Rated short-circuit making capacity (Icm)
The rated short-circuit making capacity of a circuit-breaker is the value of short-circuit capacity assigned to that circuit-breaker by the manufacturer for the rated operational making voltage at rated frequency and at a specified power factor for A.C., or time constant for D.C. It is expressed as the maximum prospective peak current. For a c the rated short-circuit making capacity of a circuit-breaker shall be not less than its rated ultimate short-circuit breaking capacity, multiplied by the factor n of table as below .
For d c the rated short-circuit making capacity of a circuit-breaker shall be not less than its rated ultimate short-circuit breaking capacity. A rated short-circuit making capacity implies that the circuit-breaker shall be able to make the current corresponding to that rated capacity at the appropriate applied voltage related to the rated operational voltage.
Rated short-circuit breaking capacities
In case of circuit breakers rated Short-Circuit capacity is the values of short-circuit breaking capacity of that circuit-breaker assigned by the manufacturer for the rated operational voltage, under specified conditions.
A rated short-circuit breaking capacity requires that the circuit-breaker shall be able to break any value of short-circuit current up to and including the value corresponding to the rated capacity at a power-frequency recovery voltage corresponding to the prescribed test voltage values and:
a) For alternating current at any power factor not less than as per table below;
b) For direct current, with any time constant not greater than as per table given above.
For power-frequency recovery voltages in excess of the prescribed test voltage values , no short-circuit breaking capacity is guaranteed
Now what are Power frequency recovery voltages?
All tests shall be made at the rated frequency of the circuit-breaker. For all short-circuit tests, if the rated breaking capacity is essentially dependent on the value of the frequency, the tolerance shall not exceed +-5%. If the manufacturer declares the rated breaking capacity to be substantially unaffected by the
value of the frequency, the tolerance shall not exceed +-25 %.
For alternating current the circuit-breaker shall be capable of breaking a prospective current corresponding to its rated short-circuit breaking capacity and the related power factor given in table above, irrespective of the value of the inherent D.C. component, on the assumption that the AC, component is constant.
(a) Rated service short-circuit breaking capacity of a circuit-breaker
A breaking capacity for which the prescribed conditions according to a specified test sequence include the capability of the circuit-breaker to carry its rated current continuously
(b) Ultimate short circuit breaking capacity of circuit breaker:-
A breaking capacity for which the prescribed conditions according to a specified test sequence do not include the capability of the circuit-breaker to carry its rated current continuously
The rated short-circuit breaking capacities are stated as
(i) Rated ultimate short-circuit breaking capacity;
(ii) Rated service short-circuit breaking capacity.
(i) Rated ultimate short-circuit breaking capacity (Icu)
The rated ultimate short-circuit breaking capacity of a circuit-breaker is the value of ultimate short-circuit breaking capacity assigned to that circuit-breaker by the manufacturer for the corresponding rated operational voltage. It is expressed as the value of the prospective breaking current, in kA (RMS. value of the a.c. component in the case of a,c. ),
(ii) Rated service short-circuit breaking capacity (Ics.)
The rated service short-circuit breaking capacity of a circuit-breaker is the value of service short-circuit breaking capacity assigned to that circuit-breaker by the manufacturer for the corresponding rated operational voltage. It is expressed as a value of prospective breaking current, in kA, corresponding to one of the specified percentages of the rated ultimate short-circuit breaking capacity, in accordance with table as given below, and rounded up to the nearest whole number. It may be expressed as a % of Icu (for example Ics = 25 % Icu).
Alternatively, when the rated service short-circuit breaking capacity is equal to the rated short-time withstand current , it may be stated as that value, in kA, provided that it is not less than the relevant minimum value of table as below.
Where Icu exceeds 200 kA for utilization category A, or 1000 kA for utilization category B, the manufacturer may declare a value Ics of 50 kA.
Standard relationship between short-circuit making and breaking capacities and related power factor, for a.c. circuit-breakers
The standard relationship between short-circuit breaking capacity and short-circuit making capacity is as given in table below.
The rated short-circuit making and breaking capacities are only valid when the circuit-breaker is operated in accordance with the requirements as given below in Operating conditions of circuit breakers. For special requirements, the manufacturer may assign a value of rated short-circuit making capacity higher than that required by table above. Tests to verify these rated values shall be the subject of agreement between manufacturer and user.
Rated short-time withstand current (Icw)
The rated short-time withstand current of a circuit-breaker is the value of short-time withstand
current assigned to that circuit-breaker by the manufacturer under the test conditions
For A.C., the value of this current is the R.M.S value of the A.C. component of the prospective short-circuit current, assumed constant during the short-time delay.
