Circuit Breakers

DC and AC circuit breakers, from 6A on a DIN rail up to 200A bolted straight to the battery.

The mistake worth avoiding first: an AC breaker will not do a DC job. Alternating current passes through a zero voltage point a hundred times a second, so the arc that forms as the contacts open puts itself out. Direct current never reaches zero, the arc keeps burning across the opening contacts, and it will go on burning until something melts. A DC rated breaker is built to stretch that arc out and quench it, which is why the DC version of the same breaker on the same rail will cost more. Putting an AC breaker on the battery side is not a saving, it is a catastrophe waiting to happen.

Three sorts here for three jobs. The Nomad NMCB is a single pole thermal breaker that bolts onto two cables and needs no rail, 50A to 200A at 12V, 24V or 48V. It doubles as the battery on and off switch, it resets with a lever instead of a fuse that you need a spare for, and it meets SAE J1171 for external ignition protection, so it can be mounted aboard a petrol powered boat. The Suntree SL7-63N is the DIN rail breaker, two pole, 6kA, in 6A to 50A at 415V AC or 10A to 63A at 550V DC. The Suntree SM1 is a moulded case thermal magnetic breaker in 100A and 150A, two pole, 25kA, which is what goes between a large battery bank and the inverter or charge controller.

The Suntree breakers are made to IEC 60947-2, the industrial standard for polluted, humid, harsh and outdoor use, and the AC and PV DC ones also hold SAA certification. That is deliberately not IEC 60898-1, the switchboard standard for clean dry indoor work. An off grid installation is a shed, a bach, a boat or a pole, and none of those is a clean dry switchboard.

Size the breaker on the maximum expected current that will actually flow under normal circumstances. On 12V a 2000W to 2200W inverter wants the 150A Nomad and a 3000W to 3400W inverter wants the 200A, because a 12V inverter draws a very large current for the power it puts out. The same inverter at 24V draws half of that and at 48V a quarter, so the breaker gets smaller as the system voltage goes up. If you think these sizes are a little low, the circuit breaker range that we have here has an enormous surge ability to go many more times above its nominal rate and for an extended period of time. This will allow you full use of the inverter without restriction. And also ask yourself: what do I want to happen first, the circuit breaker to trip or the inverter to break?

Two boxes to put them in. The most common fireproof distribution box is surface mounted and takes 6 DIN modules, which is three two pole breakers, we also have two DIN and 12 DIN size boxes at hand. The 250 mm enclosure is IP66 to EN 60529 and IK10 for impact, painted cold rolled steel, lockable with two keys, with a mounting plate and an M6 earth point, for anything outdoors. The larger surface-mounted Suntree breakers can be placed in this box if you need greater protection.

A breaker is not an isolator and not a fuse. The PV DC isolator switches and the MC4 inline fuses are in the Solar Connectors category, and the dual battery isolators are in Battery Isolators. If you are not sure what you need, tell us the battery voltage, the inverter size and what the array is, and we will advise you what to put where.