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.

 

[dpc]

I would still have the breaker and trip unit tested to verify that it is working within specs.

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.

 

[jraef]

rockman,

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
For the best use of Eng-Tips, please click here -> faq731-376

 

[Zogzog]

I would call someone else that is unbiased (Preferably NETA certified) to come test your breakers, unless of couse Siemens offers to do the testing free of charge to remedy the problem.

Find a local NETA testing company at

http://www.netaworld.org

 

[davidbeach]

jraef, your advice is undoubtedly good, but it would have been appropriate in your second paragraph to make note of who pays your salary. Those of us who pay any attention know, but some people might not.

 

[jraef]

Point taken. For those who don't know, I work for Siemens, in fact I work for the Circuit Protection and Controls Division. I didn't intend for my comments to be an advertisement, I mentioned it because I have to deal with the RL breakers a lot and have found that local NETA service people and even our distributors end up turning them over to the SIS people in the end anyway because it is difficult to order parts other than through them now.


"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

 

[Zogzog]

"it is difficult to order parts other than through them now"

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.

 

[Zogzog]

Jraef,

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.

 

[rockman7892]

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.

 

[rockman7892]

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?

 

[dpc]

The ghost of ABB maybe. I think you need to get Siemens on site ASAP and have them try to figure out what is going on.

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.

 

[JPGQC]

Hi Rockman

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.

 

[rockman7892]

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.

 

[itsmoked]

Sounds pretty lame to me.

Hey rockman, what kind of plant is this? Is it in a lightning area?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[oldfieldguy]

There's a company called "Utility Relay" that makes a very capable replacement, very programmable trip unit that will swap with your Static-Trip whatevers. Ten of them will cost you a lot less than $37K. Google them and see it they might meet your needs.

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

 

[rockman7892]

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.

 

[itsmoked]

Thanks for the info.

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

 

[dpc]

We have several clients who use the Utility Relay trip units and I have not heard of any problems with them.

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.

 

[rcw retired EE]

A comment on the original failure - Size 5 staters at 480V are prone to connection problems at the brekaer to bus connection. This is especially true if a draw out MCC bucket is used.

We found many with loose connections during PM shutdowns and caught others through infared insepctions.

Check the connections when you get a chance.

 

[LionelHutz]

The "Utility Relay" trips sound like an interesting solution.

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 $$.

 

[jraef]

Follow up on the Static Trip III and the Utility Relay Co. replacement.

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
For the best use of Eng-Tips, please click here -> faq731-376