Machine grounding

Hello ESD Expert,

I would like asking regarding required limits for machine ground. As we knew that there was no compliance verification for it on ESD S20.20. Standard, some person suggest to me to use <1.0 ohm impedance, but I think this different thing, as I know that based on Standard statement that this required limit purposed for Verify Equipment conductor grounding (AC Ground) to nuetral at bus panel.

Could you please advise the required limit that I can apply for machine.

Thank you very much.


Expert might be a stretch :smirk: but am going to try and help. If I understand your question correctly you are simply stating that S20.20 has no requirement for equipment ground verification in Table.2 or 3. So if you wanted to do your own verification what limit would be used?

S20.20 Table 1 identifies the requirements for equipment grounding with a required limit of <1 ohm. This of course would normally be verified at installation. If you wished to make this a routine verification I see no reason why you would not just use the same limit value. There is certainly nothing wrong with exceeding the requirements if your company choses to do so.

Was this helpful? or Could you reword the question and maybe we could do more to help?


ANSI/ESD S20.20 grounding requirements are for the ESD grounding system only. The less than 1 ohm impedance it simply the ground wire or the protective earthing system in the facility. If you use an auxiliary grounding system then the maximum resistance between that and ground has a resistance not to exceed 25 ohms. These are based on North American standards and my differ in other countries.
For a machine ground you need to consult with the equipment manufacturer and the electrical code of your country.
These are typically safety concerns and outside the scope of ESD requirements.

Hi Csmithtrw,

Thanks a lot for the advise, ya actually we would like to do same required limit such as table 1, but I need rationale reason for it, because I think that spec not suitable for mc grounding, because machine is one of part ESD item at EPA Area. If we compare to soldering iron (<10 ohms) or non monitor personel grounding wrist strap (<2 ohms) and also others on table 3, all of them are connected to ac Ground right, but the Spec is not <1.0 ohm. Beside that I have been observating that some machines that they can’t get <1.0 ohm. Then the solution we added wire jumper from ac Ground to the mc body, some of them Ok but the another still out spec. I think some material such metal of body machine, cables
and other are having resistance that effect it can’t get <1.0 ohm. What do you think if we apply <10 ohms for it?

Warm Regards.

Hi JhonK,

Thanks a lot for the advise, same with my comment previously to Csmithtrw, my oppinion is the machine also one part of ESD item at EPA area because ESDS item also contacting on them, so what do you think if we apply the required limits <10 ohms?

I’m only beginner that still learning on ESD, I’m sorry that with all my respect, hopefully ESDA can consider and defind this item with required limits in the future.


Warm Regards.


While I understand your concern, this is more of a safety and machine performance issue. For those reasons, I doubt there will be a general machine requirement. For ESD, if there is no limit or requirement then you must define it. The only powered equipment that there is a limit for are soldering irons. The compliance verification limit there is less than 10 ohm to ground.

What you need to consider it what do you want the limit to accomplish? For ESD consideration, most of the requirements are less than 1x10*9 ohms to ground. This is based on how long it would be to discharge an item (or person) within a short time and keep them at a ground potential. For a machine, you need to consider what the ground is for. If it is for safety (for example you are worried about a power fault) then a low impedance to ground might be what you want.

Hi JhonK,

Thank you very much for your advise and clearly explaination. I got it.


Warm Regards.