Is it a bad idea to plug the other end of ESD strap to wall ground?

Question

I have gone through this answer on Superuser, while it provides good information, I still have a specific question from electrical/electronics engineering point of view.

Is it wrong and unsafe to connect the other end of the ESD wrist strap to the ground point on the wall socket?

What harm can it do and if it’s wrong, why?

I was watching a Youtube video about assembling computers by a user who seems to be a professional and he says he has been in the business for over 2 decades.

The video discourages connecting ESD strap to wall socket ground, it also says that when you connect ESD strap to metallic part of cabinet, remove the power plug from the PSU. However except a reasoning that you could get electrocuted with such a setup, no further technical explanation is given.

I have worked in an IT company that was TL9000 quality certified. This standard demands ESD protection deployment wherever necessary. All relevant test benches there had ESD mats connected to common ground. All terminals had a point to plug the ESD strap’s other end and all those were connected to ground and eventually going to the earth pit. So discouraging ESD strap connection to wall socket ground (that ultimately goes into the Earth pit) is a totally new concept to me.

Answer 1

If your ground strap has a proper current limiting resistor, then you would be perfectly fine connecting it directly to ground.

If your ground strap does not have a resistor, then it greatly increases your chances of dying if you connect it directly to ground. If you don't have a resistor in your ground strap, and you touch a live wire while wearing your ground strap, then you will get full line voltage and current applied to your body - and you won't be able to break the ground connection.

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Ground straps are there to drain and prevent a build up of static charge. You don't need a fast discharge.

A ground strap should have a certain resistance (usually 1 megohm or more.) That drains any charge on your body, but limits the current in the event you accidentally touch a live wire.

A 1 megohm resistance will drain the charge on your body quickly enough - your body's capacitance is only some few 10s of picofarads. If you were charged to 10000 V (can easily happen) then it would take less than 1millisecond to discharge your body through a 1 megohm resistor. Fast enough.

If you are grounded through a 1 megohm resistor, and touch a live wire (say, 220V ac) then a maximum current of 0.2 milliamperes will flow through your body. It might tingle if you are exceptionally sensitive, but it won't injure or kill you.

Answer 2

If your wrist strap has built-in resistance, there's nothing wrong with connecting it to mains ground or PSU chassis.

However, if your wrist strap is just a conductor and you happen to touch a live wire, the strap will provide a low-resistance path to ground. This will result in a very intense shock, compared to cases where the current has to go through significant resistance along its path (like your shoes/clothes) and thus is much smaller.

Answer 3

In addition to the caveat of having a resistor built-in so that you don't electrocute yourself if you happen to find a live wire, there's also the question of whether the grounding point that you've found is actually ground.

Consider old construction that's been upgraded a few times and grandfathered into the electrical code. It's grossly illegal to build anything new like that, but it's allowed to stay because it was to code when it was built. So you may have anything inside the walls depending on its age, some of which don't have a ground wire at all, and may be difficult to tell at a glance which is live and which is neutral.

Technically, it's illegal in most codes to create a "bootlegged ground" - that is, to connect the neutral and ground terminals of an outlet both to neutral - but it's frighteningly common anyway, and a cheap 3-prong tester won't show it as anything wrong. But the load current makes the neutral wire not ground anymore, and if it were to break, then the end attached to the outlet would suddenly become live via the load. And if it's bootlegged, then the "ground" also becomes live along with it.

Now consider a polarity-reversed bootlegged ground. If it's difficult to tell which wire is what and it takes more effort to test them than the sparky can give, then you have a 50/50 chance of having it backwards. Now the bootlegged "ground" is connected directly to live! And a cheap 3-prong tester will still think that it's okay! (because the neutral and ground terminals are still connected to each other, which is all it cares about)

Then you plug your grounding strap into what you think is ground...

Answer 4

The issue is your mains ground may be rubbish.

Mains grounding starts at the Ufer-in-foundation, grounding rods, or water pipe connection (Oh, the things that can go wrong with water pipe connections!!)

From there, it goes to your Grounding Electrode System, the above plus the wiring which carries the grounding to the main service panel.

In the main panel is the Neutral-Ground equipotential bond, which clamps neutral near earth and assures the transformer secondary doesn't float at high voltages due to capacitive coupling or leakage. It also gives fault current on ground a way back to neutral. This thing breaking can cause problems.

From the main panel, safety ground is carried onward to any subpanels and to the individual branch circuits. Then, up each branch circuit one by one, each connection daisychaining off the next.

Where can this go wrong? Oh, so many places.

• The grounding source may have an issue because the water company installed a plastic smart meter.
• The grounding electrode cables may be disconnected or corroded.
• The ground wire from the panel may be broken.
• The ground wire for a subpanel circuit may have been landed on a neutral bar, because that's allowed in the main panel (since it's just a funny way to do the equipotential bond).
• The ground wire may be broken.

• A group of receptacles may have islanded grounds, when a 2-wire circuit is extended with 3-wire (w/ground) cable; in the addition area, people typically connect grounds to each other, but don't bring the grounds back to the main panel. In this island of grounds, any ground fault lights up all the grounds in the island, because it can't carry current back to the main panel and the N-G equipotential bond.

• The ground may not even be present at that receptacle. Pre-1960s wiring didn't have ground wires. Sometimes people just slap a 3-prong outlet to replace a 2-prong, so they can plug in their equipment, and leave the ground to dangle. It's the "islanded grounds" problem but just within that receptacle.

• Ditto, but protected by a GFCI, which is a legal way to leave 3-prong outlets ungrounded. This requires a sticker stating "no equipment ground", but people tear off the sticker because it's ugly. Anyway, this "ground" is useless, but at least the circuit is less likely to kill you.

• Bootlegged ground. Sometimes for lack of ground, people connect ground to neutral so that 3-lamp testers will "pass" the receptacle. This is nasty, because if the neutral wire breaks, the ground is energized with mains voltage.

• Someone installing a workshop subpanel off an old NEMA 10 (hot-hot-neutral) circuit that went to a dryer or range. No grounds here, so the whole subpanel's grounds are either islanded or bootlegged!

But yeah, if your grounding is tip-top, then grounding your ESD mat/strap is the right thing provided you do it through a 1-megaohm resistor. The point of that is so if the worst happens and either the ground is energized or you touch mains, current flow is impeded below dangerous levels.

Answer 5

ESD Straps have to be connected to ground. The electrostatic energy of your body shall be discharged to ground instead to your device under test. Be sure to use correct (certified) ESD straps as they have a defined resistance. Inappropriate ESD straps with too less or zero resistance are a safety risk. If you would touch a voltage source a current would flow through your body to ground.