ANSI C119.6-2018 bus bar 950A current cycling testing - how to reduce resistance swing?

[BrandtK]

We are developing a bus bar for a utility customer that wants a C119.6 certification. The test spec they require involves 500 four-hour current cycles at 950 amps, 2 hours on followed by 2 hours off. Resistance is measured in micro Ohms and once a baseline resistance for the bar assembly (bar, receivers, set screws) is established, the bar must remain within 5% of that value throughout the test, as measured in micro Ohms. All of the bar components are CNC milled aluminum, 6061 and 2024.

We have gone through the test twice now. In each case one bar (out of four tested each time) has dropped outside the 5% micro-Ohm range, i.e. with an out of bounds low resistance. In each case the resistance has subsequently increased through the cycling to return to the acceptable range.

We are wondering if there is anything we can do to make it more likely that we get the test result our customer requires. We are assembling the bar 'dry' without conductive grease. Could a conductive grease help, or would it just make the situation worse? Any other wisdom we should apply to get 'er done?

best,

Brandt

 

[waross]

First: Is the subject resistance the bar itself or a connection?
Second: Dry aluminum: Not good.
Aluminum quickly forms an invisible oxide skin that protects it from further corrosion.
The oxide skin has quite a high resistance.
I was on a project with a lot of aluminum welding.
Two of the welders had been factory trained to weld high current aluminum bus bars for a previous project.
There welds had all been subject to x-ray verification.
Any mark on the x-ray was a failed weld.
Voids, inclusions, whatever, perfect or cut it out.
Some tips that they passed on, supported by x-ray testing.
Do not use aluminum tools for any other material.
Angle grinders, with either wire brush or grinding wheel attached:
Two sets, one set for aluminum, one set of tools for all other materials.
Cut off saws, aluminum only. In some cases it was necessary to cut aluminum with a general purpose saw. In that case, the cut surface would be cleaned with an aluminum only grinder or wire brush before welding.
These workers had been shown the x-ray results of poor techniques during their factory training.

Surface cleaning. Immediately before welding, the weld surfaces would be ground or buffed.
IMMEDIATELY. A coffee break or a quick trip to the john were too long. The surfaces would be cleaned again if anything delayed welding.

Try wire brushing your surfaces before applying the conductive grease.
As the worker is putting his buffer down with one hand he should be picking up his conductive with his other hand.
If the worker has to make a quick trip to the john, clean the surface again.
Good luck.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BrandtK]

hi,

The subject resistance is the system of an aluminum conductor supplied by the test lab, plus the bus bar assembly (bar plus the wire mounting hardware which are a sliding receiver and a big set screw). We have no control over the preparation of the aluminum conductor; all we can control is our bus bar assembly, how much we tell the lab to torque the set screws to clamp the conductor, and any prep (such as conductive grease) required to assemble the parts.

Measuring this assembly in micro ohms makes no sense to us, but that's what the customer wants. Would a conductive grease on the threads of the set screws (threaded into the receivers) tend to reduce or increase resistance swings in a test of this nature?

 

[waross]

Working with aluminum I try to lubricate everything in sight.
Set screws and any threaded hardware have a bad tendency to gall badly in aluminum.
I use conductive grease on anything that is subject to occasional use.
A warning about conductive grease.
Some formulations include some hard grit to help scrub the surface film on aluminum.
This is not a problem with most hardware but it may not be a good idea on hardware that is used frequently.
For frequent use I would use a good quality chassis grease.

It's hard to say what may be happening.
I have never seen aluminum with low resistance problem.
High resistance joints are an ongoing issue with aluminum that is not properly cleaned.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BrandtK]

Thanks - do you have a grease you would recommend for this purpose? The set screws in use would be tightened very occasionally.

 

[waross]

My first choice of a conductive grease is Penetrox from Burndy, because that's what I was brought up on.
Noalox from Ideal is a second choice.
I have never seen a performance comparison between the two products but I have never had an issue with either one of them.

Bill
--------------------
"Why not the best?"
Jimmy Carter