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Need advice on powering and grounding DIY case
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Need advice on powering and grounding DIY case
tema
Hi! I’m building my first DIY plywood case now, 6U/84HP. And I need some advice about powering it.

I’ve already ordered a power supply, Mean Well PD-2512, which has +12V, -12V and COM (0V) outputs and I want to power up and ground my system as good as I can.

One of the possible problems is that I live in Russia, and usually our power outlet ground wire isn’t connected to anything resembling ground, so some local engineers advised me not to use it at all in any way.

So, my power supply have:
1. L/N inputs
2. -12V output
3. +12V output
4. COM output
5. Mounting holes that should be connected to chassis (as far as I understand they mean power supply chassis, which I don’t have) grounding according to power supply datasheet

And my supply doesn’t have an input pin to connect to a power outlet ground.

Also, according to the datasheet COM output is internally connected to the power supply ground via resistor and capacitor in parallel.

So, everything seems easy with AC inputs and +-12V outputs, but what do I do with a COM pin and ground holes?

I’ve read somewhere out there that COM output should be connected to ground somehow. But I don’t understand, what in this case counts as ground (assuming that my power outlet isn’t grounded and its ground isn't connected to PSU)? Is existing connection via resistor and capacitor to power supply ground is “enough”, or I need to connect COM output to somewhere else manually?

Also I’ve read somewhere that front panel should be grounded somehow. So, do I need to connect a wire from power supply ground hole to the rail? Or what should I do?

One of mine concerns is that I want to buy modules such as Pressure Points by MN in future, and as far as I understand they don’t usually pair well with Mean Well power supplies because of some grounding issues, so I want to do everything as right, as it possible.

I would really appreciate if someone would explain this topic to me and tell me what to do, because while there is plenty of useful info in another threads, I don’t know how well all of it applies to my case, keeping in mind my power outlet problems and this particular PSU specifications, such as lack of ground input pin and presence of a connection from COM to ground.

Thank you!
Graham Hinton
tema wrote:
One of the possible problems is that I live in Russia, and usually our power outlet ground wire isn’t connected to anything resembling ground, so some local engineers advised me not to use it at all in any way.


That isn't very useful advice. If you do not connect equipment that is intended to be grounded to a proper Ground/Earth then you lose the safety feature and you may be making something that is potentially lethal.

Can you find out exactly what the wall outlet ground pins are connected to?

I believe that you have type C and type F mains plugs, like in Europe, which can both reverse Live and Neutral. This means that you should double fuse and have a double pole switch on your case mains input.

Quote:

And my supply doesn’t have an input pin to connect to a power outlet ground.


That PSU has a Frame Ground (FG) input via its mounting screws for connection to a protective cover. This is to ensure that a fault will blow a fuse.

Quote:

Also, according to the datasheet COM output is internally connected to the power supply ground via resistor and capacitor in parallel.

So, everything seems easy with AC inputs and +-12V outputs, but what do I do with a COM pin and ground holes?

I’ve read somewhere out there that COM output should be connected to ground somehow. But I don’t understand, what in this case counts as ground (assuming that my power outlet isn’t grounded and its ground isn't connected to PSU)?


If you don't have a source of Mains Earth you can't ground anything. That sort of PSU would normally have a metal cover that should be grounded so that it cannot ever become live under fault conditions.

Quote:

Is existing connection via resistor and capacitor to power supply ground is “enough”, or I need to connect COM output to somewhere else manually?


That is just to give it a default reference, but if you don't have a Ground/Earth it will just be floating. It is best to connect the mains Earth directly to the 0V of your power distribution, rather than to the PSU COM output. The two ends of the wire between COM and 0V will be at different voltages due to the PSU current. If the COM is grounded then the 0V voltage will change according to the 0V current.

You will be touching 0V on every patch cable screen. If you don't ground the 0V it could become live under a fault condition. How do you feel about grabbing a cable one day and getting electrocuted? Or watching a child or pet die due to your work? That is how serious this is.


Quote:

Also I’ve read somewhere that front panel should be grounded somehow. So, do I need to connect a wire from power supply ground hole to the rail? Or what should I do?


Correctly designed audio equipment should have a grounded chassis including the panels and any bare metal that can be touched. Eurorack is not a correctly designed system. The rails and panels are often insulated and jacks randomly connect 0V to panels. The panels don't make a good enough contact with the rails, if at all. Anodised aluminium is an insulator, but it may be accidently pierced. You can't redesign every module, so it is best to leave it, just ground the 0V at the distribution.
tema
Graham Hinton
Thank you very much for the reply! Everything is much more clear now.

Quote:
Can you find out exactly what the wall outlet ground pins are connected to?


Well, usually it's just floating, sometimes it's the real earth. As far as I know in a place I live most of the time it's floating.

So, do I get it right, it's best to connect mains earth to both FG at power supply and 0V at distribution board, and hope that the outlet is really grounded this time?
Rex Coil 7
tema wrote:
Graham Hinton
Thank you very much for the reply! Everything is much more clear now.

