The actual electrical device can be designed such that it depends on exactly which direction is live and which is neutral.
Imagine a circuit loop that, as you follow along the circuit, has an AC power source, then a switch, and then the electrical appliance, leading back to the AC source it started from.
If you design the circuit so that you know for sure that the live wire goes to the switch first before the actual load, then your design ensures that if there is a fault or a short somewhere in the appliance, it won’t let the live power leak anywhere (because the whole device is only connected to the neutral line, not the hot live voltage that alternates between positive and negative voltage). It’s safer, and is less likely to damage the internals of a device. Especially if someone is going to reach inside and forgets to unplug it or cut power at the circuit breaker.
In practice tons of outlets are wired the wrong way around.
F actually has a convention for the socket, which is probably ignored even more often, but I would never trust live and neutral not to have been swapped somewhere regardless of outlet.
Just forcing plug designers to consider live/neutral being randomized in a very obvious manner might be safer in the long run than working on a partially broken system where someone manufacturer might be fooled into trusting it.
You have both live and neutral lines in the cable, coming up to a switch, which can either open the circuit on the live line or the neutral line. Then, the lamp itself has a single light bulb as the load.
If you place the switch on the live line, then the energy of the live line stops at the switch, with only whatever lower voltage is in the neutral line to actually be connected to the light bulb and lamp assembly.
But if you place the switch on the neutral line, you’re leaving the entire lamp on the voltage of the live line, which gives the voltage more places to potentially short circuit. If you were to take a non-contact voltage detector, you’d be able to detect a live voltage in the line leading up to the bulb, even when it’s not turned on.
You generally do this with the in-wall wiring and switches, too, and make the wall switches break open the circuit on the live line, not the neutral line. It’s just a better practice overall.
And no, the neutral line is not totally grounded, so it can still pose a danger, too. But safety is exercised in layers, and putting the switch on the live line is the better practice.
This is an entirely theoretical problem that just doesn’t exist in practice. Just to be clear, for it to short circuit, it’s needs to find a path to ground. It can’t just “go somewhere”. Just because the line is longer didn’t make it more dangerous for it to “just exist”. There are regulations for wires, which include frankly absurd safety margins, regulations for the electrical devices that are not optional either (CE compliance for example). It just complicates this for basically no reason to have keyd outlets.
It’s not electricity, exactly, but it is a higher voltage that is different from the average of everything around it. Electricity needs a closed loop to flow, and breaking open the loop with a switch means that no electricity flows, but the voltage of the live line goes up and down, creating an electric potential with anything that might be at a different voltage, if a conductor touches both.
it’s a bad practice to design appliance in such a way to assume that neutral will have low voltage, because in case of neutral failure in three-phase circuit you can get full voltage there, and there can be a couple of volts difference (sometimes more) between neutral and ground even in normal circumstances
it’s better to cut off both live and neutral at the same time anyway, especially if there’s no standard which is which. also, as device designer you don’t know if it’ll be used on a circuit that has neutral and phase where you think it’ll go or not. (ie british appliance used on unpolarized circuit, like type F. adapters exist)
it’s a bad practice to design appliance in such a way to assume that neutral will have low voltage, because in case of neutral failure in three-phase circuit you can get full voltage there,
Who’s using three phase in a setting where these types of plugs are used? In the US, at least, three phase circuits use very different receptacles and plugs.
The fact of the matter is that the switch has to be placed somewhere. And it’s safer to place the switch between the load and the live wire, rather than between the load and the neutral wire. Designing a system where the live and neutral can easily be known makes it easier to do the safer thing.
you don’t have to have three phase circuit to be affected by floating neutral in three-phase substation upstream. in some places in us there are 208v interphase three-phase circuits, which give 120v phase to neutral, which is distributed as a pair of wires as single-phase circuit. this is also normal way to deliver single-phase power in europe, as it’s most efficient use of conductor. (from 400v three-phase circuits) in case more power is needed than single-phase circuit can deliver, three-phase circuit is installed
if there’s switch on device, it’s 2p1t meaning both phase and neutral are switched. if it’s permanent, non-pluggable circuit, like lightning, it’s okay if only phase is switched (neutral is connected permanently)
This is entirely an US problem created by the “center tap” nonsense. Nobody else I’m aware of uses that, let alone with that other. The outlet in question (type F) I’m only aware of being in use with one phase of a there phase supply plus the neutral, or just “the” phase and neutral. Note that in the second case, even if a house or apartment only has one phase wired to it, it’s still generally part of a three phase supply, but the other phases just aren’t wired to that particular place (incredibly rare these days, but might be the case for very old homes/installations).
Now the real reason for it being safe: The neutral is required to be wired to ground at the main breaker panel. With installations newer than 2000-something, every circuit has to be GFCI protected. With even newer installations having even more granular requirements (not sure on the specifics).
in many flats even recently built you don’t get three-phase power, just single phase, but building divides single three-phase supply into three groups of single phase circuits like you say (do you really need 20kW in residential flat? one that doesn’t use EV charger, built in 90s-10s?) i guess it depends on country also. separate houses tend to get three phase connection where i live
floating neutral will also be a problem in american-type two-phase installation, might be even worse (more frequent) on account of large number of lightly maintained transformers used (why on gods green earth there’s few-kV medium voltage line going down every street, americans make it make sense)
Yeah, you’re probably right. I’m in over my head on this discussion.
I am reminded of my first day in an electrical engineering circuit theory class, when the professor made very, very clear that he was teaching us theory and fundamentals, and that the real world of electricity required a lot more safety built into the procedures and designs, because not everything behaves the way the undergrad textbook describes.
The actual electrical device can be designed such that it depends on exactly which direction is live and which is neutral.
