Grounding of metal devices - Page 1
Forum section: electronics / beginners

author: TNb
author_rank: Regular Contributor
timestamp: July 05, 2014, 05:59:10 pm
content: Hi!I found an old USSR vinyl player(part of it) and want to repair it.I have some questions and mostly about the grounding.As you can see from the photos the power cable has only 2 pins, so no ground pin of course. I know that devices often get grounded on the metal casing itself, but I don't really get how this thing works - I mean it's 220 volts going straight to the metal casing, how do you not get electrocuted when you touch it.I tried to google, but not many articles tell about that and I still don't get itPlease, can you explain me how the whole technique of grounding on the metal casing works and how to do it so you will not set yourself on fire, thank you a lot in advance!And also does someone has the idea of what the hell is this green thing on the photo could be? Is it a resistor? No marking on this one....

author: XOIIO
author_rank: Super Contributor
timestamp: July 05, 2014, 06:35:49 pm
content: I thought that vinyl players had the chassis biased at the 200+ volts or whatever the tubes need?

author: TNb
author_rank: Regular Contributor
timestamp: July 05, 2014, 06:41:54 pm
content: XOIIO, yes, it's 220 volt as I said in first post. That is actually just the part for the spinning of record, without amp and other stuff.Anyway, question is more about grounding of the devices like that(straight to mains AC without ground pin).

author: liquibyte
author_rank: Frequent Contributor
timestamp: July 05, 2014, 06:48:40 pm
content: The green thing looks like a power resistor to me. Anyone else concur?

author: fubar.gr
author_rank: Unknown
timestamp: July 05, 2014, 06:49:21 pm
content: Maybe it originally had a polarized plug, so that the metal parts always connect to the neutral.Or maybe the soviets had a floating electrical grid?

author: XOIIO
author_rank: Super Contributor
timestamp: July 05, 2014, 06:50:22 pm
content: Quote from: TNb on July 05, 2014, 06:41:54 pmAhh, well you could "bootstrap" it which from what I recall is connecting ground to neutral, something done by shite electricians in old houses without grounds, not sure if it has any benefit or not. I'd just replace the cord with something more modern.

author: TNb
author_rank: Regular Contributor
timestamp: July 05, 2014, 07:09:26 pm
content: I mostly want to know why the hell the mains wire that is connected to the casing(photo 2) does not electrocute you. Not for this specific device, seen that million times(for example guitar amps).

author: edy
author_rank: Super Contributor
timestamp: July 05, 2014, 07:46:49 pm
content: I found a link that might help:http://www.worldstandards.eu/electricity/plugs-and-sockets/f/It says on the website (as well as others) that the ground is supposed to be on the top and bottom of the plug where there are "slots" that engage in the plug that go to ground. In this case the 2 prongs are not related to ground at all... they are active and neutral most likely. However based on the reversibility of the plug, if you put it in the wrong way you will have an "active" chassis" and could potentially short it to ground if you touched it.... Unless the paint is considered sufficient to insulate the chassis.Perhaps the Soviets used the chassis as a common terminal (to save on internal wiring)... they can just connect all the components to the chassis, like what happens in your car when you can ground things back via any exposed metal to loop back to the 12 V battery.We need more pictures and detailed wiring diagrams to understand what is going on.

author: TNb
author_rank: Regular Contributor
timestamp: July 05, 2014, 07:55:12 pm
content: well.... maybe I wrote it in a weird way...I'm NOT talking about this specific old piece of crap, this is just example which I happen to have here teared down. I'm talking about very common practice to have the chassis used as a ground point. Not soviet, not old. You can still buy a new tube guitar amp and have a wire soldered to the chassis. Or any other AC stuff.

