Non-Earth Grounded, Oscilloscope Observation [video] - Page 1
Forum section: electronics / beginners

author: BrooklynNerd
author_rank: Newbie
timestamp: July 17, 2013, 12:24:51 am
content: I'm wondering if anyone could help me understand what is going on here.Here's the video:http://youtu.be/S9u1J4L3Cy8In a nutshell, power in my apartment building is not up to code. Most of my wall outlets are not earth grounded. I've plugged a new oscilloscope into one of those outlets and the results are interesting.The probe laying on a table picks up a 60hz signal, yes- I know it's picking up the mainsThe the signal is amplified when I grab the probe w/o touching the metal prong on the probe .When I touch the metal prong, Vmax jumps even more to ~1.70When I earth ground the plug to my heater things change:Resting probe on the table, or me holding the without touching the metal no longer picks up 60hzWhen I touch the metal probe, I get a much lower 60hz Vmax ~180mVMy question is, what exactly is happening when my wall outlet is not earth grounded? Why is there so much extra noise? I thought earth ground was a safety feature. Does it also help discharge unintended capacitance?Is it normal to see 1.2 Vrms going through my body?Is it safe to earth ground it by attaching the third prong to my heater?Thanks everyone.. and Thanks Dave for your awesome videos! I'm a CS guy and am trying to take the leap from micro-controllers to more serious EE.

author: c4757p
author_rank: Super Contributor
timestamp: July 17, 2013, 01:23:32 am
content: The filter capacitors in the power supply are connected to ground, so when the ground pin isn't connected, they apply voltage to the chassis and the shields on the input connectors (possibly quite a lot, up to half the mains voltage). Because the oscilloscope reads (Vtip- Vshield), the relatively large voltage now present on the shield becomes a term in that expression, and unless Vtipfollows it closely, you're going to see noise.It's not reallyunsafe to ground it like that forsignal purposes onlyIMHO (others may disagree on this topic, we've had lots of arguments about it...), but you must not trust it for safety, because you don't know how good of a ground it is.You really need to get proper grounding installed for your lab. As I'm sure many people will point out, grounding isn't absolutely essential for safety, but grounding items that aredesigned to be groundedis.Keep in mind that when the chassis isn't grounded, that voltage comes through the PSU filter capacitors, so the only thing separating you from the equivalent of sticking your finger in a light socket is a little capacitor. Do you trust it? Granted, they're probably good ones in that brand-new scope, but that doesn't necessarily mean they're safe.

author: IanB
author_rank: Super Contributor
timestamp: July 17, 2013, 01:50:08 am
content: A possible way to imagine the earth ground on an oscilloscope is like a big heavy object that damps out vibrations. If you wanted a very stable table free of vibrations you would put a big heavy metal slab on it (this is the kind of thing people do when they are making holograms where everything needs a very stable platform to avoid messing up the image). The earth ground on an oscilloscope is kind of similar--it is an electrical equivalent of a big heavy metal slab, anchoring your oscilloscope to the stable voltage of the earth so that it can then measure the tiny voltage fluctuations you are interested in without electrical noise getting in the way.

author: BrooklynNerd
author_rank: Newbie
timestamp: July 17, 2013, 01:54:21 am
content: Thanks all, that is helpful. This apartment building is 100+ years old and sometimes when I run into these sort of problems, I am never certain how what I am experiencing compares to the norm.It also serves as a learning experience.

