How to connect live and neutral and earth connection in AC devices plugged into socket?

Question

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.

^{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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

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