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


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.

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

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

Best Answer

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.

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.