MCB breaker – short circuit current & Let through energy class

MCB breaker – short circuit current & Let through energy class


energy class, let through energy, and short circuit current capacity of the MCB, today’s topic is, energy class and let through energy of the MCB, you will find a small symbol here on the MCB, some thing like this, I have shown same symbol in bigger size, I will explain about this, there are 2 things are written here, one is 10000, and one is 3, 10000 is Icn, rated short circuit breaking capacity, and the 3 is, energy limiting class, or I square t, I have shown a simple circuit here, this is AC supply, this is MCB circuit breaker, this is load, this supply comes, from the generating station, there will be long wires, now, suppose; there is a short circuit here, so I will create a short circuit here, now what happens?, between this generator, and this load or short circuit point, there will be long wires, there will be transformer, they will have some resistance & inductance, so, I will show that R & L here, this is R and L, what is this?, this is R & L of these wires, and of the transformer or inductor, or whatever is here, now, current flowing here, is shown here with dotted line, this current is limited, by the R & L of this whole system, this current is called, the prospective fault current, this is peak, RMS value may be some where here, this 10000 amps, is not this current, this is fault breaking capacity, of the breaker, this current, here, can be 1000 amps. can be 2000, can be 5000, can be 10000 amps, depending on the value of R & L, and voltage here, this MCB can break up to 10000 amps. because this has, 10000 A breaking capacity, if this current, or RMS value of this current, becomes more than 10000 amps, then this breaker may fail or, will not be able to open, so this the limitation, of the current breaking capacity, of this MCB, now this plot was, I vs t, this is I square vs t, negative half cycle, is not shown here, and this area is I square t, this is I square, and this is time, if you multiply, these 2, I square & time, you will get I square t, this area, now, if short circuit is here, this MCB will open, so I will open it, so this current, was the current flowing here, if MCB was not there, or it does not open, but what happens? as soon as current increases, this MCB will sense the current, and will start opening, it does not open immediately, it takes some time, as soon as it starts opening, this current will start reducing, so the current, which was going like this, will go like this, and current becomes zero after some time, when MCB becomes fully open, current flowing without MCB, will be like this, but with MCB, current will become like this, so this current, is coming down, in the system this was the I square, if MCB does not trip, or there is no MCB, but if MCB here, then it will open, so current will reduce from this, to this, if we take I square of this, we will get like this, this is the I square without MCB, this is I square with MCB, I square is reduces, current is also reduced due to MCB, It does not becomes zero, this area=I square * t this area is called, the let through energy, this is not actual energy, I will explain further, it is I square t actually, so without MCB, I square t will be this area, with MCB, I square t will be this, this 3, gives the information, about this I square t, this area should be less as possible, class 3 defined based on, how much I square t is allowed by MCB, now why I square, I told I sqaure so many times, this is AC supply, this is short circuit, this is R & L of the wire and transformer, heavy current is flowing here, here there will will be loss, power loss here=I square R, this I square R multiply by time, you get the energy, energy=I square * R if t, you bring this side, then this becomes I square t * R if R is fixed, then energy will be proportional to I square t, now If heavy current flows here, there will be losses here, thermal energy is proportional to I square rms, then again. we write I square, as we are discussing short circuit current, we are concentrating on current I, if current I becomes double here, then thermal energy will become 4 times, If there are 2 wires, and current is flowing in them, they will have force of attraction or repulsion, depending on the direction of current, that is magnetic force, that is proportional to I square peak, so square of the current flowing here, is very important, for thermal energy, and also for magnetic force now we will discuss again, about energy limit class 3, there are some examples given here, permissible let through energy=I square t, suppose we have a MCB, has a 10000 amps, short circuit breaking capacity, and class indicated is 3, MCB type is C, rated for 32 amps, then I square t=110000 amp square secs, this means, this MCB, will allow this much I square t before tripping, similarly for another MCB type C, with 10 amp rating, I square t=80000 amp. square secs., this is as per standard for class 3, so, class 3 MCB, of type C and 10 amp rating will allow, I square t=80000 amp square secs, before tripping, this value should be, less as much as possible, now, this is another example, of class 2 MCB, Class 3 MCB allowed, 110000 amp square secs. same C32 MCB, but of class 2, will allow 370000 amp square secs, this is more, this value should be less ,if possible, so this one is better, this is bad, this is as per standards, if you want exact value for MCB, you have to refer the catalog of the mcb, and find the value written there, lower the value of let through energy, better the quality of the MCB, if very high let through energy is there, then in case of short circuit, very heavy current will flow, during short circuit, this is not a normal operation, now advantages of current limiting, I told you, that this should be low as possible, 1st advantage is lower temperature, 2nd is lower mechanical force, when short circuit happens, high current flows, so there will be a sudden force, in many equipment, if current I is less, I square t will be less, then lower mechanical force will be there, now 3rd advantage, if very high current flows, then high flux will be there ( around wires), so electromagnetic wave will be generated, that will create noise, if i square t is less, then noise will be less, that will help to reduce noise in, measuring instruments, electronic control circuits, signal transmission, and communication system, as all these will be effected by noise, so MCB indirectly helps, in reducing the noise, today we will close now,

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