- #1
Click For Summary
Discussion Overview
The discussion revolves around the "maximum interrupting time" of FW/HFW Molded-Case Circuit Breakers (MCCBs), focusing on the interpretation of time-current curves and their application in protecting motors. Participants explore the implications of various current settings and the coordination between circuit breakers and other protective devices.
Discussion Character
- Technical explanation
- Debate/contested
- Mathematical reasoning
Main Points Raised
- Some participants suggest that the maximum interrupting time can be read directly from the time-current curves provided, emphasizing the need to know the rated current of the breaker.
- Others inquire about the availability of a specification sheet for the specific model to clarify the maximum interrupting time.
- One participant explains that the maximum time corresponds to a curve and that superimposing various elements onto one curve can help visualize coordination between upstream fuses and downstream devices.
- A participant provides specific MCCB data and calculates potential maximum instantaneous fault clearing times, presenting two values (5.5 secs at 10X and 4.6 secs at 9X) and seeks validation of these calculations.
- Another participant questions whether 5.5 seconds can be considered instantaneous, discussing the typical time frames for overload and short circuit conditions, and emphasizing the need for coordination with motor characteristics.
- One participant acknowledges the misunderstanding regarding the term "instantaneous" and provides an alternative current value for consideration, suggesting a conservative approach to the instantaneous region of operation.
- A later reply raises concerns about the implications of a long locked rotor current duration, arguing that a small motor would likely not survive extended periods of high current.
Areas of Agreement / Disagreement
Participants express differing views on the interpretation of the time-current curves and the definitions of instantaneous versus long-time trip settings. There is no clear consensus on the maximum interrupting time or the appropriateness of the calculated values.
Contextual Notes
Participants highlight the importance of understanding the coordination between circuit breakers and motor characteristics, as well as the potential for confusion regarding the definitions of instantaneous and locked rotor current durations. There are unresolved assumptions regarding the specific application and conditions under which the calculations are made.
- #2
zoki85
- 1,198
- 230
What seems to be problem? Max. interrupting time vs multiple of the nominal current curve is given in the graph. Just read it.
You need to know only the rated (nominal) current of the breaker.
You need to know only the rated (nominal) current of the breaker.
- #3
- 11,326
- 8,755
Isn't there a spec sheet for that specific model?
- #4
Windadct
- 1,452
- 411
The Max time is the curve(line) - for any current level you then have to find the corresponding point on the curve. However - these curves are most useful when you superimpose the various elements onto one curve, you can then see the coordination between the various elements. For example -- if there are up stream fuses they will have a similar Time Curve - you would want the down stream items ( cables, motors) to be protected by this device and ideally not the fuses.
- #5
Adecypher
- 3
- 0
Windadct said:
Thanks
I have MCCB data as follows:
MCCB Rating = 40 Amps (i.e. Maximum Continuous Ampere Rating at 40 degree C)
Thermal Range = 32 - 40 Amps
Magnetic Range = 200 - 400 Amps (i.e. Instantaneous Pickup Range)
Horizontal axis = Multiple of current setting (i.e. Multiples of 40 Amps - Breaking Rating).
Based on this info, what will be the maximum instantaneous fault clearing time (secs)?
I came up with:
1) 5.5 secs corresponds to 10X (400 Amps)
2) 4.6 secs corresponds to 9X (360 Amps)
which one of the above two values in (secs) are close to being correct or else?
Note: Not to confuse you regarding 9X (360 Amps) value I choose in the graph.
I selected 9X because I selected this MCCB to protect a Motor whose Lock Rotor Current = 143 Amps, the Service Factor of this Motor is 1.0, therefore I used 360 Amps magnetic setting > (2.5 X LRC) = 357. 5 Amps.
Regards
Attachments
- #6
Windadct
- 1,452
- 411
Does 5.5 Sec sound like it is instantaneous? Not mocking your post - but asking you to think about the info. The Long time will be for general overloads and often based around 3 to 5 x the FLA. ( It has been a while since I did these) - however you need to ensure that even at 150% the breaker will eventually trip to prevent the motor from overheating. Smaller motors the shorter the time they can be in overload.
As for the instantaneous(see note below) - you need to make sure that the LRA hits that part of the curve, but noting some motors ( particularly high efficiency) can hit LRA on hard line starts.
The motor OEM will often provide the same format time current curve to help with this coordination.
Often the Locked Rotor coordination really is referred to a Short Time (not instantaneous) - in general terms Long Time is for Motor overload, Short time ( locked / seized motor) has a small fixed amount of time to allow for starts, and true Instantaneous is for electrical short circuits / faults ( so there is no intentional time delay on the trip).
As for the instantaneous(see note below) - you need to make sure that the LRA hits that part of the curve, but noting some motors ( particularly high efficiency) can hit LRA on hard line starts.
The motor OEM will often provide the same format time current curve to help with this coordination.
Often the Locked Rotor coordination really is referred to a Short Time (not instantaneous) - in general terms Long Time is for Motor overload, Short time ( locked / seized motor) has a small fixed amount of time to allow for starts, and true Instantaneous is for electrical short circuits / faults ( so there is no intentional time delay on the trip).
- #7
Adecypher
- 3
- 0
@ Windadct LOL you are right it is not "instantaneous"; I have attached another curve with 0.013 Amps as Inst current shown; actually you can conservatively choose 0.016 Amps around the slope region (which is the instantaneous region of operation of this breaker).
- #8
Windadct
- 1,452
- 411
So you mean 0.013 Seconds ? Wont that be at 80X or 3200A.. as you pulled the line out. --
If the LRA is 143A = 3.6X ( of the 40A) --- as I read this you are up to 40 Secconds - a small motor like this probably can not survive 40 Sec locked rotor. Another way to think of this for a 480V 3PH motor...( 143 x 480 x Srt(3) )= 119 KW of heat ! being dumped into the windings of the motor!...
I would only want LRA to exist for 1-2 Seconds at most in a typical application.
If the LRA is 143A = 3.6X ( of the 40A) --- as I read this you are up to 40 Secconds - a small motor like this probably can not survive 40 Sec locked rotor. Another way to think of this for a 480V 3PH motor...( 143 x 480 x Srt(3) )= 119 KW of heat ! being dumped into the windings of the motor!...
I would only want LRA to exist for 1-2 Seconds at most in a typical application.