Paul, I got in real trouble last time i mentioned to use a 2 prong instead of 3 prong plug. One of the moderators didn't like it....even though most commercial equipment today has a 2 prong plug, that equipment (hopefully) has been thoroughly tested to pass UI? specs. If something goes wrong, you definitely want the case grounded to main earth. Whether or not your power supply ground is connected to case...I don't know whats best. I'm still learning. BE CAREFUL. Later!
-Matthew K. Olson
-Matthew K. Olson
Mattyo5 said:
Hi Matthew,
The moderators are not being ninnies about this.
The point of the 3rd prong is to provide a return path to force the mains circuit breaker to trip in the event of a wiring fault or malfunction that puts mains voltage on the enclosure. Without that 3rd prong, a metal enclosure with mains AC in it is an electrocution waiting to happen. Vintage tube guitar amps often lack this 3rd ground prong and they end up zapping a musician every so often. http://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=guitar+amp+electrocution
Modern, commercially made equipment with 2 prong line cords either uses an external power supply (that is double insulated), have a plastic case to prevent finger contact with energized metal or are internally double insulated (like a hand power tool).
I work for a large ($1B US annual sales) corporation designing electronic scales and we use wall warts power supplies for just about all of our low end consumer products just to avoid these sorts of issues. When we bring mains power into the product we use a 3 wire power cord. We could deal with double insulating the product, but the 3rd wire is cheaper and ultimately safer.
For a DIY project, if you need to bring AC mains power inside a metal enclosure, then the 3rd power prong is the only safe way to go. There are other, saner ways to deal with ground loop problems.
Phil
I understand completely Phil, thanks for the explaination though. I'm just warning Paul that if he does start suggesting to do a 2 prong approach that ...well...its not a good idea and that i have had previous experience on this board w/ regards to that suggestion. I understand that the moderators are right on and taking this stand for a reason, and a good reason at that. No arguments here. Paul, be careful in all you do, Phil, thanks for the thorough explaination 
-Matthew K. Olson
actually, phil, question for you...suppose the case is grounded by the 3rd prong, but the center tap (from toroid 2ndary) is not grounded to the case...ie...floating ground...is there anything wrong w/ this? As I see it, if there somehow was a fault that charges the chassis, the voltage would just go right through to the 3rd prong and not hurt anyone....is this correct? thanks!
-Matthew K. Olson
actually, phil, question for you...suppose the case is grounded by the 3rd prong, but the center tap (from toroid 2ndary) is not grounded to the case...ie...floating ground...is there anything wrong w/ this? As I see it, if there somehow was a fault that charges the chassis, the voltage would just go right through to the 3rd prong and not hurt anyone....is this correct? thanks!
Mattyo5 said:
Floating your power supply ground from earth ground is not a safety issue in any way. You will probably want some kind of a bleeder resistor connecting the two grounds to prevent static charge from building up in the circuitry.
I've designed electronics used with vehicle scales which have to survive direct lightning strikes. One of the things we do to help accomplish this is to float the electronics from power ground (actually we tie logic ground to earth ground with a 10K resistor in parallel with a small cap). The issue here is that when a lightning strike occurs, the radiating e-fields around the strike will generate different voltage potentials at each end of the interconnect cable. This is especially likely with vehicle scales since the part that the trucks sits on while being weighed is normally outdoors and the operator interface (the part that is plugged into mains power) can be located upwards of 1000 feet away from the scale.
Floating the power supply ground from earth ground permits any induced potential on the electronics inside the vehicle scale and operator display to equalize to an average value (half way between the potentials at each end) without damaging anything. This potential is then bled off to earth ground via the 10K resistor. If the power supply ground was connected to earth ground then destructive currents would flow through the interconnect cable during a lightning strike and fry everthing.
At the lighting test lab we dumped 6,000 A surges into the equipment to simulate the effects of a lightning strike. Very impressive to watch.
FYI, do you know why cow and horses are more vunerable to lighting than other animals? Their front feet and back feet are too far apart. If lighting strikes sufficiently close, the induced potential at one end of a cow is enough different from that at the other end to cause current flows lengthwise through the cow's body which stops it's heart.
Phil - a endless wealth of useless details at my fingertips -
CowPow!
"FYI, do you know why cow and horses are more vunerable to lighting than other animals? Their front feet and back feet are too far apart. If lighting strikes sufficiently close, the induced potential at one end of a cow is enough different from that at the other end to cause current flows lengthwise through the cow's body which stops it's heart."
My father in law raises cows. Should I visit his farm and install ground straps around his cows in the middle, to be on the save side?
