Life safety question

[Benedict]

Hi all, I'm designing a PSU(s) for my audio amps, and I'm wondering about isolation and earthing. I've seen people leaving audio commons floating, and I've seen earthed grounds, and ground loop breaker elements etc. If I earth the amp case and signal grounds, I risk ground loops, which can be resolved. However at the same time, the PSU "live" output pin becomes earth referenced, as the other psu pin is connected to the amp case in the amp itself (now earthed), so the psu becomes 21 volt ac earth referenced, and any amp output becomes earth referenced too. Can't touch this, I think.
If I leave it all floating, it's all good, you can (stupidly) lick the contacts and the case all you wish, there's almost no way ever the amp will ground something inside due to fault, and I think this is the way it's supposed to be. However, I don't trust the transformer with my life. It's new, it's toroid, it's fused, it's protected with transils, but in the event that somehow primary live wire comes into contact with secondary, the case or the contacts could become 230 volt. In that case, would the thing I drew down there save my life? Would it somehow hurt the amp while working normaly? The ground in the pic is earth ground.

Thx for the replies, they might save someone's life. 

 


Important edit : the ground is always earth, I just drew it on neutral cause I have MEN - my ground and neutral are together on the panel. Reversing plug pins won't do anything.

[Benedict]

Whoops, green transils should be a bit higher voltage, this way they'd actually clamp the thing all the time. The idea is that the tsv's will surge current to earth ground so that the circuit breaker at the panel activates and stops it. If it's a normal situation, no current should flow through them, as there is no circuit. I'm also not sure if this setup would actually somehow reference everything to earth again or cause some other unnecessary things. This is relatively low voltage, only 20 volts ac, but I have another amp that does +/- 30 volts dc, so it applies to that too. I wouldn't want to touch 30 volts of referenced DC, let alone 60.

[calzap]

Are any of the inputs of "inputs/outputs" likely to  derive power from mains?  If yes, then there may be a "back door" to the supposedly isolated side.

Mike in California

[Benedict]

Thx for the response! Good thinking there. The only inputs will be 1) an usb connection from the PC into DAC (built into the same amp). I believe it's shield isn't connected to anything in the amp, but not sure. Data pins certainly are though... i have no clue if that matters. power pins are connected too, as they give power to dac even if the amp is switched off. It certainly gets its power from the 5V rail of the pc psu through the usb controller... The other potential input comes from battery powered devices, so it isn't an issue. Outputs are into headphones and another amp, speaker amp, which I planned on isolating in this same way.

[dom0]

an usb connection from the PC into DAC (built into the same amp).

• Your device is now earth-referenced

A lot of consumer gear is a class II appliance (not earthed), to avoid / lessen ground loops. These are however often the result of poor internal construction of the device and not inherent per se (although the often used symmetric interconnections in pro gear does help a lot here).

However if you aren't proficient with mains voltage and how to built stuff safely (e.g. you're not killing the device operator if a wire comes loose or a screw gets stuck somewhere) I strongly recommend to earth the secondary side of your PSU. Ground loops will likely not be an issue, if your device is constructed correctly.

[Benedict]

You sure about that? When I probe the amp's case, outputs, inputs, shields, it shows 200 volts AC on the scope. And I don't die when touching it. It must mean that, even though the usb is inserted, the amp is still floating, right? Am I missing something? :/
Also, while earthing the case gives a good amount of protection, as I said, it actually leaves potentially up to 30 volts AC unprotected on speaker cables, waiting for someone to touch them. While running them at that voltage would probably tear up the building first, it's still possible - not cool at all.

Any thoughts on using the stuff I drew?

Thx for the response!

[DimitriP]

One of us is misunderstanding the concept of a "floating ground" .
What you are doing has a lot of potential (!) for disaster if the polarity of the plug is reversed.
More reading here: https://en.wikipedia.org/wiki/Floating_ground


Pictures worth a thousand words and all that...here is one:
https://en.wikipedia.org/wiki/Floating_ground#/media/File:Instruments_with_floating_ground.svg

[Benedict]

Nah, we use 3 pin connectors, live, neutral and earth.

