What is the Main Difference from AC MCB and DC MCB?

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Our guide will let you discover all the vital things about it, its purpose, and its various important applications.

You will also take a short journey back in time to learn about a piece of history and meet the amazing inventors responsible for the DC MCB’s development.

In addition, you will be provided with significant information about its advantages, and how it is compared to AC MCB and fuses.

1.Who is responsible for the development of the DC MCB?

Edison is most famous for his incandescent light bulb invention as well as the motion picture camera and the phonograph.

And in 1879, he thought of creating a solution for short circuits was a rampant problem during that time and he called it the circuit breaker.

The fast expansion of electrical networks provided electricity to many streets in large cities.

However, this electrical revolution has its own problems like short circuits or power overloading.

This problem permanently damages the filament inside the bulb of the street lights and when this happens, the bulbs need to be replaced and sometimes more work has to be done a the result of the damage.

The constant replacing of bulbs and paying for repair services were expensive.

This is why there is an immediate need to find a practical and inexpensive solution.

Edison has thought about a couple of ways to alleviate short circuits and these are:

  • Use a wired fuse that will self destruct when power surges happen.
  • Build a mechanical device that will automatically spring open the moment that it detects overloading then can be reset manually.

Edison made a lot of sketches of these methods which he kept in his scientific journals.

After successfully developing his concepts, he immediately patented them.

However, Edison decided on using the first method instead which is the use of fuse to deal with the short circuit problem.

In 1876, Edison’s industrial research lab was established where he continued to create and improve his inventions.

Then in 1898, the Boston Electric Light Company installed the first circuit breaker in the L-street station.

This circuit breaker has an oil tank and upward breaking contacts that can be manually activated.

When the arc forms between the two open contacts, the oil diminishes the heat generated.

It featured upward breaking contacts that are manually activated and an oil tank.

The automatic circuit breaker was developed by Granville Woods, an African-American inventor, in 1900.

He is known for his contributions to the railroad industry such as the technology that enabled the train station to directly communicate to the conductor of the train.

Then in 1904, circuit breakers were first mass produced by the Cutter Manufacturing Company in Philadelphia.

These circuit breakers were named the ITE breaker which stands for Inverse Time Element.

In the following years, progress in technology was contributing to the development of the DC MCB that we know today.

Boston Electric Light Company and the oil circuit breaker

2.How did the DC MCB evolve?

The following enlists the significant eras that briefly detail the development of the predecessors of the DC MCBs.

  • 1960’s
  1. Air circuit breakers

Air has been always used as an insulator in a switching device present in all ranges of voltage from miniature circuit breakers at low voltage to transmission voltages of air-blast circuit breakers rated at 800kV.

The term air circuit breaker is used to define the 3 phase circuit breaker where the insulators used are the air and atmospheric pressure.

Switches in the early times are only able to do a plain break of the forming arc.

The arc is stretched between the moving contact and the stationary contact and this means that there’s no arc control.

Furthermore, arcing times are long and there are limited current and voltage ratings.

Because of this, there were developments of different arc control devices and probably the best among them all is the arc chute.

The arc chute is composed of several plain metal plates that are organized in right angles to the arc chute’s length.

There are spacers in between the metal plates that cause the arc to separate into smaller arcs.

Because of this, the arc’s heat is extracted, and eventually, it diminishes.

  • 1970’s
  1. Vacuum circuit breakers

The development of an interrupter that is capable of more fault detection and interruption resulted in the creation of vacuum interrupters.

However, the vacuum interrupter is sealed in a bottle for life and cannot be maintained.

The high vacuum’s dielectric strength is such that only 6mm contact travel is used at a 12kV rated voltage.

This made manufacturers halt any further developments in medium voltage air circuit breaker technology.

Vacuum interrupters were more expensive when compared to oil or air interrupters and this could be one of the reasons why they are not widely used even today even at low voltage.

Manufacturers designed maintenance free vacuum interrupters to offset its expensive cost.

This reduced its complexity and its size as a week.

But the operating mechanisms will still require regular lubrication, random checks, and resetting.

  1. Rotating arc SF6

The rotating arc interrupter is also an affordable device comparable to air arc control mechanisms but with similar satisfactory performance as the vacuum interrupters.

SF6 is a type of nontoxic gas that has high insulating properties.

During arcing, the SF6 decomposes and immediately recombines once the current is absent and returns to its dielectric properties.

During contact separation, the arc current is transported to a solenoid which is a copper strip turned several times.

