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I just received a 120VAC to 5VDC (20A) switching power supply (the kind with a passively ventilated metal casing) for use with a microcontroller project I'm working on (driving long LED strips).

I've wired up Line-Neutral-Ground correctly to a 3-prong plug. It tests out fine (and the case IS bonded to ground internally) but I see the outputs are labeled DC- and DC+ and indeed DC- is not bonded to ground so the outputs are floating (though I'm not detecting a high voltage differential).

Is there any danger in bonding the DC- output to Ground with a short wire, to ensure my circuit ground is also referencing the home ground? I'm not sure simply letting the outputs float is wise, but I don't want any dangerous side-effects if I ground DC-. (The circuit sharing this power will at times be connected to my PC which is itself quite thoroughly grounded so I'm inclined to emulate that behavior.)

Side-note: I have an after-market laptop supply that's two-prong... it works either way, but if I plug it in one way the metal trim on the laptop has an interesting "buzz" to the touch. Not shocking, but definitely noticeable. I suspect that when it's plugged in the right way the output ground is weakly referencing neutral, and the "buzzy" way has the output ground weakly referencing line voltage (it's floating - it'd be quite a bit more than buzzy otherwise). The OEM supply with the 3-prong plug? Fully grounded to the shield of the DC power cord.

So I suspect that when it comes to AC to DV power, getting Line and Neutral wired up wrong is far more risky than letting the output float, and that grounding the output is preferable to letting it float if you're working in an environment where you'll interface directly to grounded equipment. I just want to be sure...

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2 Answers

'Ground' is a relative thing. In the case of home electrics it's either literally a stake in the mud (earth) or the neutral of the three phase to the local sub station (or sometimes both, called PME - Protective Multiple Earth).

Live is relative to the neutral (which is sometimes earth/ground - but not in all countries)

An 'isolated' DCDC, like yours has no connection to live, neutral or ground at the output.

In the real world this is impossible. There has to be a capacitance and resistance between the + and - of the DCDC output and all of the mains connections (live, neutral and earth), however this might (and should be) very insignificant, with resistances in 10's of mega Ohms and capacitances in picoFarads

In your DCDC where you get a buzz, I expect these 'parasitics' are not insignificant, something is breaking down, lifting the DCDCs output common mode voltage through a high impedance to the live. In effect it is still 5V, but it has a common mode AC component relative to ground, on which you are standing.

(BTW this 'earth leakage' is often caused by poorly designed power factor correction circuits)

A simple test for your 'isolated' DCDC is to connect a milliammeter between each of the outputs (+ and -) one at a time to ground (earth). You should see very little current if any. Any more than 1mA or so and there is a problem with earth leakage.

If you see little current then connecting the + or the - to ground is OK.

However I will add that not connecting ground is a really good idea!

A high resistance (or low capacitance) route to ground is a really good way of preventing electrical shock in the event of a fault.

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Your statement about not grounding being safer surprises me. What sort of fault would be less dangerous because the ground reference for 0VDC was a poor (high resistance) or absent (floating)? I'd think the opposite - all other things being equal, a grounded device is safer. The only cons I've seen are that some hum could get introduced into the system (considering audio or ADC applications). –  MartyMacGyver Jan 17 '13 at 10:36
    
And to add, I read no current between DC+ or DC- and the ground or case. –  MartyMacGyver Jan 17 '13 at 10:55
    
@MartyMacGyver: if you are touching just the live half of an AC outlet, you get a little tingle, but if you also have a good connection to ground, you will get the full power of the outlet, limited by your body's resistance or the circuit breaker. Making a good connection to ground unavailable is one way to avoid this. A GFCI is another way. –  Phil Frost Jan 17 '13 at 12:45
    
So... why the trend towards three prong plugs and grounded electronics (e.g., computers, laptops and monitors)? By this logic those things must be more dangerous by virtue of their "earth"-grounded chassis and circuitry. –  MartyMacGyver Jan 17 '13 at 19:19
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As you are only talking about a 5V supply, then there is little risk from it either way, but in principle; For a current to flow there needs to be a circuit and if the negative is connected to Earth on your mains system then it is also connected to anything else earthed, and to the earth itself, so if you hold the live a circuit will be made through you to the earth. Not a problem with a 5V system and you wearing shoes/ standing on flooring or otherwise quite well insulated, But were it a higher voltage system and you touched the live while leaning on a sink or touching your computer or barefoot outdoors, then a current (relative to the supplied Voltage and the resistance to earth) would run through you.

If you do not connect the floating negative to earth then you can only get a shock by making contact with Both the + and - connections as otherwise there will be no circuit.

Mains powered equipment (in the UK) used to always have a 3 pin connection so that the metal casing could be earthed and any Loose Live wires inside could not make the case Live, but would short to the earthed casing, blowing the fuse. Now most things have Plastic cases, and modern circuit breakers are much more sensitive than fuses, this is not needed and mains connections are often 2 pin.

Grounded metal cases also have a shielding effect against magnetic waves and cannot suffer from Static build-up, which would be a major reason for your computer's metal casing and earth connection.

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I see what you mean, except that most common DC voltages are relatively low (24V and below). I'm not sure how a lack of a ground is safer in this low-voltage regime. And if there were a fault (mains voltage on the 5V line or the casing) then you'd be having a pretty bad day if the casing was ungrounded and you touched it (since mains is ground referenced to begin with). And while not everything has a grounded plug they seem prevalent on consumer electronics here in the States (e.g., PCs). –  MartyMacGyver Jan 23 '13 at 18:18
    
As I mentioned, devices with exposed metal where there is a possibility of a loose live mains wire reaching it are usually grounded, but plastic casings are common as are floating dc supplies. In the UK, mains power tools are always run from an isolating transformer, specifically to float the power and avoid electrocution. –  SamB Jan 23 '13 at 22:12
    
Got it, though I have never heard of isolation transformers being in common use in UK households for normal power tools and such. Double-insulated wiring for two-wire systems like powertools is another thing altogether safety-wise but that's beyond the scope of this question. I've concluded that grounding the DC- of my power supply (tested for leakage current) is safe & reasonable (also my next "bench" supply will be a re-purposed PSU, which will by default be grounded internally anyway). But in isolated projects for deployment I will consider leaving things floating. –  MartyMacGyver Jan 24 '13 at 9:27
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