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I am reading a manual for the installation of a structured light scanner. It says that the chassis must be grounded well.

This for me means that I have to connect a ground cable from a grounding screw to the socket ground. But I don't understand what it means that the power supply (of the 45 V master unit where RJ45 cables are connected) must be isolated from AC ground. Does it mean that I have to use two different sockets?

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    \$\begingroup\$ Could you please add a description of the item in detail - manufacturer, part number, and if they have an online manual, could you please post a link? That would help us understand what is going on. \$\endgroup\$
    – Smith
    Feb 20, 2023 at 16:39
  • \$\begingroup\$ @Smith Thanks for your comment this is the page where is written: manualslib.com/manual/1337249/… \$\endgroup\$
    – G M
    Feb 20, 2023 at 17:18
  • \$\begingroup\$ I think what they're meaning to say in the document is that the OUTPUT(s) of the power supply must be isolated from AC ground. \$\endgroup\$
    – SteveSh
    Feb 20, 2023 at 17:24
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    \$\begingroup\$ it tells you in the next sentence what it means ... This means that AC ground and DC ground are not connected \$\endgroup\$
    – jsotola
    Feb 20, 2023 at 20:55
  • \$\begingroup\$ @jsotola how can I implement it practically? \$\endgroup\$
    – G M
    Feb 23, 2023 at 8:16

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It means the output of the PSU must be "floating", with isolation from both the AC mains and from local ground.

This generally means:

  • There is an isolating transformer (iron for mains frequency, ferrite for a switch mode power supply) inside the PSU, between the input and the output
  • Neither the -ve nor the +ve terminal (nor anything in between!) are connected to local ground.
  • If the case is metal, both the +ve and -ve terminals are isolated from the case.

It does not mean that the power supply mains lead must lack an earth pin. For safety, a metal cased PSU must in general be grounded.

Most modern SMPS and bench supplies are floating, both those with metal and plastic cases.

And as suggested in another answer, a modern PSU with just a two pin AC lead is almost certain to be floating.

However, if you've ever worked on an old valve TV or radio, you are well aware of the dangers of non-isolated power supplies. Often the DC of these units comes straight from a bridge rectifier, and sometimes the DC negative is connected to the metal internal chassis to reduce the need for return wires. This means the chassis is effectively floating up and down with the mains supply, and touching the internal metal work whilst taking measurements is then very "exciting"!

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    \$\begingroup\$ Indeed, and it's better to have a power supply with earthed (grounded) inlet, as it is very typical that power supplies with only two-prong inlet have significant common mode leakage current, which many people feel as tingling or sting when handling their laptops, or mains frequency vibrations when sliding a finger over the metal surface. What's even worse is plugging a power supply requiring earth (ground) to a socket which does not provide earth (ground). \$\endgroup\$
    – Justme
    Feb 20, 2023 at 22:04
  • \$\begingroup\$ @Justme Absolutely agree. I've always hated the tingle from those PSUs, which is particularly bad if you are also touching true ground (stone floor, grounded metal work, metal shield on a shielded comms line, etc). I much prefer my Dell PSU with the trefoil lead including ground! \$\endgroup\$
    – colintd
    Feb 20, 2023 at 22:17
  • \$\begingroup\$ On the positive side, many years ago I used a 'scope to show our company's Toshiba rep the 240V present on my brand new laptop (which was particularly bad given the location of the grounded token ring cable), and he gave me $250 compensation! \$\endgroup\$
    – colintd
    Feb 20, 2023 at 22:19
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It means that the power supply's output must not have a connection to the AC socket's ground pin (internally to the power supply). It has nothing to do with using different sockets.

In other words: Use a power supply that comes with with a two-prong power plug, not a three-prong one.

The third prong on a power plug is the ground connection.

If you're from Europe, you have to use a power supply that comes with an Euro plug (which has only two contacts), not a Schuko plug (which has three contacts).

Note that you should not just try to disconnect the ground connection from a power supply that's usually connected to a three-prong (or Schuko) plug. Doing that is dangerous; you may get a shock.

The reason why the power supply should not be grounded is fairly simple: The device itself is already grounded. If you ground it both at the device and at the power supply, you create a ground loop. (If any point of a connected circuit is grounded, the entire circuit is grounded, and you don't really want to ground it twice.)

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    \$\begingroup\$ @GM That's a good point! I'll update the answer. \$\endgroup\$ Feb 20, 2023 at 17:48
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    \$\begingroup\$ You can't judge if a power supply has earth referenced outputs just by looking at the mains plug. If it has a grounded Schuko plug, it does not mean the output is earth referenced, it can still be floating. A lot of power supplies are like this. So the procedure to not use safe power supplies with earthed input is wrong. \$\endgroup\$
    – Justme
    Feb 20, 2023 at 18:44
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    \$\begingroup\$ @MattS All laptop power supplies I have ever measured and have had a grounded inlet for mains input, have used the ground for mains filtering; they have never connected inlet earth to output common (negative) rail. Neither have any lab power supply - they have a separate green banana socket for providing the mains earth separately if needed. No, capacive connection does not count, sometimes, it is a must for EMI purposes, so no ground DC loops go through that. \$\endgroup\$
    – Justme
    Feb 20, 2023 at 19:40
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    \$\begingroup\$ The need for a floating output is entirely separate for the need to ground a metal case. \$\endgroup\$
    – colintd
    Feb 20, 2023 at 21:41
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    \$\begingroup\$ @MattS Yeah, and one looking exactly like that from another manufacturer has 470 ohms resistance between mains earth input and output DC common. I don't think it was mentioned anywhere either, but a surprise mishap with ground loop (i.e. DC positive touching earthed chassis of another device) could blow up that 470 ohm resistor. Your power supply says 3000V withstand between primary and secondary, so it should not have earthed output negative. \$\endgroup\$
    – Justme
    Feb 20, 2023 at 21:51

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