Maximum Interrupting Time - Circuit Breaker

[Cypher57]

Hi,

I need to know the "maximum interrupting time" of the FW/HFW Molded-Case Circuit Breakers (see the attached time-current curves).


Thanks

 

[davidbeach]

Looks like it is something in excess of 200 seconds.

That's probably not what you're looking for, you probably need to frame the question more clearly.

 

[Cypher57]

Thanks for your response. I have another curve which clearly tells you this info (please see attached) and I was wondering how to interpret the same for the other breaker.

 

[jraef]

Fault Clearing Time maybe? That would be the maximum trip time at the given current level, plus the maximum opening time of the breaker, which is not expressly shown on that curve. But sometimes on LV breakers, the manufacturer's TCC is assumed to includes the opening time in the curve. You would have to ask them.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[davidbeach]

Well, there's a difference between a horizontal line and a sloped line. That "maximum interrupting time" in the second plot is for the instantaneous element, it is NOT the breaker's maximum interrupting time; that's the time it takes to clear the lowest magnitude current it responds to.

Maximum interrupting time is much more meaningful when referring to a breaker tripped by an external relay; time between receipt of trip and arc extinguished, that's the interrupting time. Once you put an inverse time element into the breaker, "maximum interrupting time" becomes rather meaningless. Minimum interrupting time would provide more information there.

That said, given the second example, I'd probably use the time at about 12.5 times In for the first curve. Without knowing further context, it's difficult to know if that's really the right answer.

 

[Cypher57]

Thanks. You are correct I am looking for the "Maximum Instantaneous Fault Clearing Time". I guess I have to ask the vendor.

 

[Cypher57]

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)

@jref & @davidbeach
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

 

[LionelHutz]

The short circuit current is calculated from the system data, not using the motor data. I doubt the short circuit current is only 400A.

 

[waross]

The first curve looks like a response time curve for a thermal-magnetic breaker.
A magnetic only breaker will have just the lower portion of the curve.
The 5X to 10x settings are for let-through, not clearing time, to allow the motor to start without tripping the breaker.
For setting, the CEC limits the instantaneous setting of a magnetic only breaker to 2.15% of LRC. Not sure about the NEC.
For tripping time use the actual fault current. That may put you in the portion of the curve labelled c or d.
Motors are typically protected from overload conditions by a device which will trip on an inverse time basis if the running current is above the rated current.
The supply and conductors for a motor are protected against fault conditions by over current protection. This is instantaneous protection which clears an already faulted motor as fast as possible to provide protection for the supply conductors and overload devices and to prevent arcing damage to an already failed motor..
Typical molded case thermal magnetic breakers are seldom suitable to provide both types of protection.
A Motor Circuit Protection breaker normally does not have a thermal section for overload conditions. BUT, thermal only breakers may not be field applied. They are only permitted as part of a tested and certified assembly, such as an MCC cell.
That may be why you are having difficulty finding the curves for magnetic only breakers.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Cypher57]

Please see attached the Maximum Instantaneous conservatively will be taken as 0.016. Bill, I have a TOL for thermal protection.

 

[waross]

I spent a little time and googled up the original time current curves.
First, what current do you want to consider?
If the current is above the instantaneous setting of the breaker the clearing time will be shown by the lower portion of the curve. The time will be between about .015 seconds and .03 seconds, depending on the actual fault current and the setting of the magnetic trips (5X to 10 X).
If the current is lower than the instantaneous setting then we look at the upper portion of the curves again depending on the actual current.
This breaker has an adjustable thermal trip as well as an adjustable magnetic trip. The two upper curves are for the maximum termal setting and the minimum thermal setting. For example, on a breaker rated for 16 to 20 Amps, use the minimum trip time curve when the thermal trip is set for 16 Amps. Use the maximum time setting when the thermal trip is set for 20 Amps. For settings between 16 Amps and 20 Amps you must extrapolate the trip curve.
In your example with a setting of 9X (360 Amps) and a current of 360 Amps the trip time will be about .03 seconds. If the current is greater the trip time will be a little less.
If the actual current is less than 360 Amps then the upper part of the curve must be used depending on both the actual current and the setting of the thermal trips.
Hope this is what you are looking for.

Bill
--------------------
"Why not the best?"
Jimmy Carter