I have several 800A Micrologic 2 MCCBs with only three adjustable settings Ir,tr and Isd. Ir is set at 0.7, tr is set at 4s at x6 Ir and Isd is set at 5. Is this not incompatible? My reckoning is that the Ir is 560A, tr holds an overload of 3360A for 4s but the short time pick up pulls the breaker out instantaneously at 2800A. All the devices are similarly set. There are no issues as a result but just wondering.
I can see where the confusion arises but there is a simple explanation (which I will probably make a mess of explaining and make it far more complicated so apologies in advance if the below doesn't help.....)
The long time delay, often referred to as the overload setting or thermal trip curve, is an inverse time current curve such that the higher the current the shorter the operation time (and vice versa). As this is a defined curve with the inverse of the time approximately proportional to the square of the current. In order to set the whole of the curve it is therefore only necessary to define one point on the curve. The 4s at 6 x Ir therefore defines the whole of the curve from Ir upwards. The fact that the defined point is beyond the instantaneous trip point is irrelevant (though does present challenges in demonstrating correct operation since to demonstrate it does trip at 4s at 6 x Ir it will be necessary to switch off the instantaneous trip....)..
I can see where the confusion arises but there is a simple explanation (which I will probably make a mess of explaining and make it far more complicated so apologies in advance if the below doesn't help.....)
The long time delay, often referred to as the overload setting or thermal trip curve, is an inverse time current curve such that the higher the current the shorter the operation time (and vice versa). As this is a defined curve with the inverse of the time approximately proportional to the square of the current. In order to set the whole of the curve it is therefore only necessary to define one point on the curve. The 4s at 6 x Ir therefore defines the whole of the curve from Ir upwards. The fact that the defined point is beyond the instantaneous trip point is irrelevant (though does present challenges in demonstrating correct operation since to demonstrate it does trip at 4s at 6 x Ir it will be necessary to switch off the instantaneous trip....)..
