November 16, 2018, 04:48:40 am
content: Quote from: tggzzz on November 15, 2018, 11:46:38 am*Surely the primary warning is, if you are working on something offering unusual dangers DO NOT WORK ALONE!!Over 40 plus years, I have only seen this done once (floating an Oscilloscope in this manner), & this was done by very senior people, very gingerly, & with great care, AND with a staff member standing by with their hand on the handle of the main circuit breaker, ready to operate it instantly if required.This was looking for some waveform distortion problems at the output of an ac Automatic Voltage Regulator.In a linear system, the 'scope could be set up off line to trigger correctly on a 50Hz signal.Modern switchmode supplies means much more fiddling with the controls would be necessary, with additional hazard.The next time I was where someone had the occasion to look at a similar signal, they bought a Tek TH720A specially because of its isolated inputs.

author: bitseeker
author_rank: Super Contributor
timestamp: November 16, 2018, 05:22:58 am
content: Quote from: HooRide on January 02, 2014, 09:24:09 pmYes, they're made by Sapphire. Likewise with the EEVblog HVP-70 (70 MHz vs. 25 MHz, but same OEM). Good stuff.For a more budget-friendly option, there's the Micsig DP10013. (However, note the difference in the multiplier, which may impact your use case.)https://www.amazon.com/Micsig-DP10013-Differential-Attenuation-Tektronix/dp/B074K4XPW3/Even cheaper on eBay.

author: tggzzz
author_rank: Super Contributor
timestamp: November 16, 2018, 09:03:25 am
content: Unusually for me, I've snipped the context because in this case it doesn't matter.Quote from: vk6zgo on November 16, 2018, 04:48:40 amI prefer avoiding the problem to (possibly) fixing the problem after it has occurred. In other words, know and use the right tool for the job.Nowadays, as you say, one suitable technique would be:QuoteOr you could do this: