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Back around 2010, there were incidents where the surface of laptops have conducted 20-100V and have electrically shocked users. There is no doubt most of the newer laptops still conduct a continuous current on the surface (not static), though probably a smaller voltage. This is most prevailant with apple macbooks, and any dell laptop today.

I have had an intermediate training in physics but still struggle to understand this overall problem. I would like to attempt to make this thread helpful for people with the same struggles, and increase awareness.

Firstly, I cannot find any example of a plastic laptop shocking people. It is a 'common fact' that plastic does not conduct. And yet there are sources saying that anything can conduct given enough voltage. This begs a few simple questions. Is it significantly safer/more comfortable to use a plastic laptop? What kind of voltage would be conducted along plastic if 20V or 230V was behind it? What if 0.35mA was behind it (the typical current leakage of a macbook)?

Secondly, I have measured 1V AC from the surface of my hardwood floorboards relative to the soil outside. How can that be explained, or even quantified?

PS: Those who are electrosensitive have been known to measure their 'body voltage' and found that it increases as they touch or put their hand near laptops without touching it. Since conduction most likely requires contact without arcing to occur, I would say body voltages are not relevant here. The phenomena can possibly be explained by the highly electrical nature of the body, work done by an indian scientist many years ago and the relationship between current and EM fields.

minusatwelfth
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  • It sounds like you're confusing the difference between voltage and current. For example if I stuck a pair of 10 megaohm resistors in a power point and touched the other ends I wouldn't expect to be able to feel it, but they'd still be a 230V AC voltage there. – PeterJ Jun 28 '14 at 07:57
  • Wouldn't there be a current running through it equal to 230/10^7 amps? – minusatwelfth Jun 28 '14 at 08:20
  • Math error. Pair of, therefore twenty, not ten. We wouldn't expect to be able to feel 230/20^7 amps. But don't test this without much research and expert review. – James Cameron Jun 28 '14 at 08:27
  • So where do you think my confusion was exactly? – minusatwelfth Jun 28 '14 at 08:31
  • I mentioned a pair of resistors. But as @James said I wouldn't try it as an experiment, there can be other issues. I was just illustrating that a high voltage at low current normally can't be felt, let alone be dangerous. – PeterJ Jun 28 '14 at 08:35
  • That would seem reasonable. But how do you explain the fact that voltage kills, and not current. How would you calculate the voltage applied to your hand in that case? – minusatwelfth Jun 28 '14 at 08:41
  • The open circuit voltage remains 230V. The voltage with a hand present is different. You calculate this voltage by measuring the resistance of your hand, and applying Ohms Law and circuit theory, treating the hand as one low resistance in a network of three resistances. However, a hand does not react linearly to voltage. The effects on the nervous system of a human correlate to voltage once a minimum current is reached. While it is common to say that voltage kills, it is a mild inaccuracy. – James Cameron Jun 28 '14 at 08:43

3 Answers3

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The incidents are common, but there was a burst of reporting around 2010.

Some plastics are conductive, so it is not enough to look only at plastic versus metal.

Many laptops with a plastic casing have a metallic paint on the inside for electromagnetic compatibility (EMC) radiofrequency (RF) shielding, and for electrostatic discharge (ESD) protection. This is an intentional reduction of resistance and therefore insulation. The laptops made by OLPC where I work have this.

Plastics may conduct better than air, it depends.

Almost no laptops with LED backlighting generate dangerous voltages inside them. Older cold cathode fluorescent lamp (CCFL) backlight assemblies used dangerous voltages. As most laptops are now LED backlit displays, there are no longer dangerous voltages generated.

So you have to think of where the dangerous voltages may come from. Answer: whatever the laptop is connected to. The most likely source of danger is the power adapter. Other sources are network cables, and USB cables attached to devices which have their own power adapter.

If dangerous voltages reach the laptop from outside it, then chances are it won't do much to prevent these voltages from reaching the user. While some plastics may hinder the voltage, there are usually exposed metal parts like audio sockets, USB, network, or screws that will bypass the plastic.

I'll leave your second question and speculative postscript to others. (Please, one question at a time).

