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The basics of device circuit breakers | Why device circuit breakers ?
U
U N
A
s
A
ms
I N I StatB . I DynB . I
Fig . 2 : Power Supply U / I characteristic curve
Fig . 3 : Shutdown of overload currents in the seconds range
Fig . 4 : Shutdown of short-circuit currents in the milliseconds range
Overload Overload currents occur if end devices unexpectedly require a higher current than the intended rated current . Such situations can arise , for example , due to a blocked drive . Temporary starting currents from machines are also considered to be overload currents . The occurrence of these can be calculated in principle , but nonetheless can vary depending upon the machine load at startup time . When selecting suitable fuses or circuit breakers for these types of circuits , these conditions must be taken into account . Safe shutdown in the event of an overload should occur in the seconds range ( Fig . 3 ).
Short circuit Damage to the insulation between conductors which carry operating voltage , e . g . a damaged load feed , can cause short circuits . In the past , typical protective devices for shutting down short-circuit currents included fuses or miniature circuit breakers with a variety of tripping mechanisms . Since then , electronic protective devices have come
into general use . Short-circuit currents need to be safely shut off within a few milliseconds in order to prevent any effect on the remaining loads in the system ( Fig . 4 ). This is because a short circuit generally leads to a voltage dip , which can also cause other unaffected circuits and loads to fail .
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