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Old 18th December 2012, 07:28 AM   #1
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Default grounding

I have been thinking (never good) about grounding amplifiers.I always do the safety earth to chassis, but then the active and line go thru the transformer and I use the line as negative. So the Line is isolated from the Earth.
Should the Line be connected to Earth after the transformer, as I assume they are connected back at the power station etc. It sounds dangerous...
Some thoughts on Grounding, Earth and Line
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Old 18th December 2012, 07:46 AM   #2
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Hi bullpeters,

your use of "active" and "line" is a bit confusing to me but then English is not my first language.
I assume, you are wondering how to ground your power supply after the transformer?!?
The ground of your power supply is basically identical with signal ground.
It is a good idea to have a single point where this potential is referenced to safety ground level. This is a single connection to your chassis but does not necessary have to be at the same spot where safety ground is connected.
In most cases, it is adventitious to have some distance between the two connections.
I have read, that many people use the input as point for grounding the signal circuit.
I have very good experience using the negative terminal of the last filter cap of the power supply as point for a star ground and having a single wire going from there to the chassis. This chassis connection is separated by some distance from the safety ground connection (green/yellow here in Germany; connected to the chassis by a low resistance bond i.e. star washer, etc.). This way you are sure, your signal is actually referenced to ground level.

There are a few threads about avoiding ground loops etc here on the forum.
So, I am sure you will find a plethora of information if my advice is too convoluted.

Cheers,
Martin
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Old 18th December 2012, 08:21 PM   #3
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LINE NEUTRAL EARTH. Sorry very tired at present
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Old 18th December 2012, 09:43 PM   #4
DF96 is offline DF96  England
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Neutral may be connected to earth at the substation, but you should not connect it at home as huge currents could flow especially under fault conditions. You should regard neutral as being potentially almost as dangerous as live. It may have highish voltage, or it may get swapped with live.

Live and neutral go to the transformer primary, via switch and fuse. Ground/earth goes to the chassis (and the transformer frame). The signal ground will also connect to the chassis somewhere.
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Old 18th December 2012, 11:01 PM   #5
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And it's ok to connect the signal ground to the chassis cause its isolated from "neutral" by the transformer???
Thanks
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Old 19th December 2012, 12:07 AM   #6
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Kind of, Neutral should be viewed as a live conductor (The names are cannonically Line, Neutral and Earth, with both line and neutral being considered to be live conductors).

The way this is supposed to work is that both line and neutral are isolated from your chassis and connect via a fuse and possibly switch and filter, to the primary side of the transformer. You do NOT make any connection between these conductors and the case (No not always strictly true, but if you are asking the question you dont want to go there).

The safety earth is connected directly to your chassis and (directly or otherwise) to the main neutral bar back at the utility substation, in normal use it carries no current to speak of, but if there is a fault to chassis then a large current can flow blowing the fuse quickly.

Neutral must never be connected to chassis unless you **REALLY** know what you are doing as it can become live under fault conditions (Hence why it is viewed as a live conductor).

It is instructive to consider what would happen if the neutral was connected to chassis and the neutral and earth came adrift in the socket, current would then flow from the live, through the transformer and would then flow to the chassis, making it live relative to earth, this condition would be potentially deadly.

The safety earth serves two purposes really, firstly it provides an independent low impedance path to ensure that a live chassis fault will blow fuses or trip breakers in a fraction of a second, second it can limit the voltage reached by the case in the event of a live chassis fault, this connection is important. It is separate (within a consumer appliance) so that a fault in the neutral cannot cause a dangerous condition.

Regards, Dan.
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Old 19th December 2012, 12:31 AM   #7
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Ok, that contradicts the other posts , which state o voltage signal (neutral?) be connected to the chassis. Not being a pain here but trying to understand the Correct way.its not my first rodeo but the more I know the less I understand, or something like that
Mick
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Old 19th December 2012, 07:48 AM   #8
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No, there is no contradiction. You are confusing "neutral" of your power line with "signal ground" of your power supply (at least that is what I think where the apparent contradiction comes from).

I was not aware of which side of your power transformer we are talking about. To make things clear:

On the primary side: As stated above by others, both "line" and "neutral" go to the transformers (via filter, fuse, power switch). ONLY safety earth is connected to the chassis here. Use a low resistance (star washer), fail-safe connection for that.

German law requires that line and neutral use shorter wires to go to the first connection than safety earth inside the chassis if you do not use an IEC plug. The idea behind that is, in case of an external force on the power cable and should the cable anchorage fail, safety earth will be the last cable to be disconnected.
No idea how the requirements are in other parts of the world.

Now, on the secondary side of the power transformer the situation is different.
Here, you would like to have your power supply 0-rail grounded (that's why it's called "ground"). That is what my first post was referring to.

I hope this makes things clearer.
In any case, I would strongly advice to pick up a standard textbook on electric installation and safety regulations of your country.
Internet forums (even excellent ones like this one) are just not a good source for information regarding safety issues.

Best,
Martin

Last edited by bayermar; 19th December 2012 at 07:56 AM.
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Old 19th December 2012, 07:59 AM   #9
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Thanks Martin
I only ask this question after reading " building valve amplifiers.
Thanks for all your input. I have been previously attaching the ground to the chassis as close as I can, star washer etc. I then attach the neutral after the transformer to the earth at the first filter cap. I don't really "understand" why, hoping to learn why. Thanks for all your help
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Old 19th December 2012, 09:13 AM   #10
DF96 is offline DF96  England
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Quote:
Originally Posted by bullpeters
I then attach the neutral after the transformer to the earth at the first filter cap.
No! You are getting three quite different things mixed up.
Neutral: part of the incoming mains supply - only goes to the transformer primary
Earth/ground: safety connection for the chassis
Signal ground: part of the circuit - will usually be connected in one and only one place to the chassis

Get the first two wrong and you have serious safety issues. Anybody who tells you to connect the incoming neutral to chassis is an idiot and must be ignored. If you have misunderstood what they say, then maybe you are not yet ready to build mains-powered equipment. It is not enough to blindly follow a rule; you need to understand what you are doing in order to keep you and your family safe.

Get the last one wrong and you get hum/buzz.
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