MCC Failure, any ideas?

Status
Not open for further replies.
J

jstrick2

Member
Hello all, got a troubleshooting issue I am trying to figure out.

I have a switchboard feeding 480V MCC sections with 800A main lugs via a parallel feed. This section includes 3 - 150HP pump motors, then a 40HP and some random 5HP buckets.


Essentially, we had A & C phase fusing blow on the feeders for this MCC section. Initially, the feeders (cable running from the switchboard to the MCC section, just for clarity) were megged bad with the first try, then good with the second. We brought in a seperate testing contractor to megger everything again and the cables passed, but were in the Mega-ohm range whereas most of our good rated cable touches tera-ohm. Megger tests were done with all three phases at once, then each phase individually, all test "satisfactory".

Replaced fusing, threw the switch and checked voltage/current at the 800A main lugs for the section. Voltage checked fine on all three phases. However, there is an amperage difference on both the A & C runs of cable.

IE, one of the parallel runs is roughly 50A higher on the A phase than the other. The same occurs on the C phase, but its closer to 30A. To make matters worse, both of the parallel runs are seperated by conduit and it seems one has an issue on A, and the OTHER has the issue on C.

So, "run 1" has A phase 50A higher than "run 2" and "run 2" has C phase 30A higher than "run 1".


I am having someone come in to do IR scans on the switch feeding the MCC section soon. Other than that, I admit I am somewhat at a loss.

I assume I have a hot spot or a breakdown in insulation on some pieces of cable, but the fact that there is no "commonality" has me wary. I hate the idea or ripping out and running new, to find the same issue.

There were no voltage issue, and I am going to try shutting off the buckets one by one, checking amperage on phases after each to see if I can find something downstream. I also think we should seperate the parallel feeds and re-megger each individual cable as well.

Fun fun. Any other ideas greatly appreciated... thanks
 
Sounds to me like your troubleshooting technique and plan is sound. I would just proceed down that path (the IR and one bucket at a time) first then go from there.
 
Anybody megger the MCC cabinet itself? It bothers me a little that the testing was limited to the feeders only, when ideally anything on the load side is suspect.

How does the fuse type in the 150hp bucket square with the fuses for the feeder, with regard to delay characteristics?
 
All loads running, loss of voltage, power returns, all motors start at same time, and fuse blows. Parallel cable current imbalance may be due to connection resistance.
 
"We brought in a seperate testing contractor to megger everything again and the cables passed, but were in the Mega-ohm range whereas most of our good rated cable touches tera-ohm. Megger tests were done with all three phases at once, then each phase individually, all test "satisfactory"."

Next time try a certified testing company, not sure who would call a few Meg good, why didnt they test the MCC's? Did they disconnect the cables from the gear and test them seperatly? Did they seperate the feeds? Did they do phase to phase readings or just phase to ground? Is this a delta system? It would be curious to see what phase A-C readings were compared to A-B and B-C if delta and A-G and C-G if a wye system.
 
bbaumer said:
Sounds to me like your troubleshooting technique and plan is sound. I would just proceed down that path (the IR and one bucket at a time) first then go from there.
Click to expand...

Have to agree with this but will elaborate a bit.

In the future megger the cable while on the reel, before installation, and then after - maintain a written record.
I have received cable with factory defects. I am also familiar with a jobsite that received 6 reels, each reel of 1000' cable was defective. Luckily they found this out before pulling all that cable. The manufacturer replaced all this cable. Meggering cables should not require a testing lab, it is a very simple procedure.

I would also get a DLRO, you can rent one, and check the bus connections in the MCC. You can do this by pulling the buckets and reading resistance from incoming point to the stab connection at the bucket. The resistance will slightly increase as more connections become involved. But you should only be reading micro ohms.
I have found bus connections that were not torqued, made up semi tight, and missed when the contractor went through to torque everything down.
I really like a DLRO - you find all the poor connections that the installer swears is perfect.
 
forgot

forgot

It seems to me that an IR temp meter is better. It's quicker, tests under actual load and displays temperature, which should correlate well to connection integrity.

