I understand how electricity works well enough to handle it safely - and, at least as important, to know when I can't handle it safely and it's time to call in a professional - but that's not the same as trusting it. Honestly, it's a lot more the same as not trusting it. I can and do prototype line-voltage filament LED lamp designs on my bench without a problem, but I think more than anything else it's maintaining a lively awareness of the danger involved that helps there not be a problem.
What's really scary is stuff that's as dangerous as those prototypes and doesn't look it - a condition that I think only ionizing radiation excels electricity in making it possible to create. All anyone has to do is screw up wiring an appliance with exposed metal parts, and the whole thing can be live at potentially lethal mains voltage - and I'll freely admit that having had my batteries very unpleasantly charged that way once, thanks to a miswired ceiling fan, strongly informs my perspective on the subject.
As with fire, so with electricity: an indispensably useful servant, but also an ineluctably dangerous one, to be regarded with respectful mistrust at all times lest a moment's carelessness end in tragedy.
Or for extra fun, the wiring in the house. Lots of people think things like changing a plug are "easy" (they are), but they're also really easy to screw up.
Now you don't need some DIY'd appliance. Your Ikea stuff is now potentially deadly.
The number of DIY jobs I've run across where the hot/neutral are reversed on the plugs... It's insidious because for all intents and purposes it will _work_. Nothing you plug in will have any issues.
But that nice shiny metal lamp you've got? It's got the neutral bonded to the body so that if the hot were to come loose and contact the body it has a path to ground. What happens when the neutral and hot are reversed? The body of your lamp is now live, and when it's off it has no path to ground. Wear rubber shoes when you're turning your lamp on and off.
Your toaster? It only switches the hot. So when your toast is done and you stick your metal knife all up in there to try and dig your toast out, it's all still live with no path to ground. Whoops.
A lot of assumptions that are essential to things being safe all fall apart as soon as someone puts those black wires under the silver screws.
Another commenter pointed this out, but I have to chime in as well: no. Not true. The neutral is NOT bonded to the exterior body of a metal lamp. Not even on old fixtures built in the 1960s.
The old fixtures, if they were metal, were often not grounded, i.e. the lamp might not be connected by a 3-prong plug (hot, neutral, ground). But the neutral was definitely not bonded to the metal body.
If you inspect a new fixture, typically this means: it is either constructed with no accessible conductive parts, OR its metal case is bonded to the ground (not neutral!) and it has a 3-prong plug, OR it has a step-down transformer that converts to lower voltage before it enters the conductive lamp.
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The thing that does happen when switching the neutral wire on a lamp is that doing so will energize the more-accessible "outer ring" of the Edison female receptacle rather than the less-accessible "center pin". This means that if you change the bulb with the switch on, you'd be more likely to contact the hot "outer ring".
Also, in most cases, the switch is only across (the wire that is supposed to be) the hot lead-- with neutral and hot swapped at the plug, you're not only energizing the outer ring of the light bulb socket, but also leaving it energized when the switch is off.
This leaves a shock hazard exposed even when the switch is off-- the lamp still won't light under normal circumstances because there's no path to ground with the switch off on the neutral line, but if you provide it one (your body) by touching it, you'll still get a shock regardless of the state of the switch.
Yeah, comment kinda got downvoted into oblivion for a slightly off example, but this was kind of my point. There are a lot of appliances that become significantly less safe when you put the power into places it's not supposed to be. And the danger is non-obvious.
oops, yes, good point.
That would be illegal where I live, and I think in most of the world too. In fact, on a standard residential outlet here, it would immediately bring the house power down.
Either the body is grounded or it's floating. It should never be linked to the neutral.
How?
They wouldn't trip in this case just by plugging a poorly wired appliance in. They detect an imbalance in current flow between the hot and neutral wires. If they detect more current going out than coming back in, they recognize that the current has found another path (which is bad) and trip.
Unless you have a grounded lamp (which is certainly not common here; I've never seen one) there'd be no alternate path until you provided one by being shocked, so there'd be no immediate trip. It would, however, increase safety by tripping once you were being shocked instead of simply allowing you to continue being shocked.
For example, if someone did indeed have a metal-cased grounded desk lamp whose body was also connected to neutral too; it would trip as soon as you plug it in, whether you turn it on or not, and you won't be able to turn the RCD back on until you unplug the lamp [1].
In the US they're called GFCIs as you point out, but this name is bad, since it implies that it only trips on ground faults. In reality they trip on /any/ current imbalance, regardless of how it's happening, over which conductors, or in which direction it is occurring (after all, we are talking about /alternating/ current here). Among other things, this means that if you take a line conductor from a non-RCD-protected circuit, and borrow the neutral from an RCD-protected circuit to power a load, that RCD will still trip, even though the fault isn't on the circuit(s) that the RCD was installed to protect.
As to tripping should the neutral-connected case develop line potential, a circuit breaker will immediately (< 40ms) trip anyway. As to tripping should the grounded-case develop line potential; circuit breakers will also do that if your earth fault loop impedance is low enough (and if it isn't, you'll have an additional single time-delayed master RCD for the entire installation, for fault protection anyway). In a properly designed, installed, and maintained electrical installation, such a fault will simply not exist for long (40-500ms), regardless of whether you have RCDs for additional protection or not. This makes it highly unlikely that you would subsequently come into contact with that mains-potential object; but on the off chance that you do, say, if it only develops a line to earth fault when you're touching it because it wasn't turned on yet; indeed, an RCD for additional protection is a nice thing to have.
[1] Sometimes, depending on design, one can manually hold the tripping latch up to prevent it from disconnecting even when it detects a fault. This is extremely bad design in my opinion, and is not the case with UK RCDs; the tripping mechanism is entirely internal, and even if you epoxy or padlock the latch in the on position, it will still trip correctly. I've seen some circuit breaker designs that suffer this flaw too, but again not in the UK.
Thanks to Big Clive, I have a vague but reasonable sense of what UK electrical systems are like. It's not safe, metaphorically or literally, to assume the US is up to the same standard. As you might find out the hard way if you did make such an assumption, North American electrical systems in a lot of ways are shockingly poor.
You can also get circuit breakers with integrated GFCIs (which is what we call RCBOs); those are usually double-width devices in the US (For example, with the Siemens "QF" product range [1]), however, again, single-width versions are available (For example, with the Square D "QO" product range [2]).
Because a GFCI needs to measure current flowing in both directions, it has an inherent need for a neutral connection too. This is what the flying lead is for. The circuit neutral would be terminated into the GFCI instead of the panel neutral bar; the flying lead is terminated into the panel neutral instead.
I'm also not assuming a ground; a neutral-bonded lamp body sitting on a conductive surface which is in contact with the mass of the earth would still be enough to immediately trip a GFCI under the same circumstances as my previous post, regardless of whether someone is touching it or not, and if it is turned on or not (assuming the switch is in the line conductor).
EDIT: However, this all goes to crap if the lamp only has a 2-prong plug, yes.
I agree that some aspects of US electrical installations are exceedingly bad compared to over here. For example, it is stupidly easy to shock yourself removing a live socket from a wall over there, compared to it being a basic impossibility over here -- yes, I know, safe isolation and all that, but name me a sparky that hasn't done this at least once.
[1] https://cdn11.bigcommerce.com/s-b8b6a/images/stencil/1280x12... [2] https://cdn11.bigcommerce.com/s-b8b6a/images/stencil/1280x12...
