Mobile Equipment Grounding

Technical Questions
#1

Hi All,
One of my customer mentioned that grounding requirement for mobile carts have been changed. Only ESD wheel are allowed and no more using grounding chain. Is that true? In fact i did not come across any update on the mobile carts grounding requirement. Thanks

Regards
Andy

#2

ESD wheels will surely provide a better contact with ground than a chain, however theres no requirement how you achieve it. Either chain, ESD wheels or both resistance of mobile cart with ground should be less than 10^9 ohms.

Regards,
George

#3

Thanks a lot for the clarification. I do agreed that ESD wheel provide better contact compare to drag chain. It performed better with loads while drag chain performence does not change with loads. Thanks again

#4

There are no new requirements proposed to say that wheels are required. As with most things, if you make the measurement and I meets the requirements, you are compliant. It does not matter how that is accomplished.

#5

I have a question on hand-held bore scopes used to inspect coated circuit cards inside an enclosure. The boards are coated however there are exposed plugs/connectors that will most likely come in contact with either the end of the bore scope or the cable of the scope. In general, would the tool be considered ESD safe if the user is holding the toll while grounded via wrist strap and heel straps on an ESD floor while the chassis of the enclosure is sitting on an ESD mat???

#6

KCline,

I have never measured the resistance of a borescope’s cable or camera but I would guess that most of them are insulators (high resistance). You could measure the resistance of the cable and camera with a 2-point probe to verify. If it is insulative the ESD risk would likely be small as an insulator can’t discharge.
If there are metal or conductive parts of the camera you could have a potential ESD risk although being in an enclosure could help to suppress any voltage present on the conductor. The best way to check this would be to measure voltage on the conductive parts of the borescope in the enclosure using a high impedance contact voltmeter. If the voltage is below your CDM limit, then the risk should be low.
If you do have a borescope with conductive cables and camera you would want to verify the resistance from the camera to the end held by the operator is conductive. This could be done with a multimeter. If it is conductive, your plan to ground the operator should provide a low risk to an ESD discharge.