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I've been following a few products that were announced recently that use conducting grid charging and they're all advertised as being safe for users, even though some of these operate at high voltages and currents. These are a few examples of what I'm talking about:

http://www.skysense.co/charging-pad-indoor/specs

http://www.edronic.com/

https://www.flicharge.com/

I'm curious as to how are these charging bases designed to distinguish between human contact and metal contact, what would happen if I were to touch the charging base while the a device is being charged on it?

horta
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afatm
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  • It is not magnetic coupling though, It's conductive charging so the transmitter is a conductive surface and it requires contact with the receiver to be able to charge the device on the other end. – afatm Feb 08 '18 at 06:12
  • Is the output up to 30V 6A unsafe? – Tony Stewart EE75 Feb 08 '18 at 06:26
  • `what would happen if I were to touch the charging base while the a device is being charged on it?` ... do you use tongs to pick up the cell phone from the charging pad? – jsotola Feb 08 '18 at 06:29
  • Tony, isn't that amount of current dangerous? – afatm Feb 08 '18 at 06:36
  • Jsotola, once again this is conductive charging and not inductive charging. Both ends of the of the transmitter and the receiver are made of conductors and it requires physical contact for the charging to happen. – afatm Feb 08 '18 at 06:39
  • *isn't that amount of current dangerous?* A human kan be killed with a lot less than 6A, **however** to make 6A flow through a human you'd need more than 100 kV. Since this only has 30 V only small currents can flow and these small currents are not dangerous. – Bimpelrekkie Feb 08 '18 at 07:48
  • I would not call that "wireless" - yeah it has no traditional wire, but it works just the same with large exposed metal contacts instead of wires. I guess that's where a lot of the confusion is coming from now. – Arsenal Feb 08 '18 at 09:32
  • 30V will give you qute a sting. There may be some mechanism like an RCD to detect current leaking through the user. – pjc50 Feb 08 '18 at 20:24
  • Yes pjc50, if they're smart, they'll detect when a voltage is detected between two charging squares because the drone is assumed to never have a completely dead battery. Only then will they turn on a switch to send power to those two squares. You could try and fool the algorithm and touch those two squares at the same time as the drone to see how much 30V stings... – horta Feb 20 '19 at 23:24

2 Answers2

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This is covered in NEC 725.21, as a power-limited low-voltage circuit.

  • voltage is 30V or less
  • total power is 100VA or less (therefore under 100W)
  • both of these things can be true at different times, i.e. A charger could output 30V capped at 3A then later 16V capped at 6A.

These are given relaxed treatment in NEC because of the low probability of starting a fire and the nil probability of electrocuting someone.

It takes more than 30V to push electricity through human skin, unless you are doing something really weird like planting your face on the power base right after getting a bunch of fresh piercings.

What's more, it isn't terribly hard to put arc-fault AFCI detection in a DC circuit. If someone drops a set of keys on the power block, that will make a distinctive electrical "sound" (you could literally hook up a set of speakers and hear the sound). So the power block can listen for energy usage that is not like a device being charged on battery. Here enters UL and the other listing agencies, who have the power to make that a requirement for a listing. You can bet a key/coin drop will be part of their testing. No UL listing, no sales in the civilized world except through eBay and AliExpress.

Hypothetically it would also be possible for the charging base to have a conversation with the device - especially in the flicharge.com example. If three devices report drawing 70ma, 200ma and 130ma respectively, except the base is seeing 900ma outflow -- uh-oh!

Harper - Reinstate Monica
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30V DC can be considered extra-low voltage and is safe to the touch under most conditions. Under worst case conditions (very low skin resistance due to moisture etc.), you may represent a resistance of as low as 1000 Ohm, so the 30mA flowing through your body could be fatal.
The answer to this question may be interesting for you: How much voltage is "dangerous"?

Manu3l0us
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