Workbench grounding - Page 1
Forum section: electronics / projects

author: Sigmoid
author_rank: Frequent Contributor
timestamp: October 22, 2013, 03:02:14 pm
content: Hey,I'm improving the ESD grounding of my mini-lab.This one:http://s15.postimg.org/kt8r8s2pn/IMG_0596.jpgThe structure is a two-shelf workbench (metal structure with dielectric coating), with a grounded ESD mat on each shelf (since the photo was made, I put one on the bottom shelf as well).I'm trying to figure out the best way of grounding the whole thing. Currently only the top ESD mat is grounded via the factory 1M grounding point, through the earth ground point of my lab power supply.I'm about to get another grounding point for the bottom mat, and planning to do something like this:Does this look good? Thanks to the dielectric paint on the bench, there will be no resistive path between each grounding point.

author: madires
author_rank: Super Contributor
timestamp: October 22, 2013, 03:18:39 pm
content: IMHO your design should be fine!

author: CodiJack
author_rank: Contributor
timestamp: October 22, 2013, 05:13:27 pm
content: From what I know 1M resistor is placing between "star" connection and ground,not between all surfaces.Have a look for this document:http://www.listaintl.com/sites/default/files/Lista-ESD-User-Handbook-esd_rev2.pdf

author: madires
author_rank: Super Contributor
timestamp: October 22, 2013, 09:05:00 pm
content: Quote from: CodiJack on October 22, 2013, 05:13:27 pmFrom an ESD point of view that would be sufficient but we should add some safety measures in case a mat or the bench is connected to mains voltage by incident. If mats, bench and wrist strap are connected directly all will have the same potential including you wearing the wrist strap or touching any mat. To mitigate the effects of an electric shock you could use additional resistors between the conducting parts for limiting current. For example, if you wear the wrist strap and somehow drop a wire or a tool and connect the mat with mains 120/240V a resistor between wrist strap and mat would save you from some pain.

author: Sigmoid
author_rank: Frequent Contributor
timestamp: October 23, 2013, 05:22:49 pm
content: Quote from: madires on October 22, 2013, 09:05:00 pmThey aren't. It's a star topology, with a single rail connected to earth ground, and separate 1MOhm resistors to each mat, to the metal structure, and to the wristband.Quote from: CodiJack on October 22, 2013, 05:13:27 pmThanks. This says metal structures should be grounded without a resistor... That's actually one of the things I've been considering.I'm wondering if that is a good idea though, in my case. It makes some sense, but could result in a ground loop if a metal screw or other bare part is touched...

author: IanB
author_rank: Super Contributor
timestamp: October 23, 2013, 10:04:23 pm
content: I was going to suggest, as noted above, that you should bond your metal workbench to ground. You can than provide grounding terminals at various places on your bench to clip mats or wrist straps to.It never hurts to bond any exposed metal structures to a good earth ground, just as the electrical code requires for piping or structural metalwork.

author: Nerull
author_rank: Frequent Contributor
timestamp: October 23, 2013, 10:10:55 pm
content: Quote from: Sigmoid on October 23, 2013, 05:22:49 pmI think the idea is that if your bench were to come into contact with high voltage, it should have an immediate path to ground. If your bench is connected to earth via a 1MOhm resistor, the bench will simply become energized instead, fuses/breakers will not blow, and the best path to ground may be through you.

author: Kjelt
author_rank: Super Contributor
timestamp: October 23, 2013, 10:35:36 pm
content: Indeed should the metal of the bench be grounded.Also the tabletop should be of an isolated material such as wood for safety issues.And what surprised me was that all the 1M resistors in the official ESD grounding points were 1W types not the standard 1/4W.If you are planning to work with high voltages also an isolation transformer will be a good safety precaution. And what is also handy is to build a DUT box, a small switch box with a make and break switch powering a larger box with relay(s) that powers your Device Under Test (mains and/or DC). Make sure that the break switch is a safety switch (the big red knob) and the make switch a bit hard to activate (sunken)

author: SeanB
author_rank: Super Contributor
timestamp: October 24, 2013, 06:16:29 pm
content: Reason for 1W resistors is voltage rating. 1/4W resistors typically will be rated to withstand 250VDC, and typically will break over at around 350VDC. Not enough safety margin to withstand mains, as a spike will possibly be enough to break it down and then it will arc over and stay conducting. A 1W resistor typically will withstand 500VAC indefinitely, and with an overload will survive longer without breaking down.

author: Lightages
author_rank: Unknown
timestamp: October 24, 2013, 06:28:33 pm
content: Also make sure that the AC outlets to your test equipment and DOT are connected with a ground fault interrupter if you want the best safety. This can cause problems sometimes when the GFI will trip on a faulty equipment and you are trying to find the fault.In this case just have a non GFI outlet to use too.