3 ms·
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 di
by 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.
- throwanem 6y agoYou're still assuming a ground where none exists, and you also seem to be assuming RCDs in the distribution panel, which is not something I have ever seen in any US residential or commercial installation - I don't think the national code would allow it. The only RCDs you see here are the GFCIs built into outlets. Those are only required in new construction, and then only in kitchens, bathrooms, and other places near water. Hell, I haven't even always lived in places that had circuit breakers instead of Edison-screw fuses, and I'm not all that old. 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.
- aaronmdjones 6y agoThe US National Electric Code does allow for GFCIs (RCDs) and AFCIs (AFDDs) in breaker panels -- they are sometimes double-width devices (especially if they are double-pole isolators) which is why they're not commonly used that way, but single-width versions are available. 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/1280x1280/products/186/378/QF115_02__98593.1358279807.jpg?c=2 https://cdn11.bigcommerce.com/s-b8b6a/images/stencil/1280x12... [2] https://cdn11.bigcommerce.com/s-b8b6a/images/stencil/1280x1280/products/230/561/QO115GFI_03__39029.1359031602.jpg?c=2 https://cdn11.bigcommerce.com/s-b8b6a/images/stencil/1280x12...