ESD Table Earth Bonding Points - Why?
We have ESD tables in the laboratory with Earth Bonding Points:
These are directly connected to PE (I measured). It is my understanding that equipment has to be connected to PE directly and ESD mats and bracelets have to be connected to PE through high impedance for both safety and controlled charge dissipation.
It seems that this invites doing something dangerous like:
Not saying one should do this, but one could, meaning at some point someone not knowing might. What is the point of providing these Earth Bonding Points on the front panel of the ESD table? What has the need to be grounded directly that does not already have a dedicated equipment grounding wire?
EDIT: I measured the band attachment and it is indeed 1MOhm. The confusion came from the fact that I had seen specific Earth Bonding Points that integrated the resistance:
Therefore, I incorrectly assumed there is no resistance within the armband itself and that the armband is directly grounded. I understand now.
1 answer
I've never been involved with the design of an ESD table, so I can only imagine some reasons to put grounding connections at the front:
- For convenience. If you've got a table specifically for ESD reasons, you want to be wearing a wrist strap or ankle strap connected to ground. That has to be connected somewhere. A socket right where you need it makes sense. Think of the alternative. You'd otherwise have to run a wire from a water pipe, the ground of a nearby electrical socket with obvious associated risk of error, drag it on a concrete floor, or whatever.
- As a reminder. Seeing it right there may remind you to connect when you might otherwise forget.
- For compliance checking. The manager or anyone else can easily see you are properly connected.
I'm not sure what your second picture is trying to show. As you say, it's important that there is some resistance between your body and ground. You want to slowly bleed off any charge, not cause a spark or sudden current or voltage transient. You also don't want significant current running thru your body in case you accidentally touch a live power wire. A good resistance is usually multiple MΩ. Commercial wrist straps usually have this resistance built-in. You are supposed to connect the far end directly to ground, as you show.
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