"FYI, do you know why cow and horses are more vunerable to lighting than other animals? Their front feet and back feet are too far apart. If lighting strikes sufficiently close, the induced potential at one end of a cow is enough different from that at the other end to cause current flows lengthwise through the cow's body which stops it's heart."
My father in law raises cows. Should I visit his farm and install ground straps around his cows in the middle, to be on the save side?
Jewilson, you need to read the rest of this thread. I have gotten in trouble w/ moderators saying that you should use an adapter to ...3 prong -->2 prong adapter...to isolate from AC ground. Its not a good idea. If the chassis becomes charged ...ac ground needs to be attached to the chassis so you don't get rail voltage when you touch the chassis. So, as has been said to me, be careful w/ what you suggest. Yes, many components have only 2 prongs, but these are carefully inspected etc. commercial components...VCR's, etc. ...and for us DIYer's....its best to ground chassis to AC ground. Just FYI. Later!
-Matthew K. Olson
Later!-Matthew K. Olson
Corona Cows
"My father in law raises cows. Should I visit his farm and install ground straps around his cows in the middle, to be on the save side? "
Obviously the answer is to have the live stock where corona hats to disipate ground traveling ion fields before they discharge.
Methinks this grooming product must have been attempting the same results:
"My father in law raises cows. Should I visit his farm and install ground straps around his cows in the middle, to be on the save side? "
Obviously the answer is to have the live stock where corona hats to disipate ground traveling ion fields before they discharge.
Methinks this grooming product must have been attempting the same results:
Attachments
Isolation
Mattyo5
A lot of us go to great trouble rid our systems and test equipment from 60Hz interference or hum. Sometime this is the only way except for purchasing transformers in bulk. I don't intend to do that. Also, it's normal for a fuse to blow before you get a hot chassis, you do have fused equipment right.
Sound like a grounding rod for cows and horses won't work either.
Later
Mattyo5
A lot of us go to great trouble rid our systems and test equipment from 60Hz interference or hum. Sometime this is the only way except for purchasing transformers in bulk. I don't intend to do that. Also, it's normal for a fuse to blow before you get a hot chassis, you do have fused equipment right.
Sound like a grounding rod for cows and horses won't work either.
Later
Jewilson, please read this thread
http://www.diyaudio.com/forums/showthread.php?s=&threadid=17566&highlight=
I'm only mentioning this because this is what I have been told. You can argue it out w/ the moderators.
Yes I use fuses. And yes, I go to great lengths to rid my system of hum as well...but removing earth bonds to chassis (not to power supply necessarily) ground is a bad idea.
-Matthew K. Olson
http://www.diyaudio.com/forums/showthread.php?s=&threadid=17566&highlight=
I'm only mentioning this because this is what I have been told. You can argue it out w/ the moderators.
Yes I use fuses. And yes, I go to great lengths to rid my system of hum as well...but removing earth bonds to chassis (not to power supply necessarily) ground is a bad idea.
-Matthew K. Olson
Re: Isolation
Hi jewilson,
The problem is what happens when your transformer or mains wiring develops a short to the chassis of your enclosure. This is not inconceivable, transformer fail (especially when run hot for prolonged periods of time). Wiring can fail, especially if you neglected to use heat shrink tubing on mains wiring connections or did not carefully route and secure the mains wiring to prevent pinch points. Also I don't understand how a fuse could provide any protection from a short in the wiring before the fuse. Fuses are to protect the expensive transformer from downstream failures, not to prevent an electrocution.
If the enclosure is grounded, then a short will result in large ground fault currents and the circuit breaker in your house will trip, protecting you and your loved ones from electrocution. This is how things are supposed to work.
If the enclosure is not grounded, then there will not be sufficient additional current flow in the hot leg to blow a fuse or trip the AC breaker. A GFI breaker may detect the unbalanced currents in the Hot and Return leads and trip, but a traditional breaker will not detect this condition. This means that the full mains voltage will still be present on the enclosure waiting for someone to touch it. Don't forget that the amount of current required to kill is well under 100 mA.
Phil
jewilson said:
Hi jewilson,
The problem is what happens when your transformer or mains wiring develops a short to the chassis of your enclosure. This is not inconceivable, transformer fail (especially when run hot for prolonged periods of time). Wiring can fail, especially if you neglected to use heat shrink tubing on mains wiring connections or did not carefully route and secure the mains wiring to prevent pinch points. Also I don't understand how a fuse could provide any protection from a short in the wiring before the fuse. Fuses are to protect the expensive transformer from downstream failures, not to prevent an electrocution.
