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My metal cased laptop seems to be emitting a small electrical discharged, most notable when my finger is lightly run over it.

I recently used this machine (bare foot standing up) for approximately 1hr and I'm concerned it has left me with physical side effects as I have red mark on my hands and a rash on my torso and feet.

Please can you review these ratings to confirm my suspicions may be correct.

Uk Mains supply 230V 50hz 3pin adaptor: 2.5A 250V (3A fuse) Charger body: 1.5A, 100-240V in. 3.65 16.5V out

Voltage recorded with multimeter from hand touching casing to bare foot at ground = 2.4V.

Note: swapping the adaptor to the manufacturers extension lead eliminates all stray current.

3pin extension: 3A 250V in, 2.5A 250V out.

Any advice welcome.

3 Answers 3

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"I'm getting an electric shock from my [insert device name]. I measured [pick a number between 50 and 100] volts with my multimeter! How can you sell something so dangerous?"

A very common question received at my previous job working for a radio manufacturer. All of the radios ran off switch-mode power supplies and therefore were not grounded; the Ground pin of the (UK) plug was plastic, so it couldn't have been.

The reason for the high volume of such complaints is the very nature of radios. The big metal extendable stick on the back is an antenna, and physics/electromagnetic theory dictates that if you put a resistance from the antenna (when the radio is powered) to earth, electrons will flow and therefore a voltage is generated. If that restance is tens of Megaohms, i.e. from a multimeter, you're going to read voltages that would make your hair stand on end. Except not literally.

So, to the question at hand relying heavily on the answer I would provide to those that were worried about possible danger involved:

Voltage does not kill. Ever. Unless it's so high that it ionises atoms in your body, but that's perhaps even beyond lightning-strike territory.

Current kills. This webpage states that death 'occurs' between 100 and 200mA. Interestingly there's a category above 'death', but that's another discussion. 'Sensation' occurs between 1 and 10mA.

Now to the Human Body Model. I used to have a really good diagram of typical resistance values from various parts of your body to your feet, which is generally the part connected to ground. Alas I cannot find this anymore, but this article from the US National Library of Medicine gives figures that are suitable. With the lowest quoted resistance for skin of 1k Ohms (wet, broken skin) and an internal resistance of 300 Ohms, the HBM resistance is 2.3k Ohms (it has to go through the skin twice). This resistance and the "2.4 volts" given in the OP shows that the current flowing is ~1mA, the "threshold of sensation" according to the first link. If you can, measure with the multimeter from the laptop to ground without you touching the laptop - please post the result.

No, the laptop is not damaging your health, but the prolonged exposure may prove uncomfortable and give you the noted symptoms. I cannot imagine the effects would be long lasting - I use a TENS machine regularly when I go to my osteopath and I once spotted in the manual "What to do if you fall asleep while using the TENS". I didn't read the section, but it was pretty inconspicous which suggests that you needn't be concerned if it happens. There were no big skull-and-crossbones images next to it! Consult a doctor for an accurate answer, this is just an educated opinion.

I doubt that the laptop is faulty, nor is the charger. Universal chargers (I'm sure yours is as it can be powered from 100-240V) will, like the radios I mentioned, be made to use in countries that have non-dedicated-ground electrical supplies. Therefore the laptop isn't grounded when plugged in, giving rise to a floating case voltage. Not as bad as the antenna scenario of course. My Sony surround sound amplifier, with only a figure-8 mains lead, has the exact same 'tiny shocks when you run your finger over it' sensation when touching its metal fascia.

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  • The measured voltage from the case to mother earth is ~2V and the measured amps is ~30mA. Also worth mentioning this is an premium grade macbook pro with magsafe power supply and the 3pin plug has a metal earthed prong not plastic. Could it be an issue with the return current @ 3.65A and plug fuse only rated to 3A?
    – Sy_UK
    Jun 6, 2015 at 18:38
  • How did you measure the current (I assume from the laptop to ground)? 30mA flowing through your body will cause definite, if not intolerable, discomfort. I have to admit I'm confused by all the terms you've used in your question; I understand what each value means, I just don't get quite what each is referring to. I am most certainly thrown by "Note: swapping the adaptor to the manufacturers extension lead eliminates all stray current." Could you perhaps draw a simple, labelled diagram of what's going on between the wall socket with all the steps to your laptop (picture of you not needed!) Jun 6, 2015 at 18:48
  • See photo of necessary connections: [img]s27.postimg.org/a50c97dvn/IMG_20150606_221452911.jpg[/img][/… [url=postimage.org/]photo uploading[/url]
    – Sy_UK
    Jun 6, 2015 at 21:41
  • Thank you so far for your input charlie and I apologise for the crude terminology. In answer to your question the voltage was measured from case to ground and from case to ground terminal of 3pin wall socket, with both measurements giving same readings (fluke multimeter). Regarding the adaptor (power supply unit) it has interchangeable heads; 1 x direct 3pin plug and 1 x 2m extension. The noticeable difference between the extension and the shorter standard connection is the wire gauge. Extension looks like 2 core 1.5mm, but the lead from the adaptor looks much smaller, maybe 2 x 0.75mm.
    – Sy_UK
    Jun 6, 2015 at 21:42
  • Metal ground pin on that plug or not, that's the standard Apple charger and it's designed for two-pin systems, which follows my hypothesis on it being a floating ground. I guarantee there is no fault with either the laptop of the charger. Jun 6, 2015 at 21:50
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Well you're not going to get reliable medical advice in this place, but I'll mention that if the ground lead is broken on a 3-wire cord or the neutral and hot are swapped on a 2-wire connection a small current can flow through the galvanically isolated output to ground, should a path be present (such as your body). Of course if your adapter is actually faulty you should stop using it at once- a truly faulty adapter could kill. I try not to bet my life on a few mils of insulation in the adapter.. so no easy path should be provided.

There may be good peer-reviewed information on effects of small currents but basically most ill effects seem to be caused by heating which would not likely be a factor unless there was pain involved. Some people put electric current through their bodies deliberately with TENS units- and there is some wacky thing where they run DC through head electrodes.

Personally I think (non-medical and non-engineering opinion) it unlikely that there are significant physiological side effects from a small (not uncomfortable) current for a few hours, but if you are concerned, a qualified medical doctor would be the place to go.

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  • TL:DR find a grounded outlet. user78147 also mentions a 3pin adaptor, implying that the power supply expects a grounded connection, but no grounded outlet is available, and thus an adaptor is used. A grounded supply not only grounds the output, but also has small filtering capacitors from ground to neutral and from ground to live. If the ground is left unconnected, the capacitors form a capacitive divider, which causes the power supply output to be at a non-ground potential, usually at half of line voltage. While unpleasant, the capacitors do limit the current to a nonlethal level.
    – jms
    Jun 6, 2015 at 13:25
  • @jms I thought that possible, hence the attempt at a more general answer. The use of 'adapter' for both the AC-DC and the plug adapter used in older homes can be a bit confusing. Of course s/he already knows how to avoid it, the question is about worries about her or his health. Jun 6, 2015 at 13:30
  • See photo of potential damage caused from this operation [img]s27.postimg.org/s9m6w6m0z/IMG_20150604_232722278.jpg[/img]
    – Sy_UK
    Jun 6, 2015 at 22:01
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just to trow this out here, do you have it on your lap? in the middle of summer in +20c then you realy need to use a table or a laptop cooler

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  • This is not an answer to the question.
    – fixer1234
    Jun 9, 2015 at 1:30

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