author: Paul Price
author_rank: Super Contributor
timestamp: July 17, 2013, 02:19:45 am
content: BrooklynNerd, See the previous topic below for more info on this topic.https://www.eevblog.com/forum/beginners/is-it-safe-to-use-a-bench-power-supply-not-grounded/https://www.eevblog.com/forum/beginners/new-lab-no-grounded-outlets/-------------------------------------------------------C4757p,There is a great difference between working with ungrounded equipment such as an oscilloscope, and sticking your finger into a light socket.If you stick you finger in a light socket you are exposing you finger to both wires of the AC mains and you can be almost sure a good shock is possible...at least across your finger, but maybe not across you body, but it could be uncomfortable. If you are standing in a bathtub of water while doing this, that could be a different story.If you touch the metal chassis of an ungrounded piece of say, an oscilloscope, you are possibly only in contact with a quite small capacitance's tiny AC current reactive leakage at the line frequency but the current has nowhere to flow and there is no shock unless you have other earth-grounded metal objects you are also in physical contact with.You could have the possibility of experiencing a mild shock from that tiny capacitor only if you can reverse a possible unpolorized socket plug and now can get the tiny current through the capacitor, but only, again if you are otherwise in contact with some grounded metal object at the same time.If you are living in an USA household that has two-wire outlets, one wire is neutral and is connected to earth ground at the power entry point breaker box of your house and does not pose a danger, if that small capacitor in the oscilloscope is connected to the neutral side of the power line.

author: c4757p
author_rank: Super Contributor
timestamp: July 17, 2013, 02:45:53 am
content: I am obviously referring to the case of capacitor failure. I understand reactance.

author: amyk
author_rank: Super Contributor
timestamp: July 17, 2013, 10:39:42 am
content: That is almost certainly going to be a Class Y rated capacitor which is designed to fail open.

author: madires
author_rank: Super Contributor
timestamp: July 17, 2013, 11:32:27 am
content: Quote from: Paul Price on July 17, 2013, 02:19:45 amDon't rely on a single RCD, if one is installed! Add more layers of protection! A proper ground adds more protection and reduces some measurement problems.

author: c4757p
author_rank: Super Contributor
timestamp: July 17, 2013, 12:36:17 pm
content: Quote from: amyk on July 17, 2013, 10:39:42 amIndeed it is. Failing short is still sonething that one should at least consider, though, in part based on the quality of the device. Hence "do you trust it?". Personally, I would, but it is something to be aware of.Quote from: AcHmed99 on July 17, 2013, 11:48:25 amAt the breaker board, usually. And Paul, it's typically one capacitor on line and one on neutral, not just a single on neutral...

author: Paul Price
author_rank: Super Contributor
timestamp: July 17, 2013, 05:03:33 pm
content: C4757p, BTW thanks for the tip on the laser printer, it was the one that solved the problem.But I must say it depends on the circuit designer, most AC Line power supplies I've seen (I've seen hundreds) seem to use just one cap to ground, some very much fewer use one from both sides of the line, like refrigerators and electric stoves, dishwashers, heavy white stuff makers like to do that.In all my many years of troubleshooting, I've never seen one of these rather funny looking capacitors (with all kinds of strange various country safety symbols printed on them) short out. Doesn't mean it can't ever-ever happen.

author: c4757p
author_rank: Super Contributor
timestamp: July 17, 2013, 05:13:36 pm
content: These very commonline inlet EMI filtersfrequently have one capacitor across the line, plus one capacitor each from line and neutral to ground. My Rigol DS1052D, with presumably the same PSU board as his, uses the exact same circuit on the board itself. I don't know about "most" power supplies, but it's very common particularly in oscilloscope power supplies.I agree, I've never seen them short out. As I said, I would trust them - but I wouldn't advise someone else to. Rather, I'd advise him to do the research and come to the same conclusion I have. If somebody tells me "don't do that, it's dangerous", I listen unless I have good reason to think he's full of shit, but if somebody tells me "oh, it's fine, go ahead", I verify first. Anything that should be an insulator protecting you from mains or high voltage shorting out is always something you should think about until you know it's not going to.

author: BrooklynNerd
author_rank: Newbie
timestamp: July 18, 2013, 01:07:51 am
content: Thanks again, this is all very good info. I should have searched the forum better and will definitely check out those other posts.

