Science desk
< May 8	<< Apr \| May \| Jun >>	May 10 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is a transcluded archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

May 9[edit]

What is the difference between live and neutral in AC current?[edit]

The title explains it all.

~~Sorry the answer is probably obvious but I'm a very stupid man.~~ WikipediaNeko (talk) 05:04, 9 May 2022 (UTC)[reply]

In an electric circuit, those parts that are electrically connected to a voltage source are called "live". Those that are not are called "neutral". Neutral parts should be safe to touch if everything is properly wired. Depending on the circuit, a part that is neutral can become live, though, by someone throwing a switch. Those that are invariably neutral should ideally be grounded (connected to "earth"). --Lambiam 05:52, 9 May 2022 (UTC)[reply]

Try working your way through Three-phase electrical power and Ground and neutral. Dolphin (t) 08:16, 9 May 2022 (UTC)[reply]

To clarify a bit more: The "live" wire (sometimes called the "hot" wire) is kept at a relatively high voltage (mains electricity in most of the world is at the 230-240 volt range, while in North America and a few other places use 110-120 V). This basically means the electrons in that wire are at a higher energy than a reference energy. In general, the reference voltage is the ground (or "earth"). Any time there is a path between conductors at different voltages, electricity will flow. If YOU are the path between the hot wire and the ground, YOU will conduct that electricity, which is generally contraindicated for your continued good health. In most two-wire systems, ground and neutral are the same wire, and use a bus at a central location at your house. For three-wire systems, you actually have two wires, the "neutral" is wired to the bus, while the "ground" is wired directly to a local ground, often a nearby pipe or conduit. This secondary ground is a safety feature that helps protect the entire house from a short-circuit on the local device by shunting any unwanted current directly to local ground instead of through the common neutral bus. --Jayron 32 13:30, 9 May 2022 (UTC)[reply]

There are various kinds of earthing systems. I'm not exactly sure what you mean by "three-wire system". Three wires where? Modern North American residential wiring has live, neutral, and ground, and ground and neutral are usually bonded together on the utility side of the building hookup (a TN-C-S earthing system). But also in many parts of NA, split-phase hookups are used, where three conductors will be coming into the building from the utility, two lives and the neutral. --47.147.118.55 (talk) 01:31, 10 May 2022 (UTC)[reply]

Don't be hard on yourself. This is a question that shows thought. Some people have given answers, but to put things more explicitly: live conductors are kept at some nominal reference voltage (mains voltage, for mains electricity). Neutral is kept at zero voltage relative to ground. For electric current to flow, you need a voltage (potential) difference between two points. The hydraulic analogy: live is the tap, with water kept under pressure, while neutral is the sewer line.

"Okay, so what's the separate earth conductor for then?" Safety. Let's say you have some appliance with a metal body (meaning the body conducts electricity). An energized wire inside it breaks and comes into contact with the body. The appliance won't work because current now has no path to flow to ground...but the body is now energized at the live voltage. If you touch it, you can become the path to ground. With earthing, any conductive parts are bonded to the separate earth conductor. If these parts somehow become energized, an electrical short to ground is formed, which will conduct very large amounts of current. In a proper installation, this will exceed the trip voltage of circuit breakers or fuses and cause them to break the circuit. So now our hypothetical appliance, when the wire breaks, shorts to ground and trips a circuit breaker. --47.147.118.55 (talk) 01:31, 10 May 2022 (UTC)[reply]

Err, "will exceed the trip ~~voltage~~ current of circuit breakers or fuse". The neutral is typically earthed at the substation since all neutrals in a given area are connected. This means that at the consumer end the neutral may be at a few volts to a few 10s of volts due to voltage drop along the line and potential differences between the substation and the premises. The "earth" wire, on the other hand, is a local earthing at the building and will also earth both the building structure (if steel) and the gas or water fittings. This latter is termed "equipotential bonding". However, local codes and regulations vary widely and what is acceptable in one country may be banned in another. Also bear in mind that most of the world uses a nominal 200-230V domestic supply which is a very different beast to the 110V supply met in the USA. Martin of Sheffield (talk) 08:15, 10 May 2022 (UTC)[reply]

(edit conflict) More specifically, it will exceed the trip current of the residual-current device a.k.a. ground fault circuit interrupter, which is a type of circuit breaker, triggered at a current of about 20mA. It will be triggered as soon at the short to earth appears. Without a separate earth wire, the RCD won't be triggered until a person touches the housing of the device where the short occurs and gets shocked. The resistance of the short to earth may be too high to trigger the fuse (normally at about 16A in 230V countries). Practices vary, sometimes an RCD protects a single wall outlet, sometimes it protects the entire house, sometimes it's absent. Furthermore, directly connecting the housing of the appliance to the neutral wire is dangerous. Not only is the neutral wire not always exactly at earth potential (which could give light shocks and spurious triggering of the RCD), but it's not unlikely that neutral and phase get swapped sometimes – in particular in countries where plugs are normally non-polarised, like the German-style Schuko. A device relying on the neutral being at earth potential would be really dangerous when exported to a country with non-polarised wall outlets. PiusImpavidus (talk) 09:51, 10 May 2022 (UTC)[reply]

earth both the building structure (if steel) and the gas or water fittings... However, local codes and regulations vary widely -> where I live (urban France), it used to be common to ground apartment breakers on the water copper pipes. Nowadays that is forbidden - among other problems, if your downstairs neighbor replaced part of their plumbing by PVC, there is no path to earth so the "grounding" does not exist. Tigraan^{Click here for my talk page ("private" contact)} 09:16, 10 May 2022 (UTC)[reply]

