Laptop case live current

  • #1
DaveC426913
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Summary:

Just realized I'm being zapped by my laptop.

Main Question or Discussion Point

I thought there was a tiny metal burr on the corner of the lappie, and I kept scraping my forearm across it.
Then I thought I had a tiny metal shaving in my skin that I kept brushing on the corner as I typed.

Turns out, that little pinprick I'm feeling is electric current. I can only feel it on thin-skinned, areas, such as the undersides of my forearms.

It goes away when I unplug the power cord. (Of course that doesn't mean it stops, it simply means it's not strong enough for me to feel it.)

If I could find my multimeter, I could figure out the voltage and amperage and figure out if it'll kill me.
 
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Answers and Replies

  • #2
anorlunda
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If you don't post for a day, we'll call 911 for you. (Just kidding.) :smile:
 
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  • #3
berkeman
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Some laptop bricks can be pretty noisy in common mode (CM), but that's the first I've heard of anybody feeling a shock from one. Is it a name-brand power brick? Did it just start happening? How long have you had that laptop and power brick?

If you ground the laptop, does the effect go away? You can ground it by using a USB cable to some grounded USB accessory, for example
 
  • #5
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As far as I know, there are no laptops in the market that are directly fed AC from the mains without an adapter. And the DC voltage should be quite low. Could it be some kind of static charge build-up?

Does your charger have an earth pin?
 
  • #7
DaveC426913
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Is it a name-brand power brick?
Yes. Acer. Original. 19V 3.42A.

Did it just start happening? How long have you had that laptop and power brick?
Hard to say. I might not normally notice it if I'm not resting my arms right.

It doesn't seem to be doing it now (an hour later), which, frankly, is even stranger.

(Have to do with it having reached being full-charge?)


As far as I know, there are no laptops in the market that are directly fed AC from the mains without an adapter. And the DC voltage should be quite low. Could it be some kind of static charge build-up?
It was doing this constantly for at least five minutes, tried both arms.


Does your charger have an earth pin?
Yes, although I can't guarantee the condition of the circuit it's plugged in to. It's plugged into a power bar which is powered from an outdoor outlet. It should be good, having been done just a few years ago by a certified electrician, but I have been having some issues with my backyard circuits futzing out on me.

Other possibly un/related anomalies: for a while the plug into the lappie was futzy. I'd have to twist the plug "just so" to get it to power the lappie. That suggests to me a twisted, worn wire inside. But there is no exposed wiring or anything else amiss, and it seems to be working fine of late.

Argh! Where is that multimeter!
 
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  • #9
berkeman
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It's plugged into a power bar which is powered from an outdoor outlet.
Okay, that's even weirder. If it's plugged into an outdoor GFCI outlet, the GFCI should trip before you feel much of a tingle/shock. In addition to checking things with your DVM, do you have one of these with the GFCI test button? Maybe worth the investment...

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  • #10
DaveC426913
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do you have one of these with the GFCI test button?
Yeah. It's with the multimeter. :sorry:
 
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  • #11
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OK, here's what is probably happening. Your power supply must not be grounded (i.e. 2 prongs, not three). PSs are required to protect the users (output) from electric shock. This is measured as no high voltages (>30Vrms normally), or if high voltages are present, then the leakage current from the output to ground must be less than a specified value. For computers that value (EN-60950-1) is something like 750uA. For medical patient contact equipment it's something like 100uA.

To achieve this safety the PS has an isolation transformer. Let's say your input voltage is 0Vac at the neutral conductor, and 120Vac at the hot conductor (w.rt. earth ground). Then the average voltage on the input circuitry is 60Vac. This is the average voltage difference across the transformer insulation. That insulation will probably act like a capacitor and let some leakage current through. It could also fail and be resistive. In PSs with a grounded plug, the transformer will usually be constructed with an electrostatic shield between the primary and secondary to divert the leakage current to ground. For loads that are grounded (not your laptop, btw), it doesn't matter, they will divert the leakage current to ground.

For PSs that don't have a ground connection, it is required that they have really good insulation (commonly called "double insulated"). In any case, the leakage voltage and current must be tested on each PS manufactured, as well as having the safety agency approve the design.

In addition, EMI filters are required to keep the PS from interfering with other stuff. Without going into the details, an easy way to filter EMI has the side effect of increasing the leakage current. So designers will sometimes trade one bad thing for another to meet the EMI & safety standards at the lowest cost. This leakage current can be a key specification that people like me look for when choosing a PS. For example, what if you make a system that has multiple PSs, each of which claims to meet the spec? OK, but your system may not meet that spec when you connect them all together.

So, the leakage current problem is sort of fundamental. The safety standards allow some, because, frankly, without a ground to send the leakage current to, there will always be some. The fact that you can feel it slightly doesn't necessarily mean that it's dangerous. Your sensory nerves are, well, sensitive, and located at your skin which is far away (electrically speaking) from your muscles. An interesting diversion, btw: the old US military safety standards (1.5mA I think?) weren't based on what it took to stop your heart, they were based on what they thought it might take to make you fall off of a ladder and smash your skull.

The leakage current that you can feel is often too small to trip a GFCI outlet. They don't want to have "nuisance tripping" that is annoying, and is also a safety hazard, because it encourages people to defeat them. GFCI is to save your life not to keep your fingers from tingling.