author: edy
author_rank: Super Contributor
timestamp: July 05, 2014, 08:06:56 pm
content: We need to see exactly the wiring diagram of a new tube guitar amp and what wires are soldered exactly, otherwise it is hard to understand the design. I'm sure an expert on the forum would know. For example, are we sure it is an active "live" wire from the mains attached to the chassis? If the chassis were metal and you touched it, it would be the same as sticking a screw-driver into the active prong in a plug... Obviously can't be a good idea. Is it the green wire from the 3rd ground pin that is attached to the chassis and connects to the ground pin on your 3-prong plug? Most likely as that is normally done to ensure the chassis doesn't charge up and shock you if you touch it and become the ground momentarily for it instead.What about the guitar that you are plugging into your amp? Is it getting any power through the connector? Perhaps the chassis is powered so it can also delivering power to the guitar as well through the phono jack. I highly doubt the entire chassis would be sitting at 120 or 220 V without any current-limiting allowing you to touch it and get a nice jolt.

author: TNb
author_rank: Regular Contributor
timestamp: July 05, 2014, 08:11:57 pm
content: I'm talking about this practice:http://electronicspani.com/chassis-ground-in-circuits/Why the current doesn't strike you when you touch that thing?

author: edy
author_rank: Super Contributor
timestamp: July 05, 2014, 08:34:19 pm
content: The webpage you linked seems to show a circuit running on a battery.In this case, the chassis just acts as a wire completing the loop back to the negative terminal of the battery. You have no danger of being shocked because the current MUST travel back to the battery.... It will not go to ground through you. Think about the 12 V battery in your car... you can tap off the positive RED of your car battery and then basically complete the circuit by touching any other part of your car metal chassis.... because it is connected to the BLACK negative terminal. The metal chassis just completes the circuit to the negative terminal because it is connected to the negative terminal.There is no danger of current jumping into you and into the ground... just like there is no danger if you touch only 1 pole of a battery.Now on the other hand, if they are plugging the chassis to the MAINS.... that is a different story. It must be the green ground wire that connects to the chassis to avoid it getting charged with respect to ground (and you which is also "grounded" essentially). The chassis is essentially equipotential to the ground. If you are NOT grounded... like when you are wearing rubber shoes and you rub your head on a blanket and get charged up with a lot of static.... you WILL ground yourself and experience a painful shock and see a spark if you touch something that is grounded (like a metal pipe, a grounded chassis, or whatever).There is no way the chassis is connected to the active "live" wire or you will ground it when you touch the chassis (unless you are wearing rubber shoes) and you will get a shock... just like when you touch the white wire from the AC mains by sticking a screw-driver into the plug.If the chassis is connected to the neutral wire, that is a different story. However that is not a good practice especially if there is a non-polarized outlet or plug, you could slip it in the wrong way and swap the neutral and active. Also, the neutral is not grounded all the time... it floats a bit with respect to ground, and you could get a bit of a zap even touching a neutral wire, so you should avoid it. But it is certainly better than touching active.

author: RazSlack
author_rank: Contributor
timestamp: July 06, 2014, 09:23:41 pm
content: I think grounding back in the age wasn't standardized, and they just used a seperate wire that they connected to the radiator, heat transfering pipe, or whatever big metal surface that's on "equipotential" with most other metal surfaces. Try looking for an unused screw hole on the chassis. This is only speculation though. I myself have an old hungarian valve-based scope that's fully metal cased and has only 2 pins as well. Needless to say I have never had the courage to try it out if it works or not, so it's just staying on my shelf.

author: DmitryL
author_rank: Regular Contributor
timestamp: July 06, 2014, 10:11:25 pm
content: Quote from: TNb on July 05, 2014, 06:41:54 pmWhy do you worry about grounding ? Motor windings are very well isolated and floating. The green thing is a wire wound resistor; don't worry about it, it's almost impossible to destroy, you can measure its resistange if you want. The brown thing is 1uF 300V paper capacitor.It must be pretty safe to use unless someone did something stupid with wiring.Unfortunately I can't see the logo on your pictures. Here is the schematics for a similar piece of garbage:http://www.rw6ase-dok1.narod.ru/arhiv/rondo201an.djvu