My ground is always earth, the third pin, but neutral and earth are together at the panel - also known as multiple earth neutral, or men for short. That's why I drew the earth on the neutral. Don't worry, reversing polarity on pins won't do anything.

Also, floating ground will float on the rms of the voltage your transformer uses (sure it might not, but usually it does), this is normal. When checking with a scope, for example, a 5 volt DC psu for a phone, you will see that both the + and - rails are actually floating on about 200 volts relative to ground, looks like normal line ac,  but there is a constant 5 volt difference between them. The volts won't kill you because for current to flow, there must be a circuit, and at the moment you touch the - rail, the + rail is only 5 volts dc relative to you and earth.

Thx for noticing that, if I tried to do this on just two pins, I'd be very much dead

[Benedict]

an usb connection from the PC into DAC (built into the same amp).

• Your device is now earth-referenced

A lot of consumer gear is a class II appliance (not earthed), to avoid / lessen ground loops. These are however often the result of poor internal construction of the device and not inherent per se (although the often used symmetric interconnections in pro gear does help a lot here).

However if you aren't proficient with mains voltage and how to built stuff safely (e.g. you're not killing the device operator if a wire comes loose or a screw gets stuck somewhere) I strongly recommend to earth the secondary side of your PSU. Ground loops will likely not be an issue, if your device is constructed correctly.

Are any of the inputs of "inputs/outputs" likely to  derive power from mains?  If yes, then there may be a "back door" to the supposedly isolated side.

Mike in California

Double checked, and indeed, plugging the usb does force the amp's case into earth. Problem solved, as now it's clear that I'll have to earth my speaker amp too, so I'm earthing the headphone amp as well, to keep all the cases and shields on earth. I'm really not happy about naked 30 volt dc connectors rel. to earth around, but whatever, it's still considered "safe"... I guess

Thanks.

[Benedict]

Agreed. It's really rare, but it does happen. Why not add a little bit extra safety just in case? It costs next to nothing. I just learned that I do have to earth ground everything anyway so it makes no difference now. Ground loop debugging awaits, I have all the confidence they will be there  I'll also have to figure something out to insulate the naked contacts on the speakers. Someone could really easily touch 30 volts dc due to their build and position. Probably won't do a thing, but you never know, sweaty hands, etc.. Better not to take any chances, right?
Thx for the info though, makes me feel a little bit easier regarding the other 2 pin wall warts.

[Benedict]

I'm using a grounded PC, so I hope all will be ok, it's just my experience to expect the worst all the time. I've done some high voltage stuff in the past, got struck by tesla coil primary, 3.5 kV @50mA, didn't do squat to me (my skin must be really proud, around 1 MOhms usually) but since then I always triple check anything remotely related to mains. Even if it wont kill me, it might hurt someone else. Better to be unnecessarily cautious than unnecessarily not. Still, many thanks for your time and information, much appreciated

[CatalinaWOW]

If safety is paramount, you have missed several opportunities.  You could transformer couple the earphones.  Put signal isolators on all of the inputs.  Don't connect the USB 5 V line.  Perhaps even eliminate USB functionality because of the increment of risk it provides.  You could put two or more layers of insulated housing around your amp, and design/specify fully isolated connectors eliminating the possibility of conductive contact with fingers.  Don't forget the tamperproof fasteners on these cases.  You could even put a second isolation transformer in series with your primary.

Some of these will affect sound quality.  Some will degrade function.   All will affect size, cost and weight.  As several have said your initial proposal has what most of the world has agreed are a reasonable set of safety precautions, but you continue to state that safety is more important to you than for most.  You will have to decide what is enough for you.

