NEC 430.52 Inverse Time breaker with Instanteous Trip

Status
Not open for further replies.
D

diakonos1984

Member
I'm working on coordinating some motor protection. I am familiar with the differences between an Instantaneous Trip breaker (a motor circuit protector--can only be used in a listed combination starter with overloads) and an Inverse Time breaker. I have read the details and exceptions listed in the NEC 2005 Article 430. What I haven't found in the code is a combination thermal-magnetic breaker with an adjustable instantaneous trip. Most inverse time (thermal) breakers have a fixed instantaneous, but a GE TKMA for example has an adjustable instantaneous trip.

My thought, and please tell me if I've missed something, is that you use both columns for the appropriate segments.

Example: 250HP induction motor on 480V. Using Table 430.250, the full load current is 302. 250% of that is 755A, so my 500A GE TKMA is well within the limits. 800% of the FLC is 2416A, and so an instantaneous setting of 5000A (the max for this breaker) is too high, but a setting of 2250A is acceptable.

Is there anything I'm missing, or is this interpolation correct?
 
You obviously have located data which I didn't find.
I thought the settings on the TKMA breaker were for the trip curve, but not necessarily "instantenous trip" settings.
 
I'm using SKM PowerTools which has breaker curves and settings adjustments built in to the graphing module, but here's a link to the GE time current curve for the TKMA breaker.

http://www.geindustrial.com/publibrary/checkout/Time%20Current%20Curves|GES-6111D|generic

You'll notice the curve has references to the high magnetic and the low magnetic settings.

It's kind of an older breaker, but there are plenty of other thermal-magnetic breakers with adjustable instantaneous segments.

Thanks for looking!
 
augie47 said:
I thought the settings on the TKMA breaker were for the trip curve, but not necessarily "instantenous trip" settings.
Click to expand...

There is no thermal-magnetic molded case circuit breaker (the black jelly bean of breakers) that I am aware of that allows you to "adjust" the Long time (thermal) portion of the breaker trip curve.

Once you get into the "200A" frame size and larger of molded case breakers almost everyone has an adjustable Instantaneous (magnetic) setting, with a typical range of about 5-10x the thermal trip rating.
 
Here's a better example that shows why it is important for me to know if a thermal-magnetic breaker with an adjustable instantaneous trip setting must conform to both columns in Table 430.52:

125HP, 460V induction motor. FLC is 156A per Table 430.250. Currently the motor is protected by a 400A Cutler-Hammer (Westinghouse) LB breaker. No overloads. The breaker curve is on Page 15 of this PDF:
http://downloads.eatoncanada.ca/downloads/Low%20Voltage%20Circuit%20Breakers/Time%20Current%20Curves/AB%20DE-ION%20Circuit%20Breakers.pdf
Table 430.52 requires the breaker setting for the inverse time (thermal portion) breaker to be 250% of FLC, which is 390A. However a 400A breaker is acceptable by Article 430.52(C)(1)Excep1 (see also Article 240.6). So no real serious problem there.

However, the range of the adjustable instantaneous trip is 2000A to 4000A. According to Table 430.52, an instantaneous breaker should be set at 800% of FLC, and never more than 1300% of FLC by Article 430.52(C)(3)Excep1. This calculates to a range from 1248A to 2028A. I would have to set the breaker all the way down to be able to get in code--and that on an exception that is intended only for use when the 800% setting interferes with motor starting. Is that what you would do? Or would you change it to a lower amp breaker, with an instantaneous that can be in the correct range?

Basically, this breaker conforms to code on the Inverse Time requirement, but not on the Instantaneous Trip requirement. I don't see where the code address both in one single breaker, is that to be inferred? Do I have to conform to both?
 
diakonos1984 said:
Table 430.52 requires the breaker setting for the inverse time...
Click to expand...

No, the NEC allows the inverse time breaker to be upto 250%, you may use a smaller breaker.

And yes, I believe you must meet the maximum INST level regardless of the breaker. I would be using something like a 225A breaker with an adjustable range of 1125-2250.
 
diakonos1984 said:
Example: 250HP induction motor on 480V. Using Table 430.250, the full load current is 302. 250% of that is 755A, so my 500A GE TKMA is well within the limits. 800% of the FLC is 2416A, and so an instantaneous setting of 5000A (the max for this breaker) is too high, but a setting of 2250A is acceptable.
Click to expand...

You also need to determine if you have an available fault current of 2250 amps. If you don't, the inst setting may not operate. Just because the code allows the 800% setting, does not mean is it correct.
 
Jim,
You are quite right in differentiating between what the standard "allows" and "requires".

Bob,
My fault currents are in the 10-20kA range, so no issues there. Additionally, the starting current is approximately 6x the FLA, so 8x the FLC is just out of nuisance trip range.

Thank you both very much for you help. I feel a lot better about this now--I'm not way out in left field somewhere...
 
Status
Not open for further replies.
Top