Equipotential bonding is not an earth, it is to ensure that the structure and fittings are themselves earthed. Consider a faulty water heater which renders the hot water system live, then without equipotential bonding someone touching the hot and cold taps could potentially(!) bridge the full 240V. Nasty! Likewise wires chafing against a steel beam might make entering or leaving a tad unpleasant. Earthing via the incoming water supply used to be common in the UK, but it was always at the point where the pipe entered the building, not a couple of stories up in the air! Martin of Sheffield (talk) 10:51, 10 May 2022 (UTC)[reply]

Yes, trip current, of course. Totally missed that. Thanks for correcting. --47.147.118.55 (talk) 23:07, 10 May 2022 (UTC)[reply]

What you describe is the operation of a circuit breaker, but in your hypothetical scenario (leak to ground in a residential installation) it is more likely that a residual-current device would trip first. (In many installations, the two functions are combined in a single element, so I guess non-electricians might call the whole thing "breaker".) The following exact values are from the French standard N FC 15-100 but I assume roughly similar stuff goes for most of Europe and probably North America as well.

The circuit breaker trips when the current passing through is over a given value - typically 63A in France (a different regulatory regime applies above that value especially when it comes to wire cross-section etc., and 63A times 230V = 14.5kW is more than enough for most homes, so that’s what is found most often). That value is called

I_{n}

.

On the other hand, the residual-current device trips when the difference in currents between the hot and neutral wire exceeds a certain value. That value should be (near) zero in a normal mode of operation and the threshold (called

I_{\Delta n}

) is usually set much lower - 30mA in new installations in France. (Note that 30mA is still above the "let go threshold" around 10mA, so you can still die from shock even with a functional RCD set at 30mA, but it would require the leakage current to be between 10mA and 30mA; the hope is that this is a rare occurrence even among situations where there is some leakage current. I suppose the value is not at 10mA because the "natural" leakage across a large installation is close to that value.)

In the case of a short-circuit to ground, the current through hot will shoot very high but the neutral wire will see none of it, so both mechanisms can catch the problem. I have not found an easy reference sheet and I am too lazy to actually read the norm, but it seems that the breaker trips in 0.1-5s, whereas the RCD trips in a few tens of milliseconds, so the RCD would go first. Tigraan^{Click here for my talk page ("private" contact)} 09:16, 10 May 2022 (UTC)[reply]

Residual-current device § North America: In the U.S./Canada, residential buildings generally do not have whole-building RCD protection. Instead, outlets with RCDs are required by the electrical code in places where wet environments are expected, such as bathrooms. What you say applies if the circuit has an RCD. --47.147.118.55 (talk) 23:14, 10 May 2022 (UTC)[reply]

360 degree camera question[edit]

I've been watching a series of TV programmes where a person walks somewhere scenic holding such a camera on a stick, which gives their view as they progress. These shots are intercut with aerial ones from a drone to give a wider view, from which it can be seen that the stick is held in one hand, dark, roughly one metre long and one centimetre in diameter. There are always some shots from the camera which show the full upper side of the hand holding it - but the stick is never visible. The universality of this makes me think that its an inherent part of how the image is constructed rather than arty editing, as why would that always be done? Can someone explain, please? >2A00:23C6:AA07:4C00:EC93:EE2B:7F99:B899 (talk) 19:53, 9 May 2022 (UTC)[reply]

Maybe the camera on the stick uses a fisheye lens with a 180 degree view, so the image includes stuff directly below the lens, but not stuff behind the lens. Fisheye lenses introduce distortion, but that can be undone by software automatically, I believe. I.e. no arty manual editing, but just transcode the video with an "undistort" feature enabled. 2601:648:8202:350:0:0:0:4671 (talk) 20:52, 9 May 2022 (UTC)[reply]

I assumed this is Walking with... (2019 TV series). Here's a set of stills from an episode of Yorkshire Walks, two of which show the phenomenon. I've cogitated over this myself, but it was years ago and I forget what I decided. The IP poster above me has a plausible sounding explanation, except I for one don't comprehend the part about "behind the lens". Omnidirectional (360-degree) camera has an example of such a lens being used with a selfie stick: it is much more distorted, so I think the inverse convolution is relevant, but I'm not sure how that would delete the very visible selfie stick from the image. In fact it looks (in the TV series) as if the camera is pointing directly down the selfie stick, as if the stick was attached to the middle of the lens. But I'm sure that isn't the case: here's an image showing the selfie stick, attached in an ordinary way below the camera. Card Zero (talk) 21:21, 9 May 2022 (UTC)[reply]

Try searching for "invisible selfie stick". Apparently there are two cameras, one each side of the stick. There is nothing that the stick hides from both cameras. catslash (talk) 21:42, 9 May 2022 (UTC)[reply]

Oh, of course. Then the stick can be identified as unlike the background, and removed during stitching. Card Zero (talk) 21:53, 9 May 2022 (UTC)[reply]