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.

 

[waross]

A couple of questions;
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

 

[dpc]

Did this occur on motor starting or while the motor was running? Is the 480 V system solidly grounded?

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.

 

[rockman7892]

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?

 

[rockman7892]

dpc

This occured while the motor was running, and the 480V system is solidly grounded.

 

[Zogzog]

When is the last time you tested your breakers?

 

[dpc]

It takes a lot of current to trip the breaker that is protecting the entire MCC. As I mentioned, an arcing fault at 480 V can burn up equipment without tripping a standard overcurrent device.

 

[rockman7892]

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?

 

[dpc]

Arcing ground fault are the most common, but other scenarios are possible.

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.

 

[rockman7892]

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?

 

[davidbeach]

The arc may well have started as phase to ground, but once the bucket is full of plasma you have a 3 phase fault, that's why the arc flash standards don't address phase to ground arc faults. A 20,000A fault for something less than the short time setting of the main breaker could be more than enough to blow open the door. After all, the door is not designed to contain an arcing fault; if it were you would have several large bolts all the way around the door to hold the door in place.

 

[dpc]

Faults can extinguish themselves. The current and voltage goes to zero twice each cycle then the arc restrikes. If the arc path lengthens or otherwise gets cooled off enough, the arc does not restrike and the fault is extinguished.

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.

 

[rockman7892]

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.

 

[Zogzog]

It means your trip unit is fried, and that would explain the breaker not tripping. We repair these all the time. There could be several failed compoents.

http://www.cbsnuclear.com

 

[itsmoked]

400A @ 480V => 200kW

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

 

[Zogzog]

Oh, about the STII, I forgot, sometimes you can reset them when you get that "disabled" message. This loss of comms can be caused by noise on the data line.

 

[rockman7892]

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

 

[itsmoked]

It could be single phase it could be three phase. The motor would think it was being soft started or stopped as a bunch of resistance suddenly appears in series.

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

 

[Zogzog]

Try to reset it, the "blip" may have come from the fault (Pretty big design flaw if you think about it), if you cant reset it your trip unit will not function and you should repair or replace it.

 

[rockman7892]

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?