author: leb120
author_rank: Contributor
timestamp: July 18, 2013, 10:03:23 am
content: For buildings where adding a ground would be too expensive or not possible a GFCI is a very good alternative.How they work:http://www.2d2c.com/gfci.phpIf you own the building and feel comfortable replacing a receptacle you can get a GFCI that will replace your existing plug.In a 100 year old building you may want to avoid this though; the wiring may have cloth insulation and may disintegrate when you touch it.Just turn off your breaker first.The other option is to get a GFCI protected power strip. Only items plugged into the strip will be protected, you can plug other power strip into this one and they will be protected as well.https://www.google.com/search?q=gfci+power+strip&tbm=shopYou could still tie the ground wire to your water or steam pipe (avoid gas piping) before of after the GFCI.GFCIs (a.k.a. GFI) look at the current coming in on the hot or neutral and compare that to the current leaving on the other wire. If there is an imbalance (because some of the current is flowing through you) of just a few milliamps it will open (disengage) both prongs of the outlet within a few tens of milliseconds. Most of the time you won't even feel the shock it just protected you from.Even a relatively poor "bonding" path back to the panel (through water piping) can still trip a GFCI meaning if a any equipment does have a ground fault (such as capacitors shorting) the GFCI will trip and protect you.

author: Spunky
author_rank: Regular Contributor
timestamp: July 19, 2013, 02:44:13 am
content: If the heater connection removes the AC wave from your scope then there must be an earth which means a GFCI will work, but only if the earth is either bonded to ground (earth rod), or there is a ground to CPC connection near to you from a nearby building, enough to carry over 5mA current.If not then you would be on an 'earth island' where a phase to earth fault simply raises the potential of the ground and no current flows (no GFCI trip), at which point of course both phases are equally live or neutral to you (take your pick), and neither can harm you unless you touch both, but dont test this though in case your ground is connected to earth. Of course if you have multiple class1 appliances then this is hazardous as 2 faults could see 2 adjacent appliances energised to each phase, waiting for you to close the circuit with your hands. And then the GFCI doesn't save you because the current killing you is matched on both phasesSo, a GFCI can't hurt, but it may not save you unless you bond the supply earth to ground.As for the scope, yes earth it to a heater. In the absence of any other earth why not?

author: Tepe
author_rank: Frequent Contributor
timestamp: July 22, 2013, 02:48:09 pm
content: Quote from: Spunky on July 19, 2013, 02:44:13 amCould you please write to the authority responsible for the thousands of pages of wiring code in my country and inform them that they have been mistaken for decades?

author: Unknown
author_rank: Unknown
timestamp: July 22, 2013, 04:29:04 pm
content: Do they seriously protect floating mains circuits with GFCIs in your country? Can you name one situation when a GFCI connected to a floating supply would trip? A GFCI detects leakage current, i.e. current flowing through one of the two wires, and then back via some path outside the GFCI (resulting in imbalance). If both wires are floating (no connection between supply earth and ground), then there is no alternate path. If you are connected to earth and touch one of the wires (say live) then not much will happen because the circuit will just float so live is at earth potential. No current will flow, and nothing will trip. If you now touch the other wire (the two class I appliances in Spunky's example), then you are connected between live and phase, and the GFCI will happily supply the full 20A indefinitely.So in this situation a GFCI is either useless and the situation is safe, or the GFCI is useless and you are being fried. Illustrating the importance of an earth-referenced mains for GFCI operation.

author: IanB
author_rank: Super Contributor
timestamp: July 22, 2013, 05:02:48 pm
content: Quote from: Tepe on July 22, 2013, 02:48:09 pmI read that Denmark uses a TT supply arrangement, where the "T" stands for "terra", or earth. The generator or transformer neutral is connected to earth at the supply, and the consumer has a local independent earth. So the electrical supply in Denmark is certainly connected to earth, and that is why an RCD will work.

