|Ask the Experts|
March 10, 2020
ESD Ground Wire Gauge
When hooking up a ground wire from an ESD bench or mat to an earth ground like a wall outlet, what is the recommended wire gauge to use? Does it matter since we're dealing with such low current?
|Expert Panel Responses|
You are right, ESD protected areas have what is called "Equipotential Bonding Systems" which tend to be very high resistance, per standard ANSI/ESD20.20. For example, all wrist straps require an 800K to 1.2M Ohm resistance in series, which is effect in maintaining and ESD equipotential between a human and the work surface, yet high enough to prevent electrocution, should the strap touch line (high) voltages. Work surfaces are required to be < 1X10^9 Ohms, a very high resistance relatively speaking, but sufficient to maintain an ESD equipotential.
So the size of the wire that connects the Equipotential Bonding System to earth is not critical.
Some issues to keep in mind however are: flexibility and durability.
Size the wire to prevent it from being damaged from movement, falling objects, or other hazards in the area. Route the wire to keep it out of harm's way. Rout it with enough extra length to run from the desk to the floor and on to the ground point to prevent other wires or objects from hanging on the wire.
Finally identify it (green/yellow insulation or sleeving) with a label indicating its purpose so others will know how to correctly reconnect it when it comes time to move things around.
Senior Project Engineer
Electronic Controls Design Inc
18 gauge stranded wire is commonly utilized for grounding. The center screw on an outlet is the preferred contact, using a ring terminal is recommended, and easiest method.
If you check any of the better know brands, Statico, SCS, Botron, you will find a wide range of grounding cords and devices.
In answer to your question, as detailed in the book "ESD From A to Z Second Edition" by John M. Kolyer and Donald E. Watson, Chapter 9 - Model Specifications - Page 81, the following sections would apply:
188.8.131.52.3 Static - grounding wire system.
The grounding lug of the work surface shall be connected by a lead wire with a resistor to a distribution wire terminating in ground. The lead wire shall be a minimum of 18 AWG, and the distribution wire shall be a minimum of 12 AWG.
There shall be no resistor between the termination and the distribution wire, but there shall be a 1 meg-ohm resistor between the distribution wire and the grounding lug of the work surface.
From your description of "a ground wire from an ESD bench or mat to an earth ground like a wall outlet" that wire would be defined as the lead wire and therefore 18 AWG should be used.
Hope this helps.
Circuit Technology Center
You are correct in assuming that it really does not matter. The ESD ground always has a series resistor of 1 megohm or so to limit current, so the resistance of the wire is extremely small in comparison.
As long as the wire is mechanically robust enough so that it doesn't break under conditions of use, the current carrying capacity is of no concern. Using stranded wire with good flexibility is important, since it is likely that the wire will see quite a bit of flexure over the course of use.
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