AdrianPerry gave good advice - If you are developing a charge that you can feel, that is in the oder of Kilovolts - USE that ESD strap!!..
Disagree with attaching the wrist strap to a painted surface. The only think that does is create a capacitor (Metal case one plate, paint the insulator, and you as the 2nd plate) and will not drain the charge (unless it arc thru the paint - LOL).
I'm on the opposite side of the fence, I fell to many under estimate ESD. In another post I found an study that indicated anywheres from 5 to 70% of returned electronic due to failures was because of ESD damage.
My response to a simular question:
Now to dispel some miss information.
(1) A “visible” arc is simply the ionization of the air this is caused when the voltage potential exceeds the ionization required for the air molecules to omit light. Example in a neon bulb the voltage required to ionize the gas is only (if memory serve me right) about 76 Volts and when the voltage across the electrodes exceeds that the bulb give off light and passes current. For air it is much higher I think, without looking up is in the KV and depends on the distance between the two points.
(2) YOU DO NOT HAVE TO DRAW AN ARC to damage a component. When two objects are at a potential difference there is an electro static field between them. The strength depends on two factors: the potential difference and the distance between them. Distance is important as the strength is inversely proportional to the square of distance. IE double the distance and the field strength is reduced to ¼ of original value. While everyone knows that if you draw an arc to a pin, other than ground, you can destroy/damage the component. What is missing is YOU do NOT have to touch the component, or draw an arc!!! Summer and High humidity reduce the ability to build up a charge. Winter time and Relative humidity (RH) of 30% and less allow the body to build up a charge very quickly!! If you have built up a charge, you HAVE an electric field that can induce a voltage in a component. Here is an example: You just received a new memory module and you pull it out of the package. You are very careful to NOT touch the edge connector, RH is 20% (winter time) – You can still induce a voltage that will damage one of the memory chips just by bringing your fingers very close – NO ARC, NO CURRENT, but you induce a low voltage say 20 Volts between two adjacent cells inside the memory chip package. OPPS, they operate at 1->2 volts. Case in point: The AF had a system that measured runway visibility, I dubbed it a Lighting detection system – If they had a lightning strike 5 Miles away the EM pulse would wipe the system out.
(3) You asked about ground. A ground is simply a point that has the ability to absorb or give up enough electrons so as not to take on a voltage level. Normally a ground is connected to earth ground, but not always as a car is NOT tied to earth ground but has a very large mass of metal. For a computer case it is tied to earth ground thru the 3rd wire on the AC input. Under normal conditions the metal case is enough to be considered ground when not plugged in, But NOT always. Case in point, again worse case, winter time and you have built up 20 KVs (AND YESS that is possible) you touch the case that is not connected to earth ground – What happens is you simply transfer electrons to the case to neutralize the potential difference between you and the case. The case is now charged to 10 KV UNLESS it has a path to earth ground. This then creates a potential difference between the case and ALL the grounds inside the case. During this last winter there were several post where people have drawn an arc to the case (Close to the USB front panel ports) and Killed the USB chip on the mother board. This was limited to only a few cases and poor shielding and/or the individual arced to one of the pins on the USB port.
And from another:
http://www.descoindustries.com/pdf/CostofESDDamage.pdf
http://www.static-sol.com/library/ [...] damage.htm
And:
http://en.wikipedia.org/wiki/Electrostatic_discharge
Quote
However, many ESD events occur without a visible or audible spark. A person carrying a relatively small electric charge may not feel a discharge that is sufficient to damage sensitive electronic components. Some devices may be damaged by discharges as small as 10 V. These invisible forms of ESD can cause outright device failures, or less obvious forms of degradation that may affect the long term reliability and performance of electronic devices. The degradation in some devices may not become evident until well into their service life.