I have bought 2 high-current Busbars (6-way, CIP100400070). One for plus and one for minus. While you can buy optional fuse holders with corresponding screws, washers and nuts for the positive side, I can’t find any suitable screws, washers and nuts for the negative side online.
According to the data sheet, the rail is made of copper (apparently tinned). The two M10 connecting screws are made of stainless steel (A2-70) and the washers look like tinned copper. The screws are screwed into a nut that looks like aluminum.
Since the current when connecting cable lugs passes less via the screw and more via the washer, the material of the screw may be stainless steel (the washers are crucial).
The rail can withstand up to 1,500 A. Let’s assume that a consumer draws 250 A at one M8 connection. How do you do that? Do you simply use Zinc plated washers or stainless steel washers between the cable lug and nut? Or pure copper screws, washers and nuts? I have concerns here, but can’t find any tin-plated washers for M8 (I’m German and live in Europe).
According to ChatGPT and Deepseek, neither stainless steel nor galvanized material is safe for high-current applications.
The intent is that the lug surface area directly in contact with the tinned busbar should conduct 99% or more of the current. Any current that conducts through the screw and washers above the lug should be negligible.
Therefore, the most important thing is to ensure a clean lug and busbar, possibly coated with an Anti-Oxidization compound if you are in a challenging environment.
Having sufficient torque on the nut/lock washer/washer above the lug is critical.
Never put anything between the lug and the busbar.
I’ve only seen stainless used for this application. There may be better choices.
Except for when a device comes with copper hardware, I use stainless steel. To prevent any corrosion or galvanization I use a small amount of NO-OX-ID between the lug and the busbar. Depending on the location (indoors/outdoors) I’ll also coat the top of the lug even though I don’t expect any current to flow through there. It’s just to keep the (tinned copper) lug “clean” from the contact with the stainless steel washer.
Most studs are stainless steel, though there are the exceptions. So stainless steel fasteners on a stainless steel stud is a good match.
Thank you for your answers, from which I conclude the following:
Due to the significantly poorer conductivity of stainless steel, almost all the current flows directly through the tinned cable lug into the tinned busbar.
It is important that the connection cable lug → busbar is (very) good so that there is no or only a low contact resistance. Otherwise a (considerable) part of the current would flow through the upper washer, which would lead to (strong) heating due to the high resistance. Presumably for this reason, Victron uses tin-plated washers for safety concerns.