Thunderbolt cable spark issue!

Probably not hazardous, but it's not good. That's resulting from something not being properly grounded, either the laptop or the device you're plugging in (if it's not bus powered). Since you say it's only happening when the charger is plugged in, I'm assuming it's probably your computer. Unplug UltraStudio, connect the charger, and then plug in the Thunderbolt cable (without anything connected to the other end). If it still sparks, it's your computer, and I would take it to the Genius Bar or call Apple Care about it.

I get the same when I plug in USB peripherals on my early '13 rMBP. Would be good to know what's going on.

Happened on my 2010 13" as well. It's no problem.

Happens on my early 2013 rMBP as well, with both powered HDDs and HDMI.

Based on the replies I'm assuming it's a combination of two things. Having a powered peripheral is creating a charge of some sort (most likely positive) in the cable. When the Mac is plugged in, the ports become powered. Normally they only provide power when they are being used, however when the computer is plugged in the ports are always powered, that's why you can charge your phone with the lid closed and charger plugged in, but not when it's unplugged. So the positive charge on the cable and the negative charge on the ports from having power on is creating a spark across the ports. This is just an educated guess, so it could be something else. To me it doesn't seem like a good thing per se, but the metal casing on the outside of cables and ports is the ground so it's probably not anything to worry about.

A couple of days ago it fried my mac pro.

Bringing up an old thread here but I've been experiencing this for a year or so. (One spoke to a person at OWC who said it's not totally abnormal.)

2013 Mac Pro being plugged into an OWC Thunderbay. Sparks also happen plugging in an imac. All units properly grounded, plugged into Furman surge protector etc. Has happened in two different locations. Not a grounding issue.

This last spark happened while thunderbolt plugged into Mac Pro, at a new location all the above same proper grounding, and tried to connect to the Thunderbay. Massive spark happened and all usb + thunderbolt and HDMI connections on Mac Pro are now shot. Brought it into the Apple store and they are fixing.

Mega unfortunate. Everyone I've spoken to at apple + OWC seems to be at a loss.

Any ideas?

cheers