I have tried searching, but couldn't really find a defintive answer to my problem.
Hopefully the attached schematic sheds a bit of light on the the issue.
I have a metallic housing in which I have put a Meanwell Power-supply which is connected to the mains on one side (L, N, PE) and outputs +24V on the other side. The power-supply housing is also metallic and is directly connected to PE.
The arduino is powered via a USB connection from a computer and its GND is connected to the - pole.
The analog inputs are connected to Arduino directly. However, due to the housing being metallic they are also connected to PE. Same goes for all switches and digital outputs. Since the connectors are all metallic, they are also connected to the housing.
My question is about grounding the Ardunio GND (at the same the Power-Supply ' - ' pole) to the PE.
Is it necessary and what would be the best way to go about this?
Or should I just get a plastic housing?
Grounding the power supply casing to PE helps prevent EMI radiation from the power supply, and nothing is gained by connecting the power supply case to other parts of your circuit.
I recommend to avoid doing so, as you might introduce ground loops, depending on whether other line powered components might be connected to your project.
Normally when AC power goes into the box, the metal case is connected to earth ground. That's for safety... If something goes wrong and there is a power connection to the case, power will be shorted to ground and it's impossible for you to get shocked (1) or electrocuted by touching the case. And this will also blow the circuit breaker.
It's not necessary to connect the Arduino ground to the case/chassis, but it's (usually) OK. That ground can be "floating" relative to earth ground. In some cases, especially with analog, you want it to be floating to prevent ground loops which can introduce noise into the analog caused by multiple grounds. (if the analog, or other circuitry is earth-grounded somewhere else).
(1) YOu can still get an ESD shock or spark if you walk across the carpet, building-up a charge on your body, which suddenly discharges when you touch ground.
Maybe I have not properly described the issue. The power-supply housing is grounded per default to the PE (internally) and is screwed to the metallic housing. So the housing is connected to PE by default anyways.
The same goes for the metallic switches and analog and digial connectors. Them being metallic means, that their negative poles are also connected to PE via the housing. Hence, the Arduino GND has to be connected to the PE as well.
If I want it to be floating GND, the easiest solution would be for me to use a plastic housing for all the switches and input and outputs since them being metallic means, that they are connected to the PE (Protective Earth).
The analog signals come from an instrument which is grounded via it's own mains cable, and the arduino is connected to a computer which is also grounded via it's own mains cable. So I basically have three parts which are all grounded somewhere else.
When I put it together like this, it seems to be wrong somehow
I don't think you have this right. I would expect that the case of the power supply be connected to PE but the output be floating, leaving you the choice of connecting the output as negative or positive ground, or floating.
I hope the metal cases of the switches and other parts are NOT in any way connected to the electrical connections.
It is only the metal body and the operating lever of the switch (if it has one) that are connected to the protective earth.
The contacts contacts are isolated from the body/lever
Similarly when mounting for example a D-connector to the earthed chassis only the metal cover, and jack posts get connected to earth, the contacts are isolated from earth.
This means that the power supply and Arduino can be floating (if required).
If you connect the Arduino to a USB port on a desktop computer the the Arduino GND does get connected to protective earth, but when connecting to a laptop via USB, the Arduino is usually floating.
This depends on the protection class of the power supply unit. If it is protection class I, the output must also be protected with PE. Only in the case of protection class II power supplies the output may have no connection to PE. If the output voltages can be touched directly - i.e. are not insulated - then the output must be insulated from PE and a protection class II power supply unit must be used.
True, but I would expect that a power supply with a metal case to have the case connected to PE not the output. It is for the designer of the equipment that the supply is going to power to decide if the equipment, and thus the power supply, be connected to PE. If the power supply came with one pole already connected to PE then that would pre judge what the supply is to be used for and what polarity with respect to earth is required.
Yes, you are right about the switches, silly of me to think otherwise. The analog inputs are bnc connectors which are also metallic. Their negative is connected to the GND and to the PE through the enclosure. I guess I should mount them separately and isolated from the enclosure then, correct?
The powersuplly is a meanwell with a metallic enclosure. It connects to the mains with three wires, of which one is the PE. The minus pole is not connected directly to the PE, only via the enclosure contacting the outside of the bnc connectors which are connected to GND of the Arduino.
As I understand now, isolating the bnc connectors from the enclosure will result in arduino floating GND, as it is connected to the - of the power supply.
Still there is the case of the laptop being connected to the arduino via USB. Do you think there will be any issues with this?
I missed that BNC connectors usually (always?) have their outer case as the ground and screen and connected to whatever case they are installed into.
I'm not sure how best to advise you in this case. As I understand it you are going to have PE connected to circuit ground indirectly anyway, so probably best to do it deliberately with a wire.
Interested to know what others think.
If I have misunderstood please provide more information, perhaps photos or a drawing.
You understood correctly. The only connection of GND to PE will be via the outer part of the BNC connectors through the enclosure.
Thank you for your comments.
You can use another type of connector. The ground for the analog input shields MUST be the Arduino GND, not the power supply case ground or PE ground.
Proper grounding is not a simple topic, especially when you are mixing AC power circuits with switching regulators and sensitive analog inputs in a single box.
