Motor Starter Fault and Explosion
Motor Starter Fault and Explosion
(OP)
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.






RE: Motor Starter Fault and Explosion
What did trip or did the MCC stay online?
What is the connection layout? Is the 1500 KVA transformer fed by long cables? Is the MCC fed by cables or close coupled with bus work?
Was most of the damage on one phase?
With the corrosion there is a possibility that a connection went open due to corrosion and heating. The damage may have been done by motor current with one phase open. This would not trip the feed breaker. It depends a lot on the construction of the MCC. This could happen with some of the better designs. Some of the economy MCCs would have a single phase arc quickly develop into a three phase arc, which should trip the feed breaker.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Motor Starter Fault and Explosion
Size 5 (and above) starters are big ugly beasts. There is a lot of current flowing across the stabs and a loose or poorly fitting connection can generate a lot of heat.
A 480 V arcing fault can sustain itself for a long time without drawing enough current to trip a phase overcurrent device. That is why the NEC requires ground fault protection on large 480 V feeders. But I'd probably have the upstream breaker tested just to make sure it is functional.
RE: Motor Starter Fault and Explosion
Waross
To answer your questions, nothing appeared to trip and the MCC did stay online. The motor did stop, probably b/c some of its control wires were melted and came loose. So when we initially when down to the MCC this starter had been damaged but the rest of the MCC and starters stayed online. We meggered the cable leads out to the motor and they appeared fine.
The transformer itself is fed from a MV switch with cables of about only 50ft or so. The cable feed from the trasformer to the MCC is probably only about 25-30ft, so these are both small distance feeds.
Most of the damage did appear to be on one phase in this case the "C" phase. Like I mentioned on this phase the wires connecting the stabs to the breaker appeared to be completley melted and the other two stab-to-phase connections stayed intact. The electricians did inform me that with the contacts inside the motor breaker two phases seemed to be damaged, I belive the B and C phases.
I'm assuming your asking if one phase was damaged more than others to determine if this was a single phasing condition? Whould a single phasing condition draw enough current to blow a door open? With a single phase condition I thought the current would multiply by a factor of 1.73 which in this case would have been about 400A? Can 400A cause this much damage? I always thought that this kind of damage was due to an extremly high fault current?
Why would a single phase condition not trip the feeder breaker upstream? Wouldn't this be determinded by the short time setting?
RE: Motor Starter Fault and Explosion
dpc
This occured while the motor was running, and the 480V system is solidly grounded.
RE: Motor Starter Fault and Explosion
RE: Motor Starter Fault and Explosion
RE: Motor Starter Fault and Explosion
dpc
When you mention an arcing fault are you talking about an arcing fault to ground? Is it the current that is associated with this arcing fault that causes the explosion type damage?
Does the current magnitude have to be large in order to blow a door open? What type of current value could this type of damage start occuring at?
RE: Motor Starter Fault and Explosion
Are you sure the door was latched prior to the fault? Any evidence of the latch being bent, broken, etc.
You say that the upstream breaker has ground fault protection at 1200 A for .25 sec. I would expect the fault current to be higher than 1200 A to blow open a latched door, but it might not have lasted 15 cycles once it got to that magnitude - blowing itself out once the current go high enough.
I would definitely want to test that upstream breaker, especially the ground fault protection. You might want to make sure the ground and neutral connections are correct as well. Let us know what you find out.
RE: Motor Starter Fault and Explosion
dpc
I agree that the door may not have been latched prior to the fault. Even without being latched I would expect the fault current to be more than 1200A to blow open the door.
If an arcing fault did indeed occur and no breakers tripped, then what would have interrupted this fault restoring the MCC to its normal state?
I will look into this feeder breaker working properly and let you know what I find.
Do you think the arcing fault could have been a ground fault created by a loose bus-stab connection?
RE: Motor Starter Fault and Explosion
RE: Motor Starter Fault and Explosion
The ground fault element on the upstream breaker was set with a delay of 0.25 sec, so no matter how high the current was, the ground fault protection would wait 15 cycles before tripping.