Quote:
Can you find out exactly what the wall outlet ground pins are connected to?


Well, usually it's just floating, sometimes it's the real earth. As far as I know in a place I live most of the time it's floating.

So, do I get it right, it's best to connect mains earth to both FG at power supply and 0V at distribution board, and hope that the outlet is really grounded this time?
I thought you said you didn't have a "mains Earth"?

And ...

tema wrote:
... So, do I get it right, it's best to connect mains earth to both FG at power supply and 0V at distribution board, and hope that the outlet is really grounded this time?
In your situation you shouldn't be hoping anything, you should be measuring to be absolutely certain of the limits and resources of what's going on and where you are.

Read what Graham said carefully, especially the part about how Eurorack isn't properly designed (from a safety aspect). Add it all up, take a look at this issue:

* You intend to make a power system for a device that wasn't designed properly from the get go (safety wise).

* You're not sure if you have an Earth ground or not.

* The entire device (Euro modular synthesizer) is literally covered in conductive material (metal).

* The entire device (Euro modular synthesizer) is literally covered with the hands of the operator during it's operation nearly the entire time it is powered up.

I see 4 issues here that are deeply intertwined that are a sure bet for deadly failure, if the proper precautions are not observed.

Take readings.

Write down your measurements.

PLEASE BE CAREFUL!
tema
Rex Coil 7 wrote:
I thought you said you didn't have a "mains Earth"?

I said, whether I have a mains earth depends on where I'm going to use my modular.
I can be sure whether I have ground or not at each time, but I know that I wont have an earth everywhere I go.
So I want to try to design my powering the way it'll work best for the both situations.
Rex Coil 7 wrote:
PLEASE BE CAREFUL!

Thank you. I'll be careful.
cornutt
My system is completely ungrounded. I know, I know. But (1) I have a wood case with wood rack rails, so panel grounding would be hit or miss anyway unless I did a lot of work to tie everything to the return, and (2) I never have ground loop problems this way. I have the system plugged into a power strip that has a GFCI (RCD to you Brits and Aussies), to take care of the safety aspect.
tema
cornutt wrote:
I have the system plugged into a power strip that has a GFCI (RCD to you Brits and Aussies), to take care of the safety aspect.

Wow, I actually never heard of GFCIs, I've only seen simple circuit breakers. Sounds like it's a good thing for safety.
So I'm thinking now, maybe it's a good idea to implement GFCI inside a eurorack case?
Rex Coil 7
Ground loops cannot exist without a ground connection ... at least to SOME sort of ground (when I say "ground" I mean actually attached to Earth .. not to Zero Volt).

Hoo boy, this thread is going to be a bucket of fun.

Hmm ... I think "I'm out" on this one. I'll just spectate from here on out.

Please, carry on.
Graham Hinton
tema wrote:

I can be sure whether I have ground or not at each time, but I know that I wont have an earth everywhere I go.
So I want to try to design my powering the way it'll work best for the both situations.


The thing that makes that difficult is that you touch the 0V on patch cables and so that should be grounded. Appliances, and I mean consumer stuff rather than audio equipment, that are not grounded are double insulated instead. How are you going to insulate all your plug sleeves and jack bushings for when you are using it ungrounded?

If you get a single mains fault in grounded equipment through a worn or loose wire or melted insulation and it touches the ground a fuse or trip will blow. If you get a single mains fault in ungrounded equipment the 0V may become live and it will remain working until you touch it and get a shock. Working on a synthesizer there is a high risk of that going from hand to hand with your heart in the middle. Which will stop.

Quote:
I actually never heard of GFCIs, I've only seen simple circuit breakers. Sounds like it's a good thing for safety.
So I'm thinking now, maybe it's a good idea to implement GFCI inside a eurorack case?


You should definitely put one on your distribution board. Note that they have different current ratings and trip when the difference between Live and Neutral exceeds that. In the UK every house has to have one that trips on 30mA difference. There has to be a margin to allow for leakage in mains filters. For just your system a lower value would be better, but also beware that you have to get a shock first to trip it whereas a grounded system would trip as soon as the fault occurs. If you do have an RCD on your mains board you would need a smaller value so that you don't trip your whole house. I have no idea what the practise is in Russia, but using Schuko plugs is not an auspicious start. Even with a regular maintenance check for worn and damaged cabling I wouldn't be happy working with that. How badly do you want a Darwin Award?

cornutt wrote:
I have a wood case with wood rack rails, so panel grounding would be hit or miss anyway unless I did a lot of work to tie everything to the return, and (2) I never have ground loop problems this way


You wouldn't. That is like saying "I have a foil lined hat and I haven't been abducted by aliens".

