is it safe to use a bench power supply not grounded?? - Page 1
Forum section: electronics / beginners

author: kasumyku
author_rank: Regular Contributor
timestamp: June 25, 2013, 10:23:41 pm
content: Hi everyone,Maybe its just a stupid question but, hey, I rather ask experienced guys like you than to risk getting a nasty shock!I just bought a power supply(ebay cheapo), and just wondering ... Is it safe to use it if its not grounded(it has only 2 prongs), or should I change it to a 3 wire cable with 3 prongs,with the ground wire connected to the metal case of the power supply?And also the funny thing is that it has 3 output connections , + , ground, and -.Why would it have the 3rd output connection(ground) when it's not even grounded that damn thing!!!Thanks!

author: Unknown
author_rank: Unknown
timestamp: June 25, 2013, 10:27:58 pm
content: You may have discovered why it's an ebay cheapo. I wouldn't use it until at least the case is grounded.

author: Paul Price
author_rank: Super Contributor
timestamp: June 25, 2013, 11:15:20 pm
content: Always practice caution when working with electronics but use common sense.If you want, you can always attach a ground connection to some piece of metal on the chassis with a screw and washer and patch wire in a 3-wire cord or plug it into an isolation transformer for double safety.I wouldn't worry about it, it is probably no less safe than using any other small appliance like a bench top PC with a two wire cord, or a speaker system, an audio amp, any two wire wall wart, and electric shaver, or a clock or a clock radio, etc, all with possibly a metal surface or case, just like millions and millions of people in the world are doing.The ground connection on the front output connection area of the supply is used to connect the chassis of the power supply to the common ground of a test circuit if the power supply + and - outputs are both floating connections to your test circuit so to minimize the small capacitive coupling of the internal power transformer from upsetting a delicate circuit.You can always test the power plug to ground with a DVM see if there is any smaller than a meg resistance to the chassis ground indicating a fault, but just because it has just a two-wire power cord doesn't mean it isn't well insulated or double insulated from the AC mains and is quite safe to use. You can always take a peek inside to see what the workmanship looks like and get some idea on how the power transformer is made. It might just have a switching power supply, and it could just as well be well isolated from the AC mains.You can even make yourself a very low current 1000V or so DC power supply and Hi-Pot test it if you are paranoid enough.I use power supplies and other equipment all the time like that, but I am careful to not have any earth grounded equipment or grounded metal structures under my feet or nearby that could cause mild shocks of create a real danger to me if I am using a piece of equipment like this. That's how I and so very many others do things in my country. You can make up your own mind.See the very recent "New Lab no Ground Outlets" post about this about same topic for more solid reason, blind belief as well as ranting, insults to posters as well as paranoia about this subject..or you could just attach a 3-wire cord and be done with it.

author: edpalmer42
author_rank: Super Contributor
timestamp: June 26, 2013, 02:59:46 am
content: Maybe someone replaced the power cord. What's the brand and model? Maybe someone here can tell you if it's supposed to have a 3 wire cord.Ed

author: pickle9000
author_rank: Super Contributor
timestamp: June 26, 2013, 03:43:57 am
content: First you did the right thing you took the cover off. Do the basics (always take it apart). Make sure the the negative output is not grounded to the chassis. If not then add a grounded plug (ground to the chassis), while your at it. Make sure the fuse holder (if it has one is solid) if not replace it and make it solid. Properly strain relief the power cord, make sure the mains switch is well soldered and all the primary stuff has heat shrink. Use an anti slip washer on the chassis ground and make sure you have exposed metal for it to connect to. Safety first then function.For the heck of it check the supply for overshoot, if you have some look for a 100-330uf cap and replace it with a 470 and check again. That will usually help.Even if the supply is utter crap you can always reuse the case/ display transformer and so on and build a new one. inside.

author: Kremmen
author_rank: Super Contributor
timestamp: June 26, 2013, 06:03:01 am
content: Quote from: kasumyku on June 25, 2013, 10:23:41 pmNot knowing your location and local code this may or may not apply, but in the EU a device like that would be basically illegal. In simple terms such a gadget appears to be a Class 0 device, not allowed any more in new production since a long time. As others have pointed out, it is possible to turn this into a Class I device by changing the power cord and plug to grounded options and _properly_ attaching the PE wire to the metal housing once the necessary double checking for other connections has been done. After that it is legal and as safe as Class I devices ever are.All current ungrounded appliances are Class II devices - safety (double) isolated and those therefore in practice preclude simple metal housings. There is no straightforward way to turn a lower class device into Class II because of the requirement of double/reinforced insulation. As per code, only Class II devices may have the CEE 7/16 or so called Euro plug (the flat 2 pronged one) and they must display the Class II symbol (double square) in the housing.The fact alone, that you have a ground post but no ground in the plug is highly suspicious, so my bet is that the gizmo observes no specific classification at all - it is just thrown together.

author: cthree
author_rank: Frequent Contributor
timestamp: June 26, 2013, 06:40:09 am
content: You need to consider it unsafe but probably fixable. Replace the cord with a grounded cord and plug. Drill and tap a hole or use a sheet metal screw somewhere solid to attach the green ground wire using an ring terminal. Check that all the connections look good and that the fuse is on the HOT (black) wire not the neutral (white) wire. Make sure the ground binding post is actually attached firmly to the chassis and that the negative post isn't.That should render it reasonably safe. There is some other good advice except for the advice of Paul Price who doesn't understand the difference between a bench power supply and a two prong, double insulated clock radio. That you should ignore.You should consider anything you buy from abroad the same even if it has a grounded plug. Chinese crap is notoriously bad and often unsafe. They get really basic stuff wrong.

