Fuses on a TCC

In Lesson 1 a breaker traced a single curve. A fuse doesn’t. A fuse plots as a band between two curves on the TCC — and once you understand why, you’ve read most of what makes fuses different.

Two curves, not one

A fuse interrupts in two physical stages:

1. Melt. Heat builds in the fuse element until it physically melts.
2. Arc. A plasma arc bridges the melted gap; the fuse keeps conducting until the arc extinguishes in the sand or quartz fill.

The first curve on the band is the minimum-melt time — the earliest a fuse could possibly start opening at a given current. The second is the maximum-clearing time — the latest it will finish, arc included.

Between the two curves, you don’t know what the fuse is doing. It might be melting; it might be arcing; it might have already opened. For coordination purposes, the rule is:

The width of the band exists for two unavoidable reasons:

• Manufacturing tolerance. Two fuses with the same part number have slightly different element geometry. The band represents the spread across a population of fuses.
• Installation effects. Ambient temperature, enclosure cooling, pre-loading from continuous current — all shift the melt point. Manufacturers publish curves that bound the realistic range.

A fuse on the plot

Below is a generic 100 A fuse. Drag the rating slider — both curves shift left or right together. Toggle the class — RK5 is the general-purpose dual-element time-delay you’d find in most motor and feeder applications; J and L are current-limiting types optimized for fault interruption; T is a fast-acting class used where short clearing time is everything.

100 A fuse

Rating A

Class RK5 · general-purpose dual-element time-delay

RK5JLT

The shaded region between the dashed line (min-melt) and the solid line (max-clear) is the operating band. Three things to notice:

1. At low overload — say 2× rating — the band is wide in time. Min-melt might be 50 s, max-clear 100 s. Tolerance dominates.
2. At high fault current the band narrows. Both curves converge toward the floor.
3. The vertical drop on the right side is the current-limiting region. Above 20–30× rating (depending on class), the fuse opens in less than half a cycle — fast enough to interrupt the fault before it reaches peak. That sub-cycle clearing is what makes fuses unique on the plot.

What “current-limiting” buys you

A current-limiting fuse doesn’t just open fast; it actively limits the peak let-through current. If the available fault is 50,000 A symmetrical, the peak available without the fuse would be roughly 2.3× that — about 115,000 A. A properly applied current-limiting fuse can chop the peak to a small fraction of that.

The let-through curves that quantify this aren’t on a TCC — they’re a separate plot (peak let-through current vs available fault current). For coordination work, what matters is just that above the current-limiting threshold, fuse clearing time goes vertical.

Class differences at a glance

Move through the four classes on the slider above. The band shape holds; only the speed and the current-limiting threshold change.

Class	Speed at 6× rating	Current-limiting threshold	Typical use
RK5	Slow (≈6 s)	~30×	General-purpose feeders, motor branch
J	Medium (≈3 s)	~25×	Compact panelboards, short-circuit ratings
L	Slow (≈12 s)	~20×	High-amp service entrance (601–6000 A)
T	Fast (≈0.5 s)	~35×	Compact mains/feeders where space is tight; fast non-motor loads

Class is the first decision in a fuse application. Rating is the second. Both choices show up as visible shifts on the plot — band position (rating) and band shape and steepness (class).

What’s next

In Lesson 3 we’ll go back to breakers and meet the LVPCB with electronic LSI(G) trip — four independent slider sets controlling four discrete regions of the curve. That’s the device you tune when coordination requires precision the thermal-magnetic breaker can’t give you.

By the end of Module 1 you should be able to look at any TCC plot — fuse band, breaker curve, or LVPCB LSI(G) staircase — and know what mechanism made each shape.