Can We Use AC MCB On DC Circuit or In place of DC MCB?

Can we use AC MCB on DC Circuit?

For Understanding, what inside an AC MCB? one thermal-based overload trip mechanism and another one are a magnetic-based trip mechanism for short circuit trips, and these two contributing to tripping.

Does Overload trip of AC MCB Works DC circuits:

Yes, overload tripping is nothing but a bimetallic strip and is designed to withstand (No physical state change) underflow of normal rated current. when the current flow is higher than the rated current, the bimetallic strip melts and it releases the tripping mechanism and the circuit will be open circuit.

The same instant will happen while flowing DC current. In fact, DC current trip the MCB better than AC current. Hence, the overload function works in DC.

But the trip work one or two times only, Look at the live testing video what happens while passing the DC current on AC MCB.

We have given 10 Amps to the 6 Amp AC MCB. But in third trip the MCB was burned out.

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Does Short circuit work on AC MCB for DC current?

No, look at the picture there is a thin copper wound coil and a small piece of the plunger is placed inside the MCB, which is used to trip the circuit on the occasion of short circuit fault. It is the magnetic tripping of the AC MCB.

AC MCB On DC Circuit
AC MCB inside

The functionality is simple, under normal rated current, the coil creates a slight alternating magnetic field but that is not enough to pull the plunger top operate a trip mechanism. But during the fault condition, the coil creates a strong magnetic field and pull the plunger, trip the MCB.

But Look at the image of DC MCB inside view,

DC MCB inside view
DC MCB inside view

But in the DC circuit, the steady magnetic field will be created. Under the fault condition, the MCB coil creates a steady magnetic field and which pulls the plunger, trip the circuit.

Here, functionality both MCB working the same. But the arc quenching technique is different. In AC the arc will be quenched at the zero point, but DC we have to install a strong extinguisher for quenching the arc. Look at the image of DC arc extinguisher.

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DC MCB arc extinguisher
DC MCB arc extinguisher
Now you get another doubt, does earth fault function works on DC?

No, if the AC MCB has earth fault tripping means, that function does not work on DC. Since for earth fault, inside the Current transformer does not generate output for DC current.

Final Word, Do not use AC MCB on DC Circuits.


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