Breaker Time Current Curve

"...but the line vertical upwards does not intersect the curve anywhere except in the long time band ."

It doesn't? I see it clearly. At 4X the ampere setting, your trip time is right down the center of the band, and could be anywhere from 100msec to 25 seconds. 130ms is on the fast end of that range, no problem from my perspective.

Are you perhaps thinking the X axis is actual amperes, not MULTIPLES of the setting?

Hi JRaef , if we see the curve at setting 5x , the vertical line intersects the green curve line at exactly at .045 sec , so i can clearly conclude that the trip time has to be lesser then or equal to 45 milli seconds ,

but in 4x case there is a band which extends to long time region as you have said upto 25 seconds , since this is instantaneous function and i am expecting tripping value in milli seconds .. but when i see the curve it extends and does not say exact value like in 5x case , in the report i am preparing for this breaker i have to fill the curve value ... I am confused what should i put under curve time , it seems like i have to assume some range . if i say upto 200 milli seconds will it be fine

The plot looks more useful for finding the over current given the trip time, rather than the other way around.

I'm just guessing here:

You said:-

with these settings the current at which it will trip as per instantaneous function will be 1600 x 0.5 x 4 = 3200

I don't understand why you've got the x 0.5 in there? Surely the instantaneous function will kick in at 1600 x 4 = 6400 A.

As it is you've set the current to 2 x set current. I believe that the y axis should read time in milliseconds, and not seconds:-

So you'd expect it to break between 85ms and 175ms

And you got 130ms. So, looks about right.

At 4x set current you'd expect the break time to be between 25ms and 50ms.

The OP is mixing up two separate but interrelated functions. If you want to test both the Long-time and Instantaneous functions, you need to be sure to understand the following:

1600A trip with 0.5x LTpu = 800A Long time pickup.

4x Instantaneous will be 4 x LTpu = 3200A.

If you want to test the long time trip response at 4x pickup, you need to get the Instantaneous function out of the way, since it will override the LT. In other words, turn the Inst up to 8x, for example, then test the trip at 4x = 3200A. The time delay to trip should be in the range of 25-50 seconds, per the trip curve.

Then to test the Instantaneous, adjust it back to 4x and inject the 3200A. It should trip "instantaneously," which according to the curve should be somewhere in the range of about 0.025 to 0.15 second, based on the "foot" of the curve. 0.130 sec. sounds about right, because he is just on the border of the instantaneous pickup.

Hope this helps.

Hi Randall ,

for this u need to see the information on the trip unit , the photo i have enclosed , in this case the instantaneous is not a function of rated amps but it is a fucntion of the long time setting , if i change the long time the instantaneous also gets changed

this is the cse in GE selec trip powerbreak breakers .

in other breakers almost all of them what you have said applies , instantaneous function is independent is in the multiples of the plug .

This is how I understand the curves to work.

1. The top section of the graph shows long time pickup at 100% ie. becomes vertical at #1 on the X axis. Setting the long time pickup to anything less than 100% requires that the curve be shifted to the left on the X axis. This shift occurs in a region called the short time pickup which is located just above the instantaneous area in the 50ms -500ms range. The instantaneous region remains static. The result is that for a setting of 50%, the top unlatch section of the curve would now become vertical at the far left (0.5) of the graph rather than at #1 on the X axis and corresponding to a current of 800 amps in your case. The mid range curve would now be vertical at the 1.75X point on the current line and corresponding to a set point of 1600 x 2 = 3200 amps

If the breaker is a 1600 amp unit and you have set 4X and 50%, then the instantaneous trip current is now 4 x 1600 x 0.5 = 3200 as you suggest. The breaker should trip within one cycle and no more than 3 cycles when fault current meets or exceeds the set point, in this case 16ms @ ≥3200 amps for 60Hz or 20ms @ ≥3200 amps for 50Hz. Total extinguishing of the arc should have completed within about 30 cycles, the time delay between trip and arc quench is the reason for the width of the curve - minimum and maximum clearing times.

You will note that the #1 vertical which always corresponds to the breaker rating shows the trip point at minimum 300s when the rated current is being passed, anything less than the rated current will not trip the unit unless the top of the curve has been shifted left by decreasing the long time trip level.

Having adjusted the graph to suit the 50% long time setting, you will now see that by projecting the 4x line upwards it is inside the green band until approximately 150ms, at 3200 amps continuous load the unit will complete its trip somewhere between 10ms and 150ms.

Below are two curves for an 800 amp unit (the pink one) that may assist. The blue curve is for a feeder breaker which has not been adjusted.

Hi Spades ,

10 milli second is the start of the curve as we begin from the bottom , i get that , but as you have said that the range will be from 10 milli second to 150 milli second , can you please explain how you arrived at the 150 milli seconds value

Please advise the outcome of the telephone call with the equipment manufacturer on this topic.