Grounding a Chassis Before Opening It

[atakioki]

I've got my ESD mat and strap to ground myself and my workbench, just curious if anyone goes to the trouble of grounding a computer chassis before they open it and start pulling boards.

The advice on the internet is just to touch the chassis to ground it, but with paint and anodizing, that doesn't seem like a reliable way to truly equalize your potential with the chassis.

Would it be worthwhile to build an IEC C13 to NEMA 5-15P cable that contains only the ground conductor as a way to dead-sure ground the chassis without energizing the system?

 

[mdog69]

I've got my ESD mat and strap to ground myself and my workbench, just curious if anyone goes to the trouble of grounding a computer chassis before they open it and start pulling boards.

The advice on the internet is just to touch the chassis to ground it, but with paint and anodizing, that doesn't seem like a reliable way to truly equalize your potential with the chassis.

Would it be worthwhile to build an IEC C13 to NEMA 5-15P cable that contains only the ground conductor as a way to dead-sure ground the chassis without energizing the system?

(For those who live in the UK "IEC C13 to NEMA 5-15P cable" = "kettle lead with a US plug")

For bench work, I generally keep power cords unplugged. Creating special lead, creates the risk that one day you might accidentally connect the wrong lead and end up with an energized computer without realising it.

If you want to earth the chassis, use a lead with a crocodile clip and bond the system to your ESD mat.

The main things are:
- ensure the computer and your body are at the same potential. Even if you are loaded with 20kV of static, as long as you, the PC, and the item you are installing are all at 20kV throughout the task, there's little chance of ESD.
- If you have two items at different potential, ensure that any discharge occurs between items that can with stand it
e.g.
20kV between your fingers and an ESD bag
20kV between the crocodile clip of your wrist band and the PC
20kV between you and the wristband connected to the PC - not an issue the 1Mohm resistor will mean you won't feel it.

If you want to connect your PC to your ESD zone via the , then chop the NEMA plug off a standard black or grey C13 cord, and wire all three conductors to a banana plug/croc clip/press stud or whatever connector your ESD kit uses. Then wrap the C13 end of the cable with some variation of green or green+yellow tape so that you can clearly distinguish your "safe" cable from a normal one.

Personally, for card install/removal, I just use a wrist strap connected to the computer chassis, and make sure that the item being replaced doesn't leave my hands between computer and ESD bag.

 

[Timo W.]

I've got my ESD mat and strap to ground myself and my workbench, just curious if anyone goes to the trouble of grounding a computer chassis before they open it and start pulling boards.

It is actually a safety hazard to ground yourself with a strap and the computer you are working on. If you wear a strap and your workbench is ESD-safe, do not ground the computer you are working on.

 

[MiaM]

For a special cord, you'd use a thinner wire that makes it clear that it's just a ground. But it might be a better idea to make ground wires connecting to for example various types of DSUB shells and other connectors (DIN and such). Almost all computers connect the shield/shell to ground.

In general almost all computers have metal screws that you can touch to ground it, or you can touch the metal part of a screwdriver if the only accessible ground is recessed screws.
Also you can easily touch the pins of an IEC13 male chassis connector.

Warning: There were mains filters that had capacitors across live-neutral, sitting before the mains switch, and if you unplugged one of those devices without pressing power on afterwards, you could get an unpleasant shock from touching the mains input. IIRC I've experienced this on 1970's cassette decks (for the then 220V part of the world).

Alligator clip grounding is at least in theory a good idea, but you have to be careful that the clip don't fall off without you noticing it.

 

[maxtherabbit]

When possible, i.e. when the computer has a physical power toggle switch, I (leave) connect(ed) the standard power lead during the service to provide grounding.

(If I needed to work on the primary side of the power supply obviously I would not do this)

 

[atakioki]

(For those who live in the UK "IEC C13 to NEMA 5-15P cable" = "kettle lead with a US plug")

Sometimes a kettle lead is C13, sometimes it's C15. I mention this only because I got burned by local sourcing giving me C13 "kettle ledes" when I asked for C15s and I had to sit around watching sheep graze for three days waiting for the correct cables to be shipped

I was already planning to use just a thin green grounding wire and nothing else between the NEMA and IEC connector as @MiaM suggested to disambiguate it from an actual power lead, but I like even better the idea of not using a NEMA at all and putting a banana clip or snap connector on the other end and terminating it on my common ground point.

This is probably overkill, but I know static is a problem in my work area. I would be sad to kill something in such an avoidable way.

 

[atakioki]

It is actually a safety hazard to ground yourself with a strap and the computer you are working on. If you wear a strap and your workbench is ESD-safe, do not ground the computer you are working on.

Can you elaborate on why this would be dangerous? I don't have any formal electrical training, I appreciate especially any safety advice you have!

 

[mark0x01]

It is actually a safety hazard to ground yourself with a strap and the computer you are working on. If you wear a strap and your workbench is ESD-safe, do not ground the computer you are working on.

I find that to be somewhat bizzare.
Having worked in hardware in DC's for many years, the equipment being worked on is always grounded through the rack, and the wrist strap is usually just connected to the chassis.

 

[atakioki]

I went ahead and made one of these and tried it out. My tester shows that it's working.. but it's sort of pointless.

Functionally, paint and anodizing seem to be irrelevant for the purposes of static voltages. I can touch off Macintosh G5s and painted chassis and they tone out fine for the purposes of ESD. I thought this might be useful from the perspective of being able to ground devices I can't fit on my workbench (such as mini-computers), but in practice it's just as easy to put a hand on the chassis of the computer and put a hand on the mat and it's grounded. The Quadra 700 is completely covered in plastic, but touching any the shells of any of the ports in the back panel and it's grounded too.

It was fun to try this, but conventional wisdom is correct. Just touch off the chassis of what you're working on for 3-4 seconds and you'll bleed off any charge.