EDIT 10/11 9:40am Central: I learned from my partner on this project I got the order of events wrong. I have edited the post accordingly.

I am working on some custom electrical hardware for modifications to a battery powered (24V) vehicle. All of my electronics live inside a painted metal box. Recently, after a laptop was connected via USB to my custom hardware, the laptop was placed onto the metal box and immediately there was a spark, and my laptop screen died without recovery. The laptop was on battery power, and the custom hardware was powered up.

I don't know exactly where the spark occurred. It could have been on the laptop metal case, or at the large data/power docking port on the bottom of the laptop (DELL precision m4800).

About the electronics:

  • The custom electronics operate at 24, 12, and 5V, all powered directly from vehicle battery with DC-DC converters.
  • All of the custom added electronics share a common ground which is the negative terminal of the 24V main battery.
  • I have chosen my 'functional earth' for Ethernet shielding etc to be the box/vehicle frame.
  • All of the custom added hardware lives in a metal box that is electrically connected to the frame of the vehicle (my functional earth). There is a 500kOhm resistance between the vehicle frame, and the negative terminal of the battery.
  • The vehicle runs on rubber wheels, indoors, on a plastic like painted floor.

I don't understand what caused this spark with enough energy to break my laptop. How could such a damaging potential difference form between the metal box and the laptop case/internals?

Most importantly, how can I safely prevent this from happening again?

Thank you for any help and feedback!

EDIT: Newly updated artwork System diagram

  • 1
    Was the laptop running on battery or a wallplug? – PlasmaHH Oct 10 at 21:29
  • Hmmm... 500000 Ohms between devices at 12v is pretty mathematically similar to them not being connected at all. Bonding and grounding connections should be as close to 0 ohms as you can get them. That said, from your diagram it appears that you touched the outer(ground) connector to the casing of your electronics box, and while a simple static shock was a good way to fry a PC in the 80s and early 90s, it's actually pretty rare to fry one that way these days, so you probably gave the thing quite a substantial jolt. – K H Oct 10 at 21:29
  • I made a mistake, and have edited my post with the correct order of events. About the resistance, I thought the resistance between the frame of the vehicle and the battery negative was a safety feature to prevent catastrophe if any positive wire broke and contacted the frame. – ztan Oct 11 at 15:06
  • In most vehicles, the negative side of the vehicle battery is solidly connected to the vehicle frame. – Peter Bennett Oct 11 at 15:36

Was it simply static build up on the vehicle?

Highly unlikely, because Laptop USB ports have protection chips against that.

But they usually connect the USB shielding directly to GND with close to zero resistance.

The following scenario seems a lot more likely:

  • USB cable screen only connected to +24V (or +12V). That would put your metal case at the same voltage, which the 500k resistor would not mind much.
  • Connecing the laptop bridges the USB shield to USB ground line, which is at zero volts (due to being connected to your car electronics).
  • Now there is a dead short of +12 or +24 volts inside your laptop, burning traces and probably leaving a quite noticable skid mark.

You could have prevented this (or at least noticed a burned fuse) by connecing the metal box directly to vehicle GND. There are isolated USB interfaces, but most of the cheaper ones only support full speed.

  • I made a mistake and edited my post with the correct order of events. So the computer USB shield will be connected to the laptop metal case? I was plugging into an STM Nucleo board which has the USB shield connected directly to ground. So that would put my laptop case at ground potential right? How then could there be so much potential between ground and the metal box with enough current conducting ability to make a spark? – ztan Oct 11 at 15:49

500k is sufficient to prevent build-up of a static charge between the circuitry and the frame, static charges build up at sub-microamp rates, but there's nothing preventing the entire vehicle gaining a charge so long as the wheels and the surface it runs on are good insulators - in fact them being insulators is a necessary condition to be able to generate static. It's not unheard of for charges in the tens of kilovolts to accumulate, and unless there's a discharge mechanism before the vehicle comes into contact with anything else, it'll share that charge on first contact - you'd have gotten a jolt if you touched it before the cord did.

There are discharge straps and chains available for carts used in ESD-sensitive environments like electronics manufacture, but if the floor coating is a good insulator this doesn't help. (there was a fad some decades ago for having these things hanging off the backs of cars too). There are (slightly) conductive floor coatings (epoxies/urethanes/films) used in these situations. You could have a sprung contact that discharges the vehicle if it comes in to dock at known locations before anything else contacts it.

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