But, it's pretty terrible marketing for your PC to make you feel like you're being electrocuted. So, good manufacturers will ensure less leakage than the standards allow. This is often done with a grounded (3-prong) power input. OTOH, ancient houses don't always have grounded outlets, so some go for convenience.

So, if I were you, I would replace that PS. Because 1) it sounds annoying, 2) it may be damaged in ways which could be scary.
 
  • #12
DaveC426913
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OK, confirmed. I can still get that zap.

It's very minor. Almost hard to tell, unless I'm paying attention. Like a burr on the metal, or a teeny metal sliver in my skin. But it's not a tingle - it's like a needle.

It happens if I ground myself on something, while the power supply is plugged in.
If I don't ground myself it's less noticeable.
If I unplug the power supply, it stops.
 
  • #13
berkeman
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Were you able to try a grounded USB cable shield to see if that gets rid of it?
 
  • #14
DaveC426913
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Were you able to try a grounded USB cable shield to see if that gets rid of it?
I have a USB phone recharger. Shall I just plug the USB end in and ... I dunno ... stick some aluminium foil in the other end, and bridge it to my rad? o0)
 
  • #15
berkeman
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So this is outdoors, so I assume you are telecommuting from your patio? :smile:

Anyway, simplest would be to find something that is Earth grounded nearby and use a big cliplead to connect the Type-A USB shield at the other end of the cable to that Earth ground.
 
  • #16
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If I unplug the power supply, it stops.
That proves the problem is due to improper grounding.
Yes, although I can't guarantee the condition of the circuit it's plugged in to. It's plugged into a power bar which is powered from an outdoor outlet. It should be good, having been done just a few years ago by a certified electrician, but I have been having some issues with my backyard circuits futzing out on me.
Is it possible to test on a different outlet? Plug it in and see whether it still zaps you. If it does, move on to the next experiment. If it doesn't, the outlet that you generally use is faulty and not the charger.

Next, wrap a wire around the earth pin and ground it manually. If you can still feel the current, it's your charger that is faulty (torn wires inside?). If you don't feel a zap, there is some problem in earthing in your house.
 
  • #17
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It's plugged into a power bar which is powered from an outdoor outlet. It should be good, having been done just a few years ago by a certified electrician, but I have been having some issues with my backyard circuits futzing out on me.
That proves the problem is due to improper grounding.
That seems like a reasonable explanation to me. Wrichik Basu's advice to plug the laptop and charger into an indoor circuit would be my next step.
I remember you saying a while back that you were having trouble with an outdoor circuit, but I don't remember the details. In any case, since you were having problems with an outdoor circuit before, and you're again having problems with the (same?) outdoor circuit, it seems to me that the insulation in the "hot" wire is damaged, and coming in contact with either the ground wire or common wire.

If it happened to me, I would use my GFCI device at the outside receptacle. It will tell you whether the receptacle is connected correctly. If it isn't, I would disconnect that outlet where it's connected to the house wiring.

One possibility is that the outdoor receptacle has the hot (black) and common (white) wires switched. Another is that the cable is connected incorrectly to the house wiring.

If neither of these turns out to be the case, I would suspect that the cable insulation is damaged, causing a short between the hot and either the common or ground. It wouldn't be difficult to check that -- get a roll of Romex of the length you need and appropriate to the amperage of the circuit -- 12AWG for a 20 A circuit, and 14AWG for a 15 A circuit. Disconnect both ends of the potentially damaged wire, but attach the new wire to one end, wrapping the connection with electrician's tape. As you pull the old wire out, you will be pulling new wire in to replace it. If the old cable is damaged, you might be able to find a worn spot on the outer insulation or a place where it was pinched.
 
  • #18
DaveC426913
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Don't seem to have the problem on an indoor circuit.
 
  • #20
berkeman
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Don't seem to have the problem on an indoor circuit.
Did you ground yourself? Are you well-grounded? :wink:
 
  • #21
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I would not recommend that you use yourself as the meter. Instead of being "well-grounded", I would recommend that you remain insulated from the ground while you experiment.
 
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  • #22
Tom.G
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Working outdoors you are probably either physically closer to the earth or on a concrete slab that is on the earth. Either of these increases your capacitive coupling to Earth Ground. Yes, being on a concrete slab makes it worse because concrete is normally damp internally.

I agree that most of the problem is some small leakage in the power brick. The following assumes that the plug on the power brick is not polarized (both prongs the same size). A temporary work-around is to try reversing the power plug when plugging into the outlet.

If that helps you can mark the plug to always plug it in the same way, or, using solder and some small wire, widen the prong that goes into the wide slot in the outlet strip. The wide slot is the Neutral connection that is eventually connected to Ground via the electrical system.

As @berkeman suggested, it would still be a good idea to use a GFCI tester on that outdoor outlet and outlet strip... even when/if you solve the computer problem.

Please keep us updated.

Cheers,
Tom
 
  • #23
DaveC426913
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I'm sitting on a wood deck that is on top of a concrete slab. Not fully insulated.

My power brick is 3-pronged. So is the power bar it's plugged into.
 
  • #24
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There's sensitive, and there's 'sensitive'...
I used to check continuity of our 'OO' model trains' 2-rail power by gently running a dry finger tip along each rail in turn. These carried bridge-rectified but un-smoothed 12 Volts, should have been below sensitivity threshold. My brother could not feel that 100 Hz 'buzz', I could...
 

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