What is the “Maximum Interrupting Time” – Breaker

Adecypher

Hi,

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

Thanks

Hello,
that depends on your expected overcurrent. In case you want to know what happens when a short circuit occurs, you need to know your loop impedance. If you want to know what happens if you have e.g. 2 times the nominal current of your breaker device then it would be about 180 seconds maximum triping time, at 2.5 times nominal current you would have maximum 95 seconds triping time. Over 10 times nominal current the curves in the diagram converges to one curve that means minimum triping time = maximum triping time.(usually in case of a short circuit and depending on your loop impedance).

Best regards

Thanks for replying,

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 - Breaker 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

Hello,

if you say the Thermal range is between 32 - 40 Amps then set it to the nominal Motor current (shown on the Name plate) and you shoul be on the safe side. Both of your readings are correct, for 10 times In (=40 Amps) it will trip at Maximum 5.5 secs and at 9 times In it will trip after max. 4.6 secs. (in fact usually it is somewhere between Maximum and Minimum time, so if your assumptions regarding the current are correct, you are on the safe side).
But you should not start with the capabilities of your switching device (MCCB) but with those of your end device (=Motor). You select the right MCCB regarding to the Name plate of the Motor. Withstanding inrush current is one thing but it should also be able to switch of an short circuit (the current depends on the Loop impedance which corresponds to your cble length).

Regards

Great response thanks!