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Electronics Forum / Basics / July 2008being shocked by Amp casing
hi all, perhaps slightly off-topic but you guys have been of great help to me in the past When I was installing a new hifi amplifier last night, I realised i was being lightly shocked each time i touched the amplifier case while I was lying on the floor trying to re-route some of the speaker cables behind the cabinet. The shock is definatly between the Amplifier case and the floor. ( no frayed wires - new installation) the amplifier is supplied by a 2 wire AC cord ( no earth ) what is causing this? is this dangerous? Can I add my own earth wire ( joined to the case, to the AC socket Earth Plug) ? will this help? > hi all, perhaps slightly off-topic but you guys have been of great > help to me in the past [quoted text clipped - 15 lines] > Can I add my own earth wire ( joined to the case, to the AC socket > Earth Plug) ? will this help? You are experiencing a ground loop. The potentials of the grounds that you are touching are not the same. The amp could be defective or just has some chassis potential. Putting everything on the same ground ("earth" across the pond) will eliminate this unless the amp is defective. Tom > You are experiencing a ground loop. I very much doubt it ! Inter-ground potentials should *never* be detectable by touch. If they are, get an electrician QUICK ! Graham > hi all, perhaps slightly off-topic but you guys have been of great > help to me in the past [quoted text clipped - 15 lines] > Can I add my own earth wire ( joined to the case, to the AC socket > Earth Plug) ? will this help? Is it just a half-mains alternating voltage buzz? ... like you get when you touch the metal casing of a two-wire-fed mains appliance like the Sony DAV-S880 home theatre system, in which internal suppression/filter capacitors connected to line and neutral appear to have their other ends connected to the exposed metalwork! If so, the current involved is too small to be dangerous - it's just a bit disconcerting. You could add an earth but if the equipment is designed for two-wire power then it probably isn't designed to have earth applied (others will probably advise in this respect in more detail), and this could give rise to an earth loop if anything feeding your amplifier is itself earthed. Chris > > hi all, perhaps slightly off-topic but you guys have been of great > > help to me in the past [quoted text clipped - 27 lines] > will probably advise in this respect in more detail), and this could give > rise to an earth loop if anything feeding your amplifier is itself earthed. Your answer is mostly corrrect in that you mention the effect of the suppresion filter which by default must create a leakage current potential. There is however, internally in the PSU typically another 'Y cap' that adds to the fun. Graham >hi all, perhaps slightly off-topic but you guys have been of great >help to me in the past [quoted text clipped - 12 lines] >Can I add my own earth wire ( joined to the case, to the AC socket >Earth Plug) ? will this help? Sounds like either your receptical is wired incorrectly or your amp doesn't have a polarized plug and is plugged in reversed or you have a problem inside your amp (system).... The tingle you feel is because you aren't fully grounded yourself as you are lying on the floor.. you are partially insulated.. But, yes, this dangerous.. If you were to grab something that is plugged in properly... You will complete the circuit causing a severe shock... as it is likely that the case of something else will/may be fully grounded. This can also cause damage to your system if you connect one item to a properly wired socket and another item in a socket that is reversed. At minimum... you'll see sparks. Inspect your wall sockets. The small blade is HOT and the large blade is neutral/Ground (For US of course). Use a meter to measure voltage in the socket. You should only get 120 (+ or - a few volts) ONLY if the small blade is connected to one of your test leads. Otherwise the socket is NOT wired correctly. Radio Shack used to sell three light outlet testers.. (Basically three neon bulbs connected to the outlet blades delta style...) not sure if they still do.... But try them out if your have one near you... One assumption that is always good to have is, if something is shocking you, then it's a bad thing. -- --------------------------------- --- -- - Posted with NewsLeecher v3.8 Final Web @ http://www.newsleecher.com/?usenet ------------------- ----- ---- -- - > >hi all, perhaps slightly off-topic but you guys have been of great > >help to me in the past [quoted text clipped - 36 lines] > delta style...) not sure