author: Tepe
author_rank: Frequent Contributor
timestamp: July 22, 2013, 05:10:22 pm
content: Quote from: alm on July 22, 2013, 04:29:04 pmI was being facetious. Of course the mains is grounded somewhere. Even if the OP should happen to be in the IT using parts of Norway, it will be grounded somewhere which makes Spunky's remark "So, a GFCI can't hurt, but it may not save you unless you bond the supply earth to ground" miss the point. Mains will not be floating freely unless you add an isolation transformer or you have serious installation flaws. The former is under your direct control and it is a fair guess that the latter is far less likely than you accidentally connecting yourself as a load between live and neutral - something neither grounded equipment nor CFGIs will protect you from.

author: Tepe
author_rank: Frequent Contributor
timestamp: July 22, 2013, 05:12:55 pm
content: Quote from: IanB on July 22, 2013, 05:02:48 pmCorrectamundo. TN-C-S is also seen.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: July 23, 2013, 02:06:25 am
content: Here it's acceptable to replace an ungrounded receptacle with one that's GFCI protected. That is to say you can use a 3 prong outlet on a ungrounded circuit so long as it's GFCI protected. Ungrounded receptacles are impossible to find anymore, and rewiring is expensive and therefore requiring it prevents improving safety. The utility and your panel do the ground bonding.But please don't quote me; you know, some guy on the internet, since at the very least it's been a long time since I've looked at the Canadian Electrical Code.

author: Spunky
author_rank: Regular Contributor
timestamp: July 23, 2013, 02:17:14 am
content: Well, I've seen plenty of these RCD problems on installations which have no supply earth (usually temporary installations in the UK), and it can be a hell of a job sometimes to get a good enough earth to trip the RCD. So it's not like I haven't been there and done it. A shiny new 4ft earth spike driven in deep with an SDS drill usually works, but not always if the ground is poor. There needs to be enough current returning to it's origin.My point was a TT earthed system might trip an RCD or GFCI, and then again it might well not, and seriously they often don't. TT is antiquated, not up to modern safety standards, and there's a good reason why most of the world regulated it out of existence for new homes.If TT and IT are good enough for us all then the rest of europe has wasted it's time and money, we can forget about earth loop impedance testing and Ze values and relax, but it's not the case. I suspect Denmarks reluctance to adopt modern standards are more about economics than welfare.Admittedly GFCI is a lot lower trip current than RCD. Which would help, but doesn't negate my point. GFCI/RCD might work without a decent earth and is better than not having one.

author: Monkeh
author_rank: Super Contributor
timestamp: July 23, 2013, 02:28:18 am
content: Quote from: Spunky on July 23, 2013, 02:17:14 amAnd if the earth is so poor it won't trip, enough current to cause damage (to equipment or persons) can't flow..QuoteIn which case the shock risk is minimal, too!QuoteTT is a perfectly acceptable method and is fully capable of meeting modern safety standards with an appropriately designed installation.QuoteA GFCI and an RCD are the same thing. They can trip at 5mA, 10mA, 15mA, 30mA, 100mA, 300mA, 1A, instantly, with a time delay, etc.

author: Tepe
author_rank: Frequent Contributor
timestamp: July 23, 2013, 04:20:27 pm
content: Quote from: Spunky on July 23, 2013, 02:17:14 amI think you suspect wrong. TT is alive and kicking outside the English speaking world. It is possibly even the most widespread system and it does have certain advantages.

author: Spunky
author_rank: Regular Contributor
timestamp: July 23, 2013, 09:57:05 pm
content: There are certainly some advantages to TT, like the reduced risk of fire, and no risk of a broken supply earth. TNS and TNCS have the advantage that they have better safety against electrocution, better safety against faulty RCD/GFCI, and fast tripping of MCBs in a phase to earth fault. Choice depends on the supply infrastructure, which as it upgrades usually results in a shift away from TT and IT systems.My point about driving an earth rod applies specifically where there are multiple class 1 appliances, to prevent 2 phase faults being dangerous. Not needed if everyone is running laptops and TVs (class2), but really important if they are running fridges and cookers.RCD and GFCI are not the same thing. RCD is passive, the current imbalance in the coil itself trips it, but a GFCI uses a sensor circuit connected to the coil, hence they usually work on lower fault currents, and depend on both phases reaching the GFCI.