Imagine a circuit loop that, as you follow along the circuit, has an AC power source, then a switch, and then the electrical appliance, leading back to the AC source it started from.
If you design the circuit so that you know for sure that the live wire goes to the switch first before the actual load, then your design ensures that if there is a fault or a short somewhere in the appliance, it won’t let the live power leak anywhere (because the whole device is only connected to the neutral line, not the hot live voltage that alternates between positive and negative voltage). It’s safer, and is less likely to damage the internals of a device. Especially if someone is going to reach inside and forgets to unplug it or cut power at the circuit breaker.
In practice tons of outlets are wired the wrong way around.
F actually has a convention for the socket, which is probably ignored even more often, but I would never trust live and neutral not to have been swapped somewhere regardless of outlet.
Just forcing plug designers to consider live/neutral being randomized in a very obvious manner might be safer in the long run than working on a partially broken system where someone manufacturer might be fooled into trusting it.
Could you give an example of such a device? Seems like a bad design
A simple lamp can demonstrate.
You have both live and neutral lines in the cable, coming up to a switch, which can either open the circuit on the live line or the neutral line. Then, the lamp itself has a single light bulb as the load.
If you place the switch on the live line, then the energy of the live line stops at the switch, with only whatever lower voltage is in the neutral line to actually be connected to the light bulb and lamp assembly.
But if you place the switch on the neutral line, you’re leaving the entire lamp on the voltage of the live line, which gives the voltage more places to potentially short circuit. If you were to take a non-contact voltage detector, you’d be able to detect a live voltage in the line leading up to the bulb, even when it’s not turned on.
You generally do this with the in-wall wiring and switches, too, and make the wall switches break open the circuit on the live line, not the neutral line. It’s just a better practice overall.
And no, the neutral line is not totally grounded, so it can still pose a danger, too. But safety is exercised in layers, and putting the switch on the live line is the better practice.
That’s a lamp in theory, but do you know of any actual lamps being sold where this matters?
This is an entirely theoretical problem that just doesn’t exist in practice. Just to be clear, for it to short circuit, it’s needs to find a path to ground. It can’t just “go somewhere”. Just because the line is longer didn’t make it more dangerous for it to “just exist”. There are regulations for wires, which include frankly absurd safety margins, regulations for the electrical devices that are not optional either (CE compliance for example). It just complicates this for basically no reason to have keyd outlets.
If there’s electricity reaching the bulb it would be lit no? So if I place the contact upside down, I wouldn’t be able to turn off my light?
It’s not electricity, exactly, but it is a higher voltage that is different from the average of everything around it. Electricity needs a closed loop to flow, and breaking open the loop with a switch means that no electricity flows, but the voltage of the live line goes up and down, creating an electric potential with anything that might be at a different voltage, if a conductor touches both.
it’s a bad practice to design appliance in such a way to assume that neutral will have low voltage, because in case of neutral failure in three-phase circuit you can get full voltage there, and there can be a couple of volts difference (sometimes more) between neutral and ground even in normal circumstances
it’s better to cut off both live and neutral at the same time anyway, especially if there’s no standard which is which. also, as device designer you don’t know if it’ll be used on a circuit that has neutral and phase where you think it’ll go or not. (ie british appliance used on unpolarized circuit, like type F. adapters exist)
Who’s using three phase in a setting where these types of plugs are used? In the US, at least, three phase circuits use very different receptacles and plugs.
The fact of the matter is that the switch has to be placed somewhere. And it’s safer to place the switch between the load and the live wire, rather than between the load and the neutral wire. Designing a system where the live and neutral can easily be known makes it easier to do the safer thing.
you don’t have to have three phase circuit to be affected by floating neutral in three-phase substation upstream. in some places in us there are 208v interphase three-phase circuits, which give 120v phase to neutral, which is distributed as a pair of wires as single-phase circuit. this is also normal way to deliver single-phase power in europe, as it’s most efficient use of conductor. (from 400v three-phase circuits) in case more power is needed than single-phase circuit can deliver, three-phase circuit is installed
if there’s switch on device, it’s 2p1t meaning both phase and neutral are switched. if it’s permanent, non-pluggable circuit, like lightning, it’s okay if only phase is switched (neutral is connected permanently)
This is entirely an US problem created by the “center tap” nonsense. Nobody else I’m aware of uses that, let alone with that other. The outlet in question (type F) I’m only aware of being in use with one phase of a there phase supply plus the neutral, or just “the” phase and neutral. Note that in the second case, even if a house or apartment only has one phase wired to it, it’s still generally part of a three phase supply, but the other phases just aren’t wired to that particular place (incredibly rare these days, but might be the case for very old homes/installations).
Now the real reason for it being safe: The neutral is required to be wired to ground at the main breaker panel. With installations newer than 2000-something, every circuit has to be GFCI protected. With even newer installations having even more granular requirements (not sure on the specifics).
in many flats even recently built you don’t get three-phase power, just single phase, but building divides single three-phase supply into three groups of single phase circuits like you say (do you really need 20kW in residential flat? one that doesn’t use EV charger, built in 90s-10s?) i guess it depends on country also. separate houses tend to get three phase connection where i live
floating neutral will also be a problem in american-type two-phase installation, might be even worse (more frequent) on account of large number of lightly maintained transformers used (why on gods green earth there’s few-kV medium voltage line going down every street, americans make it make sense)
Yeah, you’re probably right. I’m in over my head on this discussion.
I am reminded of my first day in an electrical engineering circuit theory class, when the professor made very, very clear that he was teaching us theory and fundamentals, and that the real world of electricity required a lot more safety built into the procedures and designs, because not everything behaves the way the undergrad textbook describes.
So I’ve learned something new. Thanks.