Difference Between Making And Breaking Capacity Of Circuit Breaker

Well here is one answer as no one seems to know or care. (just bating the hook)

When a circuit breaker closes an arc exists for a few milli seconds that as the contacts slam closed is queched the mechanical action is very fast because it is set up that way.

When contacts are made to open an arc is drawn and will persist until the gap is larger than the flow of current can maintain. If a large inductive load is being switched out it can cause a much bigger and, longer arc than say a pure resistive load would produce. Special breakers may be needed such as air blast types where a blast of high presure air is directed to blow out the arc.

Some use special oil to quench the spark. So the breaker must suit the load to be protected. I am sure some one will fill in any gaps for me.

In high voltage breakers, pre-insertion resistors may also be employed to reduce the duty on the contacts.

On opening, an arc may be drawn until the arc length exceeds that which the available system voltage can sustain and the arc extinguishes at the next voltage zero.

When starting a motor direct-on-line, the breaker has to withstand about 5 times the full load current for a short period (as a rule-of-thumb) while the motor comes up to speed.

When stopping a motor, the current will ordinarily be less than the full load current at the moment of breaking, though there may be some tenedency to arc as a result of the inductance of the motor and its cables.

In an overload the breaker has to disconnect more than the full load current.

So the breaker has to be selected that will survive the above repeatedly and reliably, which is why manufacturers give this information.

Do you mean automatic circuit breakers? Usually, a breaker needes to be manually reset.

If you mean automatic breakers, the breaking capacity is what the breaker is rated at. For example, a 15 amp breaker will trip very close to its' 15 amp capacity; but in the case of a inductive load, this will be delayed by a few seconds unless it reaches its' high start rating. In this example, probably 25 amp max for 3 seconds or 16 amps constant for 3 seconds.

The making capacity is a leaking current. A very small current (usually 10 milliamps) will remain on the line after the breaker has tripped. As long as the small current is flowing, the breaker thinks that the overload condition still exisis. If the current disappears, the breaker thinks that the overload has been fixed and will reset itself. IF YOU EVER WORK ON THIS TYPE OF CIRCUIT, PLEASE LOCK DOWN (OR OUT) THE BREAKER BEFORE WORKING ON THE DEVICE!!!

Yes ther is a difference because the device is doing wto seperate functions.

On making a circuit, it is closing two points with high potential. no current is flowing yet hence it needs the force to make a quick and sure contact. In addition this action ensures that the breaker does stay closed ifconditions are normal, i.e no overload or short circuit existing.

The break, imvolves opening on a circuit that may have current flowing at that time of breaking. As the circuit is opened an electric arc with significantt electrical and mecahnical properties, is created. The arc resists the opening of the contacts and hence tries to pull the contact back together, note that a lot of energy is dissipated in this action. So the breaker must have the force to pull the contacts apart and the mechanisms and medium to quench the arc. Today at low voltage that mechanism involves arc chutes and air. In higher voltages we begin to see SF6 gas, vacuum bottles and at even higher voltages SF6 Gas is the predominant medium. All these in addition do have arc chute quenchers in one form or the other.

Hope this answers your questions.

How to calculate breaking & making current by considering transformer capacity & system voltage?

making current

i.e you are asking the current cap of mccb IL

EXAMP = DG SET OF 62.5 KVA ,

IL OF MCCB = 1.5 X 62.5 KVA =93.75 AMPS APPROX 100 AMP OF MCCB IS REQUIRED

NOW BREAKING CAP OF 100 AMP MCCB = [ ( IL / Z% ) X 1/1000 ]KA

Mr.Gupta,

What is Z?

Please Explain

Mr Gupta will u explain Z?

Z=impedance voltage