author: Shredhead
author_rank: Regular Contributor
timestamp: July 07, 2014, 02:44:29 am
content: I am not an expert yet in electrical but here is my take on this (keep in mind I am in America which may be different from where you live)The white common wire and the ground wire of a house are both tied to the electrical box. They have continuity between each other. If you have an ac volt meter and put a probe in the hot wire in your outlet and the common wire, you will see 120V. If you leave the probe in the hot slot and the other in the ground hole, you will get 120V. If you leave the probe in the ground hole and put the other probe in the common slot, you SHOULD get nothing. You may get a reading of a couple volts though due to noise in your system.Chassis grounding doesn't really stop electrocution. People still die from getting shocked fairly regularly unfortunately. I think of a ground wire for a house as an alternate common wire. The idea is: giving a metal chassis a way back to the electrical panel is so that in the event of a fault, the breaker will trip instead of leaving your chassis hot.Back in the day, nothing was really grounded for the most part. Tying the metal chassis to the common is just giving it a way home to the box in the event of a fault so the breaker trips and stops the short.But like someone else already mentioned, make DAMNED sure you don't switch that plug around accidentally or else you will be on a new 220 volt diet.

author: Zepnat
author_rank: Regular Contributor
timestamp: July 07, 2014, 09:08:13 am
content: Call me stupid but I don't see any connection from the plug to chassis. Where are you guys looking?Edit: in photo two the wire looks like it goes from motor frame to chassis probably to stop static build up.

author: TNb
author_rank: Regular Contributor
timestamp: July 07, 2014, 04:19:00 pm
content: Quote from: Zepnat on July 07, 2014, 09:08:13 amAh, damn, I double checked continuity again and yeah.... maybe I wasn't careful enough last time.Well, it seems like the wire touches only the motor frame. I suppose no connection to mains, though it's little hard to see by eyes(though continuity says it's ok)So, maybe false alarmBut it is still freaking scare to touch that thing while it plugged in, maybe I should cover it with some sort of paint.I suppose guitar amps do something similar for transformer, though not sure... anyway, that's different topic.Thank you very much all for help, I highly appreciate it!

author: homebrew
author_rank: Frequent Contributor
timestamp: July 07, 2014, 06:30:54 pm
content: Quote from: TNb on July 07, 2014, 04:19:00 pmThat won't make it any safer...

author: TNb
author_rank: Regular Contributor
timestamp: July 07, 2014, 07:02:50 pm
content: why not? I thought paint and varnish is good insulator O_oof course depending on the thickness, but anyway, it's not copper cladding

author: homebrew
author_rank: Frequent Contributor
timestamp: July 07, 2014, 07:19:51 pm
content: Quote from: TNb on July 07, 2014, 07:02:50 pmBecause it would be an unreliable insulation, sensitive to scratches etc. etc.Either an insulation is trustworthy or it is better to assume it isn't there at all.Like those fake USB-chargers. They won't explode in your face immediately but the insulation is not to standard. So if a tiny thing goes wrong ... bad bad bad ...

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 07, 2014, 07:23:01 pm
content: Quote from: XOIIO on July 05, 2014, 06:35:49 pmThat is completely preposterous. I can only hope that XOIIO was joking and forgot to add the emoticon.As aparently TNb has discovered. it would have been equally preposterous to connect one side of an un-polarized mains plug to the chassis of a consumer device.The load (the motor coil) is isolated from the chassis, so NEITHER side of the mains incoming power is designed to be connected to the chassis.And that green ceramic component is indeed a power resistor. (And the tan color can is a capacitor).

author: Nerull
author_rank: Frequent Contributor
timestamp: July 07, 2014, 09:10:32 pm
content: Some early radios DID have the chassis connected directly to mains, either neutral or hot depending on which way the generally non-polarized plug was inserted. They had wooden covers to prevent you from easily touching the case, but were still pretty dangerous.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 07, 2014, 09:26:43 pm
content: Quote from: Nerull on July 07, 2014, 09:10:32 pmYes, "hot-chassis" devices are still in circulation. But they must be completely insulated from user-contact with insulating enclosures, plastic knobs and buttons, etc. etc. Mains iso-transformers original major application was on the repair benches of techs who worked on "hot-chassis" radio and television receivers. Service cords that bypassed the designed interlocks were common knows as "suicide cords".

author: DmitryL
author_rank: Regular Contributor
timestamp: July 07, 2014, 09:27:47 pm
content: Quote from: Richard Crowley on July 07, 2014, 07:23:01 pmYou tell me somethinghttp://en.wikipedia.org/wiki/All_American_Five".....By eliminating a power transformer, cost of the units was kept low; the same principle was later applied to television receivers. "

