Grounding with a wood table vs a metal table standing on a carpet floor

Neither would be grounded, as neither connect to ground. They're both just a floating potential, the same as you.

If you get static shocks when you touch the water taps or large electrical appliances - fridge, cooker etc, then I'd start to worry about it… otherwise I wouldn't bother.

Your computer's case is a faraday cage anyway. Even if you get a spike from it, it's being routed to the mains socket's potential [even if the socket's not grounded you still will reach an equilibrium through it.]
Otherwise, only worry about correct grounding if you're working inside it.

In general, this is not something you need to consider... if the shocks are distressing for you, or you're asking out of curiosity, please read on.

There are two major questions here.

• Are you a "normal user" who is just working with their computer, or are you working with bare electronics, exposed PCBs / ICs, etc...?
• Are you getting static shocks, and is this concerning / distressing for you?

For the first question, if you are a "normal user", then you shouldn't typically need to worry about it. If you open the computer to perform an upgrade (e.g: more RAM), then carefully ground yourself or match your potential to the computer's chassis by touching / holding the metalwork. But if you're just using the fully assembled equipment, then they are (or should be) designed in such a way to handle this properly.

For the second question, the shocks aren't typically problematic, but they can be very annoying and surprising. Higher energy shocks will present as an arc, possibly with a snapping / cracking sound. If you have a pacemaker or similar, then it might be sensible or more important to look into resolving this.

"Ground" is generally used as a good solid reference because it's so readily available, but it's not really the major factor - "potential difference" is (i.e: the difference in potential / voltage between two objects).

If you're in polyester socks, and polyester trousers, and shuffling around on your polyester carpet, etc... then you can work up quite a large static charge quite quickly. Touching something that is at a different potential can cause a shock regardless of whether one or other is technically "grounded". The arc or physical contact causes the potential difference between two objects to reduce.

As above, clothing and other materials can have a large impact on this effect - conductivity being the major consideration. However, other factors such as the humidity in your room can also have a large impact - low humidity (< 40%) reduces the conductivity of the air, and permits a larger buildup of static charge between two objects - conversely higher humididty will decrease the conductivity of the air, and that permits the charge to dissipate more quickly.

Do I need to feel the "shock" from touching something conductive to be grounded?

No. ESD mats are designed specifically to have a fairly high but well defined resistance. Touching one that is correctly setup for a period of a few seconds will disappate your built-up charge, but you won't feel a shock or get an arc (though you may feel it as the charge disappates - e.g: in your hairs).

The shock or arc is the problem for electronics - it is related to a rapid change in potential / transfer of energy, and can cause damage to electronics. A slower discharge brings things to the same potential, but without the damaging effects of a quick discharge.

Would I feel that shock from touching wood, and if not would I still be "grounded?"

A metal table that is directly connected to earth (e.g: via a socket) will have a low impedance path to ground. You are more likely to feel the discharge, and you are more likely to damage equipment.

A wooden table has a far higher resistance, and therefore you are quite unlikely to feel the discharge, even though it can / will still happen.

A final couple of points:

• Please do not play with mains sockets if you don't know what you're doing or have any doubts.
• Please do not connect a metal table directly to ground. A low impedance path will give shocks and can be just as damaging to equipment as a poor / non-ESD-safe environment.

Further response to your situation (upgrading components in your computer)... apologies if this is getting long and in-depth.

If it helps, try to think less about being "grounded" and more about the "potential difference" between objects. Everything is relative, so you can consider your workbench to be your reference point ("ground-ish"), and everything around it to be at some potential from that point. Placing equipment on the desk will allow the potential difference (i.e: static charge) between the desk and equipment to equalize over time - even if the desk isn't very conductive (e.g: wood).

Additionally, keep in mind that "static electricity" behaves differently to "current electricity" (ref). Static electricity is typically many thousands of volts and up.

Given that wood is (generally speaking) more conductive than air, a build up of static charge on your person will "dissipate into" the wooden desk more quickly than it will "dissipate into" the surrounding air - but it will likely be significantly slower than using a proper ESD-safe working environment.

I've quoted "dissipate into" above, because as I've said, what we really care about here is reducing or removing the potential difference between objects... "dissipate into" might not be the best way of thinking about this, but it seems to be common terminology for this effect. "Equalizing the potential difference" is really what we're interested in, and probably helps with mentally reasoning about this.

The rate of discharge (or rather, speed at which equilibrium is achieved) is an important factor when working with sensitive electronics. Discharging too fast is bad and potentially damaging (e.g: metal desk tied to ground), while discharging too slowly (e.g: wooden desk) can mean that the charge doesn't have an opportunity to fully dissipate before you let go and start work.

ESD mats and earthing points are designed with a non-zero resistance (typically about 1 MΩ for an earth bonding point). This is so ensure a controlled discharge over a period of time. They are all about dissipating the static charge (over time), not discharging it (quickly).

As above, the "potential difference" is by far the most important part. When working with electronics, it is the potential difference between your body (and thus fingers) and the sensitive components (i.e: ICs) which is the concern. Electronic devices with metal chassis (e.g: a PC) are often design with their "ground" / 0v / reference tied to the mains-ground and also connected to the chassis. By holding the metal chassis of the equipment, you will bring your body to the same potential, meaning that you are then able to safely handle the components inside. If you aren't wearing an ESD wristband, and you walk up and poke at a PCB directly (instead of touching the enclosure first), then that discharge can occur through the circuit board and/or chips... this may well cause damage.

With this conceptual view, your body can be considered a tiny capacitor (~100 pF) charged up to ~15 kV. You can model other objects the same way. With a low impedance path, the discharge can be fast (< 1 μs), which can result in a spark. There is little total energy here, but the voltage (aka: potential difference) is very significant and that small amount of energy transferred can still be damaging. If the path of that fast discharge is through sensitive electronics components (i.e: the ICs directly), then damage can definately occur.

So, in summary... if you're not working in an ESD-safe environment:

• Always touch the chassis / enclosure / metalwork before putting your hands into the equipment
  • Every time you walk off and return
  • After taking a top off (especially a fleece or similar)
  • Periodically, as a matter of course
• Pick up components by the edge of the PCB wherever possible
  • Don't grab for the board, placing your fingers on the ICs
• Maintaining a low potential difference between yourself and the equipment is more important
• The grounding of your workbench is less important