ESD alligator clip on leads while working with unplugged equipment

[jmacion]

A common scenario for field service is that someone will unplug a system they are working on before taking it apart. Then the person will typically put on ESD protection. Usually the alligator clip on ESD protection (to the wrist band) gets attached to the equipment frame but if the equipment is not plugged in, its technically not grounded. So does this really work? Does the metal frame storage some charge?

I know from working at one large medical company that the common practice was to clip into the equipment even though it was unplugged. However, this dosent mean it is right.

What are the thoughts on this?

 

[3DDave]

The goal is to have the person and the equipment at the same potential and bleed off any high-voltage static electricity which is usually on the person into the ground of the equipment. As long as the electrical potential is the same there's far less ESD risk.

I would say no risk, but depending on what they wear, etc. Now this all works much better if the floor is conductive and the workers wear a grounding button on their shoes so that they are bled to ground potential before they get to the equipment and if the equipment shares the same ground potential then ESD chances are much reduced.

 

[IRstuff]

Just an anecdote, a coworker was tasked to verify ESD compliance of work surfaces using a handheld ESD meter; he did a meter zero, walked a few steps to the work surface and found the surface to be noncompliant, at around 90V potential, solely from the static charge from walking the few steps. He had to ground himself to the local ground after the meter zeroing to kill the charge being built up from walking a few steps.

While modern chips are plausibly robust (some boards can handle kilovolt zaps on their external I/O pins, others are not, and you don't know which ones are which until they die, or you've tested them, in the wild.

Grounding yourself to an oscilloscope ground helps to spread the charge, since the oscilloscope, when unplugged, is essentially a large capacitor, like your body. However, there may be residual charges floating around on things that weren't adequately grounded prior to unplugging. I've personally gotten zapped by a piece of equipment whose grounding bar didn't make adequate contact prior to my touching the equipment. I wound up flat on my back from the zap.

My suggestion:
> turn off equipment, but don't unplug yet
> ground yourself to the equipment first
> wait 20 seconds
> unplug equipment

This allows time for you to equalize the equipment and allows time for any high voltages or charge to dissipate beyond before you touch anything inside.



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[itsmoked]

Matters not whether or not the equipment is grounded if you're going with the wrist strap gambit. It's as 3DD stated. "it's only about the potential difference", it's not at all about the potential.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

But, there's no guarantee that everything within a box is going to be at the same potential; I got zapped by a 20 kV residual charge on the inside of an ion implanter, even though the rest of the system was off and still PLUGGED IN.

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[jmacion]

thank you IRstuff, 3DDave, and itsmoked

This makes perfect sense.
Really appreciated.