Motor Starter Fault and Explosion 4

[rockman7892]

Had an interesting one here at the plant this morning. We had a size 5 starter bucket develop a serious arc flash of some sort. After electricians were told that a motor tripped they went to the MCC and noticed that the starter bucket door was blown open and the entire MCC bucket and starter was covered in black soot, and several wires inside the bucket were melted.

The starter like I mentioned is a size 5 starter with a 200hp 480V motor connected to it. The instantaneous motor circuit breaker on the line side of the starter was a 400A circuit breaker which did not trip.

Its hard to tell where the fault occured because the arc flash caused several things to melt. From what I can tell it may have occured on one of the wires that connects the busbar stabs to the line side of the breaker. These wires are melted and broken and both the stabs and the bus bar show corrosion markings.

The thing that puzzles me is why the breaker upstream of the MCC did not trip. The MCC is a 480V 2000A bus MCC which is serviced by a 4.16D-480VY 1500kVA transformer. On the secondary of the transformer there is a 480V 2000A Siemens RL breaker with a Static II trip unit. I am thinking this breaker should have tripped before this starter blew apart???

For this starter to blow apart I am thinking there had to be some serious fault current present. The instantaneous setting on the RL breaker is set for 24,000A and the ground fault setting is set for 1200A for .25s. Should this breaker should have tripped before we damaged this starter and MCC?

If more information is required let me know and I will supply it.

 

[jraef]

I should have said heat, HIGH HUMIDITY and fine cement dust.... the trifecta of electrical component death.

 

[oldfieldguy]

jraef and the gang--

I had particular client who bought a pretty good number (a couple of dozen) Siemens breakers with the Static Trip III units and we had several failures. While th environment wasn't quite as bad as a cement plant, it was metalclad outdoor switchgear in southwest Louisiana and I watched the equipment over a period of about six years.

The failed units just quietly quit. No display. No trip, no nothing.

I became a big fan of Universal Relay after talking to their people and getting them to add an extra point to the short time trip to match some legacy equipment we were getting rid of.

old field guy

 

[rockman7892]

Jraef

Thanks Alot for the great information. I'm going to throw that back to sales engineer I have been talking to and see what they say now their surge excuse is thrown out of the window.

The trip units are in an envoiornmentally controlled electricdal room. Heat shouldn't really be too much of a factor on these units since the room is climate controlled, however like you said there it is inpossible to keep dust out of this room and out of the switchgear.

Im curious now if it is indeed the capacitors in these modules like you said. The sad part is, not only is Simens wanting to charge $37,000 to replace these units but they are asking for an additional $11,000 to just test the ones that have gone bad to try to determine a problem. I would think that they were curious themselves to find out what went wrong for their own reference, but if they are now a legacy product like you said then maybe they dont care. None the less I'm going to mention this capacitor issue and ask them to check one or two of these units to see if that is indeed the case.

The utility relay sounds like something worth investigating. The only problem is that we are building a brand new plant here which has all the RL breakers using the Static III units. The plant is very strict about using like spares and its difficult a lot of times to shift direction on products due to this spare standpoint.

You mentioned that these units get their power from the CT's. Does this mean that they are not powered when there is no current on the line?

 

[itsmoked]

Just don't mention jraef in this discussion it's, "I heard from somewhere".

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Zogzog]

We retrofit hundreds of RL breakers with AC-PRO's (Utility Relay) every year, not a problem at all. We also do a ton of the arc flash reduction upgrades for all breaker types. The system is called QUICK TRIP, if you go to the website for utility relay and look at the power point on quick trip, I wrote that presentation when I was at the last company I worked for. It is a great product, I have yet to see one fail. There are 2 ways to go, direct replacement trip units or retrofit kits, you spend a little more on the retrofit kits and labor to do the work but that way the entire tripping system is a matched set.

We also had 2 STIII failures that were similar cases last month at a customer of ours (Power plant).

We have great pricing on Utility Relay trip units, just a fraction of what you were quoted for the STIII's. I also have plenty of reconditioned STIII's, fully tested with a 1 year waranty for less than you were quoted.

http://www.cbsnuclear.com

 

[jraef]

You mentioned that these units get their power from the CT's. Does this mean that they are not powered when there is no current on the line?

Yes, that is correct. Remember, these are just TRIP units, they are not continuous monitoring units.

And yes, as itsmoked said "You heard from somewhere", please...


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[rockman7892]

Jraef

Thanks A lot for the valuable information. Of course I will only mention that I heard it from "Somewhere" perhaps a little bird.

Thanks again for the info.

 

[Zogzog]

Keep in mind that CT accuracy is very poor at low levels, that is why most trip units only go down to 0.5 for LTPU. Most trip units wont even power up if the load is less than around 20%, below that they rely on the internal batteries, which are usually dead because no one ever replaces them.

 

[Laplacian]

jraef:

Do you know more about these "supercaps"? Do they have an aqueous electrolyte solution? Were they made in China or Taiwan?

Over the past 7 years or so, I have witnessed numerous electronic equipment failing due to premature capacitor failure. Our plant still has 30 year old electronic equipment with electrolytic capacitors running just fine. The capacitor issue most visibly affected the IT world, but on an industrial scale; anything with a power supply has been affected.

Could these capacitor failures also be attributed to the subject of the IEEE Spectrum arcicle "Leaking Capacitors Muck Up Motherboards"? It would be intersting to find out.

http://en.wikipedia.org/wiki/Capacitor_plague

 

[itsmoked]

What?? Are you insinuating that cheap, lowest bid, Chinese caps could fail prematurely??

Actually that phenomenon wiped out hundreds of thousands PC motherboards a few years back.

Keith Cress
kcress -

http://www.flaminsystems.com