Galvanic corrosion between silver plated MCCB terminals and aluminium

Question

We are planning to install a switchboard in one of our garages. The MCCB (moulded case circuit breaker) is used as a main incomer which has silver plated copper terminals. The aluminium busbars are used as a conductors. While reading the IEC 60943 standard, however, I came across the anodic index of aluminium when used with respect to silver. As per the standard, the galvanic cell creates a potential difference of 770 mV which is greater than the threshold of 350 mV.

While several industrial panels and switchgear recommend to use silver plated aluminium, some do not recommend it. The current scenario uses a 630 A MCCB where the elevated temperature levels may reach 105 C. I have even read that galvanic corrosion is aggravated when the temperature reaches elevated level.

Is it really okay to connect the aluminium busbar to the MCCB (silver plated copper) terminals?

If yes, is there any concrete standard to prove this, or is there any standard that recommends to conduct a test to evaluate the joints susceptible to galvanic corrosion?

I have read this question, but it doesn't answer my question.

Also almost all the manufacturers recommend to use tin plating, but tin with silver leads to potential difference of 470 mV which is greater than the 350 mV threshold value as per the aforementioned standard. Please note that we use contact aid compound such as petroleum jelly which to some extent halts the corrosion.

Please note that this question is been migrated from the home improvement site.

Adding an image of reference to IEC 61439-2 regarding operating humidity conditions:

Answer 1

I work in Telco and from time to time come across Al busbars in exchanges/central offices.

The Telcordia and regional standards all require the use of bimetal lugs when bolting on dissimilar cables due to corrosion risk. Plating (as opposed to much thicker "cladding") is generally not a solution, as it is too thin to reliably prevent issues.

Cu cable/Al busbar lugs exist but are hard to obtain, so Al cables are normally used. In fact Al busbars are most common when all the cables are Al, as they mean you can use cheaper single metal Al crimps.

The reverse (Cu busbar, Al cable) is much more common as Al is often used as the conductor for larger cables, so those lugs are fairly easy to source.

Examples of fittings for Al cable to Cu busbars can be found on many supplier websites including TE and Cembre

The lugs are produced by a friction welding process, which ensures a permanent, corrosion free, joint between the two metals.

This article gives a good summary of the issues experienced in US domestic settings as a result of the use of Al conductors without suitable bimetal connectors, and includes a number of onward links.

You can also easily do a web search to find regional standard disallowing direct bolting without special connectors, for example here is a link to Colorado code.

I would not recommend direct Cu/Al bolting (irrespective of plating) unless explicitly covered by the applicable standard. (Some standards do allow direct bolting, but with very specific prep, anti-oxidants, and operating conditions.)

In general such joints should be done using code compliant bimetal connectors.

Extension Having done further research, it is clear that the joint instability of bolted Al/Cu joints is not solely dependent on the presence of moisture.

This paper:

M. Oberst, S. Schlegel and S. Großmann, "On the Aging of Electrical Joints with a Copper and an Aluminum Contact Member," 2019 IEEE Holm Conference on Electrical Contacts, Milwaukee, WI, USA, 2019, pp. 1-8, doi: 10.1109/HOLM.2019.8923935.

has an excellent analysis of causes of joint failure, and suggests that simple oxygen penetration is sufficient, especially at elevated temperature. It also suggests the issue is strongly non-linear, with localised heating leading to potential runaway failure after a long period of apparently good operation. This fits with my own experience of poor Cu crimps (from a misadjusted machine), where once resistance starts increasing, localised heating makes it much worse.

Answer 2

First question: Is the MCCB already supplied, or can it be changed? Perhaps the terminals can be changed out for more suitable material if the MCCB is already existing, most MCCB OEMs supply many different terminal options. Please confirm MCCB terminals are silver-plated, rather than tin-plated. Beware that silver grows whiskers faster than tin in some circumstances, refer:
https://simpliengineering.com/t/silver-plated-versus-tin-plated-copper-bus/1228

Also, is the switchboard with Alu bus bars already existing? If not, then suggest select bus-bar material to suit the MCCB terminals.

If both the alu bus-bars & the MCCB cannot be changed, then suggest connecting MCCB to Alu bar using flexible Cu cables terminated with correct lugs at each end to suit the material interface, ie: Cu cable to Alu bar at the bus-bar end, or Cu cable to silver/tin plated Cu at the MCCB end.

You may already be aware that Alu requires special preparation and treatment with oxygen barrier compounds to ensure a good connection free from oxidation, but I just thought I'd mention that to be sure. Given this, I would think it much easier to fit a bi-metal lug at the end of a flexible cable to an Alu rigid bar, rather than fit a rigid MCCB terminal to an Alu rigid bar, while maintaining an oxidation-free connection.