BBU Boric Acid Fuses

BBU Boric Acid Fuses

Boric acid fuses for power transformers, feeder circuits, distribution transformers, metal enclosed and pad mounted switchgear



Data Sheet #1123

Medium Voltage Fuse Catalog # MVFL1111

Agency Cards

Catalog Number See Catalog Numbers Tab
Volts The maximum voltage rating of the BBU fuse is the highest RMS voltage at which the fuse is designed to operate. Its dielectric withstand level corresponds to insulation levels of power class equipment, thus the name “power fuse.” Maximum voltage ratings for BBU fuses are: 17kV, 27kV and 38kV.
Amps The continuous amp rating of a BBU fuse should equal or exceed the maximum load current where the fuse is applied. They are designed to carry their rated continuous current without exceeding the temperature rise outlined in NEMA and ANSI standards. The BBU is available with continuous current ratings up to 200 amps. The current ratings carry an “E” designation as defined by ANSI and NEMA. For example, the current responsive element rated 100E amps or below will melt in 300 seconds at an RMS current within the range of 200 to 240 percent of the continuous current ratings. Above 100E amps, melting takes place in 600 seconds at an RMS current within the range of 220 to 264 percent of the continuous current rating.
IR BBU fuses have interrupting capabilities from 10kA to 14kA symmetrical
Typical Applications • Power and Distribution Transformer Protection • Medium Voltage Feeder Circuit Protection • Distribution Transformers • Medium Voltage Metal-enclosed Switchgear • Medium Voltage Pad-mounted Switches
Construction Principle parts of the replaceable BBU fuse unit are shown in the cross section views. A glass epoxy tube encloses the assembly containing the silver fuse element, arcing rod, boric acid cylinder and spring. Using a pure silver element and nichrome wire strain element makes the BBU less susceptible to failure caused by vibration, corona corrosion and fuse element aging. The components are housed in a fiberglass reinforced resin tube with plated copper contacts. BBU fuses can directly replace competitive equivalent units.


  • Voltage ratings of up to 38kV coupled with ratings to 200A provide a wide range of circuit protection
  • Time-current characteristics allow for easier coordination with downstream devices
  • Provides replacement of a variety of existing systems