Overheated buss

[electricalperson]

Key words "SEEMS to be".

Classic case of loose connection.

On the installation, sometimes a bolt will appear to be tight but it's just the threads locking up for a variety of reasons. Sometimes the phone rings and distracts the mechanic before he can finish tightening it. It may have been OVER torqued and stripped.

I can't tell from the photo but generally you can clean those up.




I looked more closely at the picture and the affected terminal looked tweeked. It looks like the B and C phases of the breaker are installed BEHIND the bus while the A phase is installed in front.

Anybody?

good call. i looked closer to the picture and you are probably right

 

[ptonsparky]

It is a MC3800F with 600A trip. The breaker was installed correctly to the buss. I had checked the bolt for tightness and no apparent arcing has occured at the junction.

Thought I had posted this already but I don't know where it went.

 

[electricalperson]

could an internal breaker problem cause this overheating?

 

[mdshunk]

could an internal breaker problem cause this overheating?

You betcha. That's a good reason to use do a FOP test or use a DLRO meter when reinstalling used breakers like this one.

 

[electricalperson]

You betcha. That's a good reason to use do a FOP test or use a DLRO meter when reinstalling used breakers like this one.

i know what a DLRO will do when doing this but what is an FOP test?

 

[mdshunk]

i know what a DLRO will do when doing this but what is an FOP test?

Fall Of Potential.

You know that when you have a difference in potential, electricity will flow. In the case of a breaker, contactor contacts, and other such connections, you can use a meter set to millivolts (sometime microvolts) and check from line to load of each pole while energized. The higher millivolts you read, the more contact resistance you have from line to load.

 

[electricalperson]

Fall Of Potential.

You know that when you have a difference in potential, electricity will flow. In the case of a breaker, contactor contacts, and other such connections, you can use a meter set to millivolts (sometime microvolts) and check from line to load of each pole while energized. The higher millivolts you read, the more contact resistance you have from line to load.

thats the test i normally do to troubleshoot breakers. never known that was the test name thanks

 

[mdshunk]

thats the test i normally do to troubleshoot breakers. never known that was the test name thanks

Now you know. The exact millivolt reading isn't as important as comparing them across all three phases, or across all the similar connections for a group of breakers. If you find a bad pole of a breaker, you can check from the buss tang to the breaker tang, on each the line side and the load side. This would let you know if the resistance was at the bolted connection or inside the breaker. In the case of this OP, it doesn't matter at this point. The breaker needs rebuilt or trashed, and the buss tang is toast.

 

[electricalperson]

usually anything over 100mv is bad and over 35mv should be watched. done this method in a house that had a breaker that kept tripping. checked connections on the circuit then measured voltage drop across the breaker. measured around 98mv, replaced the breaker and it was all fixed

added in example

 

[templdl]

MC3800 has an electronic trip unit that uses a rating plug for the interchangable trip rating.
You can still examine the moving and stationary contacts by assuring that the line side if the breaker is de-energized, removing the rating plug which should trip the breaker if it is closed, removing the line and load terminal covers in applicable, and then the cover.
You should be able to grasp the arc chute and simply pull that out which should provide a good view of the line and load terminals.

 

[ptonsparky]

It is much easier to use your little handy dandy thermal scanner, $90 +-, and one heck of a lot safer.

 

[zog]

It is a MC3800F with 600A trip. The breaker was installed correctly to the buss. I had checked the bolt for tightness and no apparent arcing has occured at the junction.

Thought I had posted this already but I don't know where it went.

I have at least 2 MC3800F breakers, PM me if interested.

 

[ptonsparky]

A & B phase contacts in the breaker failed at some point. Temper is totally out of small spring on A. The plating is gone from the breaker end of the finger buss. Where does this plating go? Does it simply evaporate?

Thanks to all.

 

[cschmid]

I was going to say have the buss bar rebuilt..but if you clean it on a wire wheel on a grinder all the impurities on the wheel have the opportunity to get embedded in the bar..Having a machine shop make new one would be best and quickest..but I would have all three done..that way the resistance is totally the same as the material is all from the same batch..An once of prevention is worth a pound of cure..But I can see you have bigger problems..I would like to know the load you are serving with this tub..equipment and actual physical application maybe something has change and even if you replace equipment it could happen again in another couple years..

 

[zog]

Is this a case of a circuit breaker being used as a motor starter? Thats what it looks like to me, this last pic shows some serious contact damage. If a breaker exceeds its duty rating the contact resistance will increase as the copper is exposed from underneath the silver plating and contact surface area decreases from pitting, the increased resistance worsens the problem every operation, as resistance increased, I2R losses increase causing eventual failure.

If this is being used as a starter you need to use a device designed for that type of duty. Also most breakers are only rated for 1 or 2 fault (Not overload) interuptions before they require an overhaul. That could also be the case here.

 

[kingpb]

I'm surprised no one has recommended replacing the entire vertical section! Why mess around with simply putting lipstick on a pig.

Contact local Eaton-Cutler Hammer rep, have them get field engineer out there ASAP and take advantage of their nationwide resources. Put a 25% mark-up on whatever they do/charge and pass it on to the Owner.

Plus,they have deeper pockets if something goes wrong.

 

[weressl]

I help at this small industrial on occasion. Their electrical mainentance person is usually up to most challenges but is in hospital at the moment recovering from bypass surgery.

He installed this used breaker mid summer of last year. I don't know the details as to why or if the buss was damaged at that time or not. The buss connections seem to be tight even now so the damage could be from poor breaker contacts.

Question is do you feel I need to replace this buss section? It feeds a Size 5 part winding start 480V starter about 40-50 years old. Cutler Hammer.

Looks like a bad connection unless the heat was originated from another internal failure of the breaker and it propagated to the termination point. It seems that the heat did not conduct to the bus plugs since the insulation damage stopped within sight in the picture. I would check regardless.

Time to change that breaker.

Some comments:

• At 40-50 years that MCC has past its design life. Suggest to management to start thinking about replacement. The safety features of the MCC that were sufficient that time may not satisfy todays requirements.
• I would never use bus-stabs on this size starter, only direct bolt on. They are notorious of thermal failure.
• Whenever you modify a UL listed assembly, make sure that you change components like-for-like.

 

[ptonsparky]

Learned a bit today when we started it up. Size 6 starter, 300 hp fan, not 200 as I was told. 50+ Amp difference from hi to low at run and not steady at all. AGe 30yrs or so.

Replacing the column is not an option. The room is about 20' long, one door, with MC on both sides. East side is 1200 amp. West is your guess, I haven't looked. Separated by maybe 32". State has said nothing new goes in. The list goes on No one volunteered to be in the room at startup, me included.

I go back tomorrow for more $$.:grin:

 

[templdl]

ptonsparky,
I can't tell you when the damage starts but there certaily was some heating the occured. When you have a bad line connection where the heat is transferes to the stationary contacts to the moving contact it will reap havock on the moving contact spring.
I is not that uncommon to have a spring weakened to the point where the contacs start to arc and it just goes down hill from there,
Dave

 

[cschmid]

you need to go look at that fan..what does the fan do..How is the fan connected to the motor..Is the fan out of balance..how much vibration is there..I think there is a solution to the problem..what is the amp difference on motor with out load..