Blue Sea Systems 5191 Single MRBF Terminal Fuse Block, 58V DC, Mounts on 3/8" Battery Post or Bus Bar Stud, for Marine and Mobile DC Systems

About the Blue Sea Systems 5191 MRBF Terminal Fuse Block

The 5191 is the simplest way to get fused overcurrent protection right at the battery. It threads onto any 3/8" (M10) stud, whether that's a battery post, battery switch terminal, or bus bar, and accepts a single MRBF fuse that protects the attached circuit.

Meeting the ABYC 7" Rule

ABYC E-11 requires overcurrent protection within 7 wire inches of the battery bank (or up to 72 inches if the conductor is in a protective sheath). In practice, that's a hard number to hit with a traditional ANL or Class T fuse holder, which needs its own mounting surface, a short battery cable, and enough clearance for the holder plus wiring. The 5191 solves this by threading directly onto the battery post itself. Protection starts at the source with zero cable between the battery and the fuse. For RVs and off-grid builds, the same principle applies even where ABYC doesn't govern: fusing at the source means no unprotected cable run that can short and overheat.

The block adds roughly 2" of height to the battery post, so measure your battery compartment clearance before committing, especially in tight marine battery boxes.

How It Mounts

The MRBF fuse seats into the block, and the load cable attaches to the M8 (5/16") output stud on top of the fuse using a standard ring terminal. The isolated stud design allows stacking of multiple ring terminals on the output side without interference, so you can run more than one load circuit off a single fuse if the fuse is sized to the combined draw.

An insulating cap snaps over the assembly to prevent accidental shorts. When the cap is in place and an MRBF fuse is installed, the entire assembly is ignition protected and weatherproof.

When to Use the 5191 vs. Other MRBF Blocks

The 5191 is a single-position block, which makes it the right choice for per-battery fusing or for protecting a single high-current circuit (inverter feed, windlass, thruster) right at the source. If you need two fused circuits off one terminal, the 2151 Dual Terminal Fuse Block gives you two positions on the same mount. For three or more circuits, the surface-mount 5196 Common Source or 5194 Independent Source blocks are better suited.

Pair this block with an MRBF terminal fuse in the appropriate amperage rating. Fuses are sold separately.