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October 17, 2019

MCB for AC | How to connect and choose right MCB for AC and DC

What is MCB?

MCB is used as a short term for electrical engineering term Miniature Circuit Breaker (MCB). Miniature circuit breakers are used in different appliances like in the air conditioner as a protection circuit against the short circuit and the overload conditions. The short circuit or the overload in a circuit can occur due to a number of reasons like faulty connections or overcurrent scenario. A miniature circuit breaker is the same as the fuse. The only difference is that when a fuse is fused it needs to be replaced whereas when a MCB is overloaded it can be easily reset with a click or push of a button. 

MCB used for AC

There is number of MCB’s available in the market which can be used with the AC. But the right decision for the selection of an MCB should be made after reading the ratings for a specific air conditioner. Each AC has its own ratings in which it operates the selection of the MCB should be made on the basis of those ratings. For example, if there is an AC with capacity of 1 Ton. The MCB should be selected of 16 Amps type C MCB, this rating for the MCB is selected because the AC takes 1.5 times greater current at the time of starting as compared to the full load current. 

How to connect MCB?

There are markings on the MCBs to be connected to the AC or other appliances. The manufacturer marks the label on the terminals for the right connections with the appliances. If the terminals are not marked by the manufacturer then the connection is made on the general rules related to connections. If the terminal is marked with a label of input or line then the supply is to be attached to that end and the load is to be attached to the mentioned load terminal. If the MCB is attached to the AC in the wrong way then the MCB may not give protection from the fault conditions. If the MCB being used is a DC MCB then there are signs on the terminals of the MCB. The connections are made easier if the positive and negative signs are indicated on the terminals of the MCB. In the case of the DC MCB if the connections are wired incorrectly then there is a huge possibility of the burning of the MCB.

Difference between AC MCB and DC MCB

If the MCB is looked for the difference between AC MCB and DC MCB by only physical parameters than the AC MCB has labels of the terminals as LOAD and LINE terminals whereas the DC MCB will have positive (+) or negative (-) sign on its terminal. If the properties of the AC MCB and DC MCB are looked for the differences among them than the only noticeable difference is the breaking of AC MCB and DC MCB. It is harder to break a DC MCB as compared to an AC MCB. AC MCB is extinguished with the help of zero crossings but a DC MCB is extinguished with the help of a mechanical interruption or with cooling. DC MCB’s open faster as compared to AC MCB’s.

Can AC MCB use for DC?

The AC current or signal is continuously changing its value for each second. The voltage of the AC signal changes its polarity from positive to negative in every second of a minute. The MCB will break at 0 Volts and the arc will be extinguished and as a result, the wiring will be protected from a huge amount of current. Whereas the DC signal is not alternating, it flows in a constant state and the value for the voltage is only changed when the circuit is turned OFF or the circuit input is decreased by some value otherwise the DC circuit will provide a constant value of voltage for each second of a minute. So, as there is no 0 Volt point in a DC state so the AC MCB will not work with DC states.

How to choose the right ampere MCB?

First, calculate the load (Watt) of all appliances using this formula:
Power (W) = Voltage (V) x Current (I) x Power Factor (PF)

Current (I) =                    Watt (W)                .
                      Voltage (V) x Power Factor (PF)

Take an example, if the calculated current is 11A, the suitable MCB rating to choose is 16A. 

Type of MCBs

MCBs are classified by 3 types of tripping curves. They are:

  • Type B (trips between 3 and 5 times of full load current). This mainly used in residential or light commercial building applications where the surge current are relatively low.
  • Type C (trips between 5 and 10 times of full load current). This mainly used in industrial, machines and induction motors.
  • Type D (trips between 10 and 20 times of full load current). This mainly used in heavy industries where the current inrush can be very high. E.g winding motors.

LINE / LOAD Terminal Marking on MCB (Polarized MCB)

For AC Circuit:

If The manufacturers indicate the Input (LINE) marking on the MCB, then you have to put the supply at the “LINE” terminal and the Load at the “LOAD” terminal for the MCB to operate perfectly.

If you make a wrong connection, then the MCB will most probably not provide the intended protection for the circuit when the need arises.

If the MCB has been in a cold state or a long time, i.e., has not been put on for a long time, then the MCB may lose its functionality and will fail to deliver when required.

In the Miniature Circuit Breaker, the fixed contact is encompassed in the arc chute, and the arc products are then de-ionized, cooled, and ejected easily when the incoming power is on the “LINE” terminal i.e. when the fixed contact is “hot.” There will be less chance to restrike the arc again.

If power is applied to the moving contact, (the “Load” terminal), the flexible connector, the trip system, and all other systems will be live/hot after the arc is put off. Here, the chances of restrike/ flashovers are much higher.

For DC Circuit:

The polarized DC MCB has a marking of ‘+’ and ‘-‘symbol

If the Polarized DC miniature circuit breaker is connected or wired incorrectly, there are possibilities of problems occurring, and in case of overload or short circuit, the MCB will not be able to cut the current and put out the arc, this may lead to the breaker burning out.

Polarized DC miniature circuit breakers make use of small magnets to pull the arc away from the contacts into the arc disrupter cage. If the direction of the current flowing through the unit is reversed, then the magnets will direct the arc away from the arc disrupter cage and into the MCB mechanism thereby destroying the device.

Non-Polarized MCB

FOR AC Circuit:

If there is no particular indication by the manufacturer on any terminal marking, then we can connect the line/ load to any side we want to.

The performance of single-break MCBs is a little bit different when the “LINE” and “LOAD” feed either from the top or bottom so the IEC 60947-2 test specifies that one additional SC test is carried out with the needed connections when the terminals are not marked “LINE” or “LOAD.”

If the terminals are not marked, then you can connect the Line or Load to either the Top or Bottom of the MCB. But it’s always better to keep the fixed contact side connected to the bus bar.

For DC Circuit:

The Non-polarized DC miniature circuit breaker does not have a ‘+’ and ‘-‘ symbol marking.

These MCBs operate safely as load-breaking isolators and for protecting the circuit in cases of fault currents regardless of the direction of the current flow in the circuit.

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