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Have a nice piece of Motorola obtainium that I want to use to power a Raspberry Pi, and I also happen to have a left over ground wire (the case has a three-lead power connector). The case is 100% plastic.

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I assume the metal bit on the top, pointed at by the arrow, is a heat sink, and am wondering if it would be a good idea to connect the ground to this?

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    \$\begingroup\$ It's a heat sink (not a shield) and no, unless it is totally electrically isolated, it's usually a bad idea to connect anything to it or earth it. \$\endgroup\$ – Andy aka Mar 12 at 12:49
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    \$\begingroup\$ I've seen designs like these where the heatsink is actually live, so never assume anything. \$\endgroup\$ – Finbarr Mar 12 at 12:51
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If users can touch the heatsink, such as at the back of some linear power supplies, it should be grounded and electrically insulated from the semiconductors mounted to it.

A common way to do this is by using aluminum oxide plates, TO-220FP packages (plastic housing) or a sil-pad.
Or semiconductors, such as IGBT modules, that are mounted on a ceramic base.

The power supply you have shown is intended to be mounted inside an enclosure, where users cannot touch it.
Heatsinks inside power supplies are often not insulated, since it add costs and users are unable to touch it anyway.

So no, it would not be a good idea to ground this heatsink, as it will most likely cause a short circuit.

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Do not ground the heatsink .Keep the power supply inside its box for safety reasons .The switching transistor has lots of high voltage high frequency noise on it when the power supply is running .Even if the switching transistor is isolated from the heatsink there will be capacitance that will have low reactance at HF .This means that you will couple lots of high frequency currents into your third wire ground causing EMC problems.

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