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I want to use a normal household breaker for connecting to a 24V circuit. The input supply is a 230V AC to 24V DC SMPS. The output of the DC converter is connected to a load (rated 180 milliamps @ 24 V) in parallel with a battery bank having two batteries of 7.2 Ah each.

If I use a normal AC MCB, and if there is a short circuit in the above circuit whereby the battery supplies a higher current, will the MCB trip?

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  • \$\begingroup\$ Someone asked something very similar some days ago, I can't find the answer right now, the answer being something like "who knows?". That's rated for AC, in DC it might not work at all. DC circuit breakers should exist, or you can make your own. \$\endgroup\$ Commented Aug 5, 2014 at 7:46

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A household circuit breaker (of relatively low amp rating) can be tripped by the levels of current that the batteries are capable of. However, will that protect what you think it will protect? That is, if a short occurs, will the wiring between the short and the breaker be sufficient to carry the current required to trip the breaker? Or will the wires simply burn? This arises because with a load rated at only 180mA, the wiring to and within the load circuitry is likely to be quite fine.

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Right, AC MCB should not be used for DC application. There are many factors, at first the design of the surge/spark current suppressed material. DC is a continuous level current while AC has a point when the level goes to 'zero' which help the surge current suppression, hence DC breakers are usually more rigid and have a better suppression mechanism.

Other than that AC current has a skin effect on the contactor, while DC current has no such effect. You may see lot of DC breaker manufacturers, one of them I recommend is E-T-A, you may visit their website here: http://www.e-t-a.com/

You may find numerous type of DC breakers depending on your application.

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A standard electromagnetic or bimetallic breaker should work fine on 24V DC. Here's an example which is rated for 6A at both 250V AC and 48V DC.

ETA - 106-P30 6A - CIRCUIT BREAKER, 6A

This breaker has a resistance of 0.05 Ohms or less, so provided your power supply and/or battery can deliver at least 0.6V for 20 seconds at 12A (or 5 seconds at 30A etc.) it will trip reliably. A good 24V 7Ah SLA battery should be able to maintain ~20V for several minutes at 30A.

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