author: Monkeh
author_rank: Super Contributor
timestamp: July 23, 2013, 09:59:23 pm
content: Quote from: Spunky on July 23, 2013, 09:57:05 pmWould you care to provide some evidence of this? Because they really are the same fundamental device, and an RCD can be configured for virtually any trip current you like.

author: IanB
author_rank: Super Contributor
timestamp: July 23, 2013, 11:41:54 pm
content: Quote from: Spunky on July 23, 2013, 09:57:05 pmRCD and GFCI are just different acronyms used in different parts of the world. If you took an RCD to North America it would be called a GFCI, and if you brought a GFCI to Europe it would be called an RCD.A "residual current" -- a current remaining after you balance the current in all the conductors -- is the same thing as a "ground fault" -- a current escaping to ground and not remaining within the power conductors.

author: Spunky
author_rank: Regular Contributor
timestamp: July 24, 2013, 12:28:40 am
content: In europe we mostly use voltage independent residual current circuit protective devices for domestic use (they have to be I believe to meet EN61008), whatever your country calls it this is what I call an RCD. They are only required to detect current imbalance, whereas according to NEC regs residual current circuit protective devices must detect downstream neutral-earth faults without current flow and live neutral reversal, which as far as my knowedge goes traditionally uses an energised circuit, which leaks current from live to neutral, and a 2nd coil. This is what I (and the country that invented it) call a GFCI. Maybe some newer models can do this passively I don't know, it would only be small number of the GFCIs in use.Some places in the world may refer to either technology as GFCI or RCD and have different requirements for their devices. So, I regard EN61008 voltage independent residual current circuit protective devices as RCD, and US type devices as GFCI, maybe this causes some confusion, especially in european countries that use the term GFCI. They may or may not be voltage dependent, depending on how their required operation is defined in local regs.And no I don't have all these regs with me in writing. Download them if you really want to, I don't have the spare time.

author: Monkeh
author_rank: Super Contributor
timestamp: July 24, 2013, 12:39:05 am
content: Quote from: Spunky on July 24, 2013, 12:28:40 amNone of which is in any way relevant to the trip current or time of the device. The fundamental operation is identical.

author: Spunky
author_rank: Regular Contributor
timestamp: July 24, 2013, 01:08:04 am
content: You debated if they were the same device, not what their trip characteristics were. They are not the same device.

author: Monkeh
author_rank: Super Contributor
timestamp: July 24, 2013, 01:10:54 am
content: Quote from: Spunky on July 24, 2013, 01:08:04 amAnd you separated them based on trip characteristics, your point being, Mr. Pot?They are fundamentally the same device with the same purpose, and I would quite happily argue that zero-current neutral-earth fault detection is nothing more than a trip characteristic.I'll admit to being unaware of the added function. I can't really decide if it's useful or a waste of parts.

author: Spunky
author_rank: Regular Contributor
timestamp: July 24, 2013, 01:22:19 am
content: Quote from: IanB on July 23, 2013, 11:41:54 pmActually if you took an RCD to the US the building inspector would (or should) fail it. There are additional requirements on a GFCI in the US that an RCD does not meet. They are indeed very close, but different. Although some RCDs do incorporate more features, they are not a requirement here.

author: IanB
author_rank: Super Contributor
timestamp: July 24, 2013, 01:40:14 am
content: Quote from: Spunky on July 24, 2013, 12:28:40 amWhich is fascinating, and Today I Learned Something New.So when I get home I am going to try connecting neutral to ground on one of my GFCI outlets and see what happens (presumably it will trip).

