450hP, 480V CWP motor disconnect switch wiring and terminal flashover - Reduced Voltage Soft-started

[mayanees]

I'm looking at a system that has six 450 hP, 480V Cooling Water Pump motors powered from three networked 1500 kVA transformers. The transformers power three switchboards, that power the six motors. The soft-starts for the six motors are located approximately 60 feet from the switchboards. The motor disconnect switches are located 120-180 feet from the soft-starts, and the motors are located ten feet from each disconnect.
The problem they're experiencing is the wiring and blades in the disconnect switches are melting.
The Eaton soft-starts, model S811+V65N3S, have internal bypass contactors that engage when the motor is up to speed.

I have determined that the switches are underrated for the application because they're unfused and it's written clearly on them that they're only good for 10kA if not outfitted with or protected by Class L fuses, and they're not. But that's a fault current thing so I don't believe it's causing this problem.
Otherwise the circuit meets NEC criteria for motors as the cable is sized appropriately. The motor is rated at 531 amps fla, and the cable is two parallel 400 kCMILs for an ampacity of 670 amps.
Eaton's manual says you can have up to 100 meters between the soft-start and the motor before needing any kind of a load reactor. They have experienced this problem on a number of the six applications, including some motor failures. It's been in service since 2014, and started experiencing problems when demand dropped for the chilled water during Covid a couple years ago. Now they're getting ready for full service this coming May/June.

Any thoughts as to why this application is experiencing failures in the disconnect switches? I don't think it has anything to do with the switches only being rated for 10kA, even though the available fault current is around 35kA, since this would only show during a fault. I understand the load to have around a 2000-amp inrush at startup reaching run speed pretty quickly in a couple seconds.

Thanks for any suggestions.

 

[mayanees]

I do have an inquiry in for Eaton's Tech Support, but they're slow to respond.

 

[petersonra]

can you check the current in each leg once the motors are up to speed? it sure appears like one phase is getting a lot more current.

i doubt it has anything to do with the soft start itself as it gets bypassed after the motors get up to speed.

is it always the same phase they have problems with?

 

[Besoeker3]

I do have an inquiry in for Eaton's Tech Support, but they're slow to respond.
View attachment 2562758

It is one phase rather than all three. To me that suggests that it is a loose terminal.

 

[don_resqcapt19]

My question is if this is a poor connection at the wire terminations or a poor connection as the switch blades? Hard to tell from the picture, but it looks like the most extensive heat damage is at the blades.

 

[infinity]

My question is if this is a poor connection at the wire terminations or a poor connection as the switch blades? Hard to tell from the picture, but it looks like the most extensive heat damage is at the blades.

I agree, it doesn't appear from the photo that the issue started at the terminal. It appears that all 3 phases have terminals that are tightened to the same point on the terminal threads which would indicate that their torque values are close or the same.

 

[Besoeker3]

My question is if this is a poor connection at the wire terminations or a poor connection as the switch blades? Hard to tell from the picture, but it looks like the most extensive heat damage is at the blades.

The yellow and orange are intact. The brown are cooked at the terminals for the foot or so. How can you explain that?

 

[infinity]

The yellow and orange are intact. The brown are cooked at the terminals for the foot or so. How can you explain that?

The contact resistance was high which toasted the conductors.

 

[tom baker]

I have solved overheating issues such as yours by using Burndy penatrox e. It’s a copper based compound, great for threads and terminals. T&B makes a similar product, both are messy and get home before you do

 

[synchro]

I agree, it doesn't appear from the photo that the issue started at the terminal. It appears that all 3 phases have terminals that are tightened to the same point on the terminal threads which would indicate that their torque values are close or the same.

I agree. And if the overheating was due to poor wire connection(s) at the terminal, the extent of the wire insulation damage would not likely be essentially identical on both wires like it appears. For that to happen, they'd need to be identically bad connections having the same excess resistance in order to create the same amount of heat and temperature in the two wires.

... They have experienced this problem on a number of the six applications, including some motor failures. It's been in service since 2014, and started experiencing problems when demand dropped for the chilled water during Covid a couple years ago. Now they're getting ready for full service this coming May/June.

Any thoughts as to why this application is experiencing failures in the disconnect switches?

Perhaps the higher prior usage created just enough heating to keep moisture from condensing on the disconnect blades. But then moisture accumulated during relative disuse, and that led to some corrosion and eventual failure of the blade connections? What kind of environment is this in as far as humidity or other potentially corrosive elements?

 

[Besoeker3]

The contact resistance was high which toasted the conductors.

Yes, that one contact...............

 

[mayanees]

It's not been isolated to an individual phase, and the switch blades have required replacement in arbitrary phases. I like the idea that contact resistance could be the cause.

I may put a recording instrument on each of the three phases to verify loading, like perhaps a PQ meter that looks at voltage and current.

 

[mayanees]

There is this issue of current imbalance:

 

[mayanees]

Perhaps the higher prior usage created just enough heating to keep moisture from condensing on the disconnect blades. But then moisture accumulated during relative disuse, and that led to some corrosion and eventual failure of the blade connections? What kind of environment is this in as far as humidity or other potentially corrosive elements?

The disconnect switches are in open air and are Siemens Non-Fusible Heavy-Duty Switches rated NEMA Type 4/4X. They'll have to be replaced based on this review because they're underrated without the fuses, so we'll spec something to go back in that is fused or at least rated for the available fault current.

 

[Jraef]

What is the HORSEPOWER rating of the disconnect switch? From what I can see, a Siemens 600A disconnect is only rated 400HP @ 480V, so it would be misapplied*. It would have needed an 800A rated disconnect for a 450Hp motor.

*The NEC requires that disconnects on motor circuits have a HP rating equal to or greater than the HP of the motor. You cannot go solely on the amp rating. This exact situation is why.

 

[mayanees]

What is the HORSEPOWER rating of the disconnect switch? From what I can see, a Siemens 600A disconnect is only rated 400HP @ 480V, so it would be misapplied*. It would have needed an 800A rated disconnect for a 450Hp motor.

*The NEC requires that disconnects on motor circuits have a HP rating equal to or greater than the HP of the motor. You cannot go solely on the amp rating. This exact situation is why.

It's 800 amp and rated for 500 hP.

 

[mayanees]

And thanks everyone for looking at this and commenting.

 

[Jraef]

It's 800 amp and rated for 500 hP. View attachment 2562770

OK, so it was likely a loose terminal or bad contact resistance then. The current imbalance could be explained by the resistance issue, but that's only a 2% voltage imbalance.

Corrosion seems to be an issue in that 2nd photo too. I would be putting a corrosion inhibitor inside of there if it were mine, since an enclosure heater would be problematic on a motor disconnect switch.

By the way, ALL NF disconnects are only rated 10kAIC. In some cases they have a series rating if you have the specific fuses they are rated with upstream somewhere.