[Benedict]

 
also, put the headphones in plastic bags/cans and drill small holes for the sound

Alright, alright, I was paranoid, but you have to understand, I still think it's better to learn not to be, than to be. Things you just wrote are perhaps a bit too much. Ensuring that touching the leads doesn't harm you, and that transformer doesn't melt and do so is something else I think (actually, I've read about it on the net, so I got extra cautious). I literally had no concrete idea about proper audio grounding practices, especially since people argue about that on the net it seems. It's rare to see people in agreement. Was it so bad to ask about it? In the end, I'll just ground everything to earth and be done with it.

(lol I like the idea of transformer isolated headphones though. Maybe mount the trafo on the top of the headband, so that the EMI affects both channels equally? naturally, it'll be in 3 plastic cases, and with antennas protruding out, exactly 45 degrees opposite in the medio-sagital line)

[Benedict]

I didn't know about them until yesterday. Great things. Don't think we have them, I really should get and install some.

Edit: I have a three phase system, and my panel can't tell the returns from one another, so unless there is some three-phase version of that I'm afraid I can't use those. They check the current going live and returning, and on the smallest difference switches the circuit open, right? Well, on my panel, there are many live wires and one common neutral for all of them, so no go.

[dom0]

Even in case of a xfmr failure, leaked current will be grounded by USB connector, then trigger your GFCI.

Either you use a transformer that is basically "must not fail" (qualified for class II appliances) or you don't and then you do class I properly (fault current return via USB is not properly).

[Benedict]

Even in case of a xfmr failure, leaked current will be grounded by USB connector, then trigger your GFCI.

Either you use a transformer that is basically "must not fail" (qualified for class II appliances) or you don't and then you do class I properly (fault current return via USB is not properly).

This right here.
My question was basically if the diagram I drew, putting a bi-directional tsv over both transformer outputs and earth, could be considered as safe as earthing the case.

[dom0]

No, at least not by the standards I know. I have to look it up for exact numbers, but the thrust is basically that no larger touchable parts (e.g. a screw doesn't qualify, external heatsink does) can have more than a few volts AC on them while the protective earth of the device is sinking 20 (25?) amps or x*nominal current (whichever is greater, and x was something like 5 or so). That's why PE wiring in devices is done with the same wire gauge as all other mains wiring. (Also, there is another standard mandating that PE cross section may not be smaller than the cross section of power carrying wires in the same circuit).

Edit: Obviously I can only speak of the rules effective in my country. They are generally considered to be the safest and clearest, while some people obviously say that you'll often get away with much less than mandated by these rules, which is not wrong per se.

[Benedict]

Right, ok, thx. I have no idea why it wouldn't be safe, and keep the system isolated at the same time. Transils won't conduct at all until their lower voltage is met, so basicly until then it's an isolated device, IF putting them there doesn't reference the lead to earth, which i THINK it shouldn't. If somehow a mains comes into contact with either of the secondary wires, transils instantly open and surge current to earth, popping a fuse or a breaker. However, that's only a theory I wanted to talk about, as said, I'm earthing everything, and not playing smart with these things. Just wanted to check if it's plausible. Thank you all.

[dom0]

A 26 V TVS will blow up pretty much instantly if you put 230 V across it*. It might survive under very specific conditions, though (i.e. sensitive and fast RCD/GFCI and short begins at low voltage point)

* which may or may not trigger a RCD. A devices safety should not depend, however, if a RCD is present or not. The purpose of a RCD is to make mains safer and avoid accidents, not to allow people/manufacturers to skimp on the safety of their designs.

[Benedict]

A 26 V TVS will blow up pretty much instantly if you put 230 V across it*. It might survive under very specific conditions, though (i.e. sensitive and fast RCD/GFCI and short begins at low voltage point)

* which may or may not trigger a RCD. A devices safety should not depend, however, if a RCD is present or not. The purpose of a RCD is to make mains safer and avoid accidents, not to allow people/manufacturers to skimp on the safety of their designs.

Thanks, that's what I needed.