AC power adapter grounding

Why is it desirable for laptop computer AC power adapters with 3-wire AC cords to have their AC and DC grounds separate?

** Improved safety.

The ground on the DC side isn't connected to the ground prong of the AC cord.

** So if the ground pin ever becomes live, the computer itself does not since the output of the PSU is safety isolated from the AC side.

With desktop computers the metal case gets earth grounded,

** It has only basic insulation, so must be fully earthed.

but with a laptop not connected to any earthed peripherals, wouldn't it be possible for leakage through the AC adapter's transformer to cause a shock hazard?

** Nope.

You would probably have to soak the PSU in salt water for that to happen.

... Phil

Why do people have the obsession that should everything be grounded ?

The problem with grounding is that there is no such thing as "absolute ground potential". People incorrectly assumes such thing exists and this causes a lot of problems, when different equipment are connected with non-isolated connections such as serial RS-232 or USB.

Of course, there are reason for grounding some parts of the circuit. For example electric codes in various countries requires that some parts of the circuit is connected to a "ground". Actually, this is a question of equipotential bonding. The purpose of equipotential bonding is to prevent dangerous (_to_people_) voltages between touchable surfaces (such as device frame and water pipes), but the voltage can still be 50-100 V at least for a few seconds.

These are IEC Class 1 grounded devices, perhaps not sufficient creepage distances etc.

cause a shock hazard?

What is wrong with IEC Class 2 double isolated power supplies ?

Why ?

Having two devices with DC-grounds connected to mains grounds and inserted into separate mains sockets and connected together with serial/USB cables is just asking for troubles, due to ground potential differences.

** Feet on the ground is good ?

Man was never meant to fly ?

Naive assumptions.....

** Nothing - but it is harder to meet the EMI regs without a local ground conductor available.

FYI:

You are wandering about on the WRONG planet !!

What you ( and a great many others ) fail to figure out is WHY class 2 is inherently better than class 1.

Safety wise, that is.

You have to think about " Murphy's Law " in a very literal way.

The way that " roots, toots and electrocutes ".

... Phil

So if the power adapter leaked from the high voltage side to the low voltage side and I stood barefoot on the ground and touched the metal shell of a connector on the laptop, that wouldn't cause a shock?

SPP34-12.0/5.0-2000.

I'm sure some are grounded. The ground can cause ground loops with audio out, perhaps other devices.

Greg

snipped-for-privacy@my-deja.com

** Do you bother even reading what I wrote ?

... Phil

Yes, and that's why I still don't understand why it's safer to keep the gro= unds separate. If the AC adapter's transformer leaks to the secondary but = not to ground, but then a person touches bare metal on the laptop computer = with one hand and earth ground with the other hand, won't he get shocked be= cause the power adapter's DC ground isn't connected to its AC ground?

side

on the laptop,

If the power supply is designed according to IEC 60950 Class II (not Class 2 as I incorrectly said in an earlier post, which appears to be some provincial standard). This IEC standard allows a maximum of 250 uA leakage current, but if you consider this as a "shock" :-), then this might be a problem. This IEC standard also defines how large creepage distances must be used and how insulation material must be used between primary and secondary.

If you really intend to operate your laptop in the shower, you should check that all equipment are at least IP65 environment protection and I would expect that the maker of such devices would also recommend power supplies for that environment. At least in Europe, RCBs are required for mains sockets in the bathroom.

You need to learn the concept and real world application of the term ISOLATION.

Also, most DC dongle adapters do NOT have transformers in them.

Now, figure out why an old AC powered drill motor DOES have a three wire, ground fault protected cord which carries a fault back to ground, and why the new, PLASTIC case jobs do NOT.

Then apply that learned knowledge to a PLASTIC cased DAC powered DC power supply device for ANY human held product.

Now, go stick your finger in an open light bulb socket standing barefoot on a concrete floor. Make sure the switch is on.

Usenet line length is 72 characters.

In Britain, AFAIR, apart from isolated "shaver" sockets, mains sockets are absolutely prohibited in bathrooms. Also light switches must be ceiling-pull. I once broke one, and had to take a leak in the dark :-)

--
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over public relations, for nature cannot be fooled."
                                       (Richard Feynman)

Just pee in the sink ;-) ...Jim Thompson

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I love to cook with wine.     Sometimes I even put it in the food.

They also have fuses in the power plugs in UK.

These may help you locate the toilet in the dark:

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

d I

r e

I noticed in Germany that the bathroom light switches are outside the bathroom.

Hey, that's what I was going to say. For an insulated and sealed laptop switching power supply, you don't really need the ground wire in order to meet the various safety regulations. You do need it to meet conducted radiation (FCC Part 15) tests. The basic problem is that the input filter on the AC input needs to operate in both common and differential modes. In differential mode, no ground is needed because all the EMI junk is across the power wires. However, in common mode, the junk is between each wires and ground, thus requiring a ground return. Some old details on the topic:

Also, I have a few switchers similar to those used in laptops in my wall wart collection that use 2 wire AC line cords. Something like this:

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

That's funny. I just relocated my switch from outside to inside. It's going to take me a long time for my mind to switch.

Greg

Why would anyone put a bathroom light switch outside the bathroom?

More:

etc...

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

I rarely turn the light on when I take a leak, what I need is very easy to find! Mikek

"Fred Abse"

** I knew about the US wiring requiring leakage breakers ( aka "core balance relays " ) in domestic bathrooms - but no-one has ever mentioned a similar UK or Euro rule before.

We have 240VAC power in Australia and no such rule, bathrooms are fitted with standard 3 pin outlets and light switches on the walls. There is also no requirement to retro fit whole house leakage breakers to older buildings.

Many Aussies would have experienced the odd " tingle" from wet switches in bathrooms.

.... Phil

"Jeff Liebermann"

** In the UK, domestic AC outlets have a 32 amp rated capacity - so it is essential to fuse the plugs to protect the flexible cables attached to appliances.

Of course, the fuses are user accessible, so it becomes a bit of a joke in reality.

Plus it offers no shock protection.

... Phil