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I know that when I have a fixed lamp in my house I have to use a single switch on the live wire. But when the device is connected using a plug the live wire and the neutral wire vary depending on how I connect the plug. I found a lamp with the following schematics in my lab with also a ground connected only to the metal case of this quartz-tungsten lamp (so, to be clear, the ground is not connected to the live or the neutral but only to the case of the lamp). This lamp is connected using a plug to the socket Type F: also known as "Schuko" which does not have any distinction between live and neutral.

schematic

simulate this circuit – Schematic created using CircuitLab

Is this connection valid? What is the best connection for a lamp which is controlled by a switch and eventually plugged into a socket? Why when I have to install a lamp in my house I have to take care to interrupt the live wire while in many movable lamps the switch could be in the live or in the neutral depending on how I connect the plug?

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  • \$\begingroup\$ Your question is based on speculation and short on necessary detail. Modern wiring standards may use weakly polarized plugs (eg one blade wider than the other) in an attempt to keep the switch on the live side and the live in the center of the socket; also many plug styles which have a good ground connection are implicitly polarized by that. Most likely you should have the wiring of the outlet checked, and if you are concerned about this lamp fixture cease using it. \$\endgroup\$ Commented Sep 18, 2020 at 15:07
  • \$\begingroup\$ Possible your wall-outlet is wired wrong. That's scarily more common than you might think. Check if GND and NEUTRAL are reversed. To be clear, if you 'switch off' the live line, you should have 'no voltage' anywhere your buddy could get shocked. Or there's just something wrong with the lamp (i.e. something doesn't match the idealized circuit you have shown). \$\endgroup\$
    – Kyle B
    Commented Sep 18, 2020 at 15:08
  • \$\begingroup\$ To further be clear, your GND should not be actually connected to ANYTHING except the metal housing of the lamp. It should not have any direct metal connection to either NEUTRAL or LIVE. Use an ohmmeter to verify that. \$\endgroup\$
    – Kyle B
    Commented Sep 18, 2020 at 15:13
  • \$\begingroup\$ @KyleB but it is actually like that, the ground is connected only to the metal case as I wrote in the question \$\endgroup\$
    – G M
    Commented Sep 18, 2020 at 15:13
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    \$\begingroup\$ People do care in theory about the polarity of movable appliances though in many modern ones (eg with power supplies and a protective ground) it doesn't matter much. But the installed base means non-polarized plugs are apparently "grandfathered in" in your region. Wikipedia seems to think that IEC_60906-1 is a proposed polarized replacement for Schuko but admits it has not been widely adopted. \$\endgroup\$ Commented Sep 18, 2020 at 16:02

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This connection is valid.

As the plug is not polarised, the switch may end up in the live or the neutral wire. This could result in one of the lamp terminals being live, even when the switch is off. This is why you should disconnect such appliances from the wall before changing a lamp, not simply switching off. If your retained the instructions that came with your lamp, they should say this.

The ground connection to the metal case of the lamp is there for your safety, it's so that two rather than one thing has to go wrong to present a shock hazard. Without the ground connection, all that is needed is for a short from live (whichever one it happens to be) to case to make the case live. With the ground connection, such a short would blow the supply fuse. It takes a ground connection failure as well as the short to live to make the case live now.

Fixed installations do not have the problems that an unpolarised plug and socket do, so it's possible to insist that the switch is in the live. This means you can expect all fixed appliances to not be live when the switch is off. There are many more fixed appliances than movable ones, so presumably it's thought to be worthwhile to get them the right way round. Note that strictly speaking, you should not rely on an 'off' appliance being 'not live'. Where a technician is expected to work on fixed equipment, it should be connected with a specially rated switch, which ensures 3 mm separation between the switch blades (at least according to UK regulations, YMMV in other places) for safety against mains transients of 1500 V, rather than the fraction of 1 mm needed to physically break the circuit from standard mains voltage.

So why are mobile circuits allowed to be connected to unpolarised plugs? It's been 'grandfathered' in. The first plugs and sockets used on electrical systems were unpolarised, and there are so many about that it's thought to be too unpopular to re-equip with polarised types. Unpolarised plugs are smaller and neater as well. So few people die as a direct result of a plug being inserted with switch to neutral that there's not a strong incentive to actually do anything about it.

So be careful with your movable lamp. Don't stick your fingers in it. Don't attempt to chew through the cable.

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  • \$\begingroup\$ Thanks, but I don't understand why when I do the connection in my house I have to take care that the switch is on the live and when I do the connection for a movable lamp I don't care? this is also often handled much more compared to a fixed lamp \$\endgroup\$
    – G M
    Commented Sep 18, 2020 at 15:37
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But why when I have to put a switch in my house I have to take care it is on the live wire and for movable lamps nobody care?

Ahhhhh I understand the reason for your original post now LOL --- Here's the thing, you are right we should care, but there's an issue of what can actually be practically enforced, what's really dangerous as compared to perceived and what's already out there in the real world. And cost of course...

You're right, ideally the lamp should have a double-pole switch, and disconnect both live & neutral. But either it's not required for illuminaries in your country, or you have a non-conforming product. Illuminaries are handled differently in regulations than most other electronic products I've noticed. Our company UL guy once told me it's presumed that 'common sense' tells even the slowest of folks that lamps get hot and you should be careful with them.

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  • \$\begingroup\$ Oky so in this case for lab equipment, there should be a double-pole switch not a single pole in this way some shocks when handling the lamp when is switch off could be avoided. \$\endgroup\$
    – G M
    Commented Sep 18, 2020 at 16:21
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What you have drawn is the correct wiring for the lamp. For one little detail I would use the "earth" symbol to signify a connection to the real earth.

enter image description here

Figure 1. Image source: LEDnique Ground - earth - chassis.

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    \$\begingroup\$ Thanks, you are right I did not realize that the circuit designer had also earth symbol. However, I don't think this is an answer to my question. \$\endgroup\$
    – G M
    Commented Sep 18, 2020 at 15:35
  • \$\begingroup\$ The additional symbols are from Digikey's Scheme-It drawing package. \$\endgroup\$
    – Transistor
    Commented Sep 18, 2020 at 15:36

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