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If I were to use a slot together aluminum enclosure for a mains powered device, and I earthed one panel of the enclosure by fitting a stud, how could I ensure continuity of the earth to the whole enclosure, in spite of the oxide layer?

Slot together aluminium housing From http://www.schaeffer-ag.de/en/products/housings/

Naturally I'd want to avoid using six earth studs, which would create ground loops in any case.

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If you want to make this enclosure continuously conductive (for RF blocking purposes, or for mains safety) then you'll need conductive screws. Another thing that may need to happen is to scrape off the oxide layer (or anodization layer which is a manufactured oxide layer).

Another way to defeat the oxide layer is with lock washer screws with grabbers and a regular washer between fastener and threads. With countersunk screws (like the ones shown above) it is not desirable to use washers as it raises the height of the screws.

The continuity can be measured with a digital multi meter (and finding a point with no oxide layer for the probes).

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  • \$\begingroup\$ Was thinking safety from the live supply coming in contact with the case rather than RF blocking. Thinking about it, if the oxide layer is 4nm, 240√2V / 4nm is nearly 85 GV/m, so maybe the oxide just breaks down. I assume the oxide layer is more of a problem in the context of RF? \$\endgroup\$ – William Jul 11 at 15:39
  • \$\begingroup\$ I think they do make conical washers that fit into countersunk holes, but they're not easy to find. \$\endgroup\$ – Hearth Jul 11 at 16:04
  • \$\begingroup\$ Sorry, it also works for AC mains, If for safety reasons, you would need a low impedance pathway to wherever the ground is, I can't remember if this is actually measured by the IPC spec during regulatory. \$\endgroup\$ – Voltage Spike Jul 11 at 16:20
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In terms of the AC Mains Equipment Grounding, it would depends on the maximum current failure for obtaining a certification of the electronics to be placed inside.

If the estimated failure current to the ground through the aluminum box is small, less than 10mA, the screws for the housing panel box should serve as conductors for the whole ensemble. If they do not provide enough conduction at the screw point, a conductive paste could help.

If the estimated failure current is greater than this value, which is the generally understood situation, and the case for Panel Boxes, every metal piece should be properly grounded. Even if there is an electrostatic, non conductive painting for the whole set. Note the standard details shown in the picture is factory welded and painted, with ring wire terminals.

enter image description here

In terms of RF Blocking, this should be supported with a proper calculation for estimate the RF energy emitted outside the box. This is a bit outside the scope of the question, and will require a RF Calculation, RF Modelling or RF Laboratory Testing.

In this case, the ground looping concept is not applicable, because there is not a current loop flowing, nor electronics components exposed to that loop.

At the end, for both two cases, this requires to be in touch with your Certification laboratory in order to know if they are currently accepting using the same mounting screws plus conductive paste as conductive elements. Finally, they will be the ones approving or rejecting your product.

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