Like many I took an ATX PSU, hooked green to black, and have a nice PSU.

I am experimenting with a HP oscilloscope from 1997 and as you know, those are grounded to main's earth, just like my ATX PSU. This is risky as you probably know.

A simple solution to this would be disconnect the PSU black (v0) from the PSU case and thus disconnecting it from the main's earth. Which, as far as I understand would make the PSU "floating". I did it and it seems to work ok.

My question is why are those black wires connected to main's earth and therefore what are the risk in disconnecting them.

  • 2
    \$\begingroup\$ Why is it risky? You just need to be aware (as you are) there's continuity between the ground clip and mains earth. Floating your power supply would be more dangerous. \$\endgroup\$
    – Colin
    Oct 11, 2018 at 9:03
  • \$\begingroup\$ @Colin, I am aware of the continuity but mistakes happens and as far as I understand shorting the oscilloscope to main's earth could burn the oscilloscope. So my question is: why is the psu gourd outputs (the black wires) are connected to main's ground and what are the consequences of "floating" them by disconnecting them from the PSU metal case. \$\endgroup\$
    – Eli Konky
    Oct 11, 2018 at 9:46
  • 2
    \$\begingroup\$ Connecting PE to GND is an important safety feature here - ATX PSUs may not be double isolated, and thus need the PE<->Secondary connection in order to reliably detect mains isolation faults. Cut the connection, and mains voltage could potentially kill you. \$\endgroup\$
    – Turbo J
    Oct 11, 2018 at 10:39
  • \$\begingroup\$ Thabk you turbo. What is PE? Just to be clear, I am not disconnecting earth from the case. Only the black wires from the case. \$\endgroup\$
    – Eli Konky
    Oct 11, 2018 at 11:03
  • 1
    \$\begingroup\$ @EliKonky PE means "protective earth"; what you called "mains earth". It's called "protective" because it protects you by channeling away the electricity in the case of a fault, hopefully tripping a CGFI/RCD or circuit breaker in the process. \$\endgroup\$
    – marcelm
    Oct 17, 2018 at 23:52

2 Answers 2


The ground is tied to the chassis ground. In the event of a fault the ground protects you by shunting the dangerous current to ground, instead of through your body to ground. Disconnecting the ground will potentially make the chassis dangerous, it is best to not disconnect the ground.

Another problem would be protecting your scope, with the SMPS disconnected, the fault would run through the prove ground and scope, then to ground. With sufficient current, this would probably smoke the probe and maybe the scope, then you if you touched the chassis.

enter image description here
Source: SMPS power supply

If your talking about disconnecting the ATX ground from the motherboard connector, then the motherboard will not function and the PS won't turn on. Your right that you are creating a ground loop, with the o-scope ground, but there are better and safer ways to make a measurement like this.

The best (and most expensive way) would be to use a differential probe

A way (that can also be dangerous depending on the voltages involved) would be to use an isolation transformer:

enter image description here

This topic has been discussed multiple times before, isolation transformers are not magic safety devices - do not trust them blindly, use differential probes.

Isolation transformer has one meaningful use with oscilloscopes, but it is still dangerous.

As oscilloscope and DUT are both grounded, you can only clip oscilloscope ground clip to the earth and measure signals with respect to the earth. This may not be what you want. If you power DUT from isolation transformer, you can clip oscilloscope probe ground clip anywhere and measure what you want. However, by doing this you once again make DUT referenced to earth (floating becomes earth referenced) and DEFEAT RCD/GFCI PROTECTION FOR DUT. See simplified diagram. Note that GFCI/RCD is not effective in this configuration even if you add it after isolation transformer.

Source: EEvblog user:electr_peter

As anecdotal evidence from a reprap that has tried this see this warning (I think the spelling reflects on the smarts that it would take to do something like this) :

I just plug ghe (sic) ATX power supply to an outlet with no ground wire. During a print i acidentally touch the atx power supply then i got electric shock

Just a warning to all do not leave ground wire floating. Btw. My power is the 550w supplyed in the kit

Source: https://reprap.org/forum/read.php?340,328329

  • 1
    \$\begingroup\$ It's not what the OP says, It basically says GND is connected to Earth, he's not disconnecting the earth from the chassis, but the GND from Earth to have a floating GND. \$\endgroup\$
    – Damien
    Oct 18, 2018 at 2:35
  • \$\begingroup\$ Oh, I thought the question was this : "My question is why are those black wires connected to main's earth and therefore what are the risk in disconnecting them." \$\endgroup\$
    – Voltage Spike
    Oct 18, 2018 at 4:40

There are a few reasons to connect the GND of the PSU to the earth.

I do not believe it is for safety reason as the comments, as a floating power supply is always safer than a grounded one and the metal case is connected to the earth anyway in case of failure.

First, most of the electronics you would put on a computer, like motherboard, are grounded to the case through the screws. If the PSU had a floating ground, you may start to have current hanging around, potentially creating issue and even trip the breaker if it's a differential current breaker, especially when connecting other devices to the computer.

Another point is if you have the GND of the supply not grounded, you may start to have common mode fluctuations, which in general is not good at all and introduce noise on the circuit. This can also create electrostatics, generate EMI and so forth.

Another reason is that most connected devices (USB, HDMI, etc..,) have earth and ground tied together, like an HDMI screen, if the GND was floating, it would then be grounded through the shielding of the HDMI cable, to the screen and back to the GND lines, this also gives rise to a lot of issues.

Laptops however are usually not grounded, you can often feel the electrostatic discharge on an aluminium chassis laptop, the problems above becomes more difficult to address and issue arises.

  • 1
    \$\begingroup\$ A laptop adapter has the Y caps from output, tied to earth (here in India anyway, where all laptop adapters have 3 pin plugs for mains). This is the same as on the input side of the transformer. I have no idea why the same thing is not done for desktop computers. I believe, the desktop PSU is actually floating (except for the higher value Y caps). The connection to earth is most likely through the motherboard screws. It may just be a matter of convenience. The motherboard does need a thick groundplane, but I doubt there is any reason to actually connect it to the chasis, other than price. \$\endgroup\$
    – Indraneel
    Oct 17, 2018 at 8:16
  • \$\begingroup\$ Not all laptop are grounded, like Apple laptop are usually not but might depend on local regulation as well. apple.stackexchange.com/questions/17575/… \$\endgroup\$
    – Damien
    Oct 17, 2018 at 8:29
  • \$\begingroup\$ By ground, you mean earth, right? Without a ground, no laptop would ever work! You're right about apple, they have only 2 pin plugs here. But all others (here in India) have 3 pin. However, that only earths the power brick, not the laptop. The laptop still has only a 2 wire DC connector. But unlike in apple, the -ve of the 2 wire DC connector is earthed with the Y cap (although this is only for EMI suppression). \$\endgroup\$
    – Indraneel
    Oct 17, 2018 at 19:04
  • \$\begingroup\$ I do not know what country you come from but in the US neutral is white, Phase or hot are typically black, while ground is green or green with a yellow stripe. \$\endgroup\$
    – Gil
    May 26, 2021 at 20:05

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