This solenoid generates a magnetic field which allows the arc to go around its inner ring.

Because of the arc’s rapid movement through the SF6, the gas allows it to cool and it diminishes the moment of the first current zero even at high fault currents.

  • 1980’s

After the last invention of interrupting devices, there are no new models that were invented.

But instead, further improvements were done to the current technologies.

Vacuum interrupters’ size was decreased and has become less expensive as well.

Almost all the new circuit breakers that were developed during this time have withdrawable designs.

A motor wound spring mechanisms were used on all medium and low voltage circuit breakers to decrease the size of charges and substation batteries.

These were the power source of the circuit breaker for it to open or to trip.

In place of the earlier solenoid, this time an electric motor charges the spring with a few amps for a few seconds.

A smaller solenoid releases a latch that releases the stored spring energy.

When the spring relaxes, it controls the mechanisms, the contact pressure is compressed, spring is opened, which connects the movable and fixed contacts of the interrupter.

On the other hand, the substation battery, despite having reduced its size, has become more complex in design.

It was possible due to the use of electronic protection relays that its mechanisms were reduced in size; however, this also created some circuit breaker problems as claimed by some manufacturers.

  • 1990’s

During this decade, vacuum circuit breakers have emerged.

And by far, it was the most preferred technology for medium voltage primary substations.

This is due to the fact that it demonstrates almost zero failure rates plus it has become affordable.

The concept of fixed circuit breakers made a comeback with the emergence of air insulated SF6 circuit breakers and SF6 insulated vacuum circuit breakers

They are housed in a low voltage compartment with conventional mechanisms.

When the GVR pole mounted auto-recloser was introduced, it signaled the advancement in circuit breakers and proved to be as reliable as the interrupter.

It features a magnetic actuator mechanism plus other fundamental innovations which were the basis of new medium voltage circuit breakers developments.

  • 2000’s

Circuit breakers will continue to evolve as new designs are always made from scratch.

Most designs aim to provide a simple and dependable magnetic actuator mechanism that is trouble free just like the latest interrupter technology.

New concepts are being applied to low and medium voltage circuit breakers, more compact than the last, replacing old technology with better and reliable DC MCBs.

3.What is the Main Difference from AC MCB and DC MCB?

AC MCB and DC MCBs have the same purpose, they are made to protect the circuit and any electrical items connected to them.

However, they may work differently in some aspects, one cannot be used in replacement of the other as it may not operate properly and could cause more problems.

The main difference between the two is their de-energizing abilities, particularly arc formation and their process of extinguishing the arc.

Even when both have the same ratings, they do not have the same ability to turn on and off the circuit.

Using the AC MCB for direct current or DC MCB for alternate current could lead to cross level misoperation.

At every cycle, AC MCB has zero crossing point which makes it easier to extinguish the arc.

While DC MCB does not have it and this makes it difficult to extinguish the arc.

Therefore, it is required to install an arc extinguishing device.

This explains why the AC and DC MCBs are not interchangeable.

There are a few points to think about concerning this:

  • DC MCB has two main contacts while AC MCB has three.
  • The arc extinguishing device of DC MCB is usually a magnetic blowing type while for AC MCB; it is usually the grid type.
  • An AC open should not be used carelessly for DC operations, otherwise, it is most likely to produce short circuits with larger sparks.

Hypothetically, because AC and DC MCBs have different characteristics, they cannot be used in exchange for the other.

However, there are some special conditions where one can be used in replacement for the other temporarily but never permanently.

4. What is the difference between the DC MCBs and fuses?

Whether residential or business establishments, as long as they are using electricity, they need a circuit breaker installed.

Even when you are already using fuses, you still need DC MCBs installed as they are sometimes required by the regulations in your area.

Using DC MCBs not only protects the circuit but also protects your fuses from constantly blowing up.

Blown fuses

DC MCBs and fuses are the same when it comes to:

  • Their purpose is both created to protect the circuits from current irregularities which can cause damage.
  • Their installations since both are generally installed inside a metal box.

DC MCBs and fuses are different when it comes to:

  • Resetting as both can be reset, but blown fuses need to be replaced.
  • Both can provide the same protection for the circuits but each work differently:
    • To protect the circuit, fuses are blown out after accumulating heat that reaches the melting point hence circuits are protected from overloading.
    • DC MCBs depends on the magnetic protection device for short circuit while thermal protection device for overloading protection.
  • Replacing blown up fuses tend to be expensive in the long run while DC MCB is a more practical choice.


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