RE: Motor Starter Fault and Explosion
Ok I was just looking at the breaker and noticed something strange. Like I mentioned before this is a Siemens RL 480V Air Breaker. The tripping of this breaker is controlled by a Siemens Static Trip III unit. On this Static Trip III unit there is an LCD target indicator which presently reads "Disabled" I do not know much about this unit but I am assuming this means something may be wrong with the trip unit and therefore I do not have anthing to trip this breaker thus comprimising the downstream protection?
Is anyone familiar enough with this Static Trip unit to know what this "Disabled" display means. The book only says that this indicates protection micro-processor not functioning properly, but was hoping to get a better explanation.
RE: Motor Starter Fault and Explosion
www.cbsnuclear.com
RE: Motor Starter Fault and Explosion
That is far more than enough energy to toast a lot of stuff. We have found many small contactors; 2s ,3s completely blown off of walls, and as charred ruins, without tripping their associated breakers. It happens. Happened in your case. The arc just doesn't draw excessive current. It just runs like a high voltage arc welder for a while.
We've seen whole services burnt 200ft back to the main panels. They drew just under the tripping current until everything was gone then self extinguish. What I'm saying is what happened in your case was a relatively common event.
It was probably initiated by a high resistance connection as previously mentioned.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Motor Starter Fault and Explosion
RE: Motor Starter Fault and Explosion
itsmoked
So what you are saying is that its not necessarily the fault current value but the power or kW associated with this fault current value.
In other words a single phase condition would not have done this b/c the motor would still run just at a higher current before it failed. This was an arcing fault somehwere in the starter bucket.
Zogzog
So does this disabled message mean my unit and breaker are functioning properly or at all for that matter. Even if it is just a comm error that can be reset like you mentioned is the unit still operating properly while "disabled" is displayed?
So does this
RE: Motor Starter Fault and Explosion
Frankly the power represented here is many times more than what would be required to toast some metal bars, contacts, wire, Bakelite, etc., in a small physical volume.
Picture a typical 1200W floor heater in that space and how warm the air would get in just a few seconds. Now consider the same but using 160 of them to heat that same couple of cubic feet. A lot of thermal damage, and like, right now!
Meanwhile this large impedance suddenly thrust into series with the motor could actually reduce the current draw on the motor circuit so things just progress till there is nothing left or the geometry reaches some configuration that doesn't support the arc any longer.
Would all be fascinating to watch. (From a distance!) (If cost was no issue.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Motor Starter Fault and Explosion
RE: Motor Starter Fault and Explosion
Zogzog
I was able to reset this "Disabled" display and now the LCD display on the trip unit reads nothing as it should? Should this breaker now be providing full protection?
After seeing this I went around to all the rest of these breakers in the plant and noticed that almost all of the rest of them said "disabled". This is scary for I'm not sure if any of them had protection while they were disabled? We were able to reset all of them and hopefully they are all now providing full protection. I plan on testing all of these breakers during our next outage.
I found it very strange they they all were disabled and called Siemens. There only answer was that it could have been the result of some sort of power surge. I still find this strange and if this is the case this worries me because during a power surge we could loose all of this protection and not even know about it.
On another note, this event has caused me to go online and watch several Arc Flash videos. I viewed several of these videos both large and small flashes and was wondering:
Are these Arc Flashes the result of failed protection, and could many of them have been prevented with properly set protection or are these flashes just things that occur that are unfortunately impossible to prevent?
RE: Motor Starter Fault and Explosion
As far as arc-flash events, I would say that most are NOT the result of failed protection. Proper protection can reduce the arc-flash energy, but it won't stop the fault from happening.
But there is always a compromise between coordination and protection, even on a well-designed, properly functioning system.
RE: Motor Starter Fault and Explosion
When the Static Trip III LCD displays "DISABLED", that is an indication that the protection microprocessor's watchdog has detected a malfunction and that the trip functions are not working at all. So you are correct, you had NO protection for that MCC bus. If you have a Static Trip IIIC, IIICP, or IIICPX versions with communications, you have an option on the trip unit to allow annunciation (via an interposing relay) of that watchdog status. If you have the plain version without communications, it did not have that option. You can tell by the last 3 digits in the part number on the face of the tript unit. If the last 3 digits are 510 and higher, it is the version with communications and can be adapted with an interposing relay for the watchdog annunciation; 504 through 509, it is the non-communication version.
