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Why do some appliances need grounding and some don't? E.g. my upright vacuum does (three pronged plug) while my shop vac does not (only two prongs). I am asking because I am not an EE professional.

amphibient
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For safety, most countries in the world require that electrical items for sale must not be dangerous even in the case of a single failure, it should take two independent failures before they can give you a shock.

All items need functional insulation to work. However, what happens if something bridges this insulation?

In Class 1 devices, all the metal parts of the appliance are bonded together, and connected to a grounded conductor. If the functional insulation fails and live touches the grounded metal, a fuse blows to remove power. You need to have both failure of the insulation, and failure of the grounding conductor, before you can get a shock. The ground conductor is an essential part of the safety, and is not required for the operation of the equipment.

In Class 2 devices, there is a second independent (supplementary) layer of insulation around the device. Both layers have to be breached before the device is dangerous to the user. This is known as double-insulated, and indicated by a 'double square' symbol on the label. This does not need further grounding for safety.

Many modern Class 2 devices have parts where two layers of insulation are not feasible, for instance the opto-coupler feedback of an offline SMPS. Here, the insulation is upgraded to reinforced insulation, which is specified and tested to a higher standard. It requires larger clearances, roughly twice those needed for functional insulation. The forward transformer is still double insulated, the primary and secondary plastic former each counting as insulation from the core.

Neil_UK
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    A note: Many modern Class 2 devices use what is called reinforced insulation, which is a single insulation barrier that's beefy enough to be both the functional and the safety (double) insulation – ThreePhaseEel Jul 26 '21 at 01:38
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    @ThreePhaseEel 4 mm clearance and 7.5 mm creepage for reinforced, versus half of those figures for functional, basic or supplementary insulation, assuming certain pollution levels, is the basic difference. It might be worth trying to add that in there without confusing the answer. – Neil_UK Jul 26 '21 at 06:07
  • +1 , if insulation is failed (but appliance still doing its job) and due to which small amount of leakage Current is increased , so is there any way for user to know that there is something wrong with insulation ? – user215805 Jul 26 '21 at 06:47
  • @user215805 Unfortunately not. That's why many jurisdictions now require regular PAT (portable appliance testing) and testing of fixed installations. You'll still get many fewer deaths and fires with manufacturers providing class 1 and 2 protection and then the users not testing them, than with them not. – Neil_UK Jul 26 '21 at 09:31
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Devices must be grounded when they have a metal casing without a double insulation between it and the electric parts. They may have grounding to improve electromagnetic noise cancelling. The latter is often found with notebook power supplies.

Janka
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In the US, either the appliance is double-insulated (meaning that there have to be two independent insulating strategies, both of which have to fail before you can touch 120V), or it needs to have a grounded metal casing.

An example of double insulated is a clock radio that has a transformer that isolates the low-voltage side from the high-voltage side for one strategy, and that has all the circuitry inside a plastic casing, with plastic buttons, for the other strategy.

But this is a legal thing, so it'll vary by jurisdiction and when the appliance was built.

TimWescott
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Very nice answers, but the answer is all with a metal casing. Very nice example is any Macbook or metal phones. Macbook has 110 V of alternating current (between absolute Earth ground and ground of device, of course) while charging unless it is grounded (and you need a different USB-C PD adapter's cable (oogh) for this, British has a catch, even though its plug is grounded, there IS NO connection inside so it is not (!)). It was a very crazy revelation... Here is an article, BTW. https://habr.com/ru/company/ruvds/blog/502068/ So, ground your Macbooks, please.

Metal phones and tablets too, so please use grounded PC's USB-C PD, PC can also have up to 98% perfect PSUs! See Titanium 80 plus spec, or even more modern GaN PSUs (I have first one efficiency is 95% on 400 W out of 850 W load, that is nuts).

Of course there is a way to do proper separation. It is done by isolation transformer. But they are very big. Any SMPS is not isolated.

Another example is my LG C9 OLED TV. It is model for Russia and uses not grounded cable, but even though it has ground on the other end (small lego MOLEX-like connector, LOL) and in USA there is a grounded cable, that is not used in Russian model.

There is BTW even a specific name for devices' cables and plugs that do not need grounding: Europlug! See wikipedia, as you can see it is all being compared to Europlug (UN deliberation, I suppose).

P.S. Please remember that it is a very bad idea to use grounded device without proper grounding in your house or appartment.

Валерий Заподовников
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    "Europlug" is not the specific name for cables and plugs that do not need grounding. I can't speak for Asia, Africa, South America, or Australia but in the U.S. they are not called "europlugs". – Elliot Alderson Jul 26 '21 at 01:27
  • It is. As I said look it up on wikipedia. https://en.wikipedia.org/wiki/AC_power_plugs_and_sockets#CEE_7/16_Alternative_II_%22Europlug%22_(Type_C) – Валерий Заподовников Jul 26 '21 at 02:56
  • Also, all other consumer standards besides Europlug, CEE 7/16 are considered deprecated as you can see in the table. I suspect it is UN deliberation. https://en.wikipedia.org/wiki/AC_power_plugs_and_sockets#Comparison_of_standard_types – Валерий Заподовников Jul 26 '21 at 03:04
  • "Of course there is a way to do proper seperation. It is done by isolation transformer." Switched mode power-supplies are isolated by their switching transformer. "They are big." No, they are small. – Transistor Jul 26 '21 at 03:21
  • SMPS are not isolated. That is kinda the whole point. No, they are big and cost x2 (at least) more than SMPS. You do understand that Macbook charger is also SMPS? – Валерий Заподовников Jul 26 '21 at 03:55
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    SMPS can be non-isolated (class I) but can also be isolated (most are, notebook chargers are a common exception). they will have capacitance between primary and secondary though. – tobalt Jul 26 '21 at 05:44
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    Sorry, no, there are multiple names for ungrounded two-connector plugs and "europlug" is not "the specific name". If you had read the entire Wikipedia article you would have also seen "NEMA 1-15 ungrounded (Type A)", "CEE 7/17 unearthed plug", "Soviet standard GOST 7396 C 1 unearthed", "SANS 164-2 2-pin", and so forth. – Elliot Alderson Jul 26 '21 at 13:40
  • @tobalt So you are saying something from Corsair will be isolated and will not have 110 V without grounding?? – Валерий Заподовников Jul 26 '21 at 19:22
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    even isolated SMPS show a high mains voltage on the DC side as common mode voltage..but the impedance of this is rather high. – tobalt Jul 26 '21 at 20:12
  • So that means I am right. Of course, my PC PSU is also grounded after all. And my apartment is grounded :) – Валерий Заподовников Jul 26 '21 at 21:17