Electrical contacting on aluminium busbar

[damien22]

Hi There,

Posting here after Months of unsuccessful research and I hope there gonna be some smart guys with good idea.

In a equipment I'm building, I need to carry large current to many PCBs. To do so, I've planned to use busbar. Those busbar have a length of 1.3 meter and are 7x8mm to which the PCB are screwed with M4 screws and the current goes trough it.

Now there are two issues, one is the electrical contact, because of the oxidation of the material which is an issue for the electrical contact and another is the strength of the material to avoid the bar to be bend during manufacturing.

We are trying to use Aluminium for this, but raw aluminium cannot be directly connected due to the oxide layer.

We have looked a lot to do surface treatment such as plating or coating but it seems impossible to find anybody that can do that on bars that are 1.3 meter long and I cannot split them unfortunately.

The solution we are looking at currently is to use inserts. As the insert a pressed fit or screwed on the aluminium, it breaks the oxide layer and then stay in place. We know of some companies doing so using ARCAP inserts.

However, one of the issue, is that there isn't much space to allow much bigger hole size (M4 on 7mm wide bars), as the insert would need to be at least 2mm thick on M4 would mean a hole ~6mm, leaving only 0.5mm of material.

Then I found Helicoil inserts which have the advantage of not needing a much larger hole size and that would mechanically fit.
https://blog.baysupply.com/common-uses-for-helical-inserts-and-why-you-need-them

However, there are very little information about using this for contacting on aluminium as an insert. The best information I found is "This makes tangless inserts particularly vital to automotive and electrical applications".

Question being, is there anyone having experience with that, perhaps have recommendation or other ideas to contact those busbars, help would be very much appreciated.

[EDIT]
The equipment is for indoor and lab.

Thanks

[beanflying]

Ok I will bite.

First question is what Current? and also what Voltage (AC or DC) and are you running one or two bars for a Ground/Neutral?

What are your reasons for not using a Copper Alloy (Brass) of some sort and what are the reasons for your tight dimensional constraints?

Also will you be supporting the busbar along the length or have you or does it need to be self supporting which from your description seems your choice?

What weight does it need to support?

Reason for the questions is we are not clairvoyants

[coromonadalix]

In my shop we discarded / ditched the aluminum bars and gone to copper ones  .....  master electricians at our shop recommended this over aluminum ?? 

They said aluminum strech and expand over time, problematics for contacts  / wiring / screwing .... at they require checking time to time for eveything connected to them, but they did not talk about oxidation ??


We replaced bars in our pcb oven, and pick and place machines power distribution circuits , (they are not American made) we have around 6 kw 347vac  of power passing thru them ... not as long as your 1.3 meter, but thicker (around 3/4 inch, maybe 1 inch thick) but we dont have any pcb's connected directly on them, all wiring.


You surely have some master electrician(s) in your country who could give you more insights ??  Do's and Dont

[damien22]

Hi Beanflying.

We tried brass, the problem is that the manufacturer (we tried 2) are not able to process the bar without bending it, by handling I suppose.

Also there are questions, as brass also oxidize, what would be the long term impact. I had somehow divergent input about this and also couldn't find much solid information about that. Brass also have about half conductivity of aluminium.

The bars, once installed are supported and there is not much mechanical stress, so there is no issues on that side.

There are several voltages rails, ranging from 15V to 90V DC. The current is up to 120A at the extremity of the GND busbar but this is a pulsed current of about 500ms every 15 seconds, not continuous.

For the size constrain, the width of the bars are constrained by the PCB design and space available, for the length is due to the system design. No chance to change those unfortunately.

[damien22]

Hi coromonadalix,

We also thought of copper but as being very soft material, the bending issue we already have with brass would only be worse. Copper also oxidize and usually those are treated when used on electrical installation.

On electrical installation, and EV for example, more and more copper (or other material) plated aluminium busbar are being used as it's cheaper and lighter. We cannot do so because ours are too long.

Unfortunately electricians weren't of help as usually in electrical installation you use either copper or copper plated aluminium busbar which you can have stock.

[wraper]

Brass is fine. Aluminium and copper mixed together do not behave well. They are dissimilar metals which cause galvanic corrosion, especially if moisture is present.

[beanflying]

Not all Brass is the same. Mild surface oxidation over time on most Copper Alloys is very minor compared to the horror that is Aluminium.

There is some oddballs such as what is generically called Gunmetal and also Marine Grade Bronze it's mechanical properties are similar to mild steel. We used it a heap in the Pump industry including pumping raw seawater so it handles corrosion really well. Also used for Marine Propellers.

