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.
While you and I know what we mean, I am not sure your explanation is much clearer than mine, but the graphs certainly help.
Lyle,
Overload delay is always desired which is why it is an inverse time curve. The higher the overload the faster the trip time, since the circuit can withstand a 5% overload for far longer than a 50% overload while a 500% overload needs to be tripped much more rapidly.
The whole point about the overload region is that it is a curve (or in the case of the log-log graphs that Mike has reproduced a straight line) and so there is a significant region (from Ir up to 5 x Ir) where the overload will trip the breaker without the instantaneous trip being activated. It is probably easier to think of it as the straight line on the log-log graph, as with a straight line of known gradient, in order to set the curve on the graph, you just need to define one point on it. The fact that this point is at six times Ir and therefore will never be utilised is irrelevant as the line from Ir to 5 x Ir has now been defined. This will enable you to determine the trip time for any defined overcurrent from the (now defined) tripping characteristics.
While you and I know what we mean, I am not sure your explanation is much clearer than mine, but the graphs certainly help.
Lyle,
Overload delay is always desired which is why it is an inverse time curve. The higher the overload the faster the trip time, since the circuit can withstand a 5% overload for far longer than a 50% overload while a 500% overload needs to be tripped much more rapidly.
The whole point about the overload region is that it is a curve (or in the case of the log-log graphs that Mike has reproduced a straight line) and so there is a significant region (from Ir up to 5 x Ir) where the overload will trip the breaker without the instantaneous trip being activated. It is probably easier to think of it as the straight line on the log-log graph, as with a straight line of known gradient, in order to set the curve on the graph, you just need to define one point on it. The fact that this point is at six times Ir and therefore will never be utilised is irrelevant as the line from Ir to 5 x Ir has now been defined. This will enable you to determine the trip time for any defined overcurrent from the (now defined) tripping characteristics.
