How can a faulty USB charger kill you?

Question

This question relates to this incident here, where a women in Australia died from a faulty USB charger that allowed the full 240V mains voltage through the USB cable.

Now from my understanding you would have to be connected to the mains ground or the earth's ground in order to get an electrical shock. Otherwise there wouldn't be a potential that would allow the current to flow through her.

Now according to the article which cites the police she received the shock because she was touching her phone which was connected to the charger and her laptop which was also connected to a charger, as well as her EARPHONES which bothers me the most, apparently she had burn marks on her chest where the laptop was and in her ears. Unfortunately there are no details about the devices that were used. So I'm just assuming below.

Here the picture shown in the article explaining the connection:

Now how would this be possible? The phone must have a metal body which is also somehow connected to the power of the phone. The only phone where I can imagine that being the case are the recent iPhone models where the outer aluminum ring on the side is apparently also acting as the antenna (here and here), maybe other aluminum unibody phones as well. Still, is that even a feasible theory? Wouldn't a metal casing be connected to ground instead? How would a ground in a DC circuit act when suddenly the circuit "becomes" AC?

Then her laptop must have had an aluminum body as well which again must be connected to the ground. Again the only case where I heard about the laptop body be somewhat connected to the circuit is unibody MacBooks where users sometimes feel a tingle when touching the laptop while it is connected to the charger but is that even the case? Again it sounds weird that the laptop's body would be grounded: there is a low DC voltage coming in from the external charger so why would that be needed?

And wouldn't the laptop charger not be faulty as well then? How would it allow the current from the phone charger to flow back into the outlet and complete the circuit? Shouldn't the laptop charger isolate the mains from the DC output circuit?

Then the headphones: WHY would the outer casing of a pair of earphones be connected to the ground in any way and how would electricity even flow through the two sides of the earphones (THROUGH her head to the other side), that sounds like a really unnecessary detour for the current.

If this indeed possible like described by the police would this also be possible by a phone and laptop with plastic bodies?

I would love to see a detailed answer: I'm not that well educated when it comes to mains AC power and safety measures taken to avoid shocks.

Answer 1

This was one of those crap Chinese-made Mains-to-USB chargers that can be purchased for as little as US$1.50.

I've taken them apart, and they are bad, criminally bad. The isolation from mains to output is not taken seriously- not enough creepage distance, and in one of the samples I examined, there was debris inside that could cause a direct short if you shook the charger just right.

She could have been touching the earphone plug while unplugging or plugging the earphones into the phone, and perhaps a grounded Ethernet port on an otherwise plastic computer. Once you're connected to the mains, any grounded bit of metal can be lethal. Perhaps the computer was metal and grounded.. most laptops these days have a grounded chassis so either the metal, trim, an exposed screw or anything like that would suffice. Whatever the current path, the muscle contractions probably caused her to grip the conductive bits more tightly rather than flinging them away, and sealed her fate.

Answer 2

What likely happened was that the adapter failed such that the USB ground became hot, which, since it is tied to the phone's chassis ground, made the chassis also mains potential. Since the laptop was aluminum and grounded, she completed a circuit by touching her phone and laptop simultaneously.

Cheapo adapters often forego safety certifications that require certain clearances between main and neutral and ground lines so that they do not contact. A component failure in one of these products could easily bridge a hot line.

Seriously some of these things are scary.

Answer 3

Most of the answers here (while correctly lambasting the low-quality phone charger that energized its output cable to live/phase level) elided this (crucial) part of the question:

And wouldn't the Laptop charger not be faulty aswell then? How would it allow the current from the phone charger to flow back into the outlet and complete the circuit? Shouldn't the Laptop charger isolate the mains from the DC output circuit?

Actually, the laptop charger sometimes has ground-earth connection by design. On one of mine (HP-branded so carrying umpteen world-wide approval seals), it explicitly says "connect only to grounded outlet".

Now this particular charger (model PA-1650-02H) does not have a IEC60950-1 class II mark (square on square) on it. That strongly suggests its output ground is connected to earth ground (in order to comply with the safety regs for class I devices). And measuring with an ohm-meter the resistance between its output ground/negative pin and its input earth prong, I get a value close to 0 ohms (within the error margin for my meter). So, yeah, there are laptop adapters and laptops that have their DC supply (negative pole) earthed by design. And the DC supply's negative pole is connected to the laptop's ground plane. Even on a plastic-case laptop (like mine), the headphone and mic jacks for example have enough exposed outer metal ring to make a connection with the hand/finger possible.

Answer 4

ANYTHING that allows AC mains to connect to your body puts you at risk of death.
The diagram and method proposed may be correct as earth paths can exist in some equipment - either intentional or via components not able to withstand mains voltage or even via noise suppression capacitors.

If the path is to neutral (which it should not be but I have seen appliances connected in that manner) an RCD will not trip.

I would NEVER depend on an RCD being in place to working.
Contacting mains AC (especially 230 VAC) is an invitation to death.

I would consider failure of the primary advice and placing 230 VAC on the user as an entirely adequate explanation of probable cause of death.

Answer 5

The manner of death was not described, but both brain, nervous system, and heart are in the current path.

Ear buds may be considered a lower impedance path than laptop on chest, and so the minimum safe current is lower. It may have been lower than that required to trigger an RCD "safety switch".

Assuming no body contact with building earth, and no earth pins on either plug, an RCD would not have tripped.

In the diagram supplied, a fault in a USB charger may raise the phone to 240V AC potential, and the return path requires no fault in a laptop power supply for current to flow.

The coroner is responsible for any finding on this case, it would be unwise to speculate (as I have) before that investigation has completed. The coroner does not investigate every death. The case is not listed for the following week, we may have to wait a bit:

http://www.coroners.lawlink.nsw.gov.au/coroners/index.html

Reaction by the New South Wales Department of Fair Trading includes their press release here:

http://www.fairtrading.nsw.gov.au/ftw/About_us/News_and_events/Media_releases/2014_media_releases/20140626_safety_alert_usb_style.page

Answer 6

There are several potential points of failure:
Bad insulation between primary and secondary of the main transformer in the power supply. For instance, no double insulated construction.
EMI suppression capacitors not safety rated.
Insufficient "creepage distance" between primary and secondary.
Feedback components between primary and secondary not sufficiently insulated, for instance a defective/counterfeit optocoupler.

Then of course there is the possibility the "manufacturer" made the "charger" as cheap as possible and just forego isolation entirely, thus passing deadly AC voltage unhindered to the USB cable.

Answer 7

In my opinion this case happen because of a touching between the 240 volts main supply and the ground of the USB cable. Thus the current from the supply directly pass through the USB cable.

About the earphone, some earphone have an aluminium speakers housing which allowed the current flow through it.

Then the ground of the laptop charger was connected to the earth pin, which connected to the earth wire.

When she touch her phone, the current flow through her hand then through her ear then through the earphone than to the ground of the laptop that connected to the laptop charger.

Answer 8

My opinion is that some people will hold the hand-held phone with the finger or the hand directly touching the USB phone charging cable. If the USB charger is faulty, AC current will flow through the charging cable and pass to the human body as conductor and then goes to ground either through the feet of the human body to the earth ground ground or to the other possible connections to the ground. The laptop has PCB grounding layer, and the grounding path can bring the AC current back to the laptop power supply, and then to the earth ground.