5
\$\begingroup\$

To earth an Aluminium enclosure, is it safe to use a welded Aluminium stud and then use a nut and washer to attach a hook-ended earthing wire to the stud?

Aluminium oxidises to develop a thin insulating layer of Aluminium Oxide. Could this increase the resistance of the earth path and render this solution unsafe?

Also, galvanic corrosion between steel and Aluminium seems to be an additional potential problem at the interface between Aluminium and Steel. Does this limits the types of materials usable for the nut, washer and hook?

\$\endgroup\$
  • \$\begingroup\$ Why are you afraid of galvanic corrosion? I though you needed an electrolyte for that. \$\endgroup\$ – Vladimir Cravero Feb 24 '17 at 14:49
  • \$\begingroup\$ I am trying to catch all the possible sources of problems (increased resistance of the earthing path). As you suggest, galvanic corrosion seems unlikely inside the enclosure since there shouldn't be electrolytes. But atmospheric humidity might condensate occasionally... The oxidation layer might be a bigger problem. I am just trying to choose the best stud/washer/nut/hook wire combination for Aluminium enclosures. \$\endgroup\$ – Eric T Feb 24 '17 at 15:07
  • \$\begingroup\$ What is the environment like? In a humid environment you will have much worse galvanic corrosion issues. But inside a home or lab it will probably not be a problem for many years. Maybe UL or someone has guidelines for chassis grounding of aluminum cases? \$\endgroup\$ – mkeith Feb 24 '17 at 16:28
  • 1
    \$\begingroup\$ Related question which might provide useful info: "How can I ensure low-resistance ground contacts to an aluminum enclosure?" \$\endgroup\$ – SamGibson Feb 25 '17 at 15:09
  • \$\begingroup\$ Mkeith: our device will be used within a Hospital. We don't expect unusually high humidity levels (sorry, I realise this statement is very qualitative, we will do some more research on the issue of humidity levels in Hospitals). On the other hand our enclosure is not rated as high IP (Ingress Protection). therefore cleaning liquids used by nurses or other clinical support staff could lead to transient (but local) high humidity. \$\endgroup\$ – Eric T Feb 25 '17 at 18:30
2
\$\begingroup\$

After some more research, I have made some progress towards an answer. Here it is the key info of what we have, in case it can help others.

The surface of the Aluminium enclosure, or any other uncoated Aluminium part, is oxidised by atmospheric air, always ending up with a thin (a few nanometers) layer of Aluminium Oxide. This layer is an electrical insulator, but since it is extremely thin, it will be scratched off easily when trying to measure its resistance with a probe or when applying several volts.

Yet, it can be a problem for long-term good earthing.

This problem is obviously very important in aviation, where Aluminium is often used and electrical earthing is improtant.

The suppliers of earthing studs (for aviation and other industries) have come up with a solution. Aluminium studs are coated with a passivation layer (e.g. Titanium) that protects against oxidation but is highly conductive. This is the key bit of info.

Aluminium earthing studs (with a Titanium coating) are welded to Aluminium enclosures. Then nuts/washers/and hook-ended wires can be tightened to the stud making the connection to earth.

BTW, we have also learnt that only Aluminium welds to Aluminium. This is why I am mentioning only Ti-coated Aluminium studs when discussing ways to connect to earth an Aluminium enclosure.

PS: There is of course another possibility, to simply use a through hole, nut and bolt (and washer). No welding. But in this case the oxidation of the Aluminium enclosure (in particular the area below the nut) could lead to high resistance. It seems that welding would be the best solution.

\$\endgroup\$
1
\$\begingroup\$

I don't know about all the safety issues involved with grounding. But we do use a lot of Aluminum panels with various grounds, both screws to connect to the Earth (Ground terminal on the AC plug) and also numerous BNC (or other style) panel mount connectors. All of these have serrated washers with I assume cut through the thin oxide layer. (We do sometimes have issues with the coating on the Al panel, and will have to sand off either the plastic or anodized layer to get good contact.)

\$\endgroup\$
  • \$\begingroup\$ Are you sure BNCs and the like are Al? I would have said it is steel or something. \$\endgroup\$ – Vladimir Cravero Feb 25 '17 at 15:37
  • 1
    \$\begingroup\$ He means he uses a non-isolated panel mount BNC in aluminum panels. The BNC body is steel, but needs to electrically contact the aluminum panel. \$\endgroup\$ – Evan Feb 25 '17 at 18:54
  • \$\begingroup\$ @VladimirCravero, Sorry the BNC panel mounts are not Al, but nickle plated (something).. if they are steel underneath it is non-magnetic. \$\endgroup\$ – George Herold Feb 27 '17 at 14:31
1
\$\begingroup\$

According to The Circuit Designer's Companion (Tim Williams, 1998), the best solution is a force-fit or welded stud, or failing that a shakeproof serrated washer in contact with the aluminium. If the enclosure is in multiple parts, then you need to ensure continity between them.

Diagram from the book

I still need to find out what a force-fit stud is, and how to ensure continuity if I'm using an enclosure of slot together aluminium pieces.

\$\endgroup\$
  • 1
    \$\begingroup\$ A force-fit stud is probably a PEM type fastener that is crimped into the metal. The studs or standoffs are not aluminum so they don't have the same issues. \$\endgroup\$ – Spehro Pefhany Jul 10 at 12:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.