Hopefully never. You would likely need a short leading to the back battery or maybe past the back battery to blow the fuse.
Speaking of fuses you should have one on the positive side leaving the front battery and one on the positive side of the back battery leading to the front. Then a last one as it leaves the back battery for the amp itself. So three fuses, the reason being if the line from the front battery shorted to ground, it would blow the front fuse, but the back battery would still be shorted to ground, and that is bad, so there needs to be a fuse on each end of that line.
3 kw solar pv - XW6048 - eight L16's
4 ton GSHP - 1 ton ASHP
2003 VW TDI 200k miles - 52 mpg lifetime
EVSE level 2 - Clipper Creek HSC-40
2013 S model with QC package Mar of 2013
lost first bar @ 72k
@100k miles - 57.3Ahr - 87.62SOH - 84.13Hx - 241GID