Do bus bar connections need to be cleaned?

[Thermoelectric]

G'day,

I'm a sparky who's currently working in a switchboard manufacturing workshop, and I've been told that any joints in the bus bar need to be cleaned with methylated spirits before they're assembled.

For reference, 90% of these bars are tinned copper (the rest are untinned) Stuff left on the bar is likely just finger oils and cutting fluid from when the bars were cut. All held together by M10 bolts torque to 40Nm.

Would these "impurities" actually interfere with the joint? I had a look for any references to it and couldn't find much mention of this causing hotspots. My understanding on contact areas is that any oils etc will just be displaced into the "troughs" of the surface and not impede the electrical contact.

Attached is a photo of one such joint as an example. 2000A max, 100x10mm bar joining to 2x 50x10mm bar.

Cheers.

[andy3055]

It is mostly to remove any oily stuff. It is a good practice to clean them as any residues could create oxidation between the mating surfaces in the long run, which in turn will cause problems at those high amps.

[ConKbot]

If it's just oils and your clamping pressure is enough to exceed the film strength and get metal on metal contact, and there isn't long term corrosion issues, sure, it's fine. Are you sure it's just oil though? And no dust or debris clinging to the oil, that would hold the surfaces apart a bit and increase resistance?
Does a solvent wipe take care of both so you don't have to worry about either?
Id rather have a validated process that makes good joints every time than skip the wiping and then have to ohm out every joint with a 100/1000A microohmmeter.

[edpalmer42]

I used to work for a telephone company.  We used bus bars for our main DC power plants.  Think 4000A meter shunts at 50V DC.  When making connections, the power guys always cleaned the connections thoroughly and then used NO-OX-ID or similar to seal the connection and prevent oxidation in the joint.  It doesn't take much resistance for a joint to heat up when there's a few thousand amps flowing through it!

By the way, a thermal camera is an excellent way to inspect any electrical joint for signs of heating.  AC or DC, any voltage, any current.  Our buildings department loved to use them to inspect breaker panels.  Much better than any non-contact thermometer.

Ed

[Whales]

I'd do the cleaning if only to offset the small chance that someone had (incorrectly) put some protective coating/barrier on the bussbars.  Just like how I don't trust other people's multimeters not to have open probe leads.  Getting trapped dirt off is probably also good, albeit I'm not sure if the pressure/torque specs are enough to deal with that anyway (to a certain degree).

Our buildings department loved to use them to inspect breaker panels.  Much better than any non-contact thermometer.

I'm imagining a small team of people in grey overalls tentatively approaching people's breaker panels in apartment blocks, a look of fear in their faces as they peer over the shoulder of the one guy with the thermal camera.  Meanwhile ordinary people just walk past the panel in front of them, living in a completely different world.

That but in a switchyard?

[trobbins]

Definitely clean the mating surfaces, which may include a light scour with a poly dish scrubber type pad, to ensure no debris or pollution.

We would also apply a light smear of vaseline to mitigate moisture ingress over time (although there are specialist products around). That is especially important for certain industrial environments like plating shops, and where the joint has a relatively high current density and no programmed maintenance inspections.

Tensile rated bolts and belleville washers also become important for higher current densities.

The highest density joints require surface machining, especially as some bar is not flat enough.

Inspection of machined surfaces for high density applications like semiconductor to water-cooled bar connections often show pitting and tracking due to micro-arcing over many years.

[SeanB]

Yes clean them, seen way too many that have cooked connections, because there was a burr, or some trapped dirt, and the copper flowed over time. Clean with a cloth at least, preferably damp with some solvent, then lightly sand down to get fresh copper surface, and apply a protective grease there.  Copaslip works well, just a tiny amount per side, and on the threads of the bolts, so that you get good torque, and the joint will come apart after a decade looking as shiny as when it was done, leaving the dull rest of the bar as contrast. Belleville washers to keep constant torque are also good, as the copper will slowly flow over time, reducing the clamping force. Then you get the red glowing current indicator bolts.

[tszaboo]

Just follow instructions, it was probably written by people with more experience in the subject than most people here.
Plus, it's your liability.

[Thermoelectric]

I'm still cleaning them, I was just curious to the reasons as said engineer was rather vauge about it. He also thinks spring washers (which no replies to this topic have mentioned) work better than Belleville washers so take that as you may.

Also interesting to see the different ways it's done, we don't scour/sand/polish the contact faces, nor use any grease on the joints or bolts. I'd have imagined greasing the joints would do the opposite of cleaning, as the grease will hold more dust and whatnot.

[Faringdon]

..yes , years ago worked in a place that distributed high current PSU's...the service engineer was constantly going round to customer premises, and wire-brushing the connections, and re-assembling them, because of volt loss and heating in the joints.

[Someone]

I'd have imagined greasing the joints would do the opposite of cleaning, as the grease will hold more dust and whatnot.

In a static position with already relatively smooth surfaces, grease is just filling the minute gaps and preventing water/moisture penetration. Electrically conductive grease can allegedly marginally reduce resistance. Fretting has been reported in some cases:
https://www.nyelubricants.com/case-study-busbars

For mechanical system even with moving parts in a "dirty" environment grease is usually a net positive for wear. Despite collecting dust/sand it does a good job carrying the contaminants out of the sliding surfaces.