See also

Community
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James Cameron
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  • So if you strictly touched plastic while there is a metal screw nearby, there would be conduction? How does that work? I always thought the existence of an arc-field/zone would explain a lot. – minusatwelfth Jun 28 '14 at 08:29
  • The conduction would be from, say, the power input socket, through the chassis or mainboard of the laptop, through the screw, then through the inside metalised paint under the plastic, then through the thickness of the plastic. Nothing to do with arc fields or zones, just simple conduction dependent on resistance of the material. If the voltage fluctuates, such as during a thunderstorm, then there may be an arc through air as the last step. – James Cameron Jun 28 '14 at 08:31
  • Oh so the resistance is just lacking. Is there a physics formula that would apply here to calculate the conduction? – minusatwelfth Jun 28 '14 at 08:36
  • No, it would depend on the material, and you are unlikely to have access to the specifications of the material. A professional insulation test instrument can yield a useful result. Ohms Law can be used to calculate the current given a resistance, and a known dangerous voltage. – James Cameron Jun 28 '14 at 08:38
  • Thankyou. Also, how would one accurately measure the voltage of a surface? Do you just use a multimeter and connect one end to ground? For example, like here https://youtube.com/watch?v=wXCoq3qiBEE – minusatwelfth Jun 28 '14 at 08:47
  • You mean the voltage at a point on the surface that a finger may experience relative to ground? You would attach several test resistances over the range of human skin body resistances expected in the user base. For instance, if the humans were to be wet and naked, you would use a lower resistance than dry and fully clothed with personal protective equipment. A multimeter would then be used to measure the voltage across the resistance. Using only an air gap would give useless results, because you won't know the current available. – James Cameron Jun 28 '14 at 08:52
  • I've reviewed the video. Australian accent, so supply is 230V nominal. I expected about 90-110V with a multimeter, he only got 1V, because his meter was set for DC. Get him to try again, with the multimeter set to AC mode. – James Cameron Jun 28 '14 at 08:56
  • ignore this comment... – minusatwelfth Jun 28 '14 at 09:03
  • Wow really?... That's scary, 100V has killed an electrician before, I think. Am I correct in applying this formula here to work out the applied current to the body http://hyperphysics.phy-astr.gsu.edu/hbase/electric/shock.html? So if you touch an iphone while it's charging you are receiving 100/40000 amps? (40000 ohms assumed to be dry skin, one fingered-contact impedance) – minusatwelfth Jun 28 '14 at 09:06
  • Not at all scary. The current is usually trivial. See the edits to my answer; read the links. As the reference you gave points out, that voltage is only dangerous at more than 10mA current. No, you have calculated wrongly, by failing to take into account the resistance inside the power adapter. Measure the voltage with a resistance to determine the current. Goodnight. – James Cameron Jun 28 '14 at 09:08
  • But I want to know the current running through the body, not a resistor – minusatwelfth Jun 28 '14 at 09:17
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There are a few misconceptions here I think.

Conductivity

It is an arbitrary simplification to divide materials into conductors and non-conductors. Rather than asking whether plastic is a conductor, it is more useful to ask what is the conductivity of plastic. You also need to identify a specific plastic (e.g. Nylon, PVC, PET etc). Most plastic products are mixtures containing fillers which will affect their conductivity.

Most plastics have a very low conductivity, this means that a small thickness of plastic will not conduct a measurable current for typical voltages found in homes.

Voltage, Resistance and Current

You write that "laptops have conducted 20-100V" but voltage isn't really conducted, it is better to think that a current of charge carriers is conducted.

The meaning and relationship of these items is primarily mathematical. It is inaccurate but may be helpful to think of these terms in relation to more familiar phenomenon:

  • voltage is a bit like a force that pushes on electrical charges.
  • electrical charges are like particles that move in an electrical current.
  • current is a quantity of charge moving past a point per unit of time.

To do harm, requires that energy be transferred into the harmed material. With electricity, the amount of energy transferred is a product of time, voltage and current.

Static electricity

Because plastic is a poor conductor, a high static voltage can accumulate on its surface and can take a long time to dissipate (perhaps several minutes, perhaps longer). The voltage can be tens of thousands of volts. However this is not normally dangerous because the energy stored is tiny.

RedGrittyBrick
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Plastics, wood (and similar) are insulators. You are correct in that with high enough potential energy applied across it that current will flow - this is called breakdown.

For example, the phenomena of arcing is when the insulator air breaks down and conducts.

sherrellbc
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