Again, the problem is pass/fail temp specs; does anyone have these? I'd think anything over 50 degrees Celsius is a problem.
 
Lxnxjxhx said:
It seems to me that an IR temp meter is better. It's quicker, tests under actual load and displays temperature, which should correlate well to connection integrity.

Again, the problem is pass/fail temp specs; does anyone have these? I'd think anything over 50 degrees Celsius is a problem.
Click to expand...

IR scanning is load dependant so the results can be misleading, it is not the tempature that is so much a concern but the delta t between phases and the delta t between the connection and ambient.

Also if the scanner is not knowledgeable on emmistivisty and reflectomotry of different materials they are just playing with an expensive toy.

I have even sen some try to use those IR meters from home depot to attempt to gain some sort of information beside "It seems a little warm"
 
RHJohnson said:
Have to agree with this but will elaborate a bit.

In the future megger the cable while on the reel, before installation, and then after - maintain a written record.
I have received cable with factory defects. I am also familiar with a jobsite that received 6 reels, each reel of 1000' cable was defective. Luckily they found this out before pulling all that cable. The manufacturer replaced all this cable. Meggering cables should not require a testing lab, it is a very simple procedure.
Click to expand...

I'm sure it's obvious, but I'll ask anyway. How do you megger cable on a reel? One clamp on the wire, the other clamp goes on ???

Don't you have to megger against other conductors or a known ground? I'm just having a hard time visualizing how you would megger a single wire by itself on a reel.
 
Cow said:
Don't you have to megger against other conductors or a known ground? I'm just having a hard time visualizing how you would megger a single wire by itself on a reel.
Click to expand...

I might have jumped the gun! I was talking cable, maybe thats not what they are feeding that MCC with? Thanks for asking Cow - I may not have been thinking, but for an apprentice you were! I like that...
Now what I said should make more sense, and I bet you know what to do.

I tend to get a little upset when I think a problem is caused by poor quality work, or that proper checks are not made before energizing something. For the last 20 years I worked I did start up (commissioning to some) at new constructed power plants. It seems every circuit (including the high voltage) had errors, sometimes it was very visible that someone purposely hid the error to get done quicker, or so his boss could not see it. To energize any of those plants without completely checking every thing would have let out all that magic smoke this forum talks about.
 
Listen very closely to what I have to say. Take an amperage reading at the mdp and then take an amperage reading at the load termination to determine that what is being sent out is in fact delivered. If it is diferent you have a problem on your parallel feeder. If it is not you have a problem on a load. I will assume there is no ground fault on this service.
 
Thanks guys for your thoughts thus far, I appreciate the time.


No ground faults on this service.


The part that is confusing me the most at this point is that the voltage checks at the main feed/lugs of the MCC section are fine. This would lead me to believe we have an issue further downstream. My reasoning being that if there was an insulation issue, I would see a voltage drop at the MCC section.

However, toss in the fact that we blew the fuses back at the switch... which blatantly points at a problem with the feeders, at least in my mind.

I am working piecewise through the loads in the MCC with some offline motor testing and amp checks to try to eliminate the larger motors. Nothing solid thus far.

We also did an IR check back at the switch and there are not hotspots at this point.


Is there a decent chance that at full load the cables would "heat up" and show the failure? This on/off failure has me going back and forth. I would think an insulation fault able to blow the fuses would stay failed. Also, I would think with voltage at the switch, there may be a chance to show a current draw through the feeders even with the MCC loads off.

I am not amazingly experience in this respect for cables, hence the quotes and back and forth :D
 
Last edited:
Again

Blown fuse

If all loads were running, a brownout or loss of voltage occured, then the power returns, all motors start at the same time, locked rotor current would probably blow fuses. This would require that the run signal didn't go away with the loss of power. Are you sure this didn't happen?

