Fuses and circuit protection

What the fuse protects

A fuse is primarily there to stop the wire from becoming a heater during a fault. The fuse rating must be above normal load but no higher than the protected conductor and connected equipment allow. If a manufacturer requires a smaller fuse, obey it.

Placement

Put protection at the source end, before an unprotected wire can rub against metal. If a conductor can be fed from both ends, each source may require protection. Distance rules vary by standard; 'somewhere in the cable' is not a design.

Fuse families

Class T offers high interrupt capability and fast response for large lithium/inverter circuits. ANL is common for high DC current but confirm interrupt rating. MIDI/MEGA suit medium/high feeders. Blade fuses suit small branches. Product name alone does not prove suitability.

Interrupt rating

A battery can deliver enormous short-circuit current. The protective device must safely open the available fault current at system voltage. This is separate from its amp rating and is especially important with large LFP banks.

Breakers

Use only breakers rated for the DC or AC voltage, current and interrupt duty. Some cheap resettable devices are unreliable switches and may not have credible fault ratings. Mount and orient them as specified.

Rule of thumb

If you cannot state the expected current, full conductor length, protection rating and manufacturer limit for a circuit, the circuit is not ready to build.

CHAPTER CHECK

• I can explain the purpose of every component covered here. • I can identify the exact manufacturer limit that controls my design. • I have added the decision, rating or measurement to my schematic/worksheet.