Can I use an AC MCB for DC application. Voltage levels in both the cases are same. If no, what are the reasons that I need to go for separate MCBs for AC & DC applications

 

I would check directly with the manufacture of the MCB or refer to the specification sheet. Usually the main issue will be with the current ratings (deratings) of the devcie in a DC application vs a an AC application.

Just as you can use a suitable rated relay for ac or dc loads, there will be some derating of the current capacity.

 

Arc quenching is much more problematic with DC - and consequently, contact wear

 

Responding to M. Ramkrishna's Mon, Jul 12, 2:16pm query:

While MCBs are available for AC and DC, their being available for both AC and DC is unusual. The exception is for very special cases. Typically, such a unit will have the AC-rated contacts within the molded thermo-setting plastic enclosure, while the DC-rated contacts are mounted externally (in air) to the case.

Caution!!!
If intended for us in the USA then the MCB must be UL-rated (or the like) for dual-voltage service.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

In my experience the difference is in the arc breaking during fault currents. when a large dc current needs to be broken, the arc just seems to travel thru mid air for much greater distance than ac would at same current. I have experienced this with 48vdc control systems for HV substations, only low voltage but high currents available. My advice, use DC rated MCB.

regards Ralph

 

AC MCBs can be used in much lower DC voltage circuits, say not more than one fourth of its AC voltage rating. A 415V MCB might work with 110V DC circuit. Reason being AC reaches zero twice at every cycle and this helps to quench the arc. Whereas the DC voltage helps the arc to sustain and it would damage the MCB. The arrangement of contacts also produces magnetic field repelling the arc towards chute, lengthen the arc and make it weaker. The air gap between the contacts are so arranged that it requires higher voltage for the arc to restrike.

Schneider Electric & ABB introduced permanent magnets in it's arc quenching arrangement in their MCBs for DC Application.