When (if ever) is installing a resistor in the ground path between a workstation and either building or electrical ground allowable? Is it a common practice? what is the downside?
The quick answer is, a resistor is not requied between a workstation and electical ground. In ANSI/ESD S6.1, it states, “Metal benchtops such as stainless steel worksurfaces used in some operations must be connected directly to AC equipment ground with no added resistance. In the event of an electrical short to the conductive surface, safety devices such as fuses, circuit breakers, or a GFCI, in the electrical system, are designed to open and thereby cut off current flow through that specific path to ground.”
However, for all other workstations, while not recommended, it is allowed to add a resistor, typically 1 Mohm, in the ground path. The thought behind this was to create a higher resistance to ground path to limit the current flow from a charged device. The resistor does not prevent that nor does it help with any CDM events.
Best pratice is now to just use wires without any added resistors.
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Thanks John!.. Helps a lot for clarification…
The workstations should always be grounded through a resistor. Zero resistance creates a current spike at discharge.
According to industry grounding standards and IEC 61340‑5‑1–based guidance
“ESD grounding standards require controlled resistance paths to earth ground, usually with a 1 megohm resistor to prevent sudden discharges."
ESD is about preventing charge accumulation AND to discharge charges safely whenever a charge has accumulated.
What JohnK wrote is correct but it applies only when talking about electrical safety and conductive work stations. Not ESD. The resistance to the ground must be low enough to blow the fuse or trip the GFCI etc. For ESD the resistor is a MUST.
This is not entirely correct. If you have a low resistance on your workstation, let’s say a stainless steel worksurface, the 1 Mohm or any resistor in the ground connection will have no effect on a discharge to a part. The current spike that happens is between the item and the worksurface, reguardless of the resistance in the ground cord.
If the worksurface and the item are at different potentials, a discharge could occur. How fast, that depends on the resistance of the contact point, not the resistance in the ground cord. If a CDM type event is a concern, the worksurface point to point resistance measurement becomes the critical factor.
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