In comparison, to the detection time response of thermal circuit breakers, we can classify the detection time response of magnetic circuit breakers as “fast”.  In many cases, magnetic breakers are in fact “too fast”, and are subject to nuisance trips due to transient inrush currents.  Whereas thermal breakers can “ride through” transient inrush currents by means of their relatively long thermal time constants, magnetic breakers tend to respond to the instantaneous magnitudes of inrush currents due to their relatively low trip energy requirements once the threshold current has been exceeded.

Over the years magnetic circuit breaker designers have developed several schemes to delay the detection response to transient inrush currents.  In essence, the goal is to mimic the dual element “slow blow” fuse structure.  The ideal “dual element” magnetic breaker would have two detection mechanisms in series:  one slow, low threshold current mechanism to ride through inrush currents; and one fast, high threshold current mechanism to quickly respond to true, high level fault currents.  The detection time current response characteristic would look much like the dual-element fuse characteristic shown in Figure 3.9.

A true, almost ideal dual-element characteristic can be achieved in a magnetic circuit breaker through use of an inertial core-delay tube.  This clever device is shown in Figure 4.13a.  The drive coil core is a hollow tube which contains a moveable, but spring restrained core ferromagnetic slug.  At low coil currents the slug is spring restrained in a recessed position, x = 0.

At coil currents above a certain operating current threshold, I_th1, the attractive magnetic force (solenoid effect) of the coil is enough to overcome the spring force and to initiate movement of the slug.  The slug moves toward the coil center and begins to fill the hollow coil core with ferromagnetic material.  As the coil core fills with magnetic material the core section reluctance falls to lower values, enabling the total magnetic flux produced by the coil to increase.  When the core slug reaches a certain point in the core section, dependent on the level of drive coil current, the core reluctance is decreased to a value low enough that the magnitude of the armature gap flux is sufficient to cause the armature to breakaway from its stopped position.  The breaker then trips as described previously.

The dynamics of the slug movement can be further slowed by the addition of a viscous fluid within the hollow core section.

If the initial core current is high enough – above a second threshold level I_th2 – the gap flux will be strong enough to trip the armature without the need of flux enhancement by slug movement through the core section.  The dynamics of this high-level trip are then the fast dynamics of a pure, simple magnetic breaker, unaffected by the flow dynamics of the core slug mechanism.

In terms of a magnetic equivalent circuit, the moveable-core dual element magnetic circuit breaker can be described as shown in Figure 4.13b.  The core path reluctance R_c is now a function of the slug displacement from its restrained x = 0 position.  It is a maximum when the slug is at x = 0, and a minimum when the slug has advanced to its core-filled position at x = x_max.

The equation of motion for the slug is given by

where M_s is the mass of the core slug, F_m is the force of the magnetic attraction on the slug, D is the coefficient of friction due to the addition of a viscous fluid environment, ξ is the spring constant of the slug restraint spring, and x_o is an equivalent displacement representing the initial restraining force of the spring.  The force of magnetic attraction F_m will be proportional to the core flux ф squared (just as the armature attractive torque T_m is proportional to the gap flux ф_g squared).  A complete solution to the dynamics of the total device would then require a simultaneous solution of Equations 4.1 and 4.18, and the magnetic circuit of Figure 4.13b.

We will not attempt to solve this system of equations here.  We will only note that the “slow” behavior of the complete system is determined by the slug movement Equation 4.18; the “fast” behavior of the system is determined by the armature movement of Equation 4.1; and coupling between the two is determined by the magnetic circuit, Figure 4.13b.  The resultant combined detection time-device current characteristics are sketched in Figure 4.13c.

Other methods of desensitizing the response of magnetic breakers to inrush currents include the tailoring of the core flux reluctance path as a function of the core slug position and the addition of an inertial device, similar to a flywheel, to the armature structure.

Flux can be “bled-off” from the core-gap-armature path through use of flux shunts (sometimes referred to as flux busters) or through the use of an elongated core path which is not covered by the drive coil.  In either situation a major portion of the core flux produced by the drive coil tends not to flow  through the core-armature gap until a movable internal core slug (similar to the one in Figure 4.13a) has reached its fully advanced position.  Rather, this flux “bleeds” into leakage paths, producing no useful armature torque.  However, when the core slug is at its most advanced position these leakage paths are effectively “shorted” and the major portion of the core flux crosses the core armature gap.  Magnetic breakers with these tailored core flux paths have an enhanced, true, dual-element response characteristic.

Extra inertial mass when added to the armature mechanism increases the total effective armature moment of inertia, or equivalently, the total effective armature characteristic time.  This addition does not change the sensitivity of the detection mechanism; it only slows its response.  It slows it, however, across the board.  It does not create the desired dual-element response; rather it simply burdens the armature with additional inertia, enabling it to ride through the transient inrush currents by means of sheer sluggishness.