Have a look through this Datasheet for a start for some light reading https://www.copper.org/applications/marine/cuni/alloys/pub-206-copper-alloys-for-marine-environments.pdf

[coromonadalix]

oh  forgot to say

With the copper bus bars, we have everything connected to it made in copper,  terminal blocs / screws blocs / distributions posts,  even the bolts are in copper ... they were screwed with torque wrenchs.

[beanflying]

The only problem with pure copper is it will be to flexible given the dimensions and span required. It really should have some mid span supports if it can added. Then Copper could be reconsidered.

[damien22]

I had a look at the bronze but electrical conductivity is too low, we would have to increase the size.

I'll have a second look at using pure copper, if we can find a supplier able not to bend them... 

[dmills]

For this I would absolutely be going with copper bars, ally is just a pain in the arse for electrical connections (It can be done, but **EVERYTHING** has to be perfect or it fails high resistance after a while).

A 1.3M long copper busbar is a perfectly standard requirement, and it would not need to be anything like 56mm^2 CSA for 120A in copper (More like 20mm^2 probably, but you might want more for mechanical reasons).

Be a little careful of the brasses, and especially things like ally-bronze, they tend to be surprisingly resistive.

Any non ferrous stockholder will be able to supply raw copper bar in 1.3M lengths, no problem at all.

I would note that copper is a bugger to tap, clearance holes and bolted connections will make everyones life easier, or do the helicoil thing. Current should mostly be transferred by the contact between the face of the bar and the face of the lug anyway, the bolt should not normally be a major current path, a disk spring is useful here to take up thermal expansion and prevent loosening in the long term.

Regards, Dan.

[damien22]

Thanks for the input dmills.

In my region seems very difficult to get unfortunately.

Would stainless helicoil thing be a good solution ? Unfortunately we cannot bolt as the bar lay flat on an insulator to which they are fixed and on the other side the PCB are mounted.

The PCB does contact with the bars so most of the current would go through there.

[MagicSmoker]

Plate the aluminum bus bars with nickel; problem solved.

[wraper]

Plate the aluminum bus bars with nickel; problem solved.

Nothing solved. It's easier, cheaper and much better to just not use aluminium.

[damien22]

Plate the aluminum bus bars with nickel; problem solved.

As discussed, we couldn't find any supplier that can do this sort of plating beyond 1m length
 

[TERRA Operative]

When I used to service welders, they often used aluminium busbars and windings in the transformers.
These busbars would pass up to 2000A at 50-80v or so at full output on the larger machines.

To make terminations, standard practice was to sand the aluminium with fine grit emery paper, then immediately smear it with a good amount of jointing compound then attach the lug with a bolt as usual and torque to spec.

I never had a burnt termination doing that.
Before I started working there and made the other techs do the joints the same way, there were a number of joint failures after extended use due to the oxide layer and dissimilar metals etc.

[damien22]

then immediately smear it with a good amount of jointing compound

What can of join compound were you using ?

Had the idea of similar process, by threading the holes a bit smaller, adding grease and then when putting the screw it would break the oxide layer and the grease would avoid oxidation but I don't know how well that would go over time...

[MagicSmoker]

Plate the aluminum bus bars with nickel; problem solved.

As discussed, we couldn't find any supplier that can do this sort of plating beyond 1m length

I was able to get large aluminum bus plates for a locomotive drive system (>1m x 1m) nickel plated at a local shop for a very reasonable price and this isn't even a particularly big city; surely there is someone not too far away who can handle something larger than 1.3m... Electroless nickel is one of the most common surface treatments for a wide variety of materials, after all.

Despite wraper's opinion - which does not seem to be informed by actual experience - no alternative comes anywhere close to cost vs. ampacity of aluminum, even factoring in the nickel plating of the latter. Copper is anywhere from 4-8x more expensive than aluminum on an ampacity basis and this comparison only gets worse if you need the bus bar to be thicker/bigger than is strictly necessary for mechanical reasons, rather than ampacity. No brass or bronze should even be considered unless this is a marine application and you have, shall we say, an unrestricted budget.

Pre-posting edit - saw TERRA Operative's comment and while threading aluminum and making connections to it directly is possible if the surface is prepped well, that last bit - relying on the competence of assembly technicians now and service techs later - is often the main argument against using bare aluminum. The type of anti-oxidation grease most commonly used in the US is "NoAlOx" which is basically a suspension of zinc particles in silicone dielectric grease. Probably something similar is available over there.