My suggestion would be to contact Siemens Industrial Services at (800) 241-4453 to have them come out and test your breakers, then inquire as to upgrading those trip units if you have the non-communicating one.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Motor Starter Fault and Explosion
Find a local NETA testing company at www.netaworld.org
RE: Motor Starter Fault and Explosion
RE: Motor Starter Fault and Explosion
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Motor Starter Fault and Explosion
You can say that again, whats the deal with that? I have ordered about $10k worth of RL parts over the last 3 months and stuff is taking forever to ship, waaay past promised delivery dates.
RE: Motor Starter Fault and Explosion
Thank for the good post, and you are correct, T/S comm issues is beyond the capabilities of some NETA and OEM field offices capabilities, I just think the actual breaker testing should be done by an independent.
We have been found some comm errors of late on STIII that we are having a hard time reproducing, we have a trip unit testing lab where we can simulate about anything but this problem has shown up twice this year and it has us stumped. I would like to get my solid state guy in touch with you if possible to discuss the issue. If you could PM me in the mike holt forum I would apprciate it.
RE: Motor Starter Fault and Explosion
Jraef
Thanks for the information. Any idea what could have caused 10 out of 11 of these units in my plant to go to the DISABLED state? This has me concerned now for my plant was almost totally without protection on the 480V side for who knows how long if you are saying this means that these units had no protection?
I will look into uprading my units if they are not the type that can be connected to a indication relay however in the mean time I'm thinking about making it a weekly if not daily PM for my electricians to check these units.
RE: Motor Starter Fault and Explosion
One more thing to add to my last post.
As I mentioned in a previous post 10 out of 11 of my static trip units read DISABLED yesterday. My electricinas reset them yesterday and they appeared to return to normal state.
Today we went back and looked at them and say that they had all returned to their disabled state.
I am trying to get Siemens out here to tell me what the heck is going on because now everytime this happens I am without protection. We reset them all again and are watching them while we wait for Siemens.
Is there something that can be happenning overnight causing them to become disabled again?
RE: Motor Starter Fault and Explosion
Are these new? If still under warranty, I'd push for replacement of all of them.
Breaker trip units have to be able to deal with power system transients, brownouts, blackouts, surges, harmonics, etc - that's the world they live in. There is nothing that should be causing them to go to Disabled other than bad hardware, bad firmware, or a combination of both.
RE: Motor Starter Fault and Explosion
The relay instantaneous setting (24 kA) is too high.
With the tranformer power you gave and, let's say Z=7%, you have a short-circuit current of 25.7 kA, assuming an infinite source.
Because of the arc impedance, the actual arcing current could be as low as 50% of that value.
That's probably why the breaker did not trip.
RE: Motor Starter Fault and Explosion
Just wanted to follow up on this issue.
I have contacted Siemens and they have indeed confirmed that the 10 of my static trip units that are reading "Disabled" are indeed damaged and are not providing any protection. The cost for replacing them, which I am doing is $37,000.00. (Very Expensive)
I gave them the serial numbers for each of these devices and pressed them to come up with an answer as to why some many of these failed. The came back to me and said they looked at the serial numbers and cross referenced them against batches that were used duing that time and found that there wasn't any documentation of defects or any other complaints about failures elsewhere.
The only answer that they could come up with was that there must be some sort of transient on our system such as a surge that caused this damage. I shared the same concern that others did in this thread that these devices live in a transient enviornment and should be able to withstand surges, and the fact that these are critical deveices which should not fail. They claim that there are no other documented cases as to units failing as results of surges nor do they have any recommendations. They suggested putting surge arrestors on our incoming line, in which we have several surge arrestors throughout the plant. We do for the record have a history of surges damaging other electronics here such as plc's etc...
So as it stands I am spending an ungodfull amount of money to replace these units and still searching for answers.