Current difference between parallel cables

Check the voltage drop across a cable ternination. This can be done by checking the voltage from a bare spot on the cable to a point past the lug. (be careful) Parallel cable current imbalance may be due to connection resistance R=V/A, or in your case A imbalance=V across lug/R of connection. The resistance could be anywhere from the cable to a point past the lug. This seems logical to me.
 
dsullivan,

I suppose I understand the thought process in checking the drop across the termination. I think this could also be a possibility, however, I think the test could be tough to perform.

With the insulation, tape, termination, etc, how would I go about performing this test? Correct me if I am incorrect, but I see this as a very hard test to perform as there are not easiy to access "bare spots" on the cable.

Also, I would think that would show on an IR scan as a hotspot.


One possibility that has occurred to me is the atmosphere is an industrial one (mill) and this cable is in conduit orginally set in concrete. Areas of this plant get hosed down, plus there are different processes that can toss out different liquids.

There may be a chance that there was water in the conduit. With the XHHW, if there was an insulation fault over time, we could have had a water short and enough heat to boil the issue off.

More stuff to look at, again, thank you for your thoughts/time.
 
If you taped a cable to lug connection then you must have a lug that is bolted to the buss. This is two locations to have resistance. One location on the cable to lug and one on the lug to bus connection.

The resistance could be only .1 ohms or 5 volts, which is only 250 watts. You may not see that with IR, it could heat everything near it to the same temperature. You evidently have enough room to get a clamp-on ammeter on the cable. You should be able to get your volt meter leads on the cable and the location where the lug is connected. Sounds like you have gone through allot of trouble to find out why there is current imbalance between parallel cables when it must be resistance difference between one cable and the other cable. This resistance difference between them must either be in the cables or the terminations.

Are you saying that the moisture in the conduit blew the fuse/fuses? What happens if the conduit gets liquid in it again, blow the fuses again, you will be called back again any time in the future that they blow, just because it will have a history of blowing fuses.

Are you sure the control system didn't start enough motors at one time to blow the fuses with locked rotor current, this could happen if a control system is on DC battery or AC uninterruptible power supply and a power interruption occurs. When I design a system I make sure that I monitor the voltage with a phase loss/low voltage monitor and incorporate that into the control system to stagger start loads in a brownout or blackout scenario. I realize that you probably didn't design the system, but when fuses blow, you will be called by the owner/engineer to replace them. This may be a situation that is the engineer's fault in the design.
 
donaldsullivan said:
If you taped a cable to lug connection then you must have a lug that is bolted to the buss. This is two locations to have resistance. One location on the cable to lug and one on the lug to bus connection.

The resistance could be only .1 ohms or 5 volts, which is only 250 watts. You may not see that with IR, it could heat everything near it to the same temperature. You evidently have enough room to get a clamp-on ammeter on the cable. You should be able to get your volt meter leads on the cable and the location where the lug is connected. Sounds like you have gone through allot of trouble to find out why there is current imbalance between parallel cables when it must be resistance difference between one cable and the other cable. This resistance difference between them must either be in the cables or the terminations.

Are you saying that the moisture in the conduit blew the fuse/fuses? What happens if the conduit gets liquid in it again, blow the fuses again, you will be called back again any time in the future that they blow, just because it will have a history of blowing fuses.
Click to expand...

I am saying that liquid in the conduit helped provide the short, yes. I believe when they originally pulled this cable, there is a high likelihood of there being sand, small bits of gravel, etc entering that conduit. This would provide a good means of insulation breakdown over 10-15 years as we have seen similar issues here before. Again, i think there is a good possibility that liquid has entered this conduit.

I would think even if the IR had trouble finding hotter temps vs. its surrounding environment, it will still show a higher temp on A phase vs. B & C phase? That was my line of thinking anyways. I will make sure some further checks are done though, thanks.

I agree that there is a resistance difference, just havent found anything solid yet.


I verified with the control systems team and there is a delay of 5 seconds between the 3 200HP motors.

Thanks.
 
Last edited:
Status
Not open for further replies.
Top