View Full Version : Preventing Electro-Static Discharge

March 19th, 2008, 01:54 AM

I need to reseat the cards in one of my IBM machines. I bought a wrist strap and a grounding cord with an alligator clip on it. I figured that I would partly unscrew the plate on an electrical outlet and clip on there. But what I am wondering is if it is enough for the computer to be grounded by keeping it plugged in during the operation? In other words, can I get out of buying one of those really expensive ground mats to set it on? What are those for, anyway?

How about when I go to install a new floppy drive? Can I just touch it on the frame and handle it any way I need to after that?

The strap and cord were just a few dollars - well worth the cost of not damaging a hard to find card.


nige the hippy
March 19th, 2008, 02:09 AM
I'd be really inclined not to unscrew an electrical outlet just in case the crocodile clip wanders!

Best bet for a fairly proper solution would be to wire a plug with just an earth lead, and take it via a 1MOhm resistor to a metal tray or plate put whatever you're working on on that, & croc clip your wrist strap to it also.
You could clip a lead from the chassis of whatever it is you're working on to the tray as well.

At a push, just clip your wrist strap to the PC chassis, it is all about damaging differences in potential, so as long as you are at the same voltage as the workpiece, or anything parts will come in contact with, you're alright.

March 19th, 2008, 02:10 AM
lol, to discharge my vinyl collection, I attached an alligator clip to the ground lead of a power cable & taped it up all nice.

I'm no expert on circuitry, but IMO I think just touching anything that's grounded should do it, unless you do something like do the moonwalk on super-shaggy carpet.

Dwight Elvey
March 19th, 2008, 06:21 AM
ESD is between two objects. Gounding one self is worthless
unless both objects are grounded.
Even without a ground strap, one can do safe ESD work.
Most people have two hands. Use one hand to equalize charge
and the other to install.
Contrary to what I've see as proper procedure, have electical
contact to both the recieving equipment and the item you are
As an example. Installing an EPROM. First put one hand on the
progrmmers chassis.
Remove the EPROM but hold the part such that your hand is
touching at least one lead ( power and ground are best )
Hold it in such a manor that any discharge would need to go
through your hand and there it no direct path to the EPROMs
leads ( Faraday shield ).
Place your free hand on the chassis surface of the equipment
you are going to install the chip into. Keep it there until
you have made contact between the leads of the EPROM and
the socket ( sometimes you might need both hands to get the
leads all aligned ). Make sure this is electrical contact.
Connecting everything to a common ground is a good way
and the recommended way in most ESD protection setups.
It is not always best when the people that are handling things
don't understand were and how static charges occur.
I once watch an assembler walk across the room holding
and EPROM at the end of her out stretched arm with no electrical
connection to her body. The leads pointing way from her body.
I couldn't have imagined a worse method for ESD.
The most common source of static is induction. This is usually
seeded with a small charge from friction.
The easiest one to deal with is getting up from a chair. Always
place your hand on a grounded surface and leave it the until
you are standing up. I had one of the lab stools with arm rest.
I found it convenient to wrap wire around the rest and connect
them to ground. It was quite natural to place ones hands on the
rest while getting up.
Surprisingly, connecting one self to ground while seated will
ensure a good charge on your body once you get up. You must
provide a discharge path until you are clear of the chair!!!!
Induced charges are caused buy the separating of thing.

March 19th, 2008, 11:29 AM
There are these mats roughly the size of a place mat that are anti-static for working on things but those are usually for chip sorting and the like. You could try to look for one of those if you're concerned with ESD.


Dwight Elvey
March 19th, 2008, 04:16 PM
Mats will not work if the person doesn't have a grounded wrist strap,
ground strap on ones shoes, or conductive shoes.
As I stated before. induced charges are the worst. If there isn't
a connection between the mat and your body, you can still develop
a large charge by just lifting your foot.
It needs to be a complete solution or your just fooling yourself.
Discharging to chassis works well if done right and doesn't depend
on possible incorrect use of anti-static devices.