ALL:

Considering the convex shape of the moving part of a breaker contact, much contact area is considered to be "good" and how much "Bad"?

I recently pulled a 225 amp, Cutler Hammer breaker out of our data center, with a 40c rise on A phase. A phase was 70 amps and 66c, and C phase was at 90 amps and 27C.

There was no evidence of a loose connection at the lug, so I opened the breaker. The contacts had some carbon, in the center of which was a clean spot, which appeared to be where most of the current was being carried. THere was some pitting.

I took a photo of this area and some measuerments, and estimated this area to be no larger than a number 12ga wire, probably closer to 14ga. B and C phases had larger contact areas and less carbon.
The ampacity of 12ga is about 40 amps(depending on ambient), and we were almost twice that at 70 amps. There is also evidence in the Thermographs that heat was radiating fromt he area of the contacts.

Opinions?

Did you check the voltage at the breaker?

voltage was 220/240. A lot of single phase stuff was on the breaker, which accounts for the load differential between phases.

Voltage drop was .3 ohms on A phase

Is it possible that the shiny spot you were looking at is on the arcing contact rather than the main contact? (just checking)

Comparitive resistance measurements between the phases would be interesting. Measurement technique is important for contact resistance measurements. 4-wire measurement device for example DLRO with Kelvin probes. Or ductor check.

I'm just playing devil's advocate to check the other angles. It sounds like you may have correctly determined the source of the hot spot is the difference in contacts. 0.3 ohms sounds high if it is a correct reading.

Pete:

I don't know what you mean "arcing contact" or Main Contact"

Any contact carrying 70 amps, whose circular mils are the size of a 12ga wire is going to generate some heat.

The electricians seemed to think that the contacts didn't look that bad.. a little carbon. But my concern is the size of the effective contact area. Although we were only conducting 70 amps, this is a 225 amp breaker. The customer was trying to stay under 45% of the rated breaker load, so about 100 amps was thier max, although they exceeded that on other identical circuits.

We have lots of other breakers in the data center like this.

In addtion this breaker only had a 5c rise last year, so its' deterioration rate was high. Other breakers have the same 5c rise this one did last year. We put the on a 6 month schedule.

Most breakers have a main contact designed to carry the continuous current and a smaller contact designed only to make and break the current. During openining the arcing contact opens first and during closing the arcing contact breaks last.

This is the main contact

If the thermograph shows heating which appears coming from the contacts of one phase, you are probably correct that is where the problem is.

0.3 ohms, if it's a good measurement, is very high and would tend to confirm. Assuming only one phase is affected you should be able to confirm your resistance measurement by verifying other phases are much lower resistance.

Is the darkening worse on the suspect phase than the other phases?

As to the role of the shiny spot, is it smaller on the suspect phase than the other phases?

I hope you will patient for me to return one more time to the subject of main vs arcing contact. They take many shapes. You described a curved contact. I am picturing something like an elf's shoe starts wide at the heel and narrows to a point at the toe. Also curves up as you go to the toe. Correct?

If that's what you have, the arcing contact is towards the toe. The main contact is towards the heel. As the mechanisms closes they don't both make make contact with the stationary contact at the same time. So again I wanted to verify if the shiny area you are looking at is the main or arcing contact.

Some other general things you might look at in trying to understand the apparent failure would be the spring force that pushes the contacts closed and the degree of overtravel (wipe), as well as alignment of the moving and stationary contacts. Also that arcing contacts makes first during closing to protect the main contact.

Pete:
Your elf Shoe anaology is correct, except that the entire contact is the same width across the whole contact vertically.
The shiny spot is on the main contact, and on just one side of it to boot. ie the entire horizontal cross section of the main contact is not connecting, most of it is carbon covered/coated.

The darkening was not worse on A phase, but the shiny spot was much smaller.