The short-time delay associated with the rated short-time withstand current shall be at least 0.05 s, preferred values being as follows:
0.05 –0.1 –0.25–0.5–1 S
The rated short-time withstand current shall be not less than the appropriate values shown in table above
Operating Conditions of Circuit breakers
For a circuit-breaker to be closed safely on to the making current as a neutral pole, then all corresponding to its rated short-circuit making capacity, it is essential that it should be operated with the same speed and the same firmness as during the type test for proving the short-circuit making capacity.
(i) Dependent manual closing
For a circuit-breaker having a dependent manual closing mechanism, it is not possible to assign a short- circuit making capacity rating irrespective of the conditions of mechanical operation.
Such a circuit-breaker should not be used in circuits having a prospective peak making current exceeding 10 kA. However, this does not apply in the case of a circuit-breaker having a dependent manual closing mechanism and incorporating an integral fast-acting opening release which causes the circuit-breaker to break safely, irrespective of the speed and firmness with which it is closed on to prospective peak currents exceeding 10 kA; in this case, a rated short-circuit making capacity can be assigned,
(ii) Independent manual closing
A circuit-breaker having an independent manual closing mechanism can be assigned a short-circuit making capacity rating irrespective of the conditions of mechanical operation.
(iii) Dependent power closing
The power-operated closing mechanism, including intermediate control relays where necessary, shall be capable of securing the closing of the circuit-breaker in any condition between no-load and Its rated making capacity, when the supply voltage, measured during the closing operation, remains between the limits of 110 “A and 85 ‘A of the rated control supply voltage, and, when a c., at the rated frequency.
At 110 % of the rated control supply voltage, the closing operation performed on no-load shall not cause any damage to the circuit-breaker.
At 85 “A of the rated control supply voltage, the closing operation shall be performed when the current established by the circuit-breaker is equal to its rated making capacity within the limits allowed by the operation of its relays or releases and, if a maximum time limit is stated for the closing operation, !n a time not exceeding this maximum time limit.
(iv) Independent power closing
A circuit-breaker having an independent power closing operation can be assigned a rated short-circuit making capacity irrespective of the conditions of power closing. Means for charging the operating mechanism, as well as the closing control components, shall be capable of operating In accordance with the manufacturer’s specification.
(v) Stored energy closing
This type of closing mechanism shall be capable of ensuring closing of the circuit-breaker in any condition between no-load and its rated making capacity. When the stored energy is retained within the circuit-breaker, a device shall be provided which Indicates when the storing mechanism is fully charged, Means for charging the operating mechanism, as well as the closing control components, shall be capable of operating when the auxiliary supply voltage is between 85%. and 110 % of the rated control supply voltage. It shall not be possible for the moving contacts to move from the open position unless the charge is sufficient for satisfactory completion of the closing operation. When the energy storing mechanism is manually operated, the direction of operation shall be Indicated This last requirement does not apply to circuit-breakers with an independent manual closing operation.
Circuit-breakers which open automatically shall be trip-free and, unless otherwise agreed between manufacturer and user, shall have their energy for the tripping operation stored prior
to the completion of the closing operation,
(ii) Opening by over-current releases
Opening under short-circuit conditions The short-circuit release shall cause tripping of the circuit-breaker with an accuracy of +-20% of the tripping current value of the current setting for all values of the current setting of the short-circuit current release. Where necessary for over-current co-ordination , the manufacturer shall provide Information (usually curves) showing
àmaximum cut-off (let-through) peak current as a function of prospective current (r, m.s. symmetrical); à /2t characteristics for circuit-breakers of utilization category A and, if applicable, B for circuit-breakers with instantaneous override.
Conformity with this information may be checked during the relevant type tests in test sequences II and Ill
(iii) Opening under overload conditions
1) Instantaneous or definite time-delay operation
The release shall cause tripping of the circuit-breaker with an accuracy of+-10% of the tripping current value of the current setting for all values of current setting of the overload release.
2) Inverse time-delay operation
At the reference temperature and at 1.05 times the current setting, i.e. with the conventional non-tripping current, the opening release being energized on all phase poles, tripping shall not occur in less than the conventional time from the cold state, i.e. with the circuit-breaker at the reference temperature. Moreover, when at the end of the conventional time the value of current is immediately raised to 1.30 times the current setting, i.e. with the conventional tripping current, tripping shall then occur in less than the conventional time later.
If a release is declared by the manufacturer as substantially independent of ambient temperature, the current values of table 6 shall apply within the temperature band declared by the manufacturer, within a tolerance of 0.3%/K.
The width of the temperature band shall be at least 10K on either side of reference temperature.