Here is what happened. I have a Belkin Surge Protector. I have connected my iPhone charger (5v 1A) to this and it powers my Pi. I have also connected powered USB hub, transcend hub3k to this surge protector. I connected my Seagate External HDD to powered USB hub and when I tried to connect the USB to Pi, I suddenly felt the current in Pi. (Pi was already running) Obviously I did not connect and I tried to touch the USB port on cable of hub, I could still feel the current.

I had tested this USB hub earlier and it was working fine. So I reconnected everything this time without surge protector. When I touched the USB port, I felt the current(this time was very less). But after a few seconds, when I touched again, nothing happened. I went ahead and connected this to my Pi. And I touched again on the Pi's USB and I feel no current.

Is it common to feel some current on the powered USB hubs? both usb hub and my HDD are USB 3.0. This is my first time using powered USB hub.

I know that I shouldn't touch and play with electronics, however I just couldn't resist (as I was sure that USB hub was not faulty). I am soon gonna get a multi meter so that I will know.

Now, if I had turned off everything and connected, then I wouldn't have known this problem at all! So, if had done that, could that damage my Pi? So I mostly supplied power to Pi from USB ports, how damaging this could have been?

and also how do I make sure that surge protector here is the culprit?

  • 2 hours ago you used a cheap chinese power adapter? raspberrypi.stackexchange.com/questions/15517/… - anyways it should only be 5 volts, shouldn't damage anything, i play with mine all the time while it's running
    – Gotschi
    Apr 27, 2014 at 19:07
  • You felt current? What did it feel like? Could you post a photo of your set-up?
    – joan
    Apr 27, 2014 at 19:10
  • @Gotschi no I did not. I used genuine Apple iPhone 4 adapter (5v 1A).
    – avi
    Apr 28, 2014 at 3:29
  • @joan - let me know if anything was not clear in the question. I will update with a photo soon
    – avi
    Apr 28, 2014 at 3:30

1 Answer 1


Simple lesson on Electrical Engineering.

USB power supplies are (should be) totally isolated from the mains. They are "double insulated" which means no single failure can result in contact with mains voltage. This is usually achieved by 2 independent layers of insulation, each capable of resisting mains voltage. (There have been occasional reports of poorly designed or manufactured devices causing death). Provided the device is undamaged there should be no risk.

Because the 5V output "floats" with respect to earth it can still have a voltage, due to capacitative coupling, which can produce a noticeable tingle, although this is safe. (Provided it is only applied to the external body; medical devices have much more stringent requirements.)

I have an early touch dimmer - which I installed in my house 35 years ago. This actually has a direct mains connection through a 10MΩ resistor to the touch plate which often has a tingle - although the current is limited to 24μA.

NOTE that even though this "tingle" you feel is safe to humans, it may not be to sensitive electronic devices. It is preferable to connect independently powered devices BEFORE applying power to minimise this risk.

Surge protectors are a different issue. They have deliberate connections between the mains A, N & E. There are capacitors (much larger than the stray capacitance inside a power supply), and some combination of active overvoltage devices. These devices require an earth connection, and can be dangerous if this is faulty. It is quite normal for these devices to have some leakage to earth, although this should be (well) below the 30mA which triggers core balance devices. They can cause problems if connected to the same circuit supplying a high current device (due to the voltage drop across the neutral).

In short, the tingle is not a safety issue. You may experience this with most modern electronic devices. The best bet is to plug all interconnected devices into the same power outlet (if possible), and make all connections before applying power.

PS You really don't need a surge protector with the Pi. Provided you power through the micro-USB it is quite well well protected.

  • so I should be fine as long as I don't it?
    – avi
    Apr 28, 2014 at 15:38

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