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 07, 2014, 09:33:05 pm
content: Quote from: DmitryL on July 07, 2014, 09:27:47 pmYes, you must have missed my previous message while you were typing. "Hot-chassis" receivers were popular back 50 years ago for economy.But that is very different than what XOIIO said. The chassis was nominal "circuit ground". It is true that you have a 50/50 chance of the chassis being "hot" (vs. "neutral") depending on how you insert the non-polar mains plug. But the chassis was never the B+ node as XOIIO proposed.

author: edy
author_rank: Super Contributor
timestamp: July 08, 2014, 02:05:59 am
content: Just out of curiosity, what does use of a hot chassis have to do with eliminating a transformer? Couldn't you still eliminate the transformer but just run insulated wires to each component and still not need to make the entire chassis hot? I thought using the chassis as a common conductor was just to make it simpler to build, so I am confused what the transformer has to do with it.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 08, 2014, 03:05:40 am
content: Quote from: edy on July 08, 2014, 02:05:59 amThe term "hot chassis" is not necessarily literal. We had a hot-chassis "All American Five" in the kitchen when I was a kid. It had a one-piece (except for the back) molded white plastic case, and a fiber-board stamped back. Inside, the entire circuit was on a PC board around 10 x 18 cm which fit into slots in the molded plastic case. There actually was NO metal chassis at all. But that didn't mean that the circuit wasn't "hot". The shaft of the volume control, and the tuning control were potential paths for shock to the user even though there was no metal chassis.Dunno whether there was any equivalent in 220V countries? Here in 110V-land, by using a 50C5 (50V) output amplifier and a 35W4 (35V) rectifier tube, and three 12V tubes for converter, IF, and detector/audio, they could string all the heaters/filaments in series to equal the mains voltage. And simply rectify the direct mains voltage to generate B+. Of course, everything was enclosed within the plastic case, ferrite or air-core loop antenna, speaker, etc. No external connections except the tethered power cord. But still lethal with that non-polarized mains plug and a 50/50 chance of "hot chassis" (or "hot circuit-common" if you wish).

author: Nerull
author_rank: Frequent Contributor
timestamp: July 08, 2014, 03:13:42 am
content: Wiki suggests that 220V countries had their own hot chassis devices.Aside from cost, another "benefit" of not having a mains transformer was that the equipment would run on DC supply systems, which were not entirely extinct at the time.

author: IanB
author_rank: Super Contributor
timestamp: July 08, 2014, 05:08:30 am
content: Quote from: Richard Crowley on July 08, 2014, 03:05:40 amCertainly, yes. I used to spend time as a kid looking for big transformers in old electronics and I found that more recent items from say the 60's and 70's had valves but no power supply transformers. This was especially true of televisions. Televisions did, however, tend to have big wire wound resistors on a ceramic former that looked like they would get hot in operation. I presume these resistors were used as a voltage divider to obtain some intermediate voltages.

author: Shredhead
author_rank: Regular Contributor
timestamp: July 08, 2014, 06:02:00 am
content: I don't see the big deal about no transformers being so dangerous. Many basic stove/ovens don't have a tranny for the high current stuff and they are on a 220V line even here in the States, 30A home run no less. If you put in fuses for over current protection in consumer electronics, what is the big scare?

author: homebrew
author_rank: Frequent Contributor
timestamp: July 08, 2014, 06:14:17 am
content: Quote from: Shredhead on July 08, 2014, 06:02:00 amSure, no problem. But those devices are almost all grounded. Or at least double insulated (certainly not the ovens, however). The scare is that a 'hot' internal chassis (large metal parts being on mains potential) are a threat when servicing those devices or if the (mostly wooden) covers are not completely intact anymore. Under those conditions, any internal wiring fault has the potential to conduct mains voltage to one of the external connectors or touchable metal parts.

author: ivaylo
author_rank: Frequent Contributor
timestamp: July 08, 2014, 06:53:01 am
content: The green thing is a 1uF, +-10%, 300V capacitor (paper I believe), manufactured in Dec, 1968.