Sorry if this is a noob question... I've Googled for instructions on how to use a wrist strap, but most of the answers I've seen assume that you're going to be opening up your PC and installing or working with boards in it, so they all say to turn off the power and attach the strap to the PC case. My situation is that we have a separate board that will be connected to the PC via a USB, but I will not be opening up the PC and don't plan to turn it off. I still want to prevent ESD damage to the board. Is it still OK to attach the wrist strap to the PC case (say, to a screw on the back) while the PC is on? Or am I risking injury? (More info: This is in an office building, recent construction, 2nd floor, all carpeted, boss would prefer not to shell out money for a mat.)
As long as you have the proper type of strap you will be safe. ESD (dissipative) straps have a large resistance (~ 1 M\$\Omega\$) in series so that in the chance that the person gets shorted out that they do not end up being wired into mains voltages. The concern here isn't for the ESD , any form of conduction will solve your ESD problems, the concern is for safety. If your boss won't shell out for a mat, just make sure he hasn't cheaped out on the wrist strap too to save a few pennies.!
A wrist strap ensures you do not pick up static electricity and convenionally this plugs onto a carbonized rubber mat (conducting slightly) that goes on the work bench where your equipment is resting to be worked on. This means that your equipment is drained of static electricity and so are you.
Not using a mat means your equipment finds a potential of its own and this could be ground (assuming it's plugged into the USB and the USB isn't somehow isolated) or it could be anything between ground and thousands of volts - without your equipment sat on its mat what potential does it have?
So it's not on a conductive mat and therefore it's sat at some unknown static potential. You come up to it with your earth strap fitted and touch a sensitive circuit and it's dead.
It's a bit like "batteries not included" because it won't work as intended until you do the job properly. Having said all that I never use wrist straps but I do use a mat and ensure that when working on equipment I "equalize" myself to the case of the equipment. Mind you we don't have carpets at work so this will help.