[TERRA Operative]

I can't remember the brand of grease we used beyond it being a slightly green/brownish grey and being in a tall squeeze bottle with a red lid..... (10 years ago is too long for my brain to remember those details.... )

Also, for a bit of clarification, we didn't thread into the aluminium busbar, we used nuts and bolts (and washers and spring washers) with through-holes to enable us to really torque down the lug without tearing out weaker aluminium threads.

[damien22]

Electroless nickel is one of the most common surface treatments for a wide variety of materials

As I understand its more special for aluminium as you need to have some special treatment before hand to remove the oxide layer otherwise the plating will not hold over time.

However, to electroless nickel plate (or any other plate) onto aluminum alloys successfully with good adhesion, the oxide film must be removed and remain so during plating deposition.

https://www.pfonline.com/articles/adhesion-of-electroless-nickel-deposits-to-aluminum-alloys

and only very specialized companies does it as far as I understand.

[ConKbot]

I don't know if it gets used for bus bars or not, but Alodine or Iridite coatings can be used on aluminum for a conductive corrosion resistant surface. I've seen it used on surfaces that were for electrical shielding and grounding purposes, not as a bus-bar conductor so they may not be suitable. They are chromate conversion coatings, so hexavalent and trivalent chromium processes exist, with the former most definitely not being RoHS.

[T3sl4co1l]

Two-ish questions:
1. Why is the brass or copper so easily bent?  Both are available in hardness grades; dead-soft would be annoying (gummy) to machine anyway.  Full hard is pretty springy.
1a. Why is the shop bending them at all?  Clearly they're a bad shop that can't follow directions (you did put a flatness spec on the drawing, right?..) and don't deserve your money, find another that will.
2. Why does it matter if it's slightly bent?  You have frequent mounting brackets to secure this long span of metal, right..?

Regarding 2, it seems pretty common to use flat stock for bus bars, which will be even more flexible, which is fine because it's secured frequently, to account for flex, vibration and so on.

You may be just as well off using a more conventional approach -- #0 cable or thereabouts (or the metric equivalent, rather), with crimp lugs and bolted connections.  If you need a lot of drop connections, this isn't going to be very convenient, but it is just fine for a 1.3m length.

The other conventional approach would be a terminal block, then routing individual connections with thinner cable of lengths, say, 0.2 to 1.6m or whatever.  Instead of a multi-drop bus, you have a star connection.

Tim

[damien22]

Two-ish questions:
1. Why is the brass or copper so easily bent?  Both are available in hardness grades; dead-soft would be annoying (gummy) to machine anyway.  Full hard is pretty springy.
1a. Why is the shop bending them at all?  Clearly they're a bad shop that can't follow directions (you did put a flatness spec on the drawing, right?..) and don't deserve your money, find another that will.
2. Why does it matter if it's slightly bent?  You have frequent mounting brackets to secure this long span of metal, right..?

I agree, but unfortunately have little control over that. Tried 2 suppliers and both same result with the second being slightly better. The problem is that it's down to the operator, and even if a set is good for a particular supplier, maybe next one won't be and that can go into production hell. We'll check for higher strength brass but I don't have much hope.

For the cable approach, there are 120 boards, each requiring 3 voltage rails + GND. That would need almost 500 large section cable and would be a real mess and pain to mount. The busbare also act as the support for the boards.

[coppercone2]

read about the oxide growth on aluminum to get an idea of whats going on. finishing101.com has some insight (linked directly in one of my previous posts).

When I read that I wanted to no longer ever use aluminum and just pay the piper at 10x cost. fuck it. Not an option? its infrastructure essential to the function of a modern society!!!

its a train they are in service for like 70+ years sometimes, look at cuba or Switzerland. Why are you going cheap for that? people actually like trains. Its not a going obsolete in 2 years equipment. Transportation is important.

How much does a damn bus bar cost in the scheme of a fucking train even if you consider a order of magnitude cost increase? MTA is already going to fuck it up with unknown unpredictable means, don't make it worse. Train fires suck. They can burn in tunnels etc.

[MagicSmoker]

How much does a damn bus bar cost in the scheme of a fucking train even if you consider a order of magnitude cost increase? MTA is already going to fuck it up with unknown unpredictable means, don't make it worse. Train fires suck. They can burn in tunnels etc.

I guess you are directing this tirade at me? Economics wasn't a factor at all in *my* decision to use aluminum for the bus plates, rather, it almost entirely due to mechanical reasons. But I'm sure that article you read on finishing101.com has made you an expert now so...