Let-through is the electrical stress that a breaker, fuse, or surge arrester allows to reach downstream equipment during a fault. Because mechanical and thermal clearing takes milliseconds, some energy always escapes. Engineers calculate let-through to ensure it does not exceed the withstand rating of the protected cables and switchgear.
We need a current-limiting fuse here to keep the let-through energy below the rating of the motor contactor.
Current-limiting devices are specifically designed to cut off a fault in less than half a cycle, drastically reducing the let-through energy.
Also heard as
- let-through current
- let-through energy
Related terms
Fuse
A sacrificial protection device containing a conductive element that melts to safely break a circuit during an overcurrent event.
Circuit breaker
A mechanical switching device capable of making, carrying and breaking currents under normal and fault conditions.
Surge arrester
Device that clamps transient overvoltages by diverting surge current to earth, then returns to insulating.
Breaking capacity
Highest fault current a circuit breaker or fuse can interrupt safely at its rated voltage.