4
\$\begingroup\$

The photo below shows an antistatic wrist strap grounded to the chassis of a PC through its alligator clip:

enter image description here

I have three basic questions:

  1. If the chassis above is not earth grounded, does it make no sense to connect the alligator clip to the chassis? In other words should the alligator clip of the wrap always be grounded to the mains earth in a way?

  2. Is this type of wearing antistatic wrist strap aiming to protect the human from ESD shock or sensitive electronic circuits in general?

  3. Is this wrap also used to protect from 230V AC line hazards or used only for ESD?

\$\endgroup\$
0
15
\$\begingroup\$

should the alligator clip of the wrap always be grounded to the mains earth in a way?

Yes, the alligator clip should be earthed. As should the work surface beneath the work piece.

Is this type of wearing antistatic wrist strap aiming to protect the human from ESD shock or sensitive electronic circuits in general?

It's meant to protect the electronics, not the worker.

Is this wrap also used to protect from 230V AC line hazards or used only for ESD?

The wrist strap does nothing to protect from line hazard, for the user or the work piece.

There should be a 1 megohm resistor between the wrist strap and the connecting wire, so as not to increase the risk to the user if they contact a powered part of the work piece.

\$\endgroup\$
13
  • 12
    \$\begingroup\$ It should be earthed, but if for some reason it cannot be then you can connect it to the chassis of the computer tower. Not as good as earthing both, but better than not connecting it to the chassis at all. \$\endgroup\$
    – DKNguyen
    May 6 '19 at 4:45
  • \$\begingroup\$ Can you elaborate a bit your first point? I understand that earthing is a "nice to have" since it prevents charge buildup altogether, but to avoid ESD events connecting the strap to the case is sufficient. Earthing might even not be present/possible/easy to achieve on some devices, depending on how they are built. \$\endgroup\$ May 6 '19 at 7:23
  • 6
    \$\begingroup\$ "There is a 1 megohm resistor" - there should be, but saying there is one without inspecting particular piece may be a deadly misinformation. \$\endgroup\$
    – Mołot
    May 6 '19 at 7:37
  • \$\begingroup\$ @VladimirCravero, that's why both the strap and the working desk should be earthed. In general it's not a good idea to work on devices that are connected to mains power, so following that, also an opened computer isn't directly earthed... \$\endgroup\$
    – aschipfl
    May 6 '19 at 8:44
  • \$\begingroup\$ You mainly want there to be a low-impedance (low in the scheme of things, so 1 Mohm is 'low') between your body and the thing you are working on. If you are earthed, but the device is not, you can still give is ESD zaps if the device itself has charge w.r.t mains earth. \$\endgroup\$
    – Joren Vaes
    May 6 '19 at 12:00
2
\$\begingroup\$

The purpose of an antistatic strap is to protect sensitive electronic components (such as capacitors and MOSFETs) from damage due to an excessive potential difference. Those are particularly vulnerable due to the fact that they have no way of sinking current besides

The reason this happens is usually that one side of the component is at ground level (or close to it) and the other, in this case the one contacting the handling person, is charged and therefore has a large potential difference relative to ground, thereby creating a large voltage across the component.

An antistatic strap, if connected to something with a large capacitance, prevents this by bringing the person's potential to that of whatever it is connected to.

So, the requirements for the connected object are: 1. to be at (or sufficiently close to, i.e. well within the specified rating of the components) the electric potential level the component being handled is at; and 2. to be able to sink enough charge to bring the wearer's electric potential to that level.

This is usually mains ground because it obviously can sink the charge, and the devices being handled are assumed to be at ground level. However, anything that satisfies the above requirements can be used as well.

The computer case is an ideal candidate in this sense, but only if it can sink your charge somewhere. In any case, if you're working with equipment inside the case while connected to it via the strap, you are guaranteed to be at exactly the same electric potential as the case and therefore the equipment within (assuming the latter is also connected to the case).

You can imagine a situation in which this is the only way, i.e. working with something that is permanently charged relative to earth (although that probably means you're at risk of being electrocuted).

So, to answer your questions:

should the alligator clip of the wrap always be grounded to the mains earth in a way?

No, in the sense that it does not have to be mains earth specifically, but yes in the sense that it must exhibit similar properties to mains earth as outlined above;

If the chassis above is not earth grounded, does it make no sense to connect the alligator clip to the chassis?

Connecting the strap to the case will work, provided that: * the case is either sufficiently large or connected to something sufficiently large, and * you're only going to handle parts either already inside the case or that had been in contact with the case. Handling anything else may or may not cause harm to the parts, depending on what the case is connected to, if anything.

2 and 3) The strap is only to protect the parts, not the wearer, but does not endanger the wearer either if properly designed. Faulty or improperly constructed straps, particularly those with the wearer shorted to the connection point, have been known to be the cause of electrocutions.

\$\endgroup\$
1
  • \$\begingroup\$ +1. Surprised such a careful and detailed answer didn't have any upvotes yet. I wonder if you might consider being the 100th committer to Materials Modeling Stack Exchange about designing better materials for semi-conductor chips, more efficient solar cells, photovoltaics, better batteries, materials for energy conversion and storage, stronger and lighter materials for cars, airplanes.. \$\endgroup\$ Feb 26 '20 at 3:53

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.