if they still do.... But try them out if your have > one near you... Sorry, PURE NONSENSE. I have seen tiny sparks in low light, interconnecting pieces of IT equipment. ALL PERFECTLY LEGAL. Graham >> >hi all, perhaps slightly off-topic but you guys have been of great >> >help to me in the past [quoted text clipped - 41 lines] >I have seen tiny sparks in low light, interconnecting pieces of IT equipment. >ALL PERFECTLY LEGAL. Maybe this can happen on perfectly legal systems, but the prudent thing to do would be to check the receptacle first... there can be some pretty strange wiring out there! Many years ago, I was using a standard (back then) grounded-case electric drill near an I-beam in my basement and got sparks. After much exploratory surgery I finally found the problem: Some prior owner had added a string of outlets while finishing the basement. He had connected the run to a run from the mains, at a small junction box in the ceiling. He had *reversed* the connections there (white to black, black to white), and *cut off the grounds* !!! And then, to make life even more interesting, he had drywalled over the junction box area so everything looked normal. DUH! Best regards, Bob Masta DAQARTA v4.00 Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Sound Level Meter FREE Signal Generator Science with your sound card! > >> >hi all, perhaps slightly off-topic but you guys have been of great > >> >help to me in the past [quoted text clipped - 55 lines] > make life even more interesting, he had drywalled over the junction > box area so everything looked normal. DUH! Totally unrelated to this problem which is I will guarantee 99.99% certain to be the use of filter caps in SMPSs that cause small leakage currents. Graham > hi all, perhaps slightly off-topic but you guys have been of great > help to me in the past [quoted text clipped - 10 lines] > > what is causing this? A switched-mode power supply almost without doubt. I imagine the amp is compact and lighweight for its power rating. > is this dangerous? As long as it's been designed to IEC standards, no. It's called 'leakage current' and has allowable limits that will cause you no harm. > Can I add my own earth wire ( joined to the case, to the AC socket > Earth Plug) ? will this help? If the amp has a dedicated ground terminal do feel free to add a *sepearate* 'earth wire' tot his. Otherwise don't tinker with the mains witing AT ALL. Graham > > hi all, perhaps slightly off-topic but you guys have been of great > > help to me in the past [quoted text clipped - 27 lines] > > Graham I'll second a leaky SMPS. My 2amp SMPS 12V wall wart (double insulated) gave me a belt the other night. Turns out there's 90Vac riding on the 12V connector. The bastard took out an expensive bit of kit. Whole point of the bloody thing is to provide galvanic isolation. Never ever, had any trouble with cheap tat 50Hz transformer versions and that's what I'm going back to. I've had enough of EMI, noise, low level oscillations, expense, complexity and unreliability. Sod the SM technology :) > "Eeyore" <rabbitsfriendsandrelations@hotmail.com> wrote in message > > [quoted text clipped - 37 lines] > I've had enough of EMI, noise, low level oscillations, expense, complexity > and unreliability. Sod the SM technology :) FWIW you have my sympathies. IEC 60065 allowable leakage current off the top of my head is 2.4mA or so. What's more the MAIN culprit isn't even usually in the common mode filter but a little known part that crosses the pri-sec isolation (a Y cap) to reduce EMC compatability problems. Graham > When I was installing a new hifi amplifier last night, I realised i > was being lightly shocked each time i touched the amplifier case while > I was lying on the floor trying to re-route some of the speaker cables > behind the cabinet. > The shock is definatly between the Amplifier case and the floor. ( no > frayed wires - new installation) What devices did you have connected to the amplifier? Were they properly grounded? > hi all, perhaps slightly off-topic but you guys have been of great > help to me in the past [quoted text clipped - 10 lines] > > what is causing this? capacitance from live to the body of the amplifier, if the power cord has a reversible plug on either end try reversing it. > is this dangerous? if this is new equipment probably not. > Can I add my own earth wire ( joined to the case, to the AC socket > Earth Plug) ? will this help? almost certainly, especially if the maker has provided an earthing terminal on the amplifier, but if you have other audio equipment that is already earthed it may be better to earth your amp to that equipment. Bye. Jasen |
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