ALL THINGS ELECTRICAL
OK, I get it, most people are going to ask—what’s a MRBF? It’s an acronym for marine rated battery fuse, a fuse that should be in use far more than it currently is, in my view. Hence this article. MRBFs, commonly known by some as cube fuses, should play a more
significant role with house bank battery
Several years back the ABYC made
some major changes to its requirements for over-current protection
for house battery banks. That section now reads:
Main Battery Overcurrent Protection
Overcurrent protection devices when installed as a main battery overcurrent protection device shall meet the requirements of E- 11. 10.1.6 or E-
For batteries or battery banks with a rating of 2200 CCA or 500-amp hours
or less, battery overcurrent protection shall have a minimum ampere interrupting capacity (AIC) rating according to Table IV-A
For batteries or battery banks with a CCA rating greater than 2200 CCA,
or 500-amp hours, battery overcurrent protection shall have a minimum
ampere interrupting capacity (AIC) rating that is at least as great as the
battery manufacturer’s short circuit rating or 100 times the battery’s nominal amp hour (Ah) rating.
The bottom line here is that it doesn’t take much of a battery bank
these days to reach the 2200 CCA mentioned in ABYC E- 11 or the
500-amp hour threshold.
Historically, I’ve always used and advised using a class T fuse to
protect battery banks, or I should say protecting the rest of the boat
in the event of a short circuit that allows the entire battery bank to
unleash itself. But, these fuse and holder combinations are rather
expensive, so this recommendation has always been met with a bit of
pushback. The class T has an AIC rating of 20,000 amps, but it is
rated and tested to achieve that at 160VDC. Understand that as voltage exposure goes down, AIC rating goes up. So, at 12 volts the theoretical AIC rating of a class T fuse is over 100,000 amps. I’ve always
felt pretty safe using them for most house banks I’ve encountered.
One of the problems with all of this is that most battery manufac-
turers do not provide the short circuit current potential on their spec
sheets. That said, the 100 times a battery’s amp hour rating is a spec-
ification within the realm of reason when we look at the ratings for
ED’S ELECTRO-TECH TIPS
MRBFs—trying to find some love
BY ED SHERMAN
Vice President/Education, ABYC
the MRBF fuse. Using the criteria above, a battery rated at 100 amp-
hours would require a fuse with a 10,000-amp AIC rating.
For those of you not familiar with AIC, we are talking about
“ampere interrupting capacity.” Simply put, this rather obscure rating
is a specification that describes the amount of amperage the fuse or
circuit breaker can be confronted with and still open the circuit. Now
keep in mind that short circuit current potential is cumulative in a
paralleled battery configuration, so it is really quite easy to build a
high-capacity bank today using some of the new tech batteries available to our market that can easily end up with tens of thousands of
available short-circuit current potential. Keep in mind that a battery’s
short-circuit current potential is somewhere on the order of five to six
times its CCA rating. This is a key point that is often overlooked when
calculating the AIC rating needed in this scenario.
Enter the MRBF. These fuses are available in 30-300 amp nominal
ratings and offer a 10,000 amp AIC rating at 14 VDC and a 5000 amp
AIC rating at 32 VDC. A specification is not given at 24 VDC but it
will be around 7500 VDC assuming a fairly linear progression.
MRBFs are also rated for ignition protection and have an IP 66 (splash
proof) rating. On small battery banks (one or two batteries) you’ll
probably get away with one of these little gems installed on the positive battery terminal that connects to the main feed from the batteries
to the DC panelboard. For larger battery banks, installing these fuses
at evenly spaced intervals in the string of batteries will eliminate any
risk of excessive short-circuit current getting beyond the specification
of the cube fuse. Keep in mind that these little gems must be installed
in their matched holder to meet all the desired specifications.
MRBFs—you need these as a part of your installations today.