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Power supply internal grounding

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FoldingAddict

Intel Fanboy
Joined
May 10, 2002
Location
Montgomery, AL
Simple question that I can't seem to find any information on after about an hour of googling.

Doing an aquarium build (for the lols) and I'm not sure what to do with the ground (3rd prong) on the power connection to the PSU. I bought a new Corsair RM750X for the build and disassembled it. Inside the PSU there was a brown wire and blue wire going to the mainboard of the PSU and a green ground wire screwed down to the chassis of the PSU. Question is, is the green ground wire necessary and if it is, where can I ground it if I'm running the PSU outside of its chassis.

Thanks a bunch
~FoldingAddict
 

Ben333

Folding for Team 32!
Joined
Feb 18, 2007
If you're using a metal motherboard tray then I'd attach it to that. You may want to attach it to the PSU's DC ground (any black wire usually) for a number of small reasons that may or may not (IMO not) matter... And I think doing so would be harmless since I believe the two often meet inside the typical PC chassis, but I could be wrong on that. If you just discard that wire you'll have a floating ground on your supply. Which is fine.

If you're not using a metal tray for the MB then I'd just simply not use that AC ground wire. There's also the option of cutting the front and back off the base of your power supply's case, leaving just the bottom. Then the PCB still gets to reside on standoffs and the dangerous AC side of it has a cover over it in case anything were ever to touch it. If you did that, obviously that's where the ground goes (probably already is)
 

ATMINSIDE

Sim Racing Aficionado Co-Owner
Joined
Jun 28, 2012
Yes, it is necessary to ground the system for safety.
Connect the ground to the chassis of the system and/or the motherboard tray.

Typically the PSU is internally grounded to its casing, which is screwed in to the case in turn grounding the whole tower.
 

Ben333

Folding for Team 32!
Joined
Feb 18, 2007
The point is, if there is no "chassis" then is it better to connect the AC ground to the DC ground? Or just leave it unhooked. I do not know the correct answer. I believe in a normal PC though, they are indirectly connected.

Keyword aquarium build. No steel casing at all. (Unless he is using a MB tray)
 

ATMINSIDE

Sim Racing Aficionado Co-Owner
Joined
Jun 28, 2012
No, you find something metallic to connect it to, you already have the AC to DC ground transferred inside the PSU.
Even if you just make a plate to go behind the motherboard for the sole purpose of grounding.
 

Ben333

Folding for Team 32!
Joined
Feb 18, 2007
Incorrect. AC to DC switching computer power supplies have a floating ground when the internal AC chassis ground is disconnected. I know this because I've seen people connect the DC ground of one supply to the +12V DC line of another, in order to use two 12V supplies to get 24V. This would not be possible if they were already connected. Most aquarium (I'm thinking oil filled) builds I've seen do not have any metal sheets in them, unless you have a MB tray or keep part of the PSU casing.

My logic is that the MB probably connects the power supply's ground to its metal contacts on the PCB (the shiny spots where the screws go), and the PSU obviously does the same to it's case, and that's where the AC ground is... And either way, the MB standoffs and the power supply chassis are electrically connected because they are touching each-other. So it may not be "wrong" to connect the AC ground to the DC ground in a scenario where there isn't any metal shielding / MB tray, or PSU casing.

If there's no MB tray, maybe somehow attach it to the IO shielding :shrug: Or make the lead longer and bolt it to a single mounting hole on the motherboard or PSU (which has the metal plating around the screw hole)
 

JrClocker

AKA: JrMiyagi
Joined
Sep 25, 2015
The third prong on the AC input is "earth ground".

Modern switching power supplies have their DC ground isolated from the earth ground.

In a proper circuit, the earth ground is not needed and is simply there for safety purposes. However, as components age in a circuit, the circuit becomes "less proper". The earth ground protects YOU from a malfunctioning piece of hardware. As your power draw goes up, the risk of an unsafe potential increases as well.

Find a place to connect the earth ground.

If you choose not to connect the earth ground, then you MUST connect this unit to AC power through a GFI breaker. The GFI breaker will pop when the current coming "in" on the AC hot does not match the current going "out" on the AC neutral.

Anything less is just stupid as you have a significant shock risk and fire hazard.
 