RE: Motor Starter Fault and Explosion
Hey rockman, what kind of plant is this? Is it in a lightning area?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Motor Starter Fault and Explosion
I've installed a bunches of their products in upgrading power systems over the years. Just make sure that whoever might undertake this replacement does actual trip testing under primary current injection to validate the sensors and the trip actuator, too.
old field guy
RE: Motor Starter Fault and Explosion
Itsmoked
It is a cement plant (16MVA) located just outside of Gainesville florida. Yes it is in a lighting area, for what I am told, Florida is the lighting capital of the world. Like I mentioned we have had several other documented cases of failures due to surges.
RE: Motor Starter Fault and Explosion
BTW: I'm with the old guy. Personally I would be looking high and low to not reward the original maker with more $$ after that shabby, disgraceful, performance. I'd check into Utility Relay.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Motor Starter Fault and Explosion
However if the breakers are still under warranty, that would probably be void if you install a third-party trip unit. But since they are making you pay for the trip units, I'm assuming the equipment is out of warranty.
My feeling is that these trip units should be robust enough to deal with just about any type of surge they might see, especially considering they are operating at 480 V through a unit substation transformer.
The surge explanation sounds bogus to me and very convenient for Siemens.
RE: Motor Starter Fault and Explosion
We found many with loose connections during PM shutdowns and caught others through infared insepctions.
Check the connections when you get a chance.
RE: Motor Starter Fault and Explosion
10 out of 11 units failing isn't much for a track record. Did Siemens tell you they'd look at the failed units and determine what is causing the problem? We're going through a product recall because of new Siemens breakers not properly operating and it's still going to cost us a lot of $$.
RE: Motor Starter Fault and Explosion
I talked with a member of my local team on that issue. He has never had a single ST-III unit fail on watchdog. The ST-III has NO DIRECT CONNECTION to the line power; it takes all of it's power from the CTs. So lightning strikes etc. would not affect them (unless of course they take a direct hit). I think the "surge" argument was a red-herring thrown out by someone who probably should not have been involved.
When I told him it was at a cement plant in Florida, he said he has a Hansen Cement plant out here that has had over 100 ST-III units in place for over 7 years with only 2 failures, neither of which were watchdog issues. What he thinks it might be in an environmental issue. The ST-III uses a super cap instead of a battery for fault memory retention. If the cap dies, it loses its user setpoints, which shows up as a watchdog failure. Most likely the combination of heat and fine dust build-up has caused the caps to overheat and die, especially if they had been un-powered for a length of time. That actually fits with your experience of resetting them and they seem OK, but only for a little while. That cap is probably no longer capable of holding a charge.
All that said, he no longer sells ST-III to his customers, he sells them on the URC replacement as well! The reason is, the ST-III is now a legacy product and there is no more R&D being done on it. The URC unit is more of a universal replacement (although there are different versions for different mfrs because of connections), and URC is keeping it current. In the future, they will be releasing an "Arc Flash" option module that can be retrofitted easily in the field whoich will provide a lower trip setting mode to reduce arc flash hazard levels temporarily. The Siemens WL breaker that replaced the RL already has that feature, but with the URC trip unit, you will be able to upgrade any older breaker to do it. He highly recommends the URC relay from a quality standpoint too, it is very reliable and easier to set up than the ST-IIIs were.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Motor Starter Fault and Explosion
RE: Motor Starter Fault and Explosion
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
RE: Motor Starter Fault and Explosion
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?
RE: Motor Starter Fault and Explosion
Keith Cress
kcress - http://www.flaminsystems.com
RE: Motor Starter Fault and Explosion
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.
www.cbsnuclear.com
RE: Motor Starter Fault and Explosion
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: Eng-Tips.com Forum Policies
RE: Motor Starter Fault and Explosion
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.
RE: Motor Starter Fault and Explosion
RE: Motor Starter Fault and Explosion
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
RE: Motor Starter Fault and Explosion
Actually that phenomenon wiped out hundreds of thousands PC motherboards a few years back.
Keith Cress
kcress - http://www.flaminsystems.com