MCC Failure, any ideas?

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To be 150HP or 200HP

To be 150HP or 200HP

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.

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

FLA of 200HP is 240A +/- specifics


FLA of 150HP is 180A +/- specifics

The motors at 150hp might be ok on the 800A main fusing .... But if I understand if they are 200hp I think the calculations are shy for 800A main fusing..

Or 200hp was reported in error in your latest post...
 
What about?

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.

A check for this could be to have normal maximum load being called on (running), and then open the fused switch. Check the run signals to the starters to see if the signal is still calling on the motor. If so that is probably your culpret and you will look like a savior, if not then I am just wrong. This would be a scheduled shutdown, but blowing fuses is always a non scheduled shutdown. Or you could ask what was running when the fuses blew. A brownout may only last a few seconds and not affect everything. Lighting may not even flicker, but across the line motor starters open on voltages less than 70%-80%. Starting current when restarting a motor coasting down with the field not fully collapsed can be higher than locked rotor.

They may have 5 seconds between normal starting, but is what they are using to catch a short brownout at say 65% catching the incident? You may prove them wrong.

I don't mean to be frank (hope I don't offend any Franks out there) but I am hard headed. You did ask for opinions. I don't think it is cable shorting out, this would take 4800+/- amps to blow an 800 amp fuse (800X6=4800) rule of thumb. That kind of current phase to ground or phase to phase will definitely damage insulation. It will show up in a megger test if done properly, and would probably not be able to return to service.
 
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More questions...

More questions...

Is this a new install, and failure was startup testing related?

You haven't posted any loads for the normal running loads, has system ever run?

Has system failed after hours of operation {post startup} or at startup?

If system has been operational for some time, has pump amp load changed?

ie... motor alignment, normal ware of pumps, difference in viscosity of what is being pumped, are pumps in constant duty or are they flow controllable. I have had operators use valves to change system normal operation. A misalignment might cause pumps to work harder. Normally the overloads would catch an issue like this unless they are set improperly.
 
Donald,

I dont mind you being Frank (sorry franks), I came here asking for help as I was running low on ideas, your time is greatly appreciated.

I am always up to learn more about different aspects as I am still fairly new to some things. Again, thanks for your input as I find it very helpful.


The control system issues I am checking into currently and if I find anything I will be sure to post results. My only question thus far would be:

An issue such as a brownout would come from the source and thereby affect the transformer feeding the switchboard and its MCC set/group. So, I would think that any situation like this would affect multiple MCC sections on that same switchboard as I can't see there being different control systems setups per MCC. However, this may not be the case and I may be missing something.


This equipment has been around for awhile, roughly 8-10 yrs I would guess. Normal running loads would be ~600A per phase (1800A). Runs 24/7, 365 unless maintenance hits.
 
jstrick2 - I've been following this thread with interest. Somethings are still not clear. It's been over a week since the original post. Are you shut down? Or are you running? Have the individual motors been checked for the running amps? I take it those MCC feeds are in metal conduit, if so, and there had been water & rocks? that damaged those cables I have absolutely no doubt you would see that in the megger readings. If the megger readings were good I think you should be looking at the MCC & motor loads.
This is a stretch, but in my 40 year electrical career I only saw this once..... A motor would trip the OL's - by the time I got there to check everything would be fine, I'd start it back up, it would run perfectly. The 3rd time I got a call I was waiting for it and got there immediately - I found when the motor got hot one of the motor windings "internally to the motor", would increase resistance and cause the OL and trip. The 1st 2 times I checked this it had cooled off enough that all resistance appeared correct.
Have you thought about a DLRO for checking the connection resistance of cable to bus, and those within the MCC?
 
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