OP
FoldingAddict

FoldingAddict

Intel Fanboy
Joined
May 10, 2002
Location
Montgomery, AL
Thanks guys good information here. I will probably end up cutting the psu casing so I can use the bottom and ground to that somewhere. All my parts are acrylic or glass (no metal motherboard tray) so nothing to mount to there.

Thanks again
 

Ben333

Folding for Team 32!
Joined
Feb 18, 2007
The third prong on the AC input is "earth ground".

Modern switching power supplies have their DC ground isolated from the earth ground.

In a proper circuit, the earth ground is not needed and is simply there for safety purposes. However, as components age in a circuit, the circuit becomes "less proper". The earth ground protects YOU from a malfunctioning piece of hardware. As your power draw goes up, the risk of an unsafe potential increases as well.

Find a place to connect the earth ground.

If you choose not to connect the earth ground, then you MUST connect this unit to AC power through a GFI breaker. The GFI breaker will pop when the current coming "in" on the AC hot does not match the current going "out" on the AC neutral.

Anything less is just stupid as you have a significant shock risk and fire hazard.

Good information, and not a bad idea with the GFCI suggestion. Although shy of water contacting the AC side I can't foresee a scenario where this would offer protection from something that would actually happen.

Also, I thought maybe earth ground and the supply's DC ground were separate, however think of this; When you measure with a DMM between +12V and the steal chassis, you'll get a reading of 12VDC. Also, if you were to connect a fan to the 12V, and attach the ground to a steel part of the chassis, the fan will run. So somewhere the two do meet each other. Or is there another explanation for why my observations are the way they are?

Basically I ask this because I'm wondering if when lacking the AC ground (3rd pin, earth) are you at a greater risk for damaging the system with ESD when you physically touch the IO, or plug something into the MB? Or is the PSU's floating ground capable of discharging that ESD from your fingertip and keep the system safe?
 

JrClocker

AKA: JrMiyagi
Joined
Sep 25, 2015
Good information, and not a bad idea with the GFCI suggestion. Although shy of water contacting the AC side I can't foresee a scenario where this would offer protection from something that would actually happen.

Even though the case is not metal, there is LOTS of metal within each computer component. A capacitor is formed every time you have 2 or more pieces of metal separated by an insulator. The capacitor will "charge" (and "leak") over time. When this capacitor gets shorted (for any reason), then there will most likely be an imbalance between the hot and neutral currents...hence popping the GFI breaker.


Also, I thought maybe earth ground and the supply's DC ground were separate, however think of this; When you measure with a DMM between +12V and the steal chassis, you'll get a reading of 12VDC.

A DC reading is RMS. If you want to see the AC component, then you need an oscilloscope to see the AC noise on the DC line. If you do this, be careful as the "clip" on the scope probe is connected to earth ground...and you can toast a circuit by connecting earth ground to something it's not supposed to be connected to.


Also, if you were to connect a fan to the 12V, and attach the ground to a steel part of the chassis, the fan will run. So somewhere the two do meet each other. Or is there another explanation for why my observations are the way they are?

Yes, the fan will run...my first electronics professor had a saying: "Voltage is BS...current is what does the work". (He was Russian, so this sounds VERY cool when you say it with a Russian accent). Voltage is just a potential (to do work)...current flow is actual work being done. You have to follow the current path:

(1) Out of +12 V on power supply
(2) Into Fan + connection
(3) Through Fan motor
(4) Out Fan - connection
(5) Into steel chassis
(6) Who knows where next
(7) Somehow to power supply case
(8) Somehow to power supply DC ground

I prefer not to have the unknown of 6-8 myself.


Basically I ask this because I'm wondering if when lacking the AC ground (3rd pin, earth) are you at a greater risk for damaging the system with ESD when you physically touch the IO, or plug something into the MB? Or is the PSU's floating ground capable of discharging that ESD from your fingertip and keep the system safe?

Bingo! It's about safety...for YOU first, then your equipment.

Just take something as simple as a network cable. Side 1 is connected into a switch (which is connected to earth ground). You want to connect Side 2 into the Ethernet jack on your PC...but the case is floating...when you plug it in, you will cause a short and will have a current pulse through the network cable as the charges equalize. How much of a current pulse? Shrug...depends on the voltage potential between the two and the resistance.


Electricity is dangerous...it can hurt you...bad.