Fundamentals of High-Voltage Fuses with High Breaking Capacity Used in Substations

In the power plants and substations of various voltage levels that we operate and maintain, high-voltage current-limiting fuses are widely used, primarily for protecting voltage transformers, small transformers, and small high-voltage motors. So why are they called high-breaking-capacity fuses? And why can't ordinary fuses be used? Today, let's learn about this topic together.

High-breaking-capacity fuses, also known as high-voltage current-limiting fuses, differ from ordinary fuses in two main aspects: First, they have a strong capability to interrupt short-circuit currents—this is where the name "high-breaking-capacity" comes from. Second, they have a significant current-limiting effect. This means that when a short circuit occurs in the protected circuit, the fuse can reliably interrupt the circuit before the short-circuit current reaches its peak value. This is the so-called current-limiting effect.

In simple and plain terms, the first characteristic is reliability: ordinary fuses are like switch blades and cannot interrupt short-circuit currents, whereas high-breaking-capacity fuses are like circuit breakers, capable of reliably interrupting short-circuit currents. The second characteristic is speed: they can quickly clear short-circuit faults before the short-circuit current fully develops, and they do so without causing the fuse itself to explode.

Structurally, high-voltage current-limiting fuses are generally cylindrical, with a hard ceramic outer shell and a seven-point column (or star-shaped) frame inside to secure the fuse element. For lower rated currents, the fuse element is typically wire-shaped, while for higher rated currents, it is usually ribbon-shaped.

The ribbon-shaped element has evenly spaced notches cut in a sawtooth pattern. The spacing and shape of these notches determine the performance parameters of the fuse. The interior is filled with quartz sand to extinguish the arc generated when the fuse element melts. Additionally, some models are equipped with striker indicators. When the fuse element blows, the indicator pops out, triggering an external position switch to send an alarm, alerting operation and maintenance personnel.

Regarding the model designation of high-voltage current-limiting fuses, let's take XRNP-12/0.5-50, used for voltage transformers, as an example. The meanings of each part are as follows:

• X stands for current-limiting type
• R stands for fuse
• N stands for indoor use
• P stands for use with voltage transformers
• 12 indicates the voltage rating of 12 kV
• 0.5 indicates the rated current of the fuse element is 0.5 A
• 50 indicates the maximum short-circuit breaking capacity is 50 kA

The fifth letter code indicates the protected object:

• P for protecting voltage transformers
• M for protecting motors
• T for protecting transformers
• C for protecting capacitors
• G for protecting specified objects