How can I ensure low-resistance ground contacts to an aluminum enclosure?

Question

I am building an electronics project with an aluminum enclosure. I need to ground it—and very well—for human safety. (The system handles 300 A of current, and I want to be sure that if there is an internal failure in the unit causing a panel of the enclosure to become energized, a fuse will blow rather than the panel becoming lethal to touch.)

Of course aluminum forms a 4 nm layer of aluminum oxide within 100 picoseconds of contact with air, and aluminum oxide is an electrical insulator with a resistivity of 1x10^14 Ω·cm.

Putting those together means that there is a 31.5 megaohm resistance between a piece of aluminum and a 0.5" conductive disc (such as a metal washer) in perfect contact with it if the aluminum oxide layer is intact.

I know that (for example) mechanically tightening things, particularly using a locking washer with teeth, can easily penetrate the aluminum oxide layer, but I am looking for industry specifications or best practices on choosing washers and torquing the connectors to ensure this happens. These ground connectors might also need be removed and replaced when servicing the equipment, and I want to provide specifications for reinstalling them to make sure this is done properly to ensure the aluminum oxide layer is pierced and conductivity is maintained.

As an example, I'd like to be able to write in the repair manual—with calculations based on sources I can cite—something such as "replace the lock washer with a new one, part number XXXXX, then tighten the bolt to YYYY Newton meters of torque, which will ensure an electrical resistance to the enclosure of less than 2.5 mΩ. Verify this by taking a four-point resistance measurement between test points 17 and 29, which should be 5.0 mΩ or less."

I'd also be interested in things such conductive greases, which I could put on self-tapping studs, that might work particularly well on aluminum to prevent the formation of the oxide layer, and could then be left in place.

Can anyone offer some pointers to resources I could use to learn best practices in working with aluminum as a conductor, and which I could use to develop a set of guidelines for repair and rework of electrical conductors in contact with aluminum?

Answer 1

I am going to elaborate a bit so this becomes an answer.

Typical avionic equipment has a typical bonding requirement of 2.5 to 5\$m\Omega\$ around a chassis. There are different classes of bonding and earthing with their own set of requirements, depending on the threat and application.

There is an excellent survey from NASA on methods used over the years and the rationale behind them.

As already noted, MIL-HDBK-419A Volume 1 and Volume 2 contain a wealth of application assistance.

On the subject of oxidisation, it is common to use a chemical conversion coating to prevent aluminium from oxidising in both aircraft and on board ships; this has an advantage of reducing overall corrosion artefacts as a mated face could be the same metal as the new coating.

Note that galvanic corrosion (also known as dissimilar metal corrosion) can be a major issue in aircraft, and we seek to minimise (or eliminate) it as it adds costs to the system operator because galvanic corrosion will eventually require repairs.

It is not always apparent that a great deal of assistance can be available for this type of issue, unless you just happen to be in (or have been in) one of the industries that require it.

Answer 2

There are specialized washers designed to produce gas-tight connections through anodization layers. http://www.we-llc.com/docs/librariesprovider3/default-document-library/code-compliant-weeb-info-for-inspectors.pdf?sfvrsn=0

I would be tempted to have the enclosure anodized, as I'm not sure that the Aluminum oxide layer is the equivalent of anodization. I'm sure the makers of the WEEB washers could tell you.

Answer 3

You could also try capacitive discharge welding or short arc welding that is currently employed for steel electrical boxes and can be done also on aluminium. This is a very quick and sure system. Generally who builds boxes for electronics could do this, ask him. Remember also that alumina is very brittle and aluminium is very soft so when you tighten a bolt the local pressure breaks the layer and makes a good contact to base metal.

Answer 4

Pure aluminum despite its natural oxide, doesn't need special treatment other than rigidly fixed mechanical hardware to make proper contact.

The high resistivity is strongly field dependent and only prevails with very small bias voltage. But the field in a nm thick oxide layer will be huge, even when there is only a Volt or less across it. At field on the order of 0.1 to 1 V/nm, even the sturdiest insulators experience total dielectric breakdown, i.e. start to form metallic filaments. In practise, it will be difficult to measure anything other than complete shorts between mechanically joint aluminum parts.

So it is not really possible to create lethal voltages when passing currents through joint aluminum parts.

However, aluminum with thick anodization is something that can develop very large voltages when you don't actively make sure, that you penetrate/remove the thick oxide layer as you mate the parts. This will usually not happen "by chance" for anodized parts.

As with anodization, if there is a thick scale on the metal due to exotic environmental conditions, the same precautions apply.