author: Spunky
author_rank: Regular Contributor
timestamp: July 24, 2013, 02:18:18 am
content: Quote from: Monkeh on July 24, 2013, 01:10:54 amYes I did seperate them based on trip characteristics, GFCIs are generally 5mA, RCDs are generally 30 to 100mA. Sure you could get a 5mA RCD or a 1000mA GFCI. Both would be unlikely in an apartment don't you think?As for the other features, I never said it wasn't a waste of parts. Personally I think it is, as an RCD would trip when loaded under the same fault anyway.And my point? The original topic has already been dealt with. I think we're losing the point now.

author: Spunky
author_rank: Regular Contributor
timestamp: July 24, 2013, 02:21:33 am
content: Quote from: IanB on July 24, 2013, 01:40:14 amTest first there's no potential between them. It should trip all on it's own once connected. That's the theory.

author: Monkeh
author_rank: Super Contributor
timestamp: July 24, 2013, 02:22:06 am
content: Quote from: Spunky on July 24, 2013, 02:18:18 amI know for a fact they will. I can't think of a situation in which a combined N-E and L-E fault would desensitize even a 30mA device sufficiently, let alone a 5mA hair trigger.QuoteBut we're learning!

author: IanB
author_rank: Super Contributor
timestamp: July 24, 2013, 03:01:53 am
content: Quote from: Spunky on July 24, 2013, 02:21:33 amMy meter says there is potential between them--about 10 mV. That's how it can work, by trying to drive a current through any such ground-neutral connection. By comparison I measure 0 mV between ground and neutral on a regular socket.And it appears it does trip, but only with a dead short. I first tried connecting a 15 W night light bulb, but apparently not enough current flowed to be detected.

author: Tepe
author_rank: Frequent Contributor
timestamp: July 24, 2013, 08:06:27 am
content: Quote from: Spunky on July 24, 2013, 12:28:40 amIn this country we talk about HFI and in newer installations about HPFI. The latter can also deal with pulsating DC.

author: Spunky
author_rank: Regular Contributor
timestamp: July 25, 2013, 01:36:43 am
content: HPI would be the same as our Type A, and HPFI would be our TYPE AC, probably all derived from the same European approval.When I said it detects N-E faults without current flow that was wrong, what I should have said was that it does it without an external load by raising the neutral potential slightly so it will drive a tiny fault current N-E. This is presumably something to do with why in the US they don't stack them in series (1 on consumer unit, 1 on the socket), whereas in the UK it's fairly common.

author: Tepe
author_rank: Frequent Contributor
timestamp: July 25, 2013, 08:20:15 am
content: Quote from: Spunky on July 25, 2013, 01:36:43 amI think it is the other way around: HFI is type AC and HPFI can be either A or B.

author: Spunky
author_rank: Regular Contributor
timestamp: July 25, 2013, 08:46:08 pm
content: Quote from: Tepe on July 25, 2013, 08:20:15 amYep, quite likely. I only heard about those terms yesterday

author: madires
author_rank: Super Contributor
timestamp: July 26, 2013, 12:37:22 pm
content: Quote from: Monkeh on July 24, 2013, 02:22:06 amA too low mA value would cause problems if you got several SMPSUs or other devices with Y caps for filtering (between N and PE, and L and PE). The usual RCD for homes/apartments over here is triggered by 30mA and there's a current limit for the Y caps of 3.5mA per device (for 230V 50Hz it's max. 22nF per Y cap).

author: Spunky
author_rank: Regular Contributor
timestamp: July 26, 2013, 09:04:25 pm
content: Quote from: madires on July 26, 2013, 12:37:22 pmYes that and old motors in vacuum cleaners, washing machines, etc. Lots of things leak to ground, which is presumably why it's 30mA to protect a circuit and only 5 or 10mA on individual outlets.