Resistance to ground for ESD protection

Question

I want to set up an electronic work area. I'm worried about ESD etc. I'm confused why some aspects of ESD precautions require high resistance to ground (1 Megohm for wrist straps) and others need to be low (milliohms marketed for the soldering iron).

I know the wristband should have higher impedance to reduce the chance of electric shock, but I don't see why such a low resistance is desirable for the soldering iron, if the goal is an equipotential. Yes, when charge is suddenly introduced that needs to be dissipated to ground, a higher resistance means a higher voltage. But since static is likely to be generated by a person, and the wrist strap needs such a high resistance, then the resistance of the soldering iron is immaterial if it is already many magnitudes smaller.

The only reason I can surmise is the iron's earth is less for ESD and more just electrical safety, eg if there's a live fault, so current will surge to earth and fuse will quickly blow. This has then been mis-marketed as an ESD feature.

End then there are ESD mats, where the top layer is "dissipative", i.e. not an insulator but pretty dawned close, (gigohms?), magnitudes greater than the wristband. Why?

What's the reason for differences?

Answer 1

It's what you suspected...

Some soldering irons have the mains cable going straight into them and are safety earthed because they have exposed metal. They're not as ubiquitous as they once were, to say the least. A secondary benefit of the earthing is that it prevents static build-up.

Many modern soldering irons are part of a soldering station, having a mains powered box which delivers safety-isolated low-voltage DC to the iron itself. They don't need a safety earth on the metal body but it's earthed to prevent static build-up.

Earthing straps and so on are earthed through high resistance, for three reasons. Firstly, it stops electric shocks to the user being more hazardous because of the easy path to earth. Secondly, to reduce the discomfort or pain of a sudden discharge when a static charge is touched. Thirdly, to try and reduce the damage to components by lessening the rate of discharge through/across them when a charged part is touched.

Answer 2

Why Megohms for some earthing conductors, and milli-ohms for others?

They have different purposes, and both are for safety.

Where an electrical appliance has exposed metal, and a 3 core cable, a mains fault which connects live to the case should blow a fuse. This requires a low resistance ground connection, low enough to conduct the 100s of amps that are required to blow the mains fuse quickly.

Where a person is connecting themselves to ground to avoid building up a static charge, there is a safety issue that should they come into contact with a live conductor, a low resistance connection to ground would draw a fatal current through them. With a minimum of 1 Megohm in series, the current in this case is limited to non-fatal levels.

Both are edge cases that occur pretty rarely to any one person, but often enough in a large population to be worth legislating for.