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My system use only line and neutral connection from mains (no ground.) Inside this system it is converted to 5V using SMPS.

My enclosure is metal, I need to ground this enclosure. Since I do not have ground wire coming in my system I am going to connect my circuit return ground (5V return ground) to the enclosure, How effective the shield will be on enclosure by doing this.

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    \$\begingroup\$ Never mind shielding effects. The purpose of a ground wire is keep you from being electrocuted (killed) if something goes wrong inside the box. You need either a real earth ground on the box, or you need an insulated (plastic) box. \$\endgroup\$ – JRE May 6 '17 at 7:32
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    \$\begingroup\$ OR you need two effective independent layers of insulation between mains and the metal enclosure - "double insulation" including internal components, especially the transformer in the SMPS must be certified "double insulated". JRE's approach is probably simpler. \$\endgroup\$ – Brian Drummond May 6 '17 at 8:39
  • \$\begingroup\$ I don't have Mains ground in my system. Does it mean i can not use metal enclosure. I failure in emission test . \$\endgroup\$ – Transformer May 6 '17 at 8:41
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Motor cars are not galvanically grounded and they produce tens of thousands of volts for the spark plugs. They are "safe" by design.

My enclosure is metal, I need to ground this enclosure.

If your metal box is powered from an SMPS that is designed to the correct standards then what is the issue?

If it's an issue of EMC then sure, you need to think about how to reduce emissions. If it's a safety issue then for sure you MUST choose a suitable SMPS.

I am going to connect my circuit return ground (5V return ground) to the enclosure

It may be a good method but it might also cause other problems - it depends on how the SMPS is designed - quite a few have an EMC capacitor from the DC output wire back to the AC rectified smoothing capacitor - this reduces EMC to the DC side and can offer some protection the other way round.

How effective the shield will be on enclosure by doing this.

It may be somewhat effective or it may be worse. You need to do some measurements.

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  • \$\begingroup\$ The SMPS i am using, is in encapsulated plastic box, Its ready made module which is CE and UL certified on it. Its related to EMI issue before we have plastic enclosure where we failed on RE test so now moving to metal enclosure which can reduce some noise. \$\endgroup\$ – Transformer May 6 '17 at 10:53
  • \$\begingroup\$ And that is why I mentioned EMC issues. There is no guarantee that the isolated output from the SMPS has EMI suppression components that might alleviate your situation. You have to consider if your design is enhancing the EMI produced by the SMPS or whether your design is generating EMI and take appropriate counter-measures. \$\endgroup\$ – Andy aka May 6 '17 at 10:57
  • \$\begingroup\$ In our last measurement on lab, we found its one board (high frequency) which is radiating , this is third party board, cant do much on their broad , so think to shield via metal enclosure. \$\endgroup\$ – Transformer May 6 '17 at 11:07
  • \$\begingroup\$ Radiated emissions can come from conducted emission along internal interconnect cables - don't rule this out - a ferrite around any such cable could make a significant difference. It's nearly always better to target the main culprit locally rather than try to apply a shield to the whole product. \$\endgroup\$ – Andy aka May 6 '17 at 11:10
  • \$\begingroup\$ we Almost tried most of thing. 1. we add ferrite on Ethernet cable, did't help, even we removed Ethernet cable. 2. Add decoupling caps on each power section of circuit. 3. Ferrrite on power supply , no effect 4. All failure freq is multiple of 50 MHz, 24 MHz. that third party board has their sources 50 MHz RMMI interface, 24 MHz is for USB hub. I think to add series resistor on clock but in that board (third party board) track goes in inner layer. \$\endgroup\$ – Transformer May 6 '17 at 11:20
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First rule of electrical engineering, especially when using live power.

DO NOT ELECTROCUTE ANYONE!

When using live power, including the ground is always your best solution. For the extra cost of cable and plugs it buys you so much more peace of mind.

If grounding is completely off the table then you need to double insulate to make sure the operator can never touch a metal part. HOWEVER. This can ONLY be applied if your widget has no external connections other than power. You can not have a DB9 serial port sticking out the side of the plastic.

As for shielding within a double insulated design. You can in fact connect the casing to the neutral line for EMI reasons. The neutral is normally attached to ground somewhere, either at the breaker box or at the line transformer.

However, if you do the latter, be aware that if you open up the box there is a chance the outlet is wired backwards and the frame will be live.

As such you need that ubiquitous "DO NOT OPEN CASE. NO USER SERVICABLE PARTS" sign on the outside of the casing.

ADDITION

If you are using a metal enclosure with mains coming in with no ground you will not be able to attain a safety certificate to allow you to sell this device pretty much anywhere on the planet.

There is one exception to that where you can prove that the case is double insulated from the contents of the box. Double Insulated (Class II) Appliances. Even then, the certifier will frown rather a lot and be reluctant.

Further, in that scenario there can be no un-isolated electrical connection to anything inside the box, so connecting it to the 5V return would be a no-no.

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  • \$\begingroup\$ Reason is we may not have ground available form this system is going to install. And metal enclosure with not be connected to neutral, it will be connect to circuit ground. \$\endgroup\$ – Transformer May 6 '17 at 14:02
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    \$\begingroup\$ @Transformer If you have a metal enclosure, with mains coming in and no ground, you will not be able to certify this product pretty much anywhere on the planet. \$\endgroup\$ – Trevor_G May 6 '17 at 14:03
  • \$\begingroup\$ i have several cheap metal kitchen appliances; blender, toaster, etc that have a 2-prong plug; what gives? \$\endgroup\$ – dandavis May 6 '17 at 16:09
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    \$\begingroup\$ @dandavis that falls into the exception I mentioned making it a double insulated (Class II) appliance. en.wikipedia.org/wiki/Appliance_classes \$\endgroup\$ – Trevor_G May 6 '17 at 16:37
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    \$\begingroup\$ in the USA at least, very few consumer goods are grounded besides computers and outdoor tools. i agree they probably should be, but they aren't... \$\endgroup\$ – dandavis May 7 '17 at 2:43

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