Low-voltage fuses — what class to use?

The basics of low-voltage industrial fuses — what you need to know.

Industrial fuses are governed by the UL248 and CSA C22.2 No. 248 harmonized standards. Each “class” of fuse has its own sub-standard. For example, UL 248.2/CSA No. 248.2 are the requirements for Class C fuses. The commons fuse classes are Class L, Class RK1, Class RK5, Class C, Class CC (Midget), Class T, Class G, Class H (Renewable), Class H (Non-Renewable), Class J, and Class K.

Before learning about what is different about each class, one needs to understand the common definitions. First, the fuse amperage rating which is the amount of continuous current a fuse can carry without deterioration or exceeding temperature limits (e.g. 400A)). Then there is the interrupting rating, which is the tested maximum rms current a fuse can interrupt (e.g. 200 kA). Time delay is an intentional delay built into the fuse clearing time (typically defined — e.g. 10s at 5x amperage rating). The opposite of a time-delay is fast-acting or non-time delay.

A current-limiting fuse limits the fault current seen compared to if the fuse was not present (replaced by a bolted conductor). It limits the peak fault current to a value below the fuse’s interrupting rating. The threshold current is the point during a fault where the fuse starts to limit the current (goes from melting to arcing).

I’ve drawn the highly simplified graph in AutoCAD to show the peak let-through current and threshold rating. The area under the curve (I²t) correlates to the amount of energy and is used as a metric for fuse let-through capability in the UL standards.

Fuses can be further classified into branch circuit protection or supplementary protection. We will stay with branch protection (the most common) for our discussion. The fuses for branch protection are plug fuses, Class CC, Class CD, Class CF, Class G, Class H, Class J, Class K, Class L, Class R, and Class T. All of these fuses are defined as current-limiting fuses with the exception of Class H and plug fuses.

Let's look at an example to demonstrate the variation of let-through current between current-limiting fuses. For a 100kA, 600V power system, where a 400A fuse is required, here is how the peak let-through currents compare:

Class K — these fuses are obsolete

Class RK1 — 35kA and 1,200,000 I²t (area under curve)

Class RK1 — 50kA and 5,000,000 I²t

Class J — 30kA and 1,100,000 I²t

Class T — 30kA and 1,100,000 I²t

Class L — 30kA and 1,100,000 I²t

If one wanted to limit the potential arc flash energy released, a good start would be Class J, T or L (without the time-delay) as they have the smallest I²t. Attached is a table summarizing the various classes and what makes each one special.

Please note this is a highly simplified overview of industrial low-voltage fuses — please read further into the designs and theory when designing.