author: Unknown
author_rank: Unknown
timestamp: June 26, 2013, 06:47:52 am
content: I am honestly confused as to why there is even a debate on whether grounding is advisable or not. I understand that there are situations where it's not, but I think those are the exception rather than the rule, and if you don't have a good reason for floating, then by default it shouldn't be. It's just too dangerous otherwise. Particularly for things with metal cases.

author: Tepe
author_rank: Frequent Contributor
timestamp: June 26, 2013, 07:28:49 am
content: Quote from: duskglow on June 26, 2013, 06:47:52 amWell, if you happen to live in a place where common installations use TT earthing with a ground connection at the breaker panel (which has an RCD ) and no ground wires to the outlets, it sort of becomes an academic discussion whether an appliance like the OP's power supply ought to have a cord with a ground wire, because it won't make any difference.

author: Kremmen
author_rank: Super Contributor
timestamp: June 26, 2013, 10:50:20 am
content: Quote from: cthree on June 26, 2013, 06:40:09 amJust a note that this specific practice is expressly forbidden: a sheet metal screw isnotapproved for PE wire attachment. Ring terminal for multistrand cable is OK, otherwise a castellated washer should be used to secure a solid wire.

author: wi6971lly
author_rank: Newbie
timestamp: June 26, 2013, 02:04:49 pm
content: Quote from: Tepe on June 26, 2013, 07:28:49 amI do believe everybody right now is assuming that OP lives in a country with grounded outlets standard. Seeing as how you do not readily have an available ground connection in your plugs why bother.

author: Tepe
author_rank: Frequent Contributor
timestamp: June 27, 2013, 06:38:11 am
content: Quote from: wi6971lly on June 26, 2013, 02:04:49 pmAn assumption that is quite likely to be correct but nonetheless still an assumption.Quote from: wi6971lly on June 26, 2013, 02:04:49 pmTo challenge your (not you specifically, of course) assumptions.

author: Paul Price
author_rank: Super Contributor
timestamp: June 28, 2013, 05:34:40 am
content: Cthree, you say, "There is some other good advice except for the advice of Paul Price who doesn't understand the difference between a bench power supply and a two prong, double insulated clock radio. That you should ignore."I am an experienced electronic engineer and before that an appliance service technician and have been doing work with electrical gear for over 20-years.I do know the difference and I live in a country where two wire outlets are the exception, not the rule and people do not use three-wire cords, and if they did, they would be useless because the power outlets in most houses are two wire outlets protected by GFI breakers and do not accept three-wire cords.In any case, you assume that in a place like this that electrocutions from having two-wire outlets and using anything like a metal-cased desktop PC would be common. In fact this kind of accident is almost unheard of.I think I stated the possible issues of a safety problem very clearly in my postings here.I would repeat myself and point again outthat having a earth grounded case to a power supply and the possibility that I might touch a live mains wire could also put me in a deadly dangerous situation. The earth ground would allow the highest current to pass through my body to the the earth ground on the power supply if I touched the power supply case and a AC hot wire in a circuit. It would not necessarily be a danger if it were ungrounded, only a few microamps of AC current from a capacitor would just likely cause me a tickle.You must not assume that only the way you do things in your country is always the best, nor the most correct and safe.

author: Tepe
author_rank: Frequent Contributor
timestamp: June 28, 2013, 06:06:40 am
content: Quote from: Paul Price on June 28, 2013, 05:34:40 amPaul, I suspect you were typing too fast here.

author: Tepe
author_rank: Frequent Contributor
timestamp: June 28, 2013, 08:17:47 am
content: Quote from: Paul Price on June 28, 2013, 05:34:40 amIn 2010 there was one (1!) electricity related fatality in Denmark: a man who broke into a station and climbed up to touch the phase in a 60 kV breaker...(source: "Statistik over elulykker 2010", The Danish Safety Technology Authority)

author: vk6zgo
author_rank: Super Contributor
timestamp: June 29, 2013, 04:01:49 am
content: Quote from: Paul Price on June 28, 2013, 05:34:40 amFor many years,"Mantel" radio sets were common in Australian households.Although the Australian power system used 3 pin PE plugs for most appliances,it was common to use 2 wire "figure 8" cable on these radios,so the metal chassis was not returned to Mains Earth.They were all transformer type power supplies,so were protected by the insulation inherent in the transformer.There were obvious possibilities of breakdown between Active & chassis via the "ON" switch on the back of a "switchpot",which was necessarily at Mains potential.No extreme measures were taken to insulate the controls aswasdone with the AC/DC "transformerless" radios used in other countries.Thousands of these things were used for tens of years without incident.Occasionally,someone would bring in a "transformerless" radio from "The Old Country",& would come to grief due to it being knocked around in transit & losing some of its safety insulation,but the incidence of shocks from the locally made units was vanishingly small.I include the above to show that I understand how the use of "2 wire" devices in a system with the Neutral earthedsomewherein the power system is not inherently dangerous.Back in the 1970s,I purchased a Communications receiver with a metal case & "2 wire" lead .In my haste to "have a listen",I chucked a long wire out the window & connected an earth.causing a nice "zap" between the antenna "earth" lead & the Earthed object I connected it to.It was apparently designed bearing in mind the US 240v system,so the RF filter circuit on the Mains input had a capacitor from each side of the incoming power to chassis.In the USA,the chassis would have been a "virtual earth" ,but in Oz,it was the middle of a voltage divider!I quickly fitted a 3 wire cable --not so much that I was worried about a bit of a tiny "zap",but more that the RF filter would now work,& to remove the possibility of killing something in the front end of the radio.The old "Mantel" sets didn't have such a filter on the mains,so didn't "zap" you.All that said,Paul,I can't help feeling that your comment on the danger of 3-wire earthed equipment is a little contrived.Many thousands of people work with this type of equipment without incident,just as others work with 2-wire powered equipment.