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If you connect the negative terminal of a Class II (isolated) wall-wart-type power supply to the neutral of the mains circuit feeding the wall wart, will that cause any damage or hazards that I should be aware of, besides the wall-wart's + terminal being considered a "live" mains wire at that point (as its now mains-referenced)? Would the results be different if a switching wall-wart was used instead of an iron-core one?

Update: this application is being wired to the interconnect circuit on hardwired smoke detectors, which is the reason it's neutral-referenced -- it seems that all interconnected hardwired smoke detectors use the '9V on interconnect wire with respect to neutral = alarm' convention -- this only works if your supply for this is neutral-referenced. Obviously, this'd be in a sealed box where wayward fingers, tongues, etal couldn't get at it, and I was planning on removing the barrel plug anyway as it's a hinderance to chassis wiring.

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    \$\begingroup\$ You would lose your isolation. \$\endgroup\$ – Matt Young Jan 14 '15 at 4:46
  • \$\begingroup\$ @MattYoung -- this is a case where the circuit it is powering is referenced from the mains neutral anyway (i.e. a non-isolated supply would do the job here, but wallwarts are buyable anywhere, vs. building a non-isolated mains supply that won't emit magic black smoke, sparks, or other such nasties). \$\endgroup\$ – ThreePhaseEel Jan 14 '15 at 4:52
  • \$\begingroup\$ @ThreePhaseEel What kind of circuit is it? What does it do? Where is it deployed? Why is it referenced from (i.e. connected to) mains neutral? The latter doesn't sound right (without knowing anything else, that is). \$\endgroup\$ – Nick Alexeev Jan 14 '15 at 5:18
  • \$\begingroup\$ @NickAlexeev -- the circuit is the interconnect wire for a hardwired smoke alarm -- these are always referenced to mains neutral. \$\endgroup\$ – ThreePhaseEel Jan 14 '15 at 12:47
  • \$\begingroup\$ Also, there will be people who assume that connecting two of your PSUs in series will give the double voltage. Just saying. \$\endgroup\$ – Simon Richter Sep 24 '15 at 15:13
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It should not cause any extra hazards or problems that I can think of, beyond the output being 'live'. However there are potentially serious consequences to the output being live- see below.

Typically the output (-) is AC-connected to the neutral anyway, but galvanic isolation provides safety and the capacitor is extremely high reliability high-voltage Y-type capacitor because a direct connection (what you want to do) is potentially fatal in general purpose applications.

Keep in mind that the cord will not typically be rated for mains voltage and that will have to be dealt with. If it's all in a box and the cord is sleeved with approved sleeving, and the connector cut off, it may be okay.

If you're planning on powering a device connected to the mains with a wall wart actually plugged into a wall (rather than locked in a box) or with the original barrel connector and/or cord still attached no, don't do it.

Here, for example,

enter image description here

is a cord connected to a 9V "wall wart" that has sustained damage from a desk chair running over the cord. As you can see, the "live" wire is exposed and ready to shock someone were it not to be isolated. Same thing applies to the barrel connectors- they are not designed to be shock-proof.

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  • \$\begingroup\$ It should never be connected to the neutral, it should be connected to the earth. That the neutral and the earth are generally joined at the main junction box is irrelevant. The whole point of the protective earth is to prevent the potentially dangerous situation where the neutral connection becomes broken, and you then get mains AC on the now floating "Neutral" \$\endgroup\$ – Connor Wolf Jan 14 '15 at 5:24
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    \$\begingroup\$ @ConnorWolf I'm assuming the OP has some kind of circuit that is inherently connected to the mains (think of an SCR phase control or something like that) and wishes an auxiliary supply to power it. In such a case, it must be wired to the neutral or hot, not to the earth. The point is to get a low voltage/high current supply common with the mains, not a general-purpose power supply that children can put their tongues on. It damn well better be locked up in a box with interlock or whatever and marked so that it doesn't get mistaken for an isolated supply. \$\endgroup\$ – Spehro Pefhany Jan 14 '15 at 5:26
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    \$\begingroup\$ @SpehroPefhany To be honest, your post makes the proposed setup sound safer than it inherently is. The thought that this circuit is inaccessible for tongues and fingers is just our speculation at this point. The O.P. haven't chimed in on this yet. \$\endgroup\$ – Nick Alexeev Jan 14 '15 at 5:30
  • \$\begingroup\$ @NickAlexeev I've added an edit that points out a possible hazard. Is there anything else that should be added? \$\endgroup\$ – Spehro Pefhany Jan 14 '15 at 5:31
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    \$\begingroup\$ Sphero - That was my reading, but some of the questions I've seen asked have made me a bit cautious about making assumptions that the OP knows why they want what they want when dealing with mains. \$\endgroup\$ – Connor Wolf Jan 14 '15 at 8:43

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