4 ms·

> 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 Or for ext

> 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 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.

marcosdumay 6y ago
> But that nice shiny metal lamp you've got? It's got the neutral bonded to the body 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.
- nucleardog 6y ago
  > on a standard residential outlet here, it would immediately bring the house power down. How?
  - marcosdumay 6y ago
    There's a circuit breaker designed specifically for that. In English I think it's called "residual current device".
    - nucleardog 6y ago
      In North America those are more commonly called a GFCI (ground fault circuit interrupter). 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.
      - aaronmdjones 6y ago
        An RCD/GFCI would immediately trip if anything past it is bridging ground and neutral. As well as ground to line, or line to neutral of another circuit (on a different RCD), or line to line of another circuit (on a different RCD, in single-phase or multi-phase installations), or neutral to neutral on a different circuit (on different RCDs), etc etc. 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.
mturmon 6y ago
> But that nice shiny metal lamp you've got? It's got the neutral bonded to the body... 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. * 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".
- JonathonW 6y ago
  > 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.
  - mturmon 6y ago
    > ...you'll still get a shock regardless of the state of the switch... oops, yes, good point.
  - nucleardog 6y ago

marcosdumay 6y ago

> But that nice shiny metal lamp you've got? It's got the neutral bonded to the body 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.

nucleardog 6y ago
> on a standard residential outlet here, it would immediately bring the house power down. How?
- marcosdumay 6y ago
  There's a circuit breaker designed specifically for that. In English I think it's called "residual current device".
  - nucleardog 6y ago
    In North America those are more commonly called a GFCI (ground fault circuit interrupter). 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.
    - aaronmdjones 6y ago
      An RCD/GFCI would immediately trip if anything past it is bridging ground and neutral. As well as ground to line, or line to neutral of another circuit (on a different RCD), or line to line of another circuit (on a different RCD, in single-phase or multi-phase installations), or neutral to neutral on a different circuit (on different RCDs), etc etc. 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.

mturmon 6y ago

> But that nice shiny metal lamp you've got? It's got the neutral bonded to the body... 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. * 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".

JonathonW 6y ago
> 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.
- mturmon 6y ago
  > ...you'll still get a shock regardless of the state of the switch... oops, yes, good point.
- nucleardog 6y ago