# Motor Branch Short CCT Calc



## l0sts0ul (May 7, 2011)

Hello All, 

This may seem like a dumb question, but I Want to find out if im thinking about this properly.

Had an emergency call out this week, and turns out aftersome searching, I found a boiler booster pump motor had blown its windings, and shorted. 
This however did not trip the branch circuit breaker, (which is 100A), and tripped the main CDP breaker (1200A) and took out all the house power for the entire building.

My reasoning for the branch cct OC protection not tripping would be the IC of the breaker not being high enough?

The next question is that, they would like me to isolate this problem from happening again, and being we are in canada, I would have to read the CEC about how the IC value would be affected if I chose to install a fused d/c in between the breaker, and the motor controller.

If someone could help me with the calc again (its been a while since I used this) to figure out the fault current/asymmetrical current of this motor I would greatly appreciate it. I have all the motor info, but its pretty basic.

3p 208v 47.6A 3600RPM TEFC

Thanks


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## Tom45acp (Sep 6, 2011)

This may also be a coordination issue between the two breakers in question, so you should investigate both. To check the coordination issue, you'll need to examine the trip curves from the manufacturer of the breakers. If any part of the trip curve for the larger breaker lays to the left of the smaller breaker, there is a coordination problem.

For your fault current, I can't help you with the asymmetrical part, but here's a link for a calculator. You'll need to know the transformer kva and impedance, or, if supplied by your utility, what the available fault current is at the service. This program seems to me to be accurate, but I would lower the transformer impedance by 10% for manufacturing tolerances.

Keep in mind, motor contributions are not taken into account.

http://www1.cooperbussmann.com/software/index.aspx


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## l0sts0ul (May 7, 2011)

Thanks


I will check with the supply authority of the area to see if they can tell me the %Z of the utility transformer. They are usually pretty high in this area as they are 250kv down to 208/120. 

Thanks for the link. 

Do you think a fused disconnect with fast acting fuses would help as a moe cost effective measure to fix this between the breaker and the motor? Or is the only proper and safe solution the breaker change. The motor is the only load on he branch cct


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## JRaef (Mar 23, 2009)

The trip timing has nothing whatsoever to do with the interrupting rating of the breaker. That only has to do with whether or not the breaker safely stops the fault current or turns into a bomb.

This was a coordination issue, or rather a lack of coordination as the others have said. Nothing more. The fault should be interrupted at the lowest level possible. That means making sure your breaker closest to the load is set to trip SOONER than the next breaker upstream of it. That is what a coordination study does. Most likely, nobody ever did one when this booster pump was installed.

I would also look at the overload relay as well. Chances are that there was an earlier problem that the OL failed to pick up that eventually led to the short circuit in the windings. The breaker is supposed to be the last resort, not the first line of defense. Most likely (if anyone will admit to it), they have been having troubles with this motor for a while, so someone figured it was OK to just keep increasing the overload settings or getting larger heater elements to make the problem go away, rather than thinking about WHY the pump is overloading.


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## JRaef (Mar 23, 2009)

Duplicated post deleted.


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## JRaef (Mar 23, 2009)

Duplicated post deleted.


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## JRaef (Mar 23, 2009)

Duplicated post deleted.


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## FastFokker (Sep 18, 2012)

JRaef, could you repeat that? :laughing:


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## JRaef (Mar 23, 2009)

FastFokker said:


> JRaef, could you repeat that? :laughing:


It said there was some sort of data base error. I tried to post an explanation, that didn't go through of course.


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## l0sts0ul (May 7, 2011)

I think you hit the nail on the head. I've had this problem with this customer not doing a pm on their motors or starters at any sort of interval let alone an IR scan. I've also caught them upping the overload settings on other motors instead of just repairing or replacing them. They already replaced this starter and overload setup once before I even got there. 

I have to look into the coordination problem as I understand you are correct. I just need to get the curve for the branch cct breaker and find out why it didn't trip. 

Still no one has answered if I could use a fused dc in between the breaker and the motor before the starter with fast acting fuses to accommodate for this? There is an unfused DC there already as a service switch but that's it. I have a feeling the breaker is a ton of money and I'm pretty sure they won't go for changing it out. 

Thanks. Btw the breaker is Siemens. Just trying to get te spec sheet now off the manufacturer website.


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## JRaef (Mar 23, 2009)

l0sts0ul said:


> I think you hit the nail on the head. I've had this problem with this customer not doing a pm on their motors or starters at any sort of interval let alone an IR scan. I've also caught them upping the overload settings on other motors instead of just repairing or replacing them. They already replaced this starter and overload setup once before I even got there.
> 
> I have to look into the coordination problem as I understand you are correct. I just need to get the curve for the branch cct breaker and find out why it didn't trip.
> 
> ...


I doubt you need to change the breaker (unless it was damaged), you just need to change the settings, and usually on the UPSTREAM breaker.

The SOP would be to set up the motor breaker to the lowest possible instantaneous trip you can without causing nuisance tripping. The starting point is the value shown in table 430.52, but then read the exceptions, because the NEC allows that yo be as high as 17X the FLC of the motor, as long as you can prove it would not otherwise hold in. Once you have that dialed in, make sure the instantaneous setting of the main is HIGHER than that, so that a fault at the motor trips the MOTOR breaker before the main sees the threshold level it is set at.


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## l0sts0ul (May 7, 2011)

1 problem here is that the breaker is not adjustable instantaneous trip. It's set and its on,y 100amp on the 47.6amp motor. Here the CEC allows us to go up to 250% FLA On a standard tripe breaker and up to 1300% on an instant mag trip. 

This is why I was asking about fuses.

