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I've got a project of a DIY microcomputer used for control and measurement purposes, which gets mains power via a Meanwell IRM-10 "AC/DC converter" mounted on one of the PCBs, which are then attached to a backplane and mounted in an aluminium case.

Part of the PCB with PSU and 2-pin AC power jack

The PCB is designed for an IRM-10 (PS2) and a non-polarised 2-pin AC power jack (J3), so the PE pin is omitted. Now the datasheet of the IRM tells that it's a Class 2 module and therefore does not require a PE pin. But: Even if the 230 Vac come in through J4 and "only" run to the L and N pins of PS2, is there still a connection from the computer's housing to PE necessary?

Note: I live in Europe where regulations regarding electrical safety are somewhat stricter than in the U.S.

Neppomuk
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    While the PS module is a class 2 you still make a direct connection to the AC lines with PCB traces, which then connect to the module pins. With that arrangement there is a potential for a line to case contact. You also show the neutral line going off somewhere, a chassis switch maybe? Is J3 polarized to guarantee proper Line/Neutral connections? Overall I doubt you can make the claim that the complete system is a class 2 device and not require a PE. If this is a commercial product you should review the design with your compliance engineer, or a vendor you might use for compliance testing. – Nedd Mar 05 '23 at 04:51
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    Even if this design is for a one-off home use device a PE connection with a polarized AC plug/cord is still a safer way to go, especially if there will be any open or direct connections to an AC line. As an alternative it should be quite easy to implement the same design using an external fully compliant double corded (class 2) AC/DC desk supply. – Nedd Mar 05 '23 at 05:22
  • @Nedd Yes, the PSU brick is gonna be connected to an on / off switch mounted on the front plate. And yes, I would attach an external (laptop?) PSU, for whtever reason (operating environment?), the inventor of the machine went for an internal PSU. – Neppomuk Mar 05 '23 at 15:51
  • You could omit the AC/DC convertor and use an external power brick by adding an appropriate socket to the case instead of J4. Or for more control over the quality of the 5 V, you could use a 12 V external power brick and a 12 V to 5 V DC/DC convertor inside. Either way, there won't be mains in the enclosure. – Andrew Morton Mar 05 '23 at 16:28
  • Which country or what kind of plugs you have? The comments about polarized plugs may not apply, as most of european plugs are non-polarized. Which is why most mains switches are dual-pole, cutting both live and neutral for safety. Also, since you have metal case, and who knows what voltage it is connected to via external connections, it's usually not left floating. If you use earthed plug, and just wire earth to metal case, you get extra safety and EMI compatibility. – Justme Mar 05 '23 at 16:35
  • @Justme In Germany we use non-polarised Type F (Schuko) plugs and sockets. I'm not sure whether the rails on the PCB comply with our safety regulations, as both L, and N must be handled with utmost care (as if they were L1 and L2). – Neppomuk Mar 05 '23 at 16:39

4 Answers4

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is there still a connection from the computer's housing to PE necessary?

Not really - not with a class II supply. It already has a double/reinforced insulation to protect against shock hazard. A connection from case to PE would be another, redundant means of shock protection in that case - and an unnecessary one.

The power inlet must have reinforced insulation as well. Anything internal that's connected to mains must be insulated that way too, so that the complete system can maintain the shock protection rating afforded by the power supply.

I usually prefer Class II power bricks securely mounted in the case, sticking out a little bit, so there's no chance of any contact between external detachable power cord and the case.

Kuba hasn't forgotten Monica
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  • Yes, the brick is soldered to a PCB, and there are just 1" long rails on the PCB to run to the inlet. There are also two (isolated, of course) cables branching off the PCB to run to a 230 Vac switch mounted on the front plate. – Neppomuk Mar 05 '23 at 16:17
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    @Neppomuk Anything connected to mains **must be double or reinforced insulated**. Soldermask on PCB is *not* considered as insulation. The wires from the PCB to the switch are *not* doubly/reinforced insulated by default. The switch itself is may be doubly/reinforced insulated **but you have to prove it**. So, with all those traces and wires and extra components, you'd pretty much have to pot that part of the circuit (PCB, wires, switch) in epoxy resin to provide the insulation required. That is usually hard or impractical to do. So **no**, it's not Class II. – Kuba hasn't forgotten Monica Mar 06 '23 at 19:40
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    So, you have to provide another means of protection against electric shock. Either by potting parts of the system (adding reinforced insulation) or by protectively earthing the enclosure. *the datasheet of the IRM tells that it's a Class 2 module* It's just a module. The whole system must be Class 2! – Kuba hasn't forgotten Monica Mar 06 '23 at 19:41
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As happens, I am currently working on a project with a similar setup: aluminum housing with two isolated 230VAC/12VDC power supplies inside.

I don't know what the German TüV says nor what the American UL says. I just know I decided that the device I am building will have a protective earth ground attached to the housing.

  1. It's DIY - it'll be fiddled with, opened, and closed. Wires can break, screws can come unscrewed
  2. There's 230VAC loose inside, just like yours. Mine are soldered to the PCB instead of using screw terminals.
  3. It's a conductive housing - mine is powder coated aluminum, but is screwed to other conductive devices that will be touched when in use.
  4. There's a large MOSFET attached to the housing for a heat sink. It is isolated, but things can fail.
  5. I'm a chicken.

For DIY, I'd tend more to being overly safe. If you're an engineer designing it for production, I'd expect you to check the relevant regulations.

I don't have access to those regulations, but this (German language) summary of the rules in Germany suggests to me that a device with a metal housing that has bare wires with 230VAC inside it would require a protective earth. When my gadget is finished and in use, it will be a daily use kind of object - I shouldn't have to consider whether it is safe or not, it should be safe.

I'm in Germany as well. I have a fused connector for the AC line (C14) to accept a standard C13 (Kaltgerätestecker) cable.

JRE
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If you have mains wiring to your onboard converter running through your metal chassis, your chassis is not Class II anymore.

If the wiring is torn lose somehow, it can make contact to the chassis because it doesn't have double insulation.

This isn't based on any regulation form but is my common sense.

tobalt
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In theory, and I don't know in legal sense, but I think your circuit should be fine as it is, as there are many lamps with unconnected floating metal chassis, and unpolarized two-prong Europlugs, and these lamps also may have a power switch that cuts only one wire, which will be either Live or Neutral depending on which way the unpolarized plug is inserted.

That said, it does not mean those lamps are safe if something goes wrong. Since the metal is ungrounded, broken mains wiring may make a connection to metal case. Again, if it is neutral, nobody dies, but if plug is flipped, it's live. If you change the bulb while switch is off, you don't know if the live is wired via switch or directly to lamp connections, so they are unsafe.

Depending on what other things your circuit is supposed to connect to, or if it has other mains circuitry in it after the switcy, it might be safer to make it better.

For instance, you could use an earthed Schuko plug and wire the earth to metal chassis. And you could use a two-pole mains switch. This ensures that you know case is safe to touch and after the switch all power is cut. Unless there are faults in the switch or earth connection, but at least in theory.

Justme
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