What is a current-limiting fuse?

A current-limiting fuse interrupts a high fault within the first half-cycle, before the current reaches its prospective peak. By clearing that fast it limits the let-through energy and peak current the downstream equipment ever sees, which protects busbars and conductors during high available-fault conditions.

How is a fuse different from a breaker on a high fault?

On a high fault the fuse melts and clears in under half a cycle, while a thermal-magnetic breaker trips on its instantaneous magnetic element in roughly one to two cycles. The fuse is therefore faster on severe faults and limits energy, but a tripped breaker can be reset whereas a blown fuse must be replaced.

What do Class CC, J, and RK5 mean?

These are UL fuse classes with different sizes, interrupting ratings, and time-delay characteristics. Class CC and J are fast-acting and physically small with high interrupting ratings; Class RK5 is a time-delay fuse that tolerates motor inrush. The tool reflects their relative speeds, not exact catalog curves.

Why does the breaker trip instantly above about ten times rated?

A molded-case breaker has a thermal element for overloads and a magnetic element for short circuits. The magnetic element has an instantaneous pickup, often eight to thirteen times rated current, above which the breaker trips in one to two cycles regardless of the thermal curve. This tool models that pickup at ten times rated.

Can I use this for a real coordination study?

No. These are simplified representative curves for teaching and quick intuition. A real selective-coordination or arc-flash study must use the manufacturer's published time-current curves, the actual available fault current, and let-through energy tables, ideally in coordination software.

Fuse vs. Circuit Breaker Comparison Tool

Fuses and circuit breakers both interrupt overcurrent, but they behave very differently as the fault gets larger. A current-limiting fuse clears within the first half-cycle on a high fault and caps the energy let through, while a thermal-magnetic breaker waits for its magnetic element to pick up. This tool plots simplified time-current behavior for both at the same ampere rating so you can see where each device is faster.

How it works

For a fault current expressed as a multiple M of the device rating, the tool estimates clearing time from representative inverse-time models. The fuse uses an I²t-style overload curve at low multiples and switches to sub-cycle current limiting once M passes the class threshold:

overload:        t = k / (M^n - 1)
current-limiting: t < ~8 ms (clears in first half-cycle)

The class coefficient k sets the relative speed: Class CC is fastest, Class J close behind, and Class RK5 is the slowest because it is a time-delay fuse built to ride through motor inrush. The breaker has two regions:

thermal:        t = k_b / (M^n_b - 1)      for M below the instantaneous pickup
instantaneous:  t = ~0.025 s (1-2 cycles)  for M >= ~10x rated

Example and notes

For a 100 A device at 8× rated (800 A) with a Class J fuse, the fuse is in or near its current-limiting region and clears in a small fraction of a cycle, while the breaker is still on its thermal curve and takes noticeably longer. Push the multiple to 12× and the breaker’s instantaneous element picks up, closing much of the gap.

Read the comparison as intuition, not gospel: the relative ordering — fuses faster and energy-limiting on severe faults, breakers resettable and adjustable — is correct, but exact clearing times come only from the manufacturer’s published time-current curves used in a proper coordination study.