Ground loop problems with synthesizers are rare because modules are mainly unity gain. Other ground related problems are really common impedance coupling and nothing to do with a loop apart from the fact that all electronic circuits are loops--that is why they are called circuits. Loops need gain and are more usually found associated with phono pickups, mic amplifiers and power amplifiers. Loops also need area to pickup magnetic fields and should not be solved by making breaks in grounding, but by flattening the loop so it has no area.
glennfin
Hope I'm not "stealing" this thread by chiming in, and I'd love to hear Graham's take on this...

My large Eurorack system is powered by two Astron RS20 linear power supplies with the outputs connected to provide +/-12v. The power supplies use a stepdown transformer so the 110vac mains are isolated from the low voltage secondaries. Inside the supplies, there is one wire that connects the primary mains earth input ground to the minus DC output of the supply. I removed this connection so the outputs of the two supplies could be connected to provide the pos and neg rails for the system.

Since the two supplies are outside the rack, transformer isolated, and there is no path inside the rack back to the 110vac mains earth/ground, I don't see how this setup could be a safety issue.... EXCEPT... what if a mains powered device external to the rack is connected? (signal processor, mixer, etc) (EDIT) = that's the issue... I'm going to change over to a dual output supply where 0V can be connected to earth/mains ground... now to find one with enough current for my large system. meh

Any thoughts?
MAXTHEDOG
Hi Tema,
Are you saying that in Russia you can't automatically assume that mains wall sockets have an earth conducter?
+1 to what Graham has already said.
You could always take your own 'earth' connection - a copper spike about a metre long, hammer it as far into the ground as you can and connect your modular PSU cable to that. Not terribly practical I admit plus unless you check the earth loop impedance (google it) you won't be sure it'll give you enough of a safety margin. But it'll be better than none.
I'm only half joking, because it sometimes needs to be done (rarely now) to get the resistance of the earth path down to a low enough value that if a fault should occur enough current will flow to take the fuse out to stop you getting a mains shock. They are not nice, believe me. very frustrating
You need at least an RCD, that goes everywhere your synth does. 30mA tripping current, absolutely no more. And test it regularly.
Regards,
MTD.
Graham Hinton
glennfin wrote:
Since the two supplies are outside the rack, transformer isolated, and there is no path inside the rack back to the 110vac mains earth/ground, I don't see how this setup could be a safety issue.... EXCEPT... what if a mains powered device external to the rack is connected? (signal processor, mixer, etc) (EDIT) = that's the issue... I'm going to change over to a dual output supply where 0V can be connected to earth/mains ground... now to find one with enough current for my large system. meh


You've worked it out. If something external is connected that has a fault and has become live no fuses will blow if your system is not Earthed so your system becomes live too.

You might also have a fault in your PSUs that could make its outputs live. Something like overheating the transformers so the insulation melts and a secondary connects to a primary. This is particularly risky when connecting other un-Earthed equipment, especially if it is powered with wall warts that are easily damaged.

You have to be a Devil's Advocate when considering safety and we are not talking about faults that might be theoretically possible, most are well known and happen too frequently. All the ones I've mentioned I have actually seen.

MAXTHEDOG wrote:
You could always take your own 'earth' connection - a copper spike about a metre long, hammer it as far into the ground as you can and connect your modular PSU cable to that. Not terribly practical I admit plus unless you check the earth loop impedance (google it) you won't be sure it'll give you enough of a safety margin. But it'll be better than none.


Actually not, both are very dangerous. You don't get safety just by connecting something to Earth, the safety comes from having a breaker board and the Neutral being bonded to Earth at the power entrance to the building. That means that the Live sees either Neutral or Earth as a return path. It normally all returns via a load and the Neutral wiring, but if it connects to something grounded it will draw too much current and blow a fuse or trip.

Making a second ground rod connection is extremely dangerous. These are sometimes called "clean" grounds which is nonsense, they are not. If you have a close lightning strike there will be a potential gradient in the ground between the two rods of tens or hundreds of thousands of volts. You will be bringing that PD inside your room in close proximity with your gear inbetween. If you have no Earth on the wall outlets the neutral is still Earth referenced and you will have both wires inside your equipment. Your synthesizer would be a mess of fused metal and plastic, you would be a pair of smoking boots.
glennfin
So....... I have a power supply that I previously built for my former smaller system using two open frame 12v supplies. The positive from one supply tied to the neg of the other to make the 0v rail. It worked great, never had an issue, problem, etc.. I opened it up to see how it was wired and the 0v rail is in fact tied to mains earth/ground at the entry point.

So last night, on my present dual Astron supply setup, I went ahead and made the connection from the 0v rail to the mains earth/ground at the entry point. Hey! it didn't blow up lol .... working great.. I think I'll keep her.. Guinness ftw!

I should also mention that the Astron's are crowbar over voltage and over current protected, and of course fused.
thegrinch
thegrinch
glennfin
Oh no..... text not posting.....
glennfin
glennfin
Something in the quote prevented the text from showing.... strange..
thegrinch
tema
So, many thanks to all of you! Learned a lot of quite interesting things here.
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