MCB(Miniature Circuit Breaker): Construction,Working (Operation),Use, Tripping Curves and Types


MCB(Miniature Circuit Breaker)
MCB(Miniature Circuit Breaker)
Credit: ABB

See above picture, have you seen this type of thing in your home ??? This is MCB .. in the above picture, four nos MCB are shown.

MCB means Miniature Circuit Breaker .. Miniature dictionary means small form of some big item ..

So what a MCB does? Why u need MCB?

See the below picture where electricity reaches to lights through a MCB.


Overload

Under normal conditions, lights take only 10 amp but due to some problems, lights take 15 amp more than rated current.

This is overload of lighting circuit and MCB disconnects (or trips ) the lighting circuit, thus saving failure. lights

Now consider below picture in which wire between light and MCB becomes short or ground or develop a shot circuit than a heavy current will flow through the MCB and MCB trips immediately ..thus MCB saves wiring or circuit to complete distrcution or develop a fire.


Shot Circuit

So a MCB provide protection against overload and short circuit. So because of this , it is being used.

Construction of MCB:

See below picture in which cross section (inside view) of MCB is shown.

MCB Construction
MCB Construction

Types of MCB:

Based on the poles

  • Single Pole MCB (SP)
  • Single Pole + Neural MCB(SPN)
  • Double Pole MCB(DP)
  • Tripple/Three Pole MCB(TP)
  • Three Pole MCB +Neutral (TPN)
  • Four Pole MCB(FP)
Single Pole MCB and its symbol
Single Pole MCB with Neutral
Double Pole MCB
Three Pole MCB
Three pole MCB with Neutral (TPN)
Four Pole MCB

Difference between Three pole +Neutral MCB and Four Pole MCB : both types of MCB looks similar but in four pole MCB, protection is provided for three phases and neutral while in case of three pole + Neutral(TPN), protection is provided only in three phases, not for neutral.

Types of MCB based on Tripping Curves

Based on tripping curves or tripping characteristics, MCB are divided into following types.

  • B Curve MCB
  • C Curve MCB
  • D Curve MCB
  • K Curve MCB
  • Z Curve MCB

Confused?? What is tripping curve and what is difference in between B, C & D and Z, K type curves MCBs.

B Type MCB:

See below picture .

Tripping Curve of B type MCB

So what you see above ..its a graph ..between circuit current (In) and time(t) in seconds ..

at various values of circuit current i.e. 0.5, 1, 2, 3, 5,7 ,10 times of the circuit current, MCB behaviour is checked and time taken by MCB to trip is noted .

and a graph is drawn between current flowing through circuit or MCB and tripping time of MCB. This is known as tripping curves or tripping characteristics of MCB.

Now look at the tripping curves once again

And look how much MCB takes time to trip when 2 times of circuit current is flowing ..whats the time from graph ..its 15 seconds ..Right ?

Now check what is MCB Tripping time when 3 times current is flowing ?

Now what you see on graph ? Time is 0.03 seconds

Why so much difference ..

  • MCB tripping time at 2 times current = 15 seconds
  • MCB tripping time at 3 times current= 0.03 seconds

Because B type MCB see 2 times current as thermal overload and see 3 times current as short circuit.

C Type MCB:

C type MCB tripping curve

Now see the curves of C type MCB

It looks similar to B type MCB curve at one instance ..but look close and compare both the curves

What you have seen ?

B type MCB trips on 3 times rated current in 0.03 seconds while C type MCB takes 2 seconds to trip on 3 times of current.

C type MCB trips in 0.03 seconds on 5 times of circuit current.

So this is difference ..C type MCB consider short-circuit from 5 times of circuit current while B type consider short circuit from 3 times of current.

D type MCB:

In D type MCB, short circuit is given from 10 times of rated current.

So what you learn

  • B Type MCB see short Circuit or gives instant tripping from 3 times of rated current
  • C type gives instant tripping on and over 5 times of rated current.
  • D type gives instant tripping on and over 10 times of rated current

But in pratice, it is not so B type MCB designed and available in the market with instant tripping @ 3-5 times of rated current.

Means some B type MCB instant tripping can start from 3 or for some MCB, from 4 or for some MCB, its from 5 so each B type MCB tripping curve is not same.

Similarly, C type MCB have tripping range between 5-10 times rated current.

And D type have range of 10-20 times rated current.

Tripping curves showing instant or short circuit tripping range of B, C and D type MCBs, shown below.

B type MCB curve

As can be seen from the above picture, B type MCB instant tripping may be start from 3 to 5 times of rated current.

C type MCB curve

As can be seen from the above picture, C type MCB instant tripping may be start from 5 to 10 times of rated current.

D type MCB curve

As can be seen from the above picture, D type MCB instant tripping may be start from 10 to 20 times of rated current .

K Type MCB

K type MCB has instant tripping range from 10 to 14 times of rated current.

Z Type MCB

Z type MCB has instant tripping range from 2 to 3 times of rated current.

In the below figure, tripping characteristics of all types of MCBs are superimposed.

Combined Tripping curve of all MCBs

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