If the enclosure is grounded, then a short will result in large ground fault currents and the circuit breaker in your house will trip, protecting you and your loved ones from electrocution. This is how things are supposed to work.
If the enclosure is not grounded, then there will not be sufficient additional current flow in the hot leg to blow a fuse or trip the AC breaker. A GFI breaker may detect the unbalanced currents in the Hot and Return leads and trip, but a traditional breaker will not detect this condition. This means that the full mains voltage will still be present on the enclosure waiting for someone to touch it. Don't forget that the amount of current required to kill is well under 100 mA.
Phil
Phil, Phil. Phil
Look I am not demanding that you are anyone else remove there ac ground returns from there cables. Having said that, I have been doing it for years no ill effect yet. In fact audio equipment has not always come with these AC grounds.
The fact is like I stated if some piece of equipment short, even in the transformer it will pop the inline AC fuse. If the diode bridge fails, it will pop the fuse.
Next you might try working on equipment that has a floating ground while you scope is connected to ground. It will not work.
So if you wire you house with all home run ac out lets you will have ground loops. When you connect equipment to multiple out lets. This is called circulating ground current.
Next yea GFI are great, but put a high current amp on one and see what happens. Also, I never use my amp where it wet.
Look I am not demanding that you are anyone else remove there ac ground returns from there cables. Having said that, I have been doing it for years no ill effect yet. In fact audio equipment has not always come with these AC grounds.
The fact is like I stated if some piece of equipment short, even in the transformer it will pop the inline AC fuse. If the diode bridge fails, it will pop the fuse.
Next you might try working on equipment that has a floating ground while you scope is connected to ground. It will not work.
So if you wire you house with all home run ac out lets you will have ground loops. When you connect equipment to multiple out lets. This is called circulating ground current.
Next yea GFI are great, but put a high current amp on one and see what happens. Also, I never use my amp where it wet.
hum, I just realized that I had a tube amp running without earth. And no way to get one since my old outlet has no ground.
I just went back to my old manuals and .... Could I use an isolation transformer instead of earth ? More specifically, I've still at home one old heavy EI core stepdown transformer. It has jumpers for 1:1 and is rated for 600W (the amp is rated at 2w).
I just went back to my old manuals and .... Could I use an isolation transformer instead of earth ? More specifically, I've still at home one old heavy EI core stepdown transformer. It has jumpers for 1:1 and is rated for 600W (the amp is rated at 2w).
Re: Phil, Phil. Phil
Jim, lose the superior tone, it doesn't make your arguments more persuasive.
There are lots of failure modes that will blow the fuse: shorted power supply cap, bridge rectifer, output device whatever. But if you short an AC hot lead to an ungrounded enclosure then the fuse will not blow, instead the enclosure will have AC mains voltage on it.
The only way a fuse will blow is if you draw more current through it than it is rated for. Unless the return lead also gets shorted to the enclosure, shorting the hot lead to the enclosure will not cause any significant increase in current flow. Ground the case like you are supposed to and the fuse will blow when the hot lead is shorted no matter what is going on with the return. Also you neglected the issue of what happens if the AC wiring before the fuse shorts.
I know that people use a power ground adapter (otherwise known as a "Widowmaker" in the live sound biz) to lift power ground, and get away with it. That don't make it smart or safe. Stop suggesting that dangerous behavior is "OK".
P.S. If you are using a ground adapter with a scope be very careful were you put your hands. I use a 1:1 isolation power transformer when working on gear that doesn't have transformer isolated power supplies (like TV's).
Phil
jewilson said:
Jim, lose the superior tone, it doesn't make your arguments more persuasive.
There are lots of failure modes that will blow the fuse: shorted power supply cap, bridge rectifer, output device whatever. But if you short an AC hot lead to an ungrounded enclosure then the fuse will not blow, instead the enclosure will have AC mains voltage on it.
The only way a fuse will blow is if you draw more current through it than it is rated for. Unless the return lead also gets shorted to the enclosure, shorting the hot lead to the enclosure will not cause any significant increase in current flow. Ground the case like you are supposed to and the fuse will blow when the hot lead is shorted no matter what is going on with the return. Also you neglected the issue of what happens if the AC wiring before the fuse shorts.
I know that people use a power ground adapter (otherwise known as a "Widowmaker" in the live sound biz) to lift power ground, and get away with it. That don't make it smart or safe. Stop suggesting that dangerous behavior is "OK".
P.S. If you are using a ground adapter with a scope be very careful were you put your hands. I use a 1:1 isolation power transformer when working on gear that doesn't have transformer isolated power supplies (like TV's).
Phil
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.