Just checked and it is a Siemens HHED63B100 thermal mag breaker. No adjustments possible.


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## JRaef (Mar 23, 2009)

l0sts0ul said:


> 1 problem here is that the breaker is not adjustable instantaneous trip. It's set and its on,y 100amp on the 47.6amp motor. Here the CEC allows us to go up to 250% FLA On a standard tripe breaker and up to 1300% on an instant mag trip.
> 
> This is why I was asking about fuses.


On 99.999% of Siemens breakers, the Instantaneous trip setting on the ones that do not have an adjustment is 10X the breaker rating, so on a 100A breaker, the instantaneous trip will be 1000A. So if your upstream breaker is 1200A, there is some other problem here. Does the main breaker have Ground Fault protection, and was the fault a grounded fault? If so, the only way another down stream device, fuses or CB, was going to stop that main from tripping is if it was smaller than the GF setting of that breaker, or the downstream device has GF protection on it as well. Fuses would not help you in that case.


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## 123 (Oct 28, 2012)

I believe fused disconnect would be ok.
But I would use non time delay fuses, and your disconnect 
would have to be 48A x 3 = 144A ............200A disconnect (300% of FLA)(pricey solution)
Fast fuses open faster than circuit breaker.
Using time delay fuses may still lead to other overcurrent devices tripping first.
Because short may take out only one fuse, overload would be good to prevent single phasing.
There is a chance this fault will not happen again. I would wait and see.


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## l0sts0ul (May 7, 2011)

Hi again


Just want to make sure I'm understanding your logic correctly. Based on the CeC your way I would have to use a 200 amp disconnect with a fuse less than 142.6 amp ( being as we cannot exceed this number by code) so that would put me at a 140a fuse if available, but more likely a 125 amp fuse. The way I see it, if I was to install a 100amp disconnect with 100amp one time fast blow fuses ( the motor magnetic starter has overloads on it already) I would be meeting the code requirement above minimums and the motor should still run as required. 

The motor currently is running on a 100amp breaker and does not start under a huge load. Movingly water, but not usually under a lot off pressure at start. 

I don't think I would need a 200amp disconnect even though code specifies I could use it. I order to accomidate the space I have to work with and the time and cost that this customer cannot afford, I believe this is the best option. 

What do you think people's??
Tanks again


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## Peewee0413 (Oct 18, 2012)

Overload sounds like a fix?


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## l0sts0ul (May 7, 2011)

Sorry I don't understand your response. The motor already has overloads on the starter. If that's what your asking.


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## Peewee0413 (Oct 18, 2012)

How do you burn windings if you have an overload? Maybe I didn't read enough of the thread to understand.


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## Peewee0413 (Oct 18, 2012)

Even if windings burn up isnt there a class that will go before a fuse blows?


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## 123 (Oct 28, 2012)

You use next lower available fuse 125A.
The fuse will fit in 200A disconnect.
If you use 100 A disconnect and 100A fast acting fuse, you might be ok,
but if you need a bigger fuse, then you have to change the disconnect.
For heavy starting loads the code allows exceeding the 300%.
More then code, the reason is to avoid blowing the fuses on the start up 
by inrush current. 
If you could get a 100A disconnect and wire it temporarily to try if it holds
for repeated stop/starts.
Using time delay fuses may defeat the purpose, and still trip the main breaker.


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## Peewee0413 (Oct 18, 2012)

did it blow on start up?


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## l0sts0ul (May 7, 2011)

No, it did not blow on startup. The customer was changing the settings on the overloads (using overload relay block) and kept turning it up, probably because the motor was single phasing and tripping on overload.

The motor winding shorted out and tripped out the main breaker in the cdp, dropping the entire common area power to the building. This place was not built with much solid consideration for stuff like this at all...very poorly engineered and held with scotch tape, and bandaids the whole way. I Could go on about the problems in this place for hours.

anyways, being it is running on a 100A breaker to begin with, I figured it would be safe to run on a 100A disconnect with fast acting fuses. Im still fuzzy as to why it would trip the main before the branch OC device, but I realize the coordination is a huge issue as to why this would happen.


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## micromind (Aug 11, 2007)

A couple of things;

First, a 47.6 amp motor will blow 100 amp fast-acting (depending on just exactly how fast they are of course....) fuses every time it tries to start. The starting current of such a motor will be in the 200-250 amp range. 

Second, does the 1200 amp breaker have ground-fault protection by any chance? If so, in my experience, the ground-fault feature will almost always trip before a branch-circuit breaker will. A coordination study will help, but I've still seen the main trip on ground fault and the smaller breaker stay closed.


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## l0sts0ul (May 7, 2011)

Hello. 


That motor will not pull 250amp on startup, the motor beside it in the same system with same specs is drawing 125amp on startup. 

The main on the system is 3200amp and it is GFI protected. The 1200 amp submain is 1200amp Is not gfi protected.


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## 123 (Oct 28, 2012)

The startup current of the motor can be calculated by Ohm’s law.
At standstill the motor doesn’t generate a counter EMF, and current is 
limited only by the resistance of the winding.
Typical initial current is 10 times the FLA. 
But even the fast acting fuses don’t initially see this current.
By the time the fuses could react, the current is less than 3 times the FLA
I have to agree with micromind that 100A fast acting fuses are unlikely 
big enough to withstand the inrush current in all situations.
A soft start or VFD would fix this.
3 phase Induction motors are extremely reliable, and seldom get a short.
the reasons can be:
water,
or overheating due to increased load or to bad bearings.
If you have replaced the motor, and it is dry, and sized correctly, and protected by overload,
it should run many years with out any problems.


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