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Old 29th November 2010, 09:15 PM   #21
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Quote:
Originally Posted by Steve Eddy View Post
Typically the only reason that's done is because the chassis is used as a shield. But since the chassis is plastic and doesn't provide any shielding, it's rather moot.

se
So in this case, if I keep the case plastic, signal ground's only connection to the PSU would be through the phantom power supply's negative supply?
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Old 29th November 2010, 09:22 PM   #22
DF96 is offline DF96  England
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A low voltage transformer with a split bobbin should be fairly safe provided mains wiring is kept well away from the secondaries. The idea is that under any conceivable fault conditions there is no way the mains can connect to the rest of the circuit e.g. any wire comes loose, any component goes short circuit, any insulator breaks down. That is why double-insulation is just that: two separate things must break before there is any danger, and each of them is of a quality that failure is very unlikely.

I might be prepared to take a few minor risks on DIY test equipment or amateur radio gear, but anything 'domestic' like an amp or preamp I feel should be above reproach where safety is concerned. For me that means proper safety grounding.
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Old 29th November 2010, 09:23 PM   #23
Pano is offline Pano  United States
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Thanks.
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Old 30th November 2010, 10:06 AM   #24
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by diymixer View Post
wait, so I have to connect signal ground (carrying the negative side of my phantom power supply) to safety ground?
yes!!!!
all exposed conductive parts must be connected to Safety Earth

Quote:
Originally Posted by panomaniac View Post
DF96. Do you really see a hazard on the secondary side of a low voltage transformer?
Now I might see that in a big power amp with high voltage rails and high current. But in signal level devices? What do you think?
we are not competent to design, build, test and guarantee double insulated mains equipment. Therefore we must connect to PE, we have no choice.

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Originally Posted by DF96 View Post
If it possible for someone to touch a conductor in or around your circuit, either directly or indirectly, then that conductor must be referenced to ground so that it cannot become live.................. So for almost all DIY the signal ground must connect somewhere directly to safety ground, and everywhere else it should connect indirectly to safety ground so that fuses still blow. Floating circuits are potentially dangerous, except where is it impossible to touch them.
if the mains input hot/Live breaks loose and touches some other low voltage part then potentially the whole device and all it's interconnects become live. If there is no PE to Safety Earth to exposed parts connection then the fuse will not blow. The equipment will appear to have turned itself off and the operator goes across to touch/fiddle/die.

To me and Davenport and DF and many others find this so obvious that we don't plod along ignorantly and put our families at risk.

Yet we see hundreds of Members who do exactly that.
What can the Forum do to protect those that just plod along?
Serious disscussion invited. New Thread?
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Last edited by AndrewT; 30th November 2010 at 10:11 AM.
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Old 30th November 2010, 01:21 PM   #25
macboy is offline macboy  Canada
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Quote:
Originally Posted by AndrewT View Post
I should not have to repeat this but Mac is seriously in error.
I respectfully disagree Andrew.

Please don't misinterpret my comment. The chassis should be grounded for safety. The input jacks need not be. They can be isolated from the chassis, with the result being that the entire secondary side of the mains transformer can be floating w.r.t. ground. You can't get a shock by touching this RCA jack. If your mains wiring is coming anywhere close to your input or output connections then you have a lot more serious safety considerations to deal with than what grounding will save you from.

If the secondary side is left floating, then if you touch it, it takes on your potential, so there is no potential difference to create shock. No possibility of shock. Of course if you stick two fingers/hands in there at different points, then all bets are off... no amount of grounding can save you from yourself. If you close the conduction loop by grounding any part of the secondary side, then you can get a shock by touching only one conductor (assuming your body is grounded elsewhere... an assumption you must always make for safety).

I really would like to hear why you think I am "seriously in error".


Quote:
Originally Posted by AndrewT
if the mains input hot/Live breaks loose and touches some other low voltage part
How can it break loose and touch some low voltage part? That is virtually impossible if you give your wiring a little thought. Your mains connections should be made far away from signal parts for many reasons, including reducing hum and noise from the line, but mostly for safety. The leads connecting the power transformer to the line should be short enough to not be able to reach any low voltage part even if they break loose. You should ensure good mechanical connection (wire through eyelet then twisted, etc.) before soldering. Don't just solder the wire to the side of the terminal.

Grounding-everything-in-sight must not be used as a crutch that will save you every time. One day it won't. Never give yourself a false sense of security or you will end up missing or ignoring other important safety issues.
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Old 30th November 2010, 01:41 PM   #26
AndrewT is offline AndrewT  Scotland
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I am not going to expend any time answering Macboy.
We will simply disagree.
I can sleep at night knowing I have not put anyone's life at risk, because I have done the research to allow me to carry out my hobby safely.
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Old 30th November 2010, 01:58 PM   #27
macboy is offline macboy  Canada
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Originally Posted by AndrewT View Post
I am not going to expend any time answering Macboy.
Very helpful. Thanks.
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Old 30th November 2010, 05:39 PM   #28
DF96 is offline DF96  England
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I am not sufficiently expert in hazard analysis to guarantee safety in the absence of a piece of grounded metal between me and mains. I therefore try to ensure that wires don't come loose, and ensure that a fuse blows if they do and then touch something they shouldn't. For me this is easier to do than full double-insulation standards.

Can you guarantee the insulation integrity of your mains transformer? If so, that only counts as one insulating barrier, so you need at least one more between the secondary and the user. OK if the circuit itself is sealed inside a plastic box with no external connections, or only via mains-rated capacitors, but no good if you have a signal ground coming out as that connects to the transformer secondary.

You need at least two barriers, or at least one barrier plus grounding. You seem to want to get away with just one barrier and no grounding.
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