Resistor or low impedance path between common ground point and mains earth?

Question

A simple EPA (ESD Protected Area) consists of 3 things:

• wrist strap
• antistatic mat
• common point ground

Wrist strap and antistatic mat both should grounded to the common point ground.

ANSI/ESD S6.1 recommends a non-resistor ground cord [...] to ground worksurfaces. However, the cord may have a 1 megohm resistor for non ESD purposes.

You might think: just put a resistor between Common Ground and mains earth...

ANSI/ESD S6.1, section 6.4.2 recommends: The resistance [...] from the common point ground to the AC equipment ground shall not be greater than 1 ohm.

Both plans below don't follow ANSI/ESD S6.1, because i think the path between antistatic mat and mains earth should NOT be a low impedance path.

And if there should be a resistor between an dissipative surface (like an antistatic mat) and mains earth for user safety, then there should be a resistor between a conductive surface (unpainted inside of a PC case) and mains earth too.

In theory, is one plan more preferable over the other?

• Plan A
• Plan B The yellow marked resistors are inside the grounding plug.

Answer 1

The reason to ground the PC case has nothing to do with ESD: it's a measure against PC case going live. If the case is grounded, a live wire touching the case will create a prompt short circuit, which will (hopefully) trigger the circuit breaker and instantly remove the power.

Grounding yourself via wrist strap has nothing to do with your safety: it actually makes things worse for you. If you touch a live wire while not wearing the strap, the current you'll get will be limited by your body's capacitance and leakage via your shoes etc. Should that happen while you're wearing a grounded wrist strap (without a resistor), that current will only be limited by your body's resistance, which is not that big at high voltage. That's why you need a resistance in series with your strap.

Now, it is true that your PC case which is grounded without any resistance remains a shock hazard: if you manage to touch it with one hand, and a live wire with the other one, you'll get a bad shock. But this is quite improbable and requires you to do something very bad (touch a live wire) in the first place. A case going live, on the other hand, is not an impossible event: a simple bump on the side may deform it hard enough to touch something on the power supply PCB. Should that happen, you'll have that same very bad stuff right in front of you and without any warning: a 1MOhm resistor can withstand 230V indefinitely.

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Edit: after the exchange in comments it became clear that you don't need protective Earth. In that case, I would go for Plan A, as it has more wiring behind the resistor and less wiring connected directly to Earth. Those resistors are installed for your safety as explained above, so it's clearly better to have more touch-safe wiring on your desk.

Answer 2

If by plan B you mean to add a resistor to the mains ground, then no plan B is not an option. If you add a resistor in series you will have two problems:

1) It affects the saftey of the earth mains, if you size the resistor wrong in the event of a fault you could have an issue with the resistor blowing out or creating common mode voltage in the case

2) You create common mode voltages and cause the whole chassis ground to bounce depending on the amount of current flowing throw chassis ground which could cause problems for the supply and circuitry (if your ground bounces up and down by 10's of mV it might create digital noise, which could be possible depending on the resistor on earth ground.) You would also increase the potential between the PC and other devices creating noise problems.

You can put resistances in series with the wrist strap and the anti static mat, but not the PC. Anti static mats (all the mats I've seen already have MΩ's of resistance in the mat material itself. Most wrist straps also have a 1MegΩ resistor built in.

If your not playing around with anything over 30V you probably don't need to worry about any of this anyway. Keep the mains ground as low resistance as possible as required, and put resistors (that are probably already there, you can tell with a good DMM (voltmeter)) in between the mat and ground and the strap and ground. Shown here:

If look at this question the diagram would look like this, the grounds would be connected in the building wiring:

In the above image the wrist strap is connected to the mat, if it's the kind with a 1MΩ resistor in it, it doesn't matter where you connect it, if you connect it to the chassis (shown as wrist strap1) , it still gets grounded. If you connect the wrist strap to the mat it gets grounded, and if you connected it to the mats ground (shown as wrist strap2) it still gets connected, each time with at least 1MΩ to ground or any other potential that might affect those who might grab onto earth mains. If you take a 1MΩ resistor and poke it into an electrical outlet it's not going to kill you, but a fork might.

Answer 3

Without using the mat case:

There is no meaning for adding the R2 resistor. It is just going to make the discharge slower and it does not add any additional protection function.

When using the mat case:

According to that the discharge resistors of the mat and the wrist strap are already built in, the point of discussion should be around the PC's discharge resistor connected to the ground.

Refereing to plan B, in case you have the PC on your mat (which is the normal thing to do) the PC is already connected to ground through the mat and you dont have to add the discharge resistor between the PC and Ground. Look at the following figure.

If your PC is not on the mat you should not touch the PC's hardware unless the PC is grounded which leads us to the first case.

Additional info:

The equivalent circuit of the setup in this video with roughly estimated values of the resistors is as following:

Answer 4

An Anti-static mat and wrist strap when used for ESD have the job to not allow a potential to be develop